9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001
NIST's Investigation WTC Investigations Critique of NIST's Report
Text of NIST's Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers (DRAFT)




NIST NCSTAR 1 (Draft) 
Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster 
Final Report of the 
National Construction Safety Team 
on the Collapses of the 
World Trade Center Towers (Draft) 
For Public Comment 


NIST NCSTAR 1 (Draft) 
For Public Comment 
Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster 
Final Report of the 
National Construction Safety Team 
on the Collapses of the 
World Trade Center Towers (Draft) 
September 2005 
U.S. Department of Commerce 
Carlos M. Gutierrez, Secretary 
Technology Administration 
Michelle O’Neill, Acting Under Secretary for Technology 
National Institute of Standards and Technology 
Hratch G. Semerjian, Acting Director 

Disclaimer No. 1 
Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a 
procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is 
not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or 
equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or 
negligence by the National Institute of Standards and Technology. 
Disclaimer No. 2 
The policy of NIST is to use the International System of Units (metric units) in all publications. In this document, 
however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline. 
Disclaimer No. 3 
Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain 
evidence received by NIST in the course of this Investigation is “voluntarily provided safety-related information” that is 
“not directly related to the building failure being investigated” and that “disclosure of that information would inhibit the 
voluntary provision of that type of information” (15 USC 7306c). 
In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been 
provided to NIST under nondisclosure agreements. 
Disclaimer No. 4 
NIST takes no position as to whether the design or construction of a WTC building was compliant with any code 
since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the 
properties and condition of the materials used, or changes to the original construction made over the life of the 
buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining 
compliance when implementing building codes. Where an Investigation report states whether a system was 
designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating 
whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the 
requirement was met. 
Use in Legal Proceedings 
No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the 
National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter 
mentioned in such report (15 USC 281a; as amended by P.L. 107-231). 
National Institute of Standards and Technology National Construction Safety Team Act Report 1 (Draft) 
Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1 (Draft), 292 pages (September 2005) 
CODEN: NSPUE2 
U.S. GOVERNMENT PRINTING OFFICE 
WASHINGTON: 2005 
For sale by the Superintendent of Documents, U.S. Government Printing Office 
Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250 
Mail: Stop SSOP, Washington, DC 20402-0001 

NATIONAL CONSTRUCTION SAFETY TEAM FOR THE FEDERAL
BUILDING AND FIRE SAFETY INVESTIGATION OF THE WORLD TRADE 
CENTER DISASTER
S. Shyam Sunder, Sc.D. (NIST), Lead Investigator 
Richard G. Gann, Ph.D. (NIST), Report Editor 
William L. Grosshandler, Ph.D. (NIST), Associate Lead Investigator 
Jason D. Averill (NIST) 
Richard W. Bukowski, P.E. (NIST) 
Stephen A. Cauffman (NIST) 
David D. Evans, Ph.D., P.E. (NIST) 
Frank W. Gayle, Ph.D. (NIST) 
John L. Gross, Ph.D., P.E. (NIST) 
J. Randall Lawson (NIST) 
H. S. Lew, Ph.D., P.E. (NIST) 
Therese P. McAllister, Ph.D., P.E. (NIST) 
Harold E. Nelson, P.E. (Private Sector Expert) 
Fahim Sadek, Ph.D. (NIST) 
NIST NCSTAR 1, WTC Investigation 

National Construction Safety Team Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

CONTRIBUTORS TO THE INVESTIGATION
National Construction Safety Team for the WTC Investigation 
S. Shyam Sunder 
William Grosshandler 
H.S. LewRichard Bukowski 
Fahim Sadek 
Frank Gayle 
Richard Gann 
John Gross 
Therese McAllister 
Jason Averill 
Randy Lawson 
Harold E. Nelson 
Stephen Cauffman 
Lead Investigator 
Associate Lead Investigator; Project Leader, Project 4: Investigation of 
Active Fire Protection Systems 
Co-Project Leader, Project 1: Analysis of Building and Fire Codes and 
Practices 
Co-Project Leader, Project 1: Analysis of Building and Fire 
Codes and Practices 
Project Leader, Project 2: Baseline Structural Performance and Aircraft 
Impact Damage Prediction 
Project Leader, Project 3: Mechanical and Metallurgical Analysis of 
Structural Steel 
Project Leader: Project 5: Reconstruction of Thermal and Tenability 
Environment 
Co-Project Leader, Project 6: Structural Fire Response and Collapse 
Co-Project Leader, Project 6: Structural Fire Response and Collapse 
Project Leader, Project 7: Occupant Behavior, Egress, and Emergency 
Communications 
Project Leader, Project 8: Fire Service Technologies and Guidelines 
Fire Protection Engineering Expert 
Program Manager 
National Construction Safety Team Advisory Committee 
Dr. John Bryan Philip J. DiNenno Dr. Kathleen J. Tierney 
Dr. John M. Barsom Paul M. Fitzgerald Dr. Forman A. Williams 
David S. Collins Dr. Robert D. Hanson 
Glenn P. Corbett Dr. Charles H. Thornton 
Contributing NIST Staff 
Mohsen Altafi Dale Bentz Sandy Clagett 
Robert Anleitner Charles Bouldin Ishmael Conteh 
Elisa Baker Paul Brand Matthew Covin 
Stephen Banovic Lori Brassell Frank Davis 
Howard Baum Kathy Butler David Dayan 
Carlos Beauchamp Nicholas Carino Laurean DeLauter 
NIST NCSTAR 1, WTC Investigation v 

Contributors to the Investigation Draft for Public Comment 
Jonathan Demarest Chris McCowan 
Stuart Dols Jay McElroy 
Michelle Donnelly Kevin McGrattan 
Dat Duthinh Roy McLane 
David Evans George Mulholland 
Richard Fields Lakeshia Murray 
Tim Foecke Kathy Notarianni 
Glenn Forney Joshua Novosel 
William Fritz Long Phan 
Anthony Hamins William Pitts 
Edward Hnetkovsky Thomas Ohlemiller 
Erik Johnsson Victor Ontiveros 
Dave Kelley Richard Peacock 
Mark Kile Max Peltz 
Erica Kuligowski Lisa Petersen 
Jack Lee Rochelle Plummer 
William Luecke Kuldeep Prasad 
Alexander Maranghides Natalia Ramirez 
David McColskey Ronald Rehm 
NIST Experts and Consultants 
Paul Reneke 
Michael Riley 
Lonn Rodine 
Schuyler Ruitberg 
Jose Sanchez 
Raymond Santoyo 
Steven Sekellick 
Michael Selepak 
Thomas Siewert 
Emil Simiu 
Monica Starnes 
David Stroup 
Laura Sugden 
Robert Vettori 
John Widmann 
Brendan Williams 
Maureen Williams 
Jiann Yang 
Robert Zarr 
Vincent Dunn 
John Hodgens 
Kevin Malley 
Valentine Junker 
Department of Commerce and NIST Institutional Support 
Michele Abadia-Dalmau 
Arden Bement 
Audra Bingaman 
Phyllis Boyd 
Marie Bravo 
Craig Burkhardt 
Paul Cataldo 
Deborah Cramer 
Gail Crum 
Sherri Diaz 
Sandra Febach 
James Fowler 
Matthew Heyman 
Verna Hines 
Kathleen Kilmer 
Kevin Kimball 
Thomas Klausing 
Donna Kline 
Fred Kopatich 
Kenneth Lechter 
Melissa Lieberman 
Mark Madsen 
Romena Moy 
Michael Newman 
Thomas O'Brien 
Norman Osinski 
Michael Rubin 
Rosamond Rutledge-Burns 
John Sanderson 
Hratch Semerjian 
Sharon Shaffer 
Elizabeth Simon 
Jack Snell 
Michael Szwed 
Anita Tolliver 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Contributors to the Investigation 
NIST Contractors 
Anter Laboratories, Inc. 
Renee Jacobs-Fedore 
Daniela Stroe 
Applied Research Associates, Inc. 
Steven Kirkpatrick* Marsh Hardy 
Robert T. Bocchieri Samuel Holmes 
Robert W. Cilke Robert A. MacNeill 
Computer Aided Engineering Associates 
Peter Barrett* Daniel Fridline 
Michael Bak James J. Kosloski 
DataSource 
John Wivaag 
GeoStats 
Marcello Oliveira 
Hughes Associates, Inc. 
Ed Budnick* Matt Hulcher 
Mike Ferreira Alwin Kelly 
Mark Hopkins Chris Mealy 
Indepdendent Contractors 
Ajmal Abbasi David Parks 
Eduardo Kausel Daniele Veniziano 
John Jay College 
Norman Groner 
Leslie E. Robertson Associates 
William J. Faschan* William C. Howell 
Richard B. Garlock* Raymond C. Lai 
Claudia Navarro 
Brian D. Peterson 
Justin Y-T. Wu 
John Schoenrock 
Steven Strege 
Josef Van Dyck 
Kaspar Willam 
NIST NCSTAR 1, WTC Investigation 

Contributors to the Investigation Draft for Public Comment 
National Fire Protection Association 
Rita Fahey* 
Norma Candeloro 
Joseph Molis 
National Research Council, Canada 
Guylene Proulx* 
Amber Walker 
NuStats, Inc. 
Johanna Zmud* Christopher Frye 
Carlos Arce Nancy McGuckin 
Heather Contrino Sandra Rodriguez 
Rolf Jensen Associates 
Ray Grill* Tom Brown 
Ed Armm Duane Johnson 
Rosenwasser/Grossman Consulting Engineers, P.C. 
Jacob Grossman* 
Craig Leech 
Arthur Seigel 
Science Applications International Corporation 
Lori Ackman 
Marina Bogatine 
Sydel Cavanaugh 
Kathleen Clark 
Pamela Curry 
John DiMarzio 
Simpson Gumpertz Heger 
Mehdi Zarghamee* 
Glenn Bell 
Said Bolourchi 
Daniel W. Eggers 
Omer O. Erbay 
Heather Duvall 
John Eichner* 
Mark Huffman 
Charlotte Johnson 
Michael Kalmar 
Jacquelyn Rhone 
Ron Hamburger 
Frank Kan 
Yasuo Kitane 
Atis Liepins 
Michael Mudlock 
Della Santos 
Robert Santos 
Bob Keough 
Joseph Razz 
Cheri Sawyer* 
Walter Soverow 
Paul Updike 
Yvonne Zagadou 
Wassim I. Naguib 
Rasko P. Ojdrovic 
Andrew T. Sarawit 
Pedro Sifre 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Contributors to the Investigation 
S.K. Ghosh Associates, Inc. 
S.K. Ghosh*
Analdo Derecho 
Skidmore, Owings, Merrill 
Bill Baker 
Bob Sinn 
John Zils 
Teng, Associates 
Shankar Nair 
Thermophysical Laboratories 
Jozef Gembarovic 
David L. Taylor 
Ray E. Taylor 
Underwriters Laboratories, Inc. 
Fred Hervey * 
Joseph Treadway* 
Mark Izydorek 
University at Buffalo 
Andrei Reinhorn 
Joshua Repp 
Andrew Whitaker* 
University of Chicago Survey Lab 
Virginia Bartot 
Martha van Haitsma 
University of Colorado 
Dennis Mileti 
University of Michigan 
Jamie Abelson 
Dave Fanella 
Xumei Liang 
Aldo Jimenez 
William Joy 
John Mammoser 
NIST NCSTAR 1, WTC Investigation 

Contributors to the Investigation Draft for Public Comment 
Wiss, Janney, Elstner 
Ray Tide* 
Jim Hauck 
Conrad Paulson 
*Principal Investigator/Key Contact 
NIST NCSTAR 1, WTC Investigation 

DEDICATION
On the morning of September 11, 2001, Americans and people around the world were shocked by the 
destruction of the World Trade Center (WTC) in New York City and the devastation of the Pentagon near 
Washington, D.C., after large aircraft were flown into the buildings, and the crash of an aircraft in a 
Pennsylvania field that averted further tragedy. Three years later, the world has been changed irrevocably 
by those terrorist attacks. For some, the absence of people close to them is a constant reminder of the 
unpredictability of life and death. For millions of others, the continuing threats of further terrorist attacks 
affect how we go about our daily lives and the attention we must give to homeland security and 
emergency preparedness. 
Within the construction, building, and public safety communities, there arose a question pressing to be 
answered: How can we reduce our vulnerability to such attacks, and how can we increase our 
preparedness and safety while still ensuring the functionality of the places in which we work and live? 
This Investigation has, to the best extent possible, reconstructed the responses of the WTC towers and the 
people on site to the consequences of the aircraft impacts. It provides improved understanding to the 
professional communities and building occupants whose action is needed and to those most deeply 
affected by the events of that morning. In this spirit, this report is dedicated to those lost in the disaster, 
to those who have borne the burden to date, and to those who will carry it forward to improve the safety 
of buildings. 
NIST NCSTAR 1, WTC Investigation 

Dedication Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

ABSTRACT
This is the final report on the National Institute of Standards and Technology (NIST) reconstruction of the 
collapses of the World Trade Center (WTC) towers, the results of an investigation conducted under the 
National Construction Safety Team Act. This reports describes how the aircraft impacts and subsequent 
fires led to the collapses of the towers after terrorists flew jet fuel laden commercial airliners into the 
buildings; whether the fatalities were low or high, including an evaluation of the building evacuation and 
emergency response procedures; what procedures and practices were used in the design, construction, 
operation, and maintenance of the towers; and areas in current building and fire codes, standards, and 
practices that warrant revision. Extensive details are to be found in the 42 companion reports. The final 
report on the collapse of WTC 7 appears in a separate report. 
Also in this report is a description of how NIST reached its conclusions. This included the 
complementing of in-house expertise with private sector technical experts; the accumulation of copious 
documents, photographs, and videos of the disaster; the establishment of the baseline performance of the 
WTC towers; the computer simulation of the behavior of each tower on September 11, 2001; the 
combination of the knowledge gained into a probable collapse sequence for each tower; the conduct of 
nearly 1,200 first-person interviews of building occupants and emergency responders; analysis of the 
evacuation and emergency response operations in the two high-rise buildings; and the compilation of 
principal findings. 
The report concludes with a list of 30 recommendations for action in the areas of increased structural 
integrity, enhanced fire resistance of structures, new methods for fire resistance design of structures, 
enhanced active fire protection, improved building evacuation, improved emergency response, improved 
procedures and practices, and continuing education and training. 
Keywords: Aircraft impact, building evacuation, emergency response, fire safety, human behavior, 
structural collapse, tall buildings, wind engineering, World Trade Center. 
NIST NCSTAR 1, WTC Investigation 

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NIST NCSTAR 1, WTC Investigation 

TABLE OF CONTENTS
National Construction Safety Team for the Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster .......................................................................................................................iii
List of Figures ........................................................................................................................................... xxi
List of Tables ........................................................................................................................................... xxv
List of Acronyms and Abbreviations ...................................................................................................... xxvii
Units and Conversion Factors .................................................................................................................. xxix
Preface .................................................................................................................................................... xxxi
Executive Summary ................................................................................................................................... xli
Contributors to the Investigation.................................................................................................................. v
Dedication ................................................................................................................................................... xi
Abstract .....................................................................................................................................................xiii
Part I: September 11, 2001 
Chapter 1 
New York City’s World Trade Center ........................................................................................ 1
1.1 The Origination............................................................................................................................... 1
1.2 The World Trade Center Complex ..................................................................................................21.2.1 The Site................................................................................................................................ 2
1.2.2 The Towers.......................................................................................................................... 5
Chapter 2 
The Account of World Trade Center 1 .................................................................................... 19
2.1 8:46:30 a.m. EDT........................................................................................................................... 19
2.2 The Aircraft................................................................................................................................... 20
2.3 The Immediate Damage ................................................................................................................. 20
2.4 The Jet Fuel................................................................................................................................... 24
2.5 8:47 a.m. to 9:02 a.m. EDT............................................................................................................ 24
2.6 9:02:59 a.m. EDT........................................................................................................................... 27
2.7 9:03 a.m. to 9:57 a.m. EDT............................................................................................................ 27
2.8 9:58:59 a.m. EDT........................................................................................................................... 32
2.9 9:59 a.m. to 10:28 a.m. EDT.......................................................................................................... 32
2.10 The Outcome................................................................................................................................. 34
NIST NCSTAR 1, WTC Investigation 

Table of Contents Draft for Public Comment 
Chapter 3 
The Account of World Trade Center 2 .................................................................................... 37
3.1 8:46:30 a.m. EDT........................................................................................................................... 37
3.2 9:02:59 a.m. EDT........................................................................................................................... 38
3.3 The Immediate Damage ................................................................................................................. 38
3.4 The Jet Fuel................................................................................................................................... 42
3.5 9:03 a.m. to 9:36 a.m. EDT............................................................................................................ 43
3.6 9:36 a.m. to 9:58 a.m. EDT............................................................................................................ 44
3.7 The Outcome................................................................................................................................. 45
Chapter 4 
The Toll ..................................................................................................................................... 47
Part II: Reconstructing the Disaster 
Chapter 5 
The Design and Construction of the Towers ......................................................................... 51
5.1 Building and Fire Codes ................................................................................................................ 51
5.2 The Codes and the Towers............................................................................................................. 51
5.2.1 The New York City Building Code.................................................................................... 51
5.2.2 Pertinent Construction Provisions ...................................................................................... 53
5.2.3 Tenant Alteration Process...................................................................................................54
5.3 Building Design ............................................................................................................................. 54
5.3.1 Loads ................................................................................................................................. 54
5.3.2 Aircraft Impact ................................................................................................................... 55
5.3.3 Construction Classification and Fire Resistance Rating..................................................... 55
5.3.4 Compartmentation .............................................................................................................. 56
5.3.5 Egress Provisions ...............................................................................................................57
5.3.6 Active Fire Protection ........................................................................................................ 60
5.4 Building Innovations..................................................................................................................... 63
5.4.1 The Need for Innovations................................................................................................... 63
5.4.2 Framed Tube System.......................................................................................................... 63
5.4.3 Deep Spandrel Plates .......................................................................................................... 64
5.4.4 Uniform External Column Geometry ................................................................................. 64
5.4.5 Wind Tunnel Test Data to Establish Wind Loads .............................................................. 64
5.4.6 Viscoelastic Dampers ......................................................................................................... 65
5.4.7 Long-Span Composite Floor Assemblies ........................................................................... 65
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Table of Contents 
5.4.8 Vertical Shaft Wall Panels..................................................................................................66
5.5 Structural Steels ............................................................................................................................. 66
5.5.1 Types and Sources.............................................................................................................. 66
5.5.2 Properties........................................................................................................................... 67
5.6 Passive Fire Protection................................................................................................................... 68
5.6.1 Thermal Insulation ............................................................................................................. 68
5.6.2 Use of Insulation in the WTC Towers................................................................................ 68
5.7 Concrete ........................................................................................................................................ 74
5.8 The Tenant Spaces ......................................................................................................................... 74
5.8.1 General ............................................................................................................................... 74
5.8.2 Walls.................................................................................................................................. 75
5.8.3 Flooring .............................................................................................................................. 75
5.8.4 Ceilings.............................................................................................................................. 75
5.8.5 Furnishings ......................................................................................................................... 75
Chapter 6 
Reconstruction of the Collapses ............................................................................................. 79 
6.1 Approach....................................................................................................................................... 79
6.2 Development of the Disaster Timeline .......................................................................................... 80
6.3 Learning from the Visual Images .................................................................................................. 82
6.4 Learning from the Recovered Steel ...............................................................................................84
6.4.1 Collection of Recovered Steel ............................................................................................ 84
6.4.2 Mechanical and Physical Properties ................................................................................... 86
6.4.3 Damage Analysis................................................................................................................ 87
6.5 Information Gained from Other WTC Fires .................................................................................. 89
6.6 The Building Structural Models..................................................................................................... 90
6.6.1 Computer Simulation Software .......................................................................................... 90
6.6.2 The Reference Models........................................................................................................ 90
6.6.3 Building Structural Models for Aircraft Impact Analysis .................................................. 92
6.6.4 Building Structural Models for Structural Response to Impact Damage and Fire and 
Collapse Initiation Analysis ............................................................................................... 95
6.7 The Aircraft Structural Model ..................................................................................................... 102
6.8 Aircraft Impact Modeling ............................................................................................................ 105
6.8.1 Component Level Analyses.............................................................................................. 105
6.8.2 Subassembly Impact Analyses ......................................................................................... 106
6.8.3 Aircraft Impact Conditions............................................................................................... 106
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Table of Contents Draft for Public Comment 
6.8.4 Global Impact Analysis .................................................................................................... 107
6.9 Aircraft Impact Damage Estimates .............................................................................................. 110
6.9.1 Structural and Contents Damage ...................................................................................... 110
6.9.2 Validity of Impact Simulations ........................................................................................ 114
6.9.3 Damage to Thermal Insulation ......................................................................................... 117
6.9.4 Damage to Ceiling System ............................................................................................... 117
6.9.5 Damage to Interior Walls and Furnishings....................................................................... 118
6.10 Thermal Environment Modeling.................................................................................................. 118
6.10.1 Need for Simulation ......................................................................................................... 118
6.10.2 Modeling Approach.......................................................................................................... 119
6.10.3 The Four Cases ................................................................................................................. 124
6.10.4 Characterization of the Fires ............................................................................................ 124
6.10.5 Global Heat Release Rates ............................................................................................... 128
6.11 Data Transfer ............................................................................................................................... 128
6.12 Thermal Mapping ........................................................................................................................ 129
6.12.1 Approach .......................................................................................................................... 129
6.12.2 The Fire-Structure Interface ............................................................................................. 129
6.12.3 Thermal Insulation Properties .......................................................................................... 130
6.12.4 FSI Uncertainty Assessment.............................................................................................131
6.12.5 The Four Cases ................................................................................................................. 136
6.12.6 Characterization of the Thermal Profiles..........................................................................139
6.13 Measurement of the Fire Resistance of the Floor System ........................................................... 139
6.14 Collapse Analysis of the Towers ................................................................................................. 141
6.14.1 Approach to Determining the Probable Collapse Sequences ........................................... 141
6.14.2 Results of Global Analysis of WTC 1 .............................................................................. 142
6.14.3 Results of Global Analysis of WTC 2 .............................................................................. 143
6.14.4 Structural Response of the WTC Towers to Fire Without Impact or Insulation 
Damage............................................................................................................................ 144
6.14.5 Probable WTC 1 Collapse Sequence................................................................................ 145
6.14.6 Probable WTC 2 Collapse Sequence................................................................................ 146
6.14.7 Accuracy of the Probable Collapse Sequences................................................................. 148
6.14.8 Factors that Affected Building Performance on September 11, 2001 .............................. 149
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Table of Contents 
Chapter 7 
Reconstruction of Human Activity ........................................................................................ 151
7.1 Building Occupants...................................................................................................................... 151
7.1.1 Background ...................................................................................................................... 151
7.1.2 The Building Egress System ............................................................................................ 151
7.1.3 The Evacuation—Data Sources........................................................................................ 153
7.1.4 Occupant Demographics .................................................................................................. 154
7.1.5 Evacuation of WTC 1....................................................................................................... 154
7.1.6 Evacuation of WTC 2....................................................................................................... 156
7.2 Emergency Responders................................................................................................................ 159
7.2.1 Data Gathered................................................................................................................... 159
7.2.2 Operations Changes Following the WTC 1 Bombing on February 26, 1993 .................. 160
7.2.3 Responder Organization ................................................................................................... 162
7.2.4 Responder Access............................................................................................................. 165
7.2.5 Communications............................................................................................................... 166
7.2.6 The Overall Response ...................................................................................................... 167
7.3 Factors That Contributed to Enhanced Life Safety...................................................................... 168
7.3.1 Aggregate Factors............................................................................................................. 168
7.3.2 Individual Factors............................................................................................................. 168
Part III: The Outcome of the InvestigationChapter 8 
Principal Findings................................................................................................................... 171
8.1 Introduction................................................................................................................................. 171
8.2 Summary ..................................................................................................................................... 171
8.3 Findings on the Mechanisms of Building Collapse ..................................................................... 175
8.3.1 Summary of Probable Collapse Sequences ...................................................................... 175
8.3.2 Structural Steels................................................................................................................ 176
8.3.3 Aircraft Impact Damage Analysis .................................................................................... 177
8.3.4 Reconstruction of the Fires............................................................................................... 179
8.3.5 Structural Response and Collapse Analysis ..................................................................... 180
8.4 Findings on Factors Affecting Life Safety................................................................................... 181
8.4.1 Active Fire Protection ...................................................................................................... 181
8.4.2 Evacuation ........................................................................................................................ 183
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Table of Contents Draft for Public Comment 
8.4.3 Emergency Response ....................................................................................................... 186
8.5 Findings on Operational Codes, Standards, and Practices ........................................................... 189
8.5.1 General ............................................................................................................................. 189
8.5.2 Structural Safety ............................................................................................................... 190
8.5.3 Fire Safety ........................................................................................................................ 191
8.6 Future Factors That Could Have Improved Life Safety .............................................................. 194
8.6.1 Building Performance Factors .......................................................................................... 195
8.6.2 Human Performance Factors ............................................................................................ 195
Chapter 9 
Recommendations................................................................................................................. 197
9.1 Building Standards and Codes: Who Is In Charge?..................................................................... 197
9.2 NIST’S Recommendations for Improving the Safety of Buildings, Occupants, and 
Emergency Responders................................................................................................................ 198
9.2.1 Group 1. Increased Structural Integrity ........................................................................... 201
9.2.2 Group 2. Enhanced Fire Resistance of Structures ........................................................... 204
9.2.3 Group 3. New Methods for Fire Resistance Design of Structures .................................. 207
9.2.4 Group 4. Improved Active Fire Protection...................................................................... 209
9.2.5 Group 5. Improved Building Evacuation ........................................................................ 210
9.2.6 Group 6. Improved Emergency Response....................................................................... 214
9.2.7 Group 7. Improved Procedures and Practices ................................................................. 216
9.2.8 Group 8. Education and Training .................................................................................... 218
9.3 Opportunity for Public Comment ................................................................................................ 218
9.4 Beginning the Implementation Process........................................................................................219
Appendix A 
National Construction Safety Team Act.......................................................................... 227
Appendix B 
Subject Index of Supporting Investigation Reports ...................................................... 235
NIST NCSTAR 1, WTC Investigation 

LIST OF FIGURES
Figure P–1. The eight projects in the federal building and fire safety investigation of the WTC 
disaster. ........................................................................................................................... xxxiii 
Figure 1–1. The World Trade Center in Lower Manhattan....................................................................... 3
Figure 1–2. Lower Manhattan and the World Trade Center towers.......................................................... 4
Figure 1–3. Tower floor plans with column numbers. ..............................................................................7Figure 1–4. Perimeter column/spandrel assembly and floor structure. ..................................................... 8
Figure 1–5. Plan of the 96th floor of WTC 1 showing the core and tenant spaces. ................................... 9
Figure 1–6. Schematic of composite floor truss system.......................................................................... 10
Figure 1–7. Schematic of a hat truss. ...................................................................................................... 11
Figure 1–8. Photograph of insulated WTC trusses.................................................................................. 12
Figure 1–9. Schematic of the three-tier elevator system. ........................................................................14
Figure 1–10. Orientation of the three stairwells. ....................................................................................... 16
Figure 1–11. Views of typical WTC office floors..................................................................................... 17
Figure 1–12. A WTC trading floor............................................................................................................ 17 
Figure 2–1. Simulated impact of American Airlines Flight 11 with WTC 1. ......................................... 19
Figure 2–2. Aircraft entry hole on the north side of WTC 1, photographed 30 s after impact. .............. 21
Figure 2–3. South face damage of WTC 1 with key aircraft component locations marked.................... 22
Figure 2–4. Simulation of aircraft impact damage to the 96th floor in WTC 1 ....................................... 23
Figure 2–5. Representation of exterior views of the fires on the four faces of WTC 1 from
8:47 a.m. to about 9:02 a.m.................................................................................................. 25
Figure 2–6. Firefighters on the scene at about 9:07 a.m.......................................................................... 27
Figure 2–7. Representation of exterior views of the fires on the four faces of WTC 1 from about 
9:38 a.m. to 9:58 a.m. ..........................................................................................................28
Figure 2–8. Steel surface temperatures on the bottom chords of fire-exposed trusses, uninsulated 
and insulated with ¾ in. of BLAZE-SHIELD DC/F............................................................ 29
Figure 2–9. Temperature dependence of yield strength of structural steel as a fraction of the value 
at room temperature. ............................................................................................................30
Figure 2–10. Simulated temperatures of two adjacent trusses (left) and two adjacent perimeter 
columns (right) exposed to the fires in WTC 1.................................................................... 30
Figure 2–11. Temperature contours on the top and bottom faces of the concrete slab (96th floor, 
WTC 1) at 100 min after impact. A portion of the concrete slab on the north face 
(top) was damaged by the impact of the aircraft.................................................................. 31
NIST NCSTAR 1, WTC Investigation 

List of Figures Draft for Public Comment 
Figure 2–12. South face of WTC 1 at 10:23 a.m., showing inward buckling (in inches) of 
perimeter columns................................................................................................................ 33 
Figure 3–1. Imminent impact of United Airlines Flight 175 with WTC 2.............................................. 38
Figure 3–2. South face damage of WTC 2 with key aircraft component locations marked.................... 39
Figure 3–3. Simulation of aircraft impact damage to the 78th through 83rd floors in WTC 2 40
Figure 3–4. Representation of exterior views of the fires on the four faces of WTC 2 at about
9:20 a.m............................................................................................................................... 43
Figure 3–5. Photograph of WTC 2 tilting to the southeast at the onset of collapse 46 
Figure 4–1. The WTC site on September 17, 2001 47 
Figure 5–1. Fire Command Desk in WTC 1, as seen from a mezzanine elevator, looking west 60
Figure 5–2. Schematic of sprinkler and standpipe systems..................................................................... 62
Figure 5–3. Diagram of floor truss showing viscoelastic damper 65
Figure 5–4. Ratio of measured yield strength (Fy) to specified minimum yield strength for steels 
used in WTC perimeter columns 68
Figure 5–5. Irregularity of coating thickness and gaps in coverage on SFRM–coated bridging 
trusses.................................................................................................................................. 70
Figure 5–6. Thermal insulation for perimeter columns 71
Figure 5–7. Temperature–dependent concrete properties 74
Figure 5–8. A WTC workstation 75 
Figure 6–1. 9:26:20 a.m. showing the east face of WTC 2 83
Figure 6–2. Close-up of section of Figure 6–1........................................................................................ 84
Figure 6–3. Examples of a WTC 1 core column (left) and truss material (right).................................... 86
Figure 6–4. WTC 1 exterior panel hit by the fuselage of the aircraft...................................................... 86
Figure 6–5. WTC 1 exterior panel hit by the nose of the aircraft............................................................ 87
Figure 6–6. Structural model of the 96th floor of WTC 1........................................................................ 93
Figure 6–7. Model of the 96th floor of WTC 1, including interior contents and partitions 93
Figure 6–8. Multifloor global model of WTC 1, viewed from the north 94
Figure 6–9. Multifloor global model of WTC 2, viewed from the south 94
Figure 6–10. Finite element model of an exterior truss seat 96
Figure 6–11. Vertical displacement at 700 oC........................................................................................... 96
Figure 6–12. ANSYS model of 96th floor of WTC 1 97
Figure 6–13. Finite element model of the Boeing 767-200ER................................................................ 103
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment List of Figures 
Figure 6–14. Pratt & Whitney PW4000 turbofan engine model 104
Figure 6–15. Boeing 767-200ER showing the jet fuel distribution at time of impact............................. 104
Figure 6–16. Calculated impact on an exterior wall by a fuel-laden wing section 105
Figure 6–17. Response of a tower subassembly model to engine impact 106
Figure 6–18. Side view of simulated aircraft impact into WTC 1, Case B 108
Figure 6–19. Column damage levels 110
Figure 6–20. Case B damage to the slab of floor 96 of WTC 1 110
Figure 6–21. Case B simulation of response of contents of 96th floor of WTC 1 111
Figure 6–22. Combined structural damage to the floors and columns of WTC 1, Case A 112
Figure 6–23. Combined structural damage to the floors and columns of WTC 1, Case B 112
Figure 6–24. Combined structural damage to the floors and columns of WTC 2, Case C 113
Figure 6–25. Combined structural damage to the floors and columns of WTC 2, Case D 113
Figure 6–26. Observed and Case A calculated damage to the north face of WTC 1 115
Figure 6–27. Schematic of observed damage (top) and Case A calculated damage (lower) to the 
north face of WTC 1 116
Figure 6–28. Schematic of observed damage (above) and Case C calculated damage (right) to the 
south face of WTC 2 116
Figure 6–29. Ceiling tile system mounted on the shaking table.............................................................. 118
Figure 6–30. Eight floor model of WTC 1 prior to aircraft impact......................................................... 120
Figure 6–31. Fire test of a single workstation 120
Figure 6–32. Interior view of a 3-workstation fire test 121
Figure 6–33. Rubblized workstations...................................................................................................... 122
Figure 6–34. Three-workstation fire test, 2 min after the start................................................................ 122
Figure 6–35. Measured and predicted heat release rate from the burning of three 
office workstations............................................................................................................. 123
Figure 6–36. Upper layer temperatures on the 94th floor of WTC 1, 15 min after impact 125
Figure 6–37. Direction of simulated fire movement on Floors 94 and 97 of WTC 1 126
Figure 6–38. Predicted heat release rates for fires in WTC 1 and WTC 2 128
Figure 6–39. Simple bar dimensions (in.) 132
Figure 6–40. Tubular column dimensions (in.) 132
Figure 6–41. Truss Dimensions (in.)....................................................................................................... 133
Figure 6–42. SFRM-coated steel components prior to a test 133
Figure 6–43. Finite element representation of the insulated steel truss (blue), the SFRM (violet), 
and the ceiling (red) 134
Figure 6–44. Comparison of numerical simulations with measurements for the steel surface 
temperature at four locations on the top chord of a bare truss 135
NIST NCSTAR 1, WTC Investigation 

List of Figures Draft for Public Comment 
Figure 6–45. Comparison of numerical simulations with measurements for the temperature of the 
steel surface at four locations on the top chord of an insulated truss................................. 135
Figure 6–46. Temperatures (°C) on the columns and trusses of the 96th floor of WTC 1 at 6,000 s 
after aircraft impact, Case B. ............................................................................................. 137
Figure 6–47. Temperatures (°C) on the columns and trusses of the 81st floor of WTC 2 at 3,000 s 
after aircraft impact, Case D. ............................................................................................. 137
Figure 6–48. Frames from animation of the thermal response of columns on the 96th Floor of 
WTC 1, Case A. ................................................................................................................. 138 
Figure 7–1. Simulated impact damage to 95th floor of WTC 1, including stairwells, 0.7 s after 
impact................................................................................................................................ 152
Figure 7–2. Simulated impact damage to WTC 2 on Floor 78, 0.62 s after impact. 
Figure 7–3. Observations of building damage after initial awareness but before beginning 
............................. 152
evacuation in WTC 1 ......................................................................................................... 157
Figure 7–4. Observations of building damage from tenant spaces in WTC 2....................................... 158
Figure 7–5. Location of the radio repeater. ........................................................................................... 161
Figure 7–6. Timing of FDNY unit arrivals. .......................................................................................... 162
Figure 7–7. Fire Command Board located in the lobby of WTC 1. ...................................................... 164
NIST NCSTAR 1, WTC Investigation 

LIST OF TABLES
Table P–1. Federal building and fire safety investigation of the WTC disaster................................. xxxii 
Table P–2. Public meetings and briefings of the WTC Investigation. ............................................... xxxv 
Table 1–1. Use of floors in the WTC towers 5 
Table 2–1. Locations of occupants of WTC 1 26 
Table 3–1. Tenants on impact floors in WTC 2..................................................................................... 40
Table 3–2. Location of occupants of WTC 2 42 
Table 4–1. Likely locations of World Trade Center decedents at time of impact 48 
Table 5–1. Specified steel grades for various applications 67
Table 5–2. Types and locations of SFRM on fire floors........................................................................ 73
Table 5–3. Floors of focus 77 
Table 6–1. Times for major events on September 11,2001................................................................... 82
Table 6–2. Indications of major structural changes up to collapse initiation......................................... 85
Table 6–3. Measured and calculated natural vibration periods (s) for WTC 1 91
Table 6–4. Summary of aircraft impact conditions.............................................................................. 106
Table 6–5. Input parameters for global impact analyses...................................................................... 107
Table 6–6. Values of WTC fire simulation variables........................................................................... 124
Table 6–7. Summary of insulation on steel components 134
Table 6–8. Regions in WTC 1 in which temperatures of structural steel exceeded 600 °C 139
Table 6–9. Regions in WTC 2 in which temperatures of structural steel exceeded 600 °C 139
Table 6–10. Comparison of global structural model predictions and observations for WTC 1, 
Case B 148
Table 6–11. Comparison of global structural model predictions and observations for WTC 2, 
Case D 149
NIST NCSTAR 1, WTC Investigation 

List of Tables Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

LIST OF ACRONYMS AND ABBREVIATIONS
Acronyms 
AA American Airlines 
ARA Application Research Associates 
ASTM ASTM International 
BOCA Building Officials and Code Administrators 
BPS Building Performance Study 
FCD Fire Command Desk 
FDNY The Fire Department of the City of New York 
FDS Fire Dynamics Simulator 
FEMA Federal Emergency Management Agency 
FSI Fire Structure Interface 
IBC International Building Code 
LERA Leslie E. Robertson Associates 
NFPA National Fire Protection Association 
NIST National Institute of Standards and Technology 
NYC New York City 
NYPD New York City Police Department 
NYS New York State 
PANYNJ The Port Authority of New York and New Jersey 
PAPD Port Authority Police Department 
SFRM spray-applied fire resistive material 
SGH Simpson, Gumpertz, & Heger, Inc. 
SOM Skidmore, Owings and Merrill 
UA United Airlines 
USC United States Code 
WSHJ Worthington, Skilling, Helle and Jackson 
WTC World Trade Center 
WTC 1 World Trade Center 1 (North Tower) 
NIST NCSTAR 1, WTC Investigation 

List of Acronyms and Abbreviations Draft for Public Comment 
WTC 2 World Trade Center 2 (South Tower) 
WTC 7 World Trade Center 7 
NIST NCSTAR 1, WTC Investigation 

UNITS AND CONVERSION FACTORS
°C degrees Celsius T (ºC) = 5/9 [T (ºF) – 32] 
°F degrees Fahrenheit 
ft feet 
gal gallon 1 gal = 3.78 x 10-3 m3 
GJ gigajoule 
GW gigawatt 
in. inch 
kg kilogram 
kip 1,000 lb 
ksi 1,000 lb/in.2 
lb pound 1 lb = 0.453 kg 
m meter 1 m = 3.28 ft 
µm micrometer 
min minute 
MJ megajoule 
MW megawatt 
psi pounds per square inch 
s second 
T temperature 
NIST NCSTAR 1, WTC Investigation 

Unit Conversion Factors Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

PREFACE
Genesis of This Investigation 
Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the 
Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began 
planning a building performance study of the disaster. The week of October 7, as soon as the rescue and 
search efforts ceased, the Building Performance Study Team went to the site and began their assessment. 
This was to be a brief effort, as the study team consisted of experts who largely volunteered their time 
away from their other professional commitments. The Building Performance Study Team issued their 
report in May 2002, fulfilling their goal “to determine probable failure mechanisms and to identify areas 
of future investigation that could lead to practical measures for improving the damage resistance of 
buildings against such unforeseen events.” 
On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of 
Standards and Technology (NIST) announced its building and fire safety investigation of the WTC 
disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-231), was 
signed into law. (A copy of the Public Law is included in Appendix A.) The NIST WTC Investigation 
was conducted under the authority of the National Construction Safety Team Act. 
The goals of the investigation of the WTC disaster were: 
• To investigate the building construction, the materials used, and the technical conditions that 
contributed to the outcome of the WTC disaster. 
• To serve as the basis for: 
- Improvements in the way buildings are designed, constructed, maintained, and used; 
- Improved tools and guidance for industry and safety officials; 
- Recommended revisions to current codes, standards, and practices; and 
- Improved public safety. 
The specific objectives were: 
1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the 
aircraft and why and how WTC 7 collapsed; 
2. Determine why the injuries and fatalities were so high or low depending on location, 
including all technical aspects of fire protection, occupant behavior, evacuation, and 
emergency response; 
3. Determine what procedures and practices were used in the design, construction, operation, 
and maintenance of WTC 1, 2, and 7; and 
4. Identify, as specifically as possible, areas in current building and fire codes, standards, and 
practices that warrant revision. 
NIST NCSTAR 1, WTC Investigation 

Preface Draft for Public Comment 
NIST is a nonregulatory agency of the U.S. Department of Commerce’s Technology Administration. The 
purposes of NIST investigations under the National Construction Safety Team Act are to improve the 
safety and structural integrity of buildings in the United States, and the focus is on fact finding. NIST 
investigative teams are required to assess building performance and emergency response and evacuation 
procedures in the wake of any building failure that has resulted in substantial loss of life or that posed 
significant potential of substantial loss of life. NIST does not have the statutory authority to make 
findings of fault or negligence by individuals or organizations. Further, no part of any report resulting 
from a NIST investigation into a building failure or from an investigation under the National Construction 
Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in 
such report (15 USC 281a, as amended by Public Law 107-231). 
Organization of the Investigation 
The National Construction Safety Team for this Investigation, appointed by the NIST Director, was led 
by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as Associate Lead Investigator, 
Mr. Stephen A. Cauffman served as Program Manager for Administration, and Mr. Harold E. Nelson 
served on the team as a private sector expert. The Investigation included eight interdependent projects 
whose leaders comprised the remainder of the team. A detailed description of each of these eight projects 
is available at http://wtc.nist.gov. The purpose of each project is summarized in Table P–1, and the key 
interdependencies among the projects are illustrated in Figure P–1. 
Table P–1. Federal building and fire safety investigation of the WTC disaster. 
Technical Area and Project Leader Project Purpose 
Analysis of Building and Fire Codes and 
Practices; Project Leaders: Dr. H. S. Lew 
and Mr. Richard W. Bukowski 
Document and analyze the code provisions, procedures, and 
practices used in the design, construction, operation, and 
maintenance of the structural, passive fire protection, and 
emergency access and evacuation systems of WTC 1, 2, and 7. 
Baseline Structural Performance and 
Aircraft Impact Damage Analysis; Project 
Leader: Dr. Fahim Sadek 
Analyze the baseline performance of WTC 1 and WTC 2 under 
design, service, and abnormal loads, and aircraft impact damage on 
the structural, fire protection, and egress systems. 
Mechanical and Metallurgical Analysis of 
Structural Steel; Project Leader: Dr. Frank 
W. Gayle 
Determine and analyze the mechanical and metallurgical properties 
and quality of steel, weldments, and connections from steel 
recovered from WTC 1, 2, and 7. 
Investigation of Active Fire Protection 
Systems; Project Leader: Dr. David 
D. Evans 
Investigate the performance of the active fire protection systems in 
WTC 1, 2, and 7 and their role in fire control, emergency response, 
and fate of occupants and responders. 
Reconstruction of Thermal and Tenability 
Environment; Project Leader: Dr. Richard 
G. Gann 
Reconstruct the time-evolving temperature, thermal environment, 
and smoke movement in WTC 1, 2, and 7 for use in evaluating the 
structural performance of the buildings and behavior and fate of 
occupants and responders. 
Structural Fire Response and Collapse 
Analysis; Project Leaders: Dr. John 
L. Gross and Dr. Therese P. McAllister 
Analyze the response of the WTC towers to fires with and without 
aircraft damage, the response of WTC 7 in fires, the performance 
of composite steel-trussed floor systems, and determine the most 
probable structural collapse sequence for WTC 1, 2, and 7. 
Occupant Behavior, Egress, and Emergency 
Communications; Project Leader: Mr. Jason 
D. Averill 
Analyze the behavior and fate of occupants and responders, both 
those who survived and those who did not, and the performance of 
the evacuation system. 
Emergency Response Technologies and 
Guidelines; Project Leader: Mr. J. Randall 
Lawson 
Document the activities of the emergency responders from the time 
of the terrorist attacks on WTC 1 and WTC 2 until the collapse of 
WTC 7, including practices followed and technologies used. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Preface 
NIST WTC Investigation ProjectsNIST WTC Investigation Projects 
Analysis of 
Steel 
Structural 
Collapse 
Evacuation 
Baseline 
Performance 
& Impact 
Damage 
Analysis of 
Codes and 
Practices 
Emergency 
Response 
Active Fire 
Protection 
Thermal and 
Tenability 
Environment 
Video/ 
Photographic 
Records 
Oral History Data 
Emergency 
Response 
Records 
Recovered 
Structural Steel 
WTC Building 
Performance Study 
Recommendations 
Government, 
Industry, 
Professional, 
Academic Inputs 
Public Inputs 
Figure P–1. The eight projects in the federal building and fire safety 
investigation of the WTC disaster. 
National Construction Safety Team Advisory Committee 
The NIST Director also established an advisory committee as mandated under the National Construction 
Safety Team Act. The initial members of the committee were appointed following a public solicitation. 
These were: 
• Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety 
Team Advisory Committee Chair 
• John Barsom, President, Barsom Consulting, Ltd. 
• John Bryan, Professor Emeritus, University of Maryland 
• David Collins, President, The Preview Group, Inc. 
• Glenn Corbett, Professor, John Jay College of Criminal Justice 
• Philip DiNenno, President, Hughes Associates, Inc. 
• Robert Hanson, Professor Emeritus, University of Michigan 
NIST NCSTAR 1, WTC Investigation 

Preface Draft for Public Comment 
• Charles Thornton, Co-Chairman and Managing Principal, The Thornton-Tomasetti Group, 
Inc. 
• Kathleen Tierney, Director, Natural Hazards Research and Applications Information Center, 
University of Colorado at Boulder 
• Forman Williams, Director, Center for Energy Research, University of California at San 
Diego 
This National Construction Safety Team Advisory Committee provided technical advice during the 
Investigation and commentary on drafts of the Investigation reports prior to their public release. 
Public Outreach 
During the course of this Investigation, NIST held public briefings and meetings (listed in Table P–2) to 
solicit input from the public, present preliminary findings, and obtain comments on the direction and 
progress of the Investigation from the public and the Advisory Committee. 
NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site 
contained extensive information on the background and progress of the Investigation. 
NIST’s WTC Public-Private Response Plan 
The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed, 
constructed, maintained, and used, especially with regard to major events such as fires, natural disasters, 
and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support 
from Congress and the Administration, has put in place a program, the goal of which is to develop and 
implement the standards, technology, and practices needed for cost-effective improvements to the safety 
and security of buildings and building occupants, including evacuation, emergency response procedures, 
and threat mitigation. 
The strategy to meet this goal is a three-part NIST-led public-private response program that includes: 
• A federal building and fire safety investigation to study the most probable factors that 
contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7 
building, and the associated evacuation and emergency response experience. 
• A research and development (R&D) program to (a) facilitate the implementation of 
recommendations resulting from the WTC Investigation, and (b) provide the technical basis 
for cost-effective improvements to national building and fire codes, standards, and practices 
that enhance the safety of buildings, their occupants, and emergency responders. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Preface 
Table P–2. Public meetings and briefings of the WTC Investigation. 
Date Location Principal Agenda 
June 24, 2002 New York City, NY Public meeting: Public comments on the Draft Plan for the 
pending WTC Investigation. 
August 21, 2002 Gaithersburg, MD Media briefing announcing the formal start of the Investigation. 
December 9, 2002 Washington, DC Media briefing on release of the Public Update and NIST request 
for photographs and videos. 
April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person 
interviews. 
April 29–30, 2003 Gaithersburg, MD National Construction Safety Team (NCST) Advisory Committee 
meeting on plan for and progress on WTC Investigation with a 
public comment session. 
May 7, 2003 New York City, NY Media briefing on release of the May 2003 Progress Report. 
August 26–27, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status of WTC 
investigation with a public comment session. 
September 17, 2003 New York City, NY Media briefing and public briefing on initiation of first-person 
data collection projects. 
December 2–3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results 
and the release of the Public Update with a public comment 
session. 
February 12, 2004 New York City, NY Public meeting: Briefing on progress and preliminary findings 
with public comments on issues to be considered in formulating 
final recommendations. 
June 18, 2004 New York City, NY Media briefing and public briefing on release of the June 2004 
Progress Report. 
June 22–23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and 
preliminary findings from the WTC Investigation with a public 
comment session. 
August 24, 2004 Northbrook, IL Public viewing of standard fire resistance test of WTC floor 
system at Underwriters Laboratories, Inc. 
October 19–20, 2004 Gaithersburg, MD NCST Advisory Committee meeting on status and near complete 
set of preliminary findings with a public comment session. 
November 22, 2004 Gaithersburg, MD NCST Advisory Committee discussion on draft annual report to 
Congress, a public comment session, and a closed session to 
discuss pre-draft recommendations for WTC Investigation. 
April 5, 2005 New York City, NY Media briefing and public briefing on release of the probable 
collapse sequence for the WTC towers and draft reports for the 
projects on codes and practices, evacuation, and emergency 
response. 
June 23, 2005 New York City, NY Media briefing and public briefing on release of all draft reports 
and draft recommendations for public comment. 
• A dissemination and technical assistance program (DTAP) to (a) engage leaders of the 
construction and building community in ensuring timely adoption and widespread use of 
proposed changes to practices, standards, and codes resulting from the WTC Investigation 
and the R&D program, and (b) provide practical guidance and tools to better prepare facility 
owners, contractors, architects, engineers, emergency responders, and regulatory authorities 
to respond to future disasters. 
The desired outcomes are to make buildings, occupants, and first responders safer in future disaster 
events. 
NIST NCSTAR 1, WTC Investigation 

Preface Draft for Public Comment 
National Construction Safety Team Reports on the WTC Investigation 
This report covers the WTC towers, with a separate report on the 47-story WTC 7. Supporting 
documentation of the techniques and technologies used in the reconstruction can be found in a set of 
companion reports. This summary report is one of a set that provides more detailed documentation of the 
Investigation findings and the means by which these technical results were achieved. As such, it is part of 
the archival record of this Investigation. The titles of the full set of Investigation publications are: 
NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team 
on the Collapses of the World Trade Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September. 
NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team 
on the Collapse of World Trade Center 7. NIST NCSTAR 1A. Gaithersburg, MD, December. 
Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005. Federal Building and Fire Safety Investigation of 
the World Trade Center Disaster: Design, Construction, and Maintenance of Structural and Life Safety 
Systems. NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Design and Construction of Structural Systems. 
NIST NCSTAR 1-1A. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Ghosh, S. K., and X. Liang. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Comparison of Building Code Structural Requirements. NIST 
NCSTAR 1-1B. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Maintenance and Modifications to Structural 
Systems. NIST NCSTAR 1-1C. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 
Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and 
Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after 
Occupancy. NIST NCSTAR 1-1D. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 
Razza, J. C., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time of the 
Design and Construction of World Trade Center 1, 2, and 7. NIST NCSTAR 1-1E. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Preface 
York City Building Code Provisions. NIST NCSTAR 1-1F. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 
Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions of the New 
York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in 
Use. NIST NCSTAR 1-1G. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems 
of World Trade Center 1 and 2. NIST NCSTAR 1-1H. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 
Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety Investigation 
of the World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life 
Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR 1-1I. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 
Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Design, Installation, and Operation of Fuel System for Emergency Power in 
World Trade Center 7. NIST NCSTAR 1-1J. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 
Sadek, F. 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: 
Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center 
Towers. NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Faschan, W. J., and R. B. Garlock. 2005. Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of 
the World Trade Center Towers. NIST NCSTAR 1-2A. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 
Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson, 
R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Analysis of Aircraft Impacts into the World Trade Center Towers, NIST 
NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Gayle, F. W., R. J. Fields, W. E. Luecke, S. W. Banovic, T. Foecke, C. N. McCowan, T. A. Siewert, and 
J. D. McColskey. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Mechanical and Metallurgical Analysis of Structural Steel. NIST NCSTAR 1-3. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Contemporaneous Structural Steel 
Specifications. NIST Special Publication 1-3A. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 
NIST NCSTAR 1, WTC Investigation 

Preface Draft for Public Comment 
Banovic, S. W. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Steel Inventory and Identification. NIST NCSTAR 1-3B. National Institute of Standards 
and Technology. Gaithersburg, MD, September. 
Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Damage and Failure Modes of Structural Steel Components. NIST 
NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Luecke, W. E., J. D. McColskey, C. N. McCowan, S. W. Banovic, R. J. Fields, T. Foecke, 
T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Mechanical Properties of Structural Steels. NIST NCSTAR 1-3D. 
National Institute of Standards and Technology. Gaithersburg, MD, September. 
Banovic, S. W., C. N. McCowan, and W. E. Luecke. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Physical Properties of Structural Steels. NIST 
NCSTAR 1 3E. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Evans, D. D., E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal Building and Fire 
Safety Investigation of the World Trade Center Disaster: Active Fire Protection Systems. NIST 
NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Kuligowski, E. D., and D. D. Evans. 2005. Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster: Post-Construction Fires Prior to September 11, 2001. NIST 
NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Hopkins, M., J. Schoenrock, and E. Budnick. 2005. Federal Building and Fire Safety Investigation 
of the World Trade Center Disaster: Fire Suppression Systems. NIST NCSTAR 1-4B. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
Keough, R. J., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards 
and Technology. Gaithersburg, MD, September. 
Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
Gann, R. G., A. Hamins, K. B. McGrattan, G. W. Mulholland, H. E. Nelson, T. J. Ohlemiller, 
W. M. Pitts, and K. R. Prasad. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Reconstruction of the Fires in the World Trade Center Towers. NIST NCSTAR 1-5. 
National Institute of Standards and Technology. Gaithersburg, MD, September. 
Pitts, W. M., K. M. Butler, and V. Junker. 2005. Federal Building and Fire Safety Investigation of 
the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis. 
NIST NCSTAR 1-5A. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly, 
J. Yang, G. Mulholland, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Preface 
Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and 
Modeling of Structural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 
Ohlemiller, T. J., G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Tests of Single 
Office Workstations. NIST NCSTAR 1-5C. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 
Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhorn, and P. A. Hough. 2005. 
Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Reaction of 
Ceiling Tile Systems to Shocks. NIST NCSTAR 1-5D. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 
Hamins, A., A. Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and 
Modeling of Multiple Workstations Burning in a Compartment. NIST NCSTAR 1-5E. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
McGrattan, K. B., C. Bouldin, and G. Forney. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Computer Simulation of the Fires in the World 
Trade Center Towers. NIST NCSTAR 1-5F. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 
Prasad, K. R., and H. R. Baum. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Structure Interface and Thermal Response of the World Trade Center 
Towers. NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 
Gross, J. L., and T. McAllister. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Structural Fire Response and Probable Collapse Sequence of the World Trade Center 
Towers. NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 
Carino, N. J., M. A. Starnes, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski. 
2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Passive 
Fire Protection. NIST NCSTAR 1-6A. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 
Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005. Federal Building and 
Fire Safety Investigation of the World Trade Center Disaster: Fire Resistance Tests of Floor Truss 
Systems. NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 
Zarghamee, M. S., S. Bolourchi, D. W. Eggers, F. W. Kan, Y. Kitane, A. A. Liepins, M. Mudlock, 
W. I. Naguib, R. P. Ojdrovic, A. T. Sarawit, P. R Barrett, J. L. Gross, and T. P. McAllister. 2005. 
Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Component, 
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Preface Draft for Public Comment 
Connection, and Subsystem Structural Analysis. NIST NCSTAR 1-6C. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 
Zarghamee, M. S., Y. Kitane, O. O. Erbay, T. P. McAllister, and J. L. Gross. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Global Structural 
Analysis of the Response of the World Trade Center Towers to Impact Damage and Fire. NIST 
NCSTAR 1-6D. National Institute of Standards and Technology. Gaithersburg, MD, September. 
McAllister, T., R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, W. M. Pitts, K. R. Prasad. 2005. 
Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Structural Fire 
Response and Probable Collapse Sequence of World Trade Center 7. 2005. NIST NCSTAR 1-6E. 
National Institute of Standards and Technology. Gaithersburg, MD, December. 
Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva, 
J. Oh, K. Rubenacker. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Structural Analysis of the Response of World Trade Center 7 to Debris Damage 
and Fire. NIST NCSTAR 1-6F. National Institute of Standards and Technology. Gaithersburg, 
MD, December. 
Kim, W. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Analysis of September 11, 2001, Seismogram Data, NIST NCSTAR 1-6G. National 
Institute of Standards and Technology. Gaithersburg, MD, December. 
Nelson, K. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: The ConEd Substation in World Trade Center 7, NIST NCSTAR 1-6H. National Institute 
of Standards and Technology. Gaithersburg, MD, December. 
Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, P. A. Reneke, and 
H. E. Nelson. 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: 
Occupant Behavior, Egress, and Emergency Communication. NIST NCSTAR 1-7. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 
Fahy, R., and G. Proulx. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Analysis of Published Accounts of the World Trade Center Evacuation. NIST 
NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September. 
Zmud, J. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Technical Documentation for Survey Administration. NIST NCSTAR 1-7B. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 
Lawson, J. R., and R. L. Vettori. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 
NIST NCSTAR 1, WTC Investigation 

EXECUTIVE SUMMARY
E.1 GENESIS OF THIS INVESTIGATION 
On August 21, 2002, the National Institute of Standards and Technology (NIST) announced its building 
and fire safety investigation of the World Trade Center (WTC) disaster.1 This WTC Investigation was 
then conducted under the authority of the National Construction Safety Team (NCST) Act, which was 
signed into law on October 1, 2002. A copy of the Public Law is included in Appendix A. 
The goals of the investigation of the WTC disaster were: 
• To investigate the building construction, the materials used, and the technical conditions that 
contributed to the outcome of the WTC disaster after terrorists flew large jet-fuel laden 
commercial airliners into the WTC towers. 
• To serve as the basis for: 
- Improvements in the way buildings are designed, constructed, maintained, and used; 
- Improved tools and guidance for industry and safety officials; 
- Recommended revisions to current codes, standards, and practices; and 
- Improved public safety 
The specific objectives were: 
1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the 
aircraft and why and how WTC 7 collapsed; 
2. Determine why the injuries and fatalities were so high or low depending on location, 
including all technical aspects of fire protection, occupant behavior, evacuation, and 
emergency response; and 
3. Determine what procedures and practices were used in the design, construction, operation, 
and maintenance of WTC 1, 2, and 7. 
1 
NIST is a nonregulatory agency of the U.S. Department of Commerce. The purposes of NIST investigations are to improve 
the safety and structural integrity of buildings in the United States and the focus is on fact finding. NIST investigative teams 
are required to assess building performance and emergency response and evacuation procedures in the wake of any building 
failure that has resulted in substantial loss of life or that posed significant potential of substantial loss of life. NIST does not 
have the statutory authority to make findings of fault or negligence by individuals or organizations. Further, no part of any 
report resulting from a NIST investigation into a building failure or from an investigation under the National Construction 
Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report 
(15 USC 281a, as amended by P.L. 107-231). 
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Executive Summary Draft for Public Comment 
4. Identify, as specifically as possible, areas in current building and fire codes, standards, 
and practices that warrant revision 
E.2 APPROACH 
To meet these goals, NIST complemented its in-house expertise with an array of specialists in key 
technical areas. In all, about 200 staff contributed to the Investigation. NIST and its contractors compiled 
and reviewed tens of thousand of pages of documents; conducted interviews with over a thousand people 
who had been on the scene or who had been involved with the design, construction, and maintenance of 
the WTC; analyzed 236 pieces of steel that were obtained from the wreckage; performed laboratory tests, 
measured material properties, and performed computer simulations of the sequence of events that 
happened from the instant of aircraft impact to the initiation of collapse for each tower. 
Cooperation in obtaining the resource materials and in interpreting the results came from a large number 
of individuals and organizations, including The Port Authority of New York and New Jersey and its 
contractors and consultants, Silverstein Properties and its contractors and consultants, the City of New 
York and its departments, the manufacturers and fabricators of the building components, the companies 
that insured the WTC towers, the building tenants, the aircraft manufacturers and the airlines. 
The scarcity of physical evidence that is typically available in place for reconstruction of a disaster led to 
the following approach: 
• Accumulation of copious photographic and video material. With the assistance of the media, 
public agencies and individual photographers, NIST acquired and organized nearly 
7,000 segments of video footage, totaling in excess of 150 hours and nearly 7,000 
photographs representing at least 185 photographers. This guided the Investigation Team’s 
efforts to determine the condition of the buildings following the aircraft impact, the evolution 
of the fires, and the subsequent deterioration of the structure. 
• Establishment of the baseline performance of the WTC towers, i.e., estimating the expected 
performance of the towers under normal design loads and conditions. The baseline 
performance analysis also helped to estimate the ability of the towers to withstand the 
unexpected events of September 11, 2001. Establishing the baseline performance of the 
towers began with the compilation and analysis of the procedures and practices used in the 
design, construction, operation, and maintenance of the structural, fire protection, and egress 
systems of the WTC towers. The additional components of the performance analysis were 
the standard fire resistance of the WTC truss-framed floor system, the quality and properties 
of the structural steels used in the towers, and the response of the WTC towers to the design 
gravity and wind loads. 
• Conduct of four-step simulations of the behavior of each tower on September 11, 2001. Each 
step stretched the state of the technology and tested the limits of software tools and computer 
hardware. The four steps were: 
1. The aircraft impact into the tower, the resulting distribution of aviation fuel, and the 
damage to the structure, partitions, thermal insulation materials, and building contents. 
2. The evolution of multifloor fires. 
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Draft for Public Comment Executive Summary 
3. The heating and consequent weakening of the structural elements by the fires. 
4. The response of the damaged and heated building structure, and the progression of 
structural component failures leading to the initiation of the collapse of the towers. 
The output of these simulations was subject to uncertainties in the as-built condition of the towers, the 
interior layout and furnishings, the aircraft impact, the internal damage to the towers (especially the 
thermal insulation for fire protection of the structural steel, which is colloquially referred to as 
fireproofing), the redistribution of the combustibles, and the response of the building structural 
components to the heat from the fires. To increase confidence in the simulation results, NIST used the 
visual evidence, eyewitness accounts from inside and outside the buildings, laboratory tests involving 
large fires and the heating of structural components, and formal statistical methods to identify influential 
parameters and quantify the variability in analysis results. 
• Combination of the knowledge gained into probable collapse sequences for each tower,2 the 
identification of factors that contributed to the collapses, and a list of factors that could have 
improved building performance or otherwise mitigated the loss of life. 
• Compilation of a list of findings that respond to the first three objectives and a list of 
recommendations that responds to the fourth objective. 
E.3 SUMMARY OF FINDINGS 
Objective 1: Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of 
the aircraft. 
• The two aircraft hit the towers at high speed and did considerable damage to principal 
structural components: core columns, floors, and perimeter columns. However, the towers 
withstood the impacts and would have remained standing were it not for the dislodged 
insulation (fireproofing) and the subsequent multifloor fires. The robustness of the perimeter 
frame-tube system and the large size of the buildings helped the towers withstand the impact. 
The structural system redistributed loads without collapsing in places of aircraft impact, 
avoiding larger scale damage upon impact. The hat truss, a feature atop each tower which was 
intended to support a television antenna, prevented earlier collapse of the building core. In 
each tower, a different combination of impact damage and heat-weakened structural 
components contributed to the abrupt structural collapse. 
• In WTC 1, the fires weakened the core columns and caused the floors on the south side of the 
building to sag. The floors pulled the heated south perimeter columns inward, reducing their 
capacity to support the building above. Their neighboring columns quickly became 
overloaded as columns on the south wall buckled. The top section of the building tilted to the 
south and began its descent. The time from aircraft impact to collapse initiation was largely 
2 
The focus of the Investigation was on the sequence of events from the instant of aircraft impact to the initiation of collapse for 
each tower. For brevity in this report, this sequence is referred to as the “probable collapse sequence,” although it does not 
actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse 
became inevitable. 
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Executive Summary Draft for Public Comment 
determined by how long it took for the fires to weaken the building core and to reach the 
south side of the building and weaken the perimeter columns and floors. 
• In WTC 2, the core was damaged severely at the southeast corner and was restrained by the 
east and south walls via the hat truss and the floors. The steady burning fires on the east side 
of the building caused the floors there to sag. The floors pulled the heated east perimeter 
columns inward, reducing their capacity to support the building above. Their neighboring 
columns quickly became overloaded as columns on the east wall buckled. The top section of 
the building tilted to the east and to the south and began its descent. The time from aircraft 
impact to collapse initiation was largely determined by the time for the fires to weaken the 
perimeter columns and floor assemblies on the east and the south sides of the building. WTC 
2 collapsed more quickly than WTC 1 because there was more aircraft damage to the building 
core and there were early and persistent fires on the east side of the building, where the 
aircraft had extensively dislodged insulation from the structural steel. 
• The WTC towers likely would not have collapsed under the combined effects of aircraft 
impact damage and the extensive, multifloor fires if the thermal insulation had not been 
widely dislodged or had been only minimally dislodged by aircraft impact. 
Objective 2: Determine why the injuries and fatalities were so high or low depending on location, 
including all technical aspects of fire protection, occupant behavior, evacuation, and emergency 
response. 
• Approximately 87 percent of the estimated 17,400 occupants of the towers, and 99 percent of 
those located below the impact floors, evacuated successfully. In WTC 1, where the aircraft 
destroyed all escape routes, 1,355 people were trapped in the upper floors when the building 
collapsed. One hundred seven people who were below the impact floors did not survive. 
Since the flow of people from the building had slowed considerably 20 min before the tower 
collapsed, the stairwell capacity was adequate to evacuate the occupants on that morning. 
• In WTC 2, before the second aircraft strike, about 3,000 people got low enough in the 
building to escape by a combination of self-evacuation and use of elevators. The aircraft 
destroyed the operation of the elevators and the use of two of the three stairways. Eighteen 
people from above the impact zone found a passage through the damaged third stairway and 
escaped. The other 619 people in or above the impact zone perished. Seven people who 
were below the impact floors did not survive. As in WTC 1, shortly before collapse, the flow 
of people from the building had slowed considerably, indicating that the stairwell capacity 
was adequate that morning. 
• About 6 percent of the survivors described themselves as mobility impaired, with recent 
injury and chronic illness being the most common causes; few, however, required a 
wheelchair. Among the 118 decedents below the aircraft impact floors, investigators 
identified seven who were mobility challenged, but were unable to determine the mobility 
capability of the remaining 111. 
• A principal factor limiting the loss of life was that the buildings were only one-third occupied 
at the time of the attacks. NIST estimated that if the towers had been fully occupied with 
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Draft for Public Comment Executive Summary 
25,000 occupants each, it would have taken about 4 hours to evacuate the buildings and over 
14,000 people might have perished because the stairwell capacity would not have been 
sufficient to evacuate that many people in the available time. Egress capacity required by 
current building codes is determined by single floor calculations that are independent of 
building height and does not consider the time for full building evacuation. 
• Due to the presence of assembly use spaces at the top of each tower that were designed to 
accommodate over 1,000 occupants per floor for the Windows on the World restaurant 
complex and the Top of the Deck observation deck, the New York City Building Code would 
have required a minimum of four independent means of egress (stairs), one more than the 
three that were available in the buildings. Given the low occupancy level on 
September 11, 2001, NIST found that the issue of egress capacity from these places of 
assembly, or from elsewhere in the buildings, was not a significant factor on that day. It is 
conceivable that such a fourth stairwell, depending on its location and the effects of aircraft 
impact on its functional integrity, could have remained passable, allowing evacuation by an 
unknown number of additional occupants from above the floors of impact. If the buildings 
had been filled to their capacity with 25,000 occupants, however, the required fourth stairway 
would likely have mitigated the insufficient egress capacity for conducting a full building 
evacuation within the available time. 
• Evacuation was assisted by participation in fire drills within the previous year by two-thirds 
of survivors and perhaps hindered by a Local Law that prevented employers from requiring 
occupants to practice using the stairways. The stairways were not easily navigated in some 
locations due to their design, which included “transfer hallways,” where evacuees had to 
traverse from one stairway to another location where the stairs continued. Additionally, 
many occupants were unprepared for the physical challenge of full building evacuation. 
• The functional integrity and survivability of the stairwells was affected by the separation of 
the stairwells and the structural integrity of stairwell enclosures. In the impact region of 
WTC 1, the stairwell separation was the smallest over the building height—clustered well 
within the building core—and all stairwells were destroyed by the aircraft impact. By 
contrast, the separation of stairwells in the impact region of WTC 2 was the largest over the 
building height—located along different boundaries of the building core—and one of three 
stairwells remained marginally passable after the aircraft impact. The shaft enclosures were 
fire rated but were not required to have structural integrity under typical accidental loads: 
there were numerous reports of stairwells obstructed by fallen debris from damaged 
enclosures. 
• The fire safety systems (sprinklers, smoke purge, and fire alarms,) were designed to meet or 
exceed current practice. However, they played no role in the safety of life on September 11 
because the water supplies to the sprinklers were fed by a single supply pipe that was 
damaged by the aircraft impact. The smoke purge systems were designed for use by the fire 
department after fires; they were not turned on but they also would have been ineffective due 
to aircraft damage. The violence of the aircraft impact served as its own alarm. In WTC 2, 
contradictory public address announcements contributed to occupant confusion and some 
delay in occupants beginning to evacuate. 
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Executive Summary Draft for Public Comment 
• For the approximately 1,000 emergency responders on the scene, this was the largest disaster 
they had even seen. Despite attempts by the responding agencies to work together and 
perform their own tasks, the extent of the incident was well beyond their capabilities. 
Communications were erratic due to the high number of calls and the inadequate performance 
of some of the gear. Even so, there was no way to digest, test for accuracy, and disseminate 
the vast amount of information being received. Their jobs were complicated by the loss of 
command centers in WTC 7 and then in the towers after WTC 2 collapsed. With nearly all 
elevator service disrupted and progress up the stairs taking about 2 min per floor, it would 
have taken hours for the responders to reach their destinations, assist survivors, and escape 
had the towers not collapsed. 
Objective 3: Determine what procedures and practices were used in the design, construction, 
operation, and maintenance of WTC 1 and WTC 2. 
• Because of The Port Authority's establishment under a clause of the United States 
Constitution, its buildings were not subject to any external building code. The buildings were 
unlike any others previously built, both in their height and in their innovative structural 
features. Nevertheless, the actual design and approval process produced two buildings that 
generally were consistent with nearly all of the provisions of the New York City Building 
Code and other building codes of that time. The loads for which the buildings were designed 
exceeded the code requirements. The quality of the structural steels was consistent with the 
building specifications. The departures from the building codes and standards did not have a 
significant effect on the outcome of September 11. 
• For the floor systems, the fire rating and insulation thickness used on the floor trusses, which 
together with the concrete slab served as the main source of support for the floors, were of 
concern from the time of initial construction. NIST found no technical basis or test data on 
which the thermal protection of the steel was based. On September 11, 2001, the minimum 
specified thickness of the insulation was adequate to delay heating of the trusses; the amount 
of insulation dislodged by the aircraft impact, however, was sufficient to cause the structural 
steel to be heated to critical levels. 
• Based on four standard fire resistance tests that were conducted under a range of insulation 
and test conditions, NIST found the fire rating of the floor system to vary between 3/4 hour 
and 2 hours; in all cases, the floors continued to support the full design load without collapse 
for over 2 hours. 
• The wind loads used for the WTC towers, which governed the structural design of the 
external columns and provided the baseline capacity of the structures to withstand abnormal 
events such as major fires or impact damage, significantly exceeded the requirements of the 
New York City Building Code and selected other building codes of the day. Two sets of 
wind load estimates for the towers obtained by independent commercial consultants in 2002, 
however, differed by as much as 40 percent. These estimates were based on wind tunnel tests 
conducted as part of insurance litigation unrelated to the Investigation. 
E.4 RECOMMENDATIONS 
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Draft for Public Comment Executive Summary 
The tragic consequences of the September 11, 2001, attacks were directly attributable to the fact that 
terrorists flew large jet-fuel laden commercial airliners into the WTC towers. Buildings for use by the 
general population are not designed to withstand attacks of such severity; building codes do not require 
building designs to consider aircraft impact. In our cities, there has been no experience with a disaster of 
such magnitude, nor has there been any in which the total collapse of a high-rise building occurred so 
rapidly and with little warning. 
While there were unique aspects to the design of the WTC towers and the terrorist attacks of 
September 11, 2001, NIST has compiled a list of recommendations to improve the safety of tall buildings, 
occupants, and emergency responders based on its investigation of the procedures and practices that were 
used for the WTC towers; these procedures and practices are commonly used in the design, construction, 
operation, and maintenance of buildings under normal conditions. Public officials and building owners 
will need to determine appropriate performance requirements for those tall buildings, and selected other 
buildings, that are at higher risk due to their iconic status, critical function, or design. 
The topics of the recommendations in eight groups are listed in Table E–1. The ordering does not reflect 
any priority. 
The eight major groups of recommendations are: 
• Increased Structural Integrity: The standards for estimating the load effects of potential 
hazards (e.g., progressive collapse, wind) and the design of structural systems to mitigate the 
effects of those hazards should be improved to enhance structural integrity. 
• Enhanced Fire Resistance of Structures: The procedures and practices used to ensure the fire 
resistance of structures should be enhanced by improving the technical basis for construction 
classifications and fire resistance ratings, improving the technical basis for standard fire 
resistance testing methods, use of the “structural frame” approach to fire resistance ratings, 
and developing in-service performance requirements and conformance criteria for sprayapplied 
fire resistive materials. 
• New Methods for Fire Resistance Design of Structures: The procedures and practices used in 
the fire resistance design of structures should be enhanced by requiring an objective that 
uncontrolled fires result in burnout without local or global collapse. Performance-based 
methods are an alternative to prescriptive design methods. This effort should include the 
development and evaluation of new fire resistive coating materials and technologies and 
evaluation of the fire performance of conventional and high-performance structural materials. 
echnical and standards barriers to the introduction of new materials and technologies should 
be eliminated. 
• Improved Active Fire Protection: Active fire protection systems (i.e., sprinklers, standpipes/ 
hoses, fire alarms, and smoke management systems) should be enhanced through 
improvements to design, performance, reliability, and redundancy of such systems. 
• Improved Building Evacuation: Building evacuation should be improved to include system 
designs that facilitate safe and rapid egress, methods for ensuring clear and timely emergency 
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Executive Summary Draft for Public Comment 
communications to occupants, better occupant preparedness for evacuation during 
emergencies, and incorporation of appropriate egress technologies. 
• Improved Emergency Response: Technologies and procedures for emergency response 
should be improved to enable better access to buildings, response operations, emergency 
communications, and command and control in large-scale emergencies. 
• Improved Procedures and Practices: The procedures and practices used in the design, 
construction, maintenance, and operation of buildings should be improved to include 
encouraging code compliance by nongovernmental and quasi-governmental entities, adoption 
and application of egress and sprinkler requirements in codes for existing buildings, and 
retention and availability of building documents over the life of a building. 
• Education and Training: The professional skills of building and fire safety professionals 
should be upgraded though a national education and training effort for fire protection 
engineers, structural engineers, and architects. 
The recommendations call for action by specific entities regarding standards, codes and regulations, their 
adoption and enforcement, professional practices, education, and training; and research and development. 
Only when each of the entities carries out its role will the implementation of a recommendation be 
effective. 
The recommendations do not prescribe specific systems, materials, or technologies. Instead, NIST 
encourages competition among alternatives that can meet performance requirements. The 
recommendations also do not prescribe specific threshold levels; NIST believes that this responsibility 
properly falls within the purview of the public policy setting process, in which the standards and codes 
development process plays a key role. 
NIST strongly urges that immediate and serious consideration be given to these recommendations by the 
building and fire safety communities in order to achieve appropriate improvements in the way buildings 
are designed, constructed, maintained, and used and in evacuation and emergency response procedures— 
with the goal of making buildings, occupants, and first responders safer in future emergencies. 
NIST also strongly urges building owners and public officials to (1) evaluate the safety implications of 
these recommendations to their existing inventory of buildings and (2) take the steps necessary to mitigate 
any unwarranted risks without waiting for changes to occur in codes, standards, and practices. 
NIST further urges state and local agencies, well trained and managed, to rigorously enforce building 
codes and standards since such enforcement is critical to ensure the expected level of safety. Unless they 
are complied with, the best codes and standards cannot protect occupants, emergency responders, or 
buildings. 
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Table E–1. Topics of NIST recommendations for improved public safety in tall and high-risk buildings. 
Responsible Community Application 
Relation to 
9/11 
Outcome 
Recommendation 
Group Recommendation Topic 
PracticesStandards, 
Codes, 
RegulationsAdoption &
Enforcement 
R&D/Further 
StudyEducation & 
Training 
All Tall Buildings 
Selected Other 
High-RiskBuildings 
RelatedaUnrelatedb 
Increased Prevention of progressive collapse and failure analysis of complex systems 
9 
9 
9 
9 
9 
Structural Estimation of wind loads and their effects on tall buildings 
9 
9 
9 
Integrity Allowable tall buildings sway 
9 
9 
9 
Enhanced Fire Fire resistance rating requirements and construction classification 
9 
9 
9 
Resistance of 
Structures 
Fire resistance testing of building components and extrapolation of test data to 
qualify untested building components 
9 
9 
In-service performance requirements and inspection procedures for sprayed fire 
resistive materials (SFRM or spray-on fireproofing) 
9 
9 
9 
9 
“Structural frame” approach (structural members connected to columns carry the 
higher fire resistance rating of the columns) 
9 
9 
9 
New Methods for Burnout without local or global structural collapse in uncontrolled building fires 
9 
9 
9 
9 
9 
Fire Resistance Performance-based design and retrofit of structures to resist fires 
9 
9 
9 
9 
Design of 
Structures 
New fire-resistive coating materials, systems, and technologies 
9 
9 
9 
Evaluation of high performance structural materials under conditions expected in 
building fires 
9 
9 
Improved Active 
Fire Protection 
Performance and redundancy of active fire protection systems to accommodate 
the greater risks associated with tall buildings 
9 
9 
9 
Advanced fire alarm and communication systems that provide continuous, 
reliable, and accurate information on life safety conditions to manage the 
evacuation process. 
9 
9 
Advanced fire/emergency control panels with more reliable information from the 
active fire protection systems to provide tactical decision aids 
9 
9 
Improved transmission to emergency responders, and off-site or black box 
storage, of information from building monitoring systems 
9 
9 
9 
NIST NCSTAR 1, WTC Investigation 

Executive Summary Draft for Public Comment 
Recommendation 
Group Recommendation Topic 
Responsible Community Application 
Relation to 
9/11 
Outcome 
PracticesStandards, 
Codes, 
RegulationsAdoption &
Enforcement 
R&D/Further 
StudyEducation & 
Training 
All Tall Buildings 
Selected Other 
High-RiskBuildings 
Related aUnrelatedb 
Improved 
Building 
Public education campaigns to improve building occupants’ preparedness for 
evacuation 
9 
9 
9 
9 
Evacuation Tall building design for timely full building emergency evacuation of occupants 
9 
9 
9 
9 
Design of occupant-friendly evacuation paths that maintain functionality in 
foreseeable emergencies 
9 
9 
Planning for communication of accurate emergency information to building 
occupants 
9 
9 
9 
Evaluation of alternative evacuation technologies, to allow all occupants equal 
opportunity for evacuation and to facilitate emergency response access 
9 
9 
9 
Improved Fire-protected and structurally hardened elevators 
9 
9 
9 
Emergency Effective emergency communications systems for large-scale emergencies 
9 
9 
9 
9 
9 
Response Enhanced gathering, processing, and delivering of critical information to 
emergency responders 
9 
9 
9 
9 
9 
Effective and uninterrupted operation of the command and control system for 
large-scale building emergencies 
9 
9 
9 
9 
9 
Improved 
Procedures and 
Provision of code-equivalent level of safety and certification of as-designed and 
as-built safety by nongovernmental and quasi-governmental entities 
9 
9 
9 
Practices Egress and sprinkler requirements for existing buildings 
9 
9 
Retention and off-site storage of design, construction, maintenance, and 
modification documents over the entire life of the building; and availability of 
relevant building information for use by responders in emergencies 
9 
9 
9 
Design professional responsibility for innovative or unusual structural and fire 
safety systems 
9 
9 
9 
Continuing 
Education and 
Professional cross training of fire protection engineers, architects, and structural 
engineers 
9 
9 
9 
Training Training in computational fire dynamics and thermostructural analysis 9 
9 
a. If in place, could have changed the outcome on September 11, 2001. 
b. Would not have changed the outcome, yet is an important building and fire safety issue that was identified during the course of the Investigation. 
l NIST NCSTAR 1, WTC Investigation 

PART I: SEPTEMBER 11, 2001 

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NIST NCSTAR 1, WTC Investigation 

Chapter 1 
NEW YORK CITY’S WORLD TRADE CENTER 
1.1 THE ORIGINATION 
In 1960, American technology was on the rise, and internationalism was a prominent theme. It was in 
this technical and global political context and this year that the planning began for a World Trade Center 
(WTC) to be located in lower Manhattan. From its first conception during the 1939 World’s Fair, it now 
emerged under the powerful advocacy of the Chase Manhattan Bank’s David Rockefeller. Here was a 
grand plan that would embody the concept of New York City as a center of world commerce and provide 
a home for numerous international trade companies. 
The organization that would build the World Trade Center was The Port of New York Authority, later to 
be renamed as The Port Authority of New York and New Jersey (Port Authority, PANYNJ). Created in 
1921, under a clause in the United States Constitution, to run the multijurisdictional commercial zones in 
the region, The Port Authority built and operated facilities on the banks of the Port of New York’s 
waterways, the bridges to cross them, and the major metropolitan airports. It had the authority to obtain 
land by eminent domain and to raise funds for its projects. Now, under the leadership of its Executive 
Director, Austin Tobin, the concept for the World Trade Center grew from the grand plan of David 
Rockefeller to the grandeur of the world's largest office complex. 
To fulfill all the functional, aesthetic, and economic desires for this concept, innovative architecture was 
needed. In 1962, the firm of Minoru Yamasaki & Associates was hired to perform the architectural 
design, which was first unveiled in 1964. The team also involved Emory Roth & Sons, P.C., as the 
architect of record.1 The structural engineering was by Worthington, Skilling, Helle and Christiansen. 
(Some time after completion of the construction, Skilling, Helle, Christiansen, and Robertson, and then 
Leslie E. Robertson Associates (LERA) assumed that role.) Jaros, Baum & Bollers were hired as the 
mechanical engineers, and Joseph R. Loring & Associates were the electrical engineers. Tishman 
Construction Corporation was the general contractor. 
In 1966, the formal groundbreaking for the towers took place. Construction began in 1968, with the first 
occupancy in 1970. These dates establish the historical context for the building codes and the state of 
practice under which the complex was designed and constructed. This will be discussed further in Part II. 
1 
The functions of these entities are as follows. In New York City, a permit, issued by the building commissioner, is required to 
construct, alter, repair, demolish or remove any building. The architect who signs and generally files the plans (as part of the 
process for securing the permit) and takes the lead role of a project is the architect-of-record. Specific subsets of plans may be 
signed by the structural, electrical, and mechanical engineers, representing the separate disciplines involved in those subsets. 
The filed plans are reviewed and approved for compliance with the building code requirements by the building commissioner 
before issuance of the permit. 
The City of New York had no jurisdiction. However, The Port Authority required that all the WTC tower plans be submitted 
for their review and approval for code compliance and other architectural requirements. The responsibility of technical 
correctness rested with the architect-of-record and the engineers-of-record. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
The expected tenancy by companies involved in international trade did not materialize as conceived, so 
the State of New York, the City of New York, and The Port Authority became the principal WTC tenants 
in the 1970s. As the years passed, however, the prestige of the address grew, and the requirement that 
occupants be involved in international trade was relaxed. At the end of the twentieth century, the World 
Trade Center was nearly fully occupied by a diverse mixture of large and small businesses and federal, 
state, and city government organizations. 
1.2 THE WORLD TRADE CENTER COMPLEX 
1.2.1 The Site 
By 2001, the WTC complex had become an integral part of Manhattan. It was composed of seven 
buildings (here referred to as WTC 1 through WTC 7) on a site toward the southwest tip of Manhattan 
Island (Figures 1–1 and 1–2). Whether viewed from close up, from the Statue of Liberty across the Upper 
Bay or from an aircraft descending to LaGuardia Airport, the towers were a sight to behold. The two 
towers, WTC 1 (North Tower) and WTC 2 (South Tower), were each 110 stories high, dwarfing the other 
skyscrapers in lower Manhattan and seemingly extending to all Manhattan the definition of “tall” 
previously set by midtown's Empire State Building. WTC 3, a Marriott Hotel, was 22 stories tall, WTC 4 
(South Plaza Building) and WTC 5 (North Plaza Building) were each 9-story office buildings, and 
WTC 6 (U.S. Customs House) was an 8-story office building. These six buildings were built around a 
5-acre Plaza named in honor of Austin Tobin. WTC 7 was a 47-story office building on Port Authority 
land across Vesey Street on the north side of the Plaza complex. Built over the ConEd substation serving 
the WTC complex, it was completed in 1987 and was operated by Silverstein Properties, Inc. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
Figure 1–1. The World Trade Center in Lower Manhattan. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
Source: The Imagers Team, NASA/GSFC. 
Figure 1–2. Lower Manhattan and the World Trade Center towers. 
Below the 11 western acres of the site, underneath a large portion of the Plaza and WTC 1, WTC 2, 
WTC 3, and WTC 6, was a 6-story underground structure. The structure was surrounded by a wall that 
extended from ground level down 70 ft to bedrock. Holding back the waters of the Hudson River, this 
wall had enabled rapid excavation for the foundation and continued to keep groundwater from flooding 
the underground levels. 
Commuter trains brought tens of thousands of workers and visitors to Manhattan from Brooklyn and 
New Jersey into a new underground station below the plaza. A series of escalators and elevators took the 
WTC employees directly to an underground shopping mall and to the Concourse Level of the towers. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
1.2.2 The Towers 
The Buildings 
The focus of the complex was on the two towers, each taller than any other building in the world at that 
time. The roof of WTC 1 was 1,368 ft above the Concourse Level, 6 ft taller than WTC 2, and supported 
a 360 ft tall antenna mast for television and radio transmission. The footprint of each tower was a square, 
about 210 ft on a side (approximately an acre), with the corners of the tower beveled 9 ft 9 in. Internally, 
each floor was a square, about 206 ft on a side.2 
The superb vistas from the top of such buildings virtually demanded public space from which to view 
them, and The Port Authority responded. The 107th floor of WTC 1 housed a gourmet restaurant and bar 
with views of the Hudson River and New Jersey to the west, the skyscrapers of midtown Manhattan to the 
north, the East River and Queens and Brooklyn to the east, the Statue of Liberty to the southwest, and the 
Atlantic Ocean to the south. Similar views could be seen from observation decks on the 107th floor and 
the roof of WTC 2. 
Table 1–1 shows the use of the floors, which was similar but not identical in the two towers. 
Table 1–1. Use of floors in the WTC towers. 
Floor(s) WTC 1 WTC 2 
Roof Antenna space and window washing 
equipment 
Outdoor observation deck and window 
washing equipment 
110 Television studios Mechanical equipment 
108, 109 Mechanical equipment Mechanical equipment 
107 Windows on the World restaurant Indoor observation deck 
106 Catering Tenant space 
79 through 105 Tenant space Tenant space 
78 Skylobby, tenant space Skylobby, tenant space 
77 Tenant space Tenant space 
75, 76 Mechanical equipment Mechanical equipment 
45 through 74 Tenant space Tenant space 
44 Skylobby, cafeteria, tenant space Skylobby, tenant space 
43 Port Authority space Tenant space 
41, 42 Mechanical equipment Mechanical equipment 
9 through 40 Tenant space Tenant space 
7, 8 Mechanical floors Mechanical floors 
Concourse through 6 6-story lobby 6-story lobby 
2 
Extensive details regarding all aspects of this report are found in the supporting Investigation reports listed in the Preface. A 
subject index of those reports appears as Appendix B to this report. Those reports, in turn, cite the numerous documents made 
available to the Investigation Team. To maintain continuity, citations of the source documents are not included in this report. 
They are found in the supporting Investigation reports. 
NIST NCSTAR 1, WTC Investigation 5 

Chapter 1 Draft for Public Comment 
The Port Authority had managed the operation of the two towers since their opening three decades earlier. 
Silverstein Properties acquired a 99-year lease on the towers in July 2001. 
The Structures 
Each of the tenant floors of the towers was intended to offer a large In 1945, a B-25 bomber 
expanse of workspace, virtually uninterrupted by columns or walls. had become lost in the fog 
This called for an innovative structural design, lightweight to minimize and struck the 78th and 79th 
the total mass of 110 stories, yet strong enough to support the huge floors of the Empire State 
building with all its furnishings and people. Structural engineers refer to Building. The building 
the building weight as the dead load; the people and furnishings are withstood the impact and 
ensuing fire and was ready 
called the live load. Collectively, these are referred to as gravity loads. for reoccupancy the 
The buildings would also need to resist lateral loads and excessive following week. 
swaying, principally from the hurricane force winds that periodically 
strike the eastern seaboard of the United States. An additional load, 
stated by The Port Authority to have been considered in the design of the towers, was the impact of a 
Boeing 707, the largest commercial airliner when the towers were designed, hitting the building at its full 
speed of 600 mph. 
Skilling and his team rose to the challenge of providing the required load capacity within Yamasaki's 
design concept. They incorporated an innovative framed-tube concept for the structural system. The 
columns supporting the building were located both along the external faces and within the core. The core 
also contained the elevators, stairwells, and utility shafts. The dense array of columns along the building 
perimeter was to resist the lateral load due to hurricane-force winds, while also sharing the gravity loads 
about equally with the core columns. The floor system was to provide stiffness and stability to the 
framed-tube system in addition to supporting the floor loads. Extensive and detailed studies were 
conducted in wind tunnels, instead of relying on specific, prescriptive building code requirements, to 
estimate the wind loads used in the design of these buildings.3 This 
approach took advantage of the allowance by some state and local A grade of steel is 
building codes for alternative designs and construction if evidence characterized by its yield 
were presented that ensured equivalent performance. strength, expressed in ksi, or 
thousands of pounds per 
square inch. This is the 
There were four major structural subsystems in the towers, referred to force per unit area at which 
as the exterior wall, the core, the floor system, and the hat truss. The the steel begins to undergo 
first, the exterior structural subsystem, was a vertical square tube that a permanent deformation. 
consisted of 236 narrow columns, 59 on each face from the 10th floor Different steel strengths, or 
to the 107th floor (Figure 1–3). There were also columns on alternate grades, are manufactured by 
stories at each of the beveled corners, but these carried none of the varying the chemistry and 
processing of the alloy. 
7
gravity loads. (There were fewer, wider-spaced columns below the Higher strength steel is used th floor to accommodate doorways.) Each column was fabricated by when the design calls for 
welding four steel plates to form a tall box, nominally 14 in. on a side. more strength per weight of 
The space between the steel columns was 26 in., with a narrower, the steel column or beam. 
The studies showed that each tower affected the wind loads on the other. This effect was not accounted for in the prescriptive 
codes. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
.WTC 1 
WTC 2 
. 
Figure 1–3. Tower floor 
plans with column 
numbers. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
framed plate glass window in each gap. Adjacent columns were connected at each floor by steel spandrel 
plates, 52 in. high. The upper parts of the buildings had less wind load and building mass to support. 
Thus, on higher floors, the thickness of the steel plates making up the columns decreased, becoming as 
thin as ¼ in. near the top. There were 10 grades of steel used for the columns and spandrels, with yield 
strengths ranging from 36 ksi to 100 ksi. The grade of steel used in each location was dictated by the 
calculated stresses due to the gravity and wind loads. 
All the exterior columns and spandrels were prefabricated into welded panels, three stories tall and three 
columns wide. The panels, each numbered to identify its location in the tower, were then bolted to 
adjacent units to form the walls (Figure 1–4). The use of identically shaped prefabricated elements was 
itself an innovation that enabled rapid construction. The high degree of modularization and prefabrication 
used in the construction of these buildings and the identification, tracking, and logistics necessary to 
ensure that each piece was positioned correctly was unprecedented. 
SpandrelTruss 
Seat 
Main 
Truss 
Metal 
Deck 
StrapsKnuckle 
Connection 
Connection 
Truss 
Seat 
Spandrel 
Source: Unknown. Enhanced by NIST. 
Column 
Damper 
Damper 
Figure 1–4. Perimeter column/spandrel assembly and floor structure. 
A second structural subsystem was located in a central service area, or core (Figure 1–5), approximately 
135 ft by 87 ft, that extended virtually the full height of the building. The long axis of the core in WTC 1 
was oriented in the east-west direction, while the long axis of the core in WTC 2 was oriented in the 
north-south direction (Figure 1–3). The 47 columns in this rectangular space were fabricated using 
primarily 36 ksi and 42 ksi steels and also decreased in size at the higher stories. The four massive corner 
columns bore nearly one-fifth of the total gravity load on the core columns. The core columns were 
interconnected by a grid of conventional steel beams to support the core floors. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
101 110 159140 150130120 
259AC 
B 
50 
89 
88 
87 
90 
91 
92 
95 96 
97 
9893 
94 
501 508 
1001 
1008 
201 
210 
240 
250230220 
401 
410 
459440 
450430420 
359 350 340 330 320 310 301
WTC1 96
Note: Column numbers are shown around the perimeter. The four corner core columns (501, 508, 1001, and 1008) 
are marked for orientation. Stairwells A, B, and C are shown in red stripes. The fourth red-striped area is the tenant's 
convenience stairwell that connected the 95th through 97th floors in WTC 1; it was not considered part of the egress 
system. The remaining numbers denote specific elevators. Much of the rest of the floor was open space suited for 
offices, conference rooms, or office cubicles. The arrangement and size of the core varied among the different floors.
Figure 1–5. Plan of the 96th floor of WTC 1 showing the core and tenant spaces. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
The third major structural subsystem was the floors in the tenant spaces. These floors supported their 
own weight, along with live loads, provided lateral stability to the exterior walls, and distributed wind 
loads among the exterior walls. The floor construction was an innovation for a tall building. As shown in 
Figure 1–6, each tenant floor consisted of 4 in. thick, lightweight cast-in-place concrete on a fluted steel 
deck, but that is where "ordinary" ended. Supporting the slab was a grid of lightweight steel bar trusses. 
The top bends (or “knuckles”) of the main truss webs extended 3 in. above the top chord and were 
embedded into the concrete floor slab. This concrete and steel assembly thus functioned as a composite 
unit, that is, the concrete slab acted integrally with the steel trusses to carry bending loads. The primary 
truss pairs were either 60 ft or 35 ft long and were spaced at 6 ft 8 in. There were perpendicular bridging 
trusses every 13 ft 4 in. The floor trusses and fluted metal deck were prefabricated in panels that were 
typically 20 ft wide and that were hoisted into position in a fashion similar to the exterior wall panels. 
Figure 1–6. Schematic of composite floor truss system 
The bottom chords were connected to the spandrel plates by devices that were called viscoelastic 
dampers. Experiments on motion perception, conducted with human subjects, had shown a high potential 
for occupant discomfort when the building swayed in a strong wind. When the tower was buffeted by 
strong winds, these dampers absorbed energy, reducing the sway and the vibration expected from a 
building that tall. The use of such vibration damping devices in buildings was an innovation at that time. 
The fourth major structural subsystem was located from the 107th floor to the roof of each tower. It was a 
set of steel braces, collectively referred to as the “hat truss” (Figure 1–7). Its primary purpose had been to 
support a tall antenna atop each tower, although only WTC 1 had one installed. The hat truss provided 
additional connections among the core columns and between the core and perimeter columns, providing 
additional means for load redistribution. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
Figure 1–7. Schematic of a hat truss. 
Fire Protection Systems 
High-rise buildings in the United States are designed to meet requirements intended, among other 
objectives, to enable the building to suffer a sizable fire and still remain standing. The requirements are 
expressed in terms of fire resistance ratings, given in units of time. 
The fire resistance of a column, wall, or floor design is rated by subjecting the assembly to standard 
heating conditions. A sample section of a wall to be tested is installed as one face of a furnace; a floor 
becomes the top of the furnace. Beams are normally rated as a part of the floor test. Floor systems are 
always tested while carrying their full design load. Walls are similarly loaded if they are intended to be 
load bearing, but are not loaded if the only load they are intended to support is their own weight. In the 
United States, columns are required to be loaded during the test, however, an alternative is often used, 
whereby the columns is not loaded and the temperature of the steel is used as a limiting criterion. 
It is widely recognized in the building profession that fire resistance ratings, although expressed in hours, 
do not mean that the structure will sustain its performance for that length of time in a real fire. Actual fire 
performance may be greater or less than that achieved in the test furnace, depending on the severity of the 
actual fire exposure. Rather, these are taken as relative ratings, e.g., a wall rated at 2 hours will block the 
spread of a fire longer than a wall rated at 1 hour. 
Bare structural steel components, when exposed to a large and sustained fire, can heat rapidly to the point 
where their ability to support their load is compromised. Thus, insulation is usually employed to 
encapsulate the steel and thus delay the heating of the steel. In the WTC towers, a major fraction of the 
core columns were enclosed or protected on several sides by sheets of gypsum wallboard. The trusses, 
perimeter columns, spandrels, and one or more surfaces of the core columns were coated with one of 
three different spray-applied fire resistive materials (SFRMs). In this report, these materials are 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
collectively referred to as “insulation.”4 The thickness of the wallboard or the SFRM was selected to 
provide an intended level of thermal protection. Figure 1–8 shows the appearance of a floor truss with 
sprayed insulation. 
Figure 1–8. Photograph of insulated WTC trusses. 
Further protection of the building against a fire was provided in part by internal, nonstructural, fire-rated 
walls. These floor-slab-to-floor-slab partitions, called demising walls, separated the tenant spaces from 
each other and from the core area. Their function was to keep a fire from spreading long enough for the 
fire to be extinguished. In a 1975 fire in WTC 1, these walls significantly confined the fire. 
There were three types of nonstructural walls in the towers. The stairwells and elevator shafts were 
surrounded by 2 in. thick, tongue-and-groove, cast gypsum panels, covered with two or three sheets of 5/8 
in. gypsum board. The demising walls were made of two sheets of 5/8 in. thick gypsum wallboard on 
each side of steel studs. These are commonly regarded as providing a 2 hour fire separation. Walls in the 
interior of the tenant spaces generally extended from the floor slab to the bottom of the drop ceiling and 
were made of single sheets of 5/8 in. gypsum wallboard over steel studs. These walls were not fire-rated. 
For some conference rooms and other spaces where sound barriers were desired, the walls extended to 
bottom of the floor slab above, in which case they were regarded as providing a 1 hour fire separation. 
In addition to these methods of passive fire protection, there were components that would be activated in 
the event of a fire. Automatic sprinklers had been installed on all of the roughly 40,000 ft2-sized floors, 
capable of controlling local fires totaling an aggregate floor area of up to 4500 ft2. In addition, there were 
The materials used to insulate structural steel are sometimes colloquially referred to as "fireproofing," referring to the intent of 
the material, rather than the property it imparts. Since an important facet of this Investigation was the determination of the 
sufficiency of the insulation in protecting the steel from the heat of the fires, this report does not pre-judge the quality of the 
material by using the colloquial term. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
standpipes (for firefighters to connect their hoses) in the stairwells on each floor, gravity fed from 20,000 
gal of stored water and by three large water pumps. A multifunction fire alarm system was intended to 
alert staff at the Fire Command Station within the building and provide voice and strobe alerts 
throughout. When turned on after the building had been cleared of people, a smoke purge system was 
intended to purge the hot, opaque fire gases from the stairwells. 
However, buildings were not (and still are not) required by the building codes or designed to withstand 
the impact of a fuel-laden jetliner. Although the impact of a Boeing 707 was stated by the Port Authority 
to have been considered in the original design of the towers, only one three-page document, in a format 
typically used for talking points was found that addressed the issue. This document stated that such a 
collision would result in only local damage and could not cause collapse or substantial damage to the 
building. NIST was unable to locate any evidence to indicate consideration of the extent of impactinduced 
structural damage or the size of a fire that could be created by thousands of gallons of jet fuel. 
The Workplace 
At the beginning of the workday, many of the roughly 50,000 people who worked in the towers and 
visited to conduct business or to tour emerged from trains in the massive subterranean station. They 
would take escalators and elevators to a one-story shopping mall, then pass through revolving doors to 
enter a spacious, 6-story-high lobby on the Concourse Level. There, they would cross paths with those 
who arrived on foot or by bus or cab. 
Getting tens of thousands of people from the Concourse to their offices was no small task. This was 
accomplished by a combination of express and local elevators located within each of the building cores 
(Figure 1–9) that was novel at the time of construction. 
• People traveling to floors 9 through 40 entered a bank of 24 local elevators at the Concourse 
Level. These were divided into four groups, with each stopping at a different set of eight or 
nine floors (9 through 16, 17 through 24, 25 through 31, and 32 through 40). 
• Those going to floors 44 through 74 took one of eight express elevators to the 44th floor 
skylobby before transferring to one of 24 local elevators. These 24 were stacked on top of 
the lower bank of 24, providing additional transport without increasing the floor space 
occupied by the elevators. 
• Those going to floors 78 through 107 took one of 11 express elevators from the Concourse 
Level to the 78th floor before transferring to one of 24 local elevators. These were also 
stacked on the lower banks of 24 local elevators. 
To provide the desired high rate of people movement, this three-tier system used roughly 25 percent less 
of the building footprint than the conventional systems in which all elevators ran from the Concourse to 
the top of the building, resulting in a building core that took up as much as one-half of the floor area. In 
addition, there was even more rentable space to be gained. At the top of each elevator bank, the 
machinery to lift the cabs occupied one additional floor. From the next floor up to the bottom of the next 
bank, there was no need for an elevator shaft. The concrete floor was extended into this space, providing 
additional rentable floor area. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
Figure 1–9. Schematic of the three-tier elevator system. 
There were two additional express elevators to the Windows on the World restaurant (and related 
conference rooms and banquet facilities) in WTC 1 and to the observation deck in WTC 2. There were 
also five local elevators: three that brought people from the subterranean levels to the lobby, one that ran 
between floors 106 and 110, and one that ran between floors 43 and 44 (in WTC 1), serving the cafeteria 
from the skylobby. There were also eight freight elevators, one of which served all floors. All elevators 
had been upgraded to incorporate firefighter emergency operation requirements. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
Also within the core were three sets of stairs that extended nearly the full 
height of the tower (Figure 1–10). However, the stairwell at an upper Following the February 
floor did not continuously descend to the lobby, but rather to horizontal 26, 1993, bombing of 
corridors in the vicinity of the mechanical floors. These were enclosed WTC 1 and in light of the 
4 hours needed to 
corridors that ranged in length from about 10 ft to about 100 ft. (As a evacuate the building, 
result of these and the tiered elevator system, the core arrangements several improvements 
varied substantially from floor to floor.) After traversing each of these, had been made to the 
the pedestrians would resume their descent. The advantages of moving stairwells: battery 
stairwell locations included reclaiming core space for occupant use above operated emergency 
lighting, photoluminescent 
terminated elevator shafts and overcoming obstructions posed by floor strips indicating the 
equipment installed on mechanical floors. path to be followed, and 
explicit signs on each 
Upon exiting the elevators or stairs, the interior view was typical of high-doorway to indicate where 
rise buildings. Surrounding the rectangular core corridor was a mixture it led. 
of walls, entry doors to firms, and glass-front reception areas. Above was 
a drop ceiling. 
Many of the floors were occupied by a single tenant. Some of these tenants occupied multiple floors. By 
2001, most of these companies had moved in after the installation of automatic sprinklers, which had 
allowed the absence of internal partitions. These companies largely took advantage of Yamasaki's design 
concept of a vast space that was nearly free of obstructions. The open arrangement often included as 
many as 200 or more individual modular workstations or office cubicles, generally clustered in groups of 
six or eight (Figure 1–11). Trading floors had arrays of long tables with multiple computer screens 
(Figure 1–12). Some of these floors had a few executive offices in the corners and along the perimeter. 
Many also had walled conference rooms. It was common for the tenants occupying multiple floors to 
create openings in the floor slabs and install convenience stairs between their floors. 
Some floors were subdivided to accommodate as many as 20 firms. Some of the smaller firms occupied 
space in the core area in the spaces over the elevator shafts. 
With thousands of workers and visitors in the buildings, there needed to be food service. The Port 
Authority maintained a cafeteria on the 43rd floor of WTC 1. In addition, a number of the companies 
maintained kitchen areas on their floors, where catered food was brought in daily, making it unnecessary 
for their staff to leave the building for lunch. There was a public cafeteria on the 44th floor of WTC 1. 
The visiting public could eat at Windows on the World at the top of WTC 1, at several restaurants on the 
observation deck of WTC 2, or in the many eateries on the Concourse Level. There were hundreds of 
restrooms, in both the tenant and the core spaces. 
NIST NCSTAR 1, WTC Investigation 

Chapter 1 Draft for Public Comment 
Figure 1–10. Orientation of the three stairwells. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment New York City’s World Trade Center 
Figure 1–11. Views of typical WTC 
office floors. 
Source: Reproduced with permission of The Port 
Authority of New York and New Jersey. 
Figure 1–12. A WTC trading floor. 
Source: Reproduced with permission of The Port 
Authority of New York and New Jersey. 
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Chapter 1 Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

Chapter 2 
THE ACCOUNT OF WORLD TRADE CENTER 1 
8:46:30 A.M. EDT 
On the morning of Tuesday, September 11, 2001, a lot of people were going to be late for work in New 
York City, which for many started at 9:00 a.m. or later. It was the first day of school for many local 
children and it also was a primary election day in New York. The weather was clear and comfortable 
with little wind to speak of, so some took time to do early morning errands. As a result, only about 8,900 
of the typical 20,000 people were in WTC 1 shortly before 9:00 a.m. 
At 8:46:30 a.m. EDT, five hijackers flew American Airlines Flight 11 (AA 11) with 11 crew and 
76 passengers into the north face of WTC 1 (Figure 2–1). 
Figure 2–1. Simulated impact of American Airlines Flight 11 with WTC 1. 
What follows is the result of an extensive, state-of-the-art reconstruction of the events that accompanied 
and followed the aircraft impact. Numerous facts and data were obtained, then combined with validated 
computer modeling to produce an account that is believed to be close to what actually occurred. 
However, the reader should keep in mind that the building and the records kept within it were destroyed, 
and the remains of the towers were disposed of before congressional action and funding was available for 
this Investigation to begin. As a result, there are some facts that could not be discerned, and thus there are 
uncertainties in this accounting. Nonetheless, NIST was able to gather sufficient evidence and 
documentation to conduct a full investigation upon which to reach firm findings and recommendations. 
The reconstruction effort, the uncertainties, the assumptions made, and the testing of these assumptions 
are documented in Part II of this report. 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
2.2 THE AIRCRAFT 
The Boeing 767-200ER was a twin-engine, wide-body aircraft, 159 ft 2 in. 
long, with a wingspan of 156 ft 1 in. Empty, it weighed 183,500 lb. It could The 767-200ER 
carry 181 passengers in its three-class seating configuration and 23,980 gal aircraft had two fuel 
tanks that extended 
(158,200 lb) of jet fuel as it covered its maximum cruising range of through most of the 
6,600 miles. The maximum total weight the plane could carry was specified at interior of the wings 
395,000 lb; the typical cruising speed was 530 mph. and a center tank 
between the wings 
On that day, AA Flight 11 was much lighter. Bound from Boston for Los in the bottom of the 
Angeles, some 3,000 miles away, it carried only about half the full load of jet fuselage. A full fuel 
load would have 
fuel. When it hit the north tower, it likely contained about 10,000 gal 
filled all three tanks. 
(66,000 lb), evenly distributed between the right and left wing tanks. Because 
of the tight maneuvers as the plane approached the tower, the baffles in both tanks had directed the fuel 
toward the inboard side of each wing. The passenger cabin was more than half empty. The cargo bay, 
carrying less than a full load of luggage, contained five tons of luggage, mail, electronic equipment, and 
food. The total weight of the aircraft was estimated to be 283,600 lb. 
2.3 THE IMMEDIATE DAMAGE 
The aircraft flew almost straight toward the north tower, banked approximately 25 degrees to the left (i.e., 
the right wing elevated relative to the left wing) and descended at an angle of about 10 degrees at impact. 
Moving at about 440 mph, the nose hit the exterior of the tower at the 96th floor. The aircraft cut a gash 
that was over half the width of the building and extended from the 93rd floor to the 99th floor (Figures 2–2 
and 2–3). All but the lowest of these floors were occupied by Marsh & McLennan, a worldwide 
insurance company, which also occupied the 100th floor. Marsh & McLennan shared the 93rd floor with 
Fred Alger Management, an investment portfolio management company. 
There was relatively little impact damage to the 93rd floor, hit only by the outboard 10 ft of the left wing. 
Containing no jet fuel, the wing tip was shredded by the perimeter columns. The light debris did minimal 
damage to the columns or to the thermal insulation on the trusses of the composite floor system 
supporting the 94th floor.5 The trusses supporting the 94th floor were impacted by flying debris on the 
93rd floor. 
The 94th floor was more severely damaged. The midsection of the left wing, laden with jet fuel, and the 
left engine cut through the building façade, severing 17 of the perimeter columns and heavily damaging 
four more. The pieces of the aircraft continued inward, severing and heavily damaging core columns . 
The insulation applied to the floor trusses above and the columns was scraped off by shrapnel-like aircraft 
debris and building wall fragments over a wedge almost 100 ft wide at the north face of the tower and 
50 ft wide at the south end of the building core. 
5 
The reader should bear in mind that the described damage to the building exterior comes from eyewitness and photographic 
evidence. The described damage to the aircraft and the building interior was deemed most likely from the computer 
simulations and analysis carried out under the Investigation. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
Figure 2–2. Aircraft entry hole on the north side of WTC 1, photographed 30 s 
after impact. 
The aircraft did the most damage to the 95th and 96th floors. (The calculated damage to the 96th floor is 
shown in Figure 2–4). The fuel-heavy inner left wing hit the 95th floor slab, breaking it over the full 60 ft 
depth of tenant space and another 20 ft into the building core. The fuselage was centered on the 96th floor 
slab and filled the 95th and 96th floors top to bottom. The severity of the impact was clear. A wheel from 
the left wing landing gear flew through multiple partitions, through the core of the building, and became 
embedded in one of the exterior column panels on the south side of the tower. The impact severed the 
bolts connecting the panel to its neighbors, and the panel and tire landed on Cedar Street, some 700 ft to 
the south. A second wheel landed 700 ft further south. Within the two floors, 15 to 18 perimeter columns 
and five to six core columns were severed, and an additional one to three core columns were heavily 
damaged. A 40 ft width of the 96th floor slab was broken 80 ft into the building. The insulation was 
knocked off nearly all the core columns and over a 40 ft width of floor trusses from the south end of the 
core to the south face of the tower. 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
Figure 2–3. South face damage of WTC 1 with key aircraft component locations marked. 
The right wing of the aircraft was fragmented by the perimeter columns on the 97th floor. In the process, 
12 of those columns were severed. The debris cut a path through the west and center array of trusses and 
core columns, stripping the insulation over a 90 ft wide path. The insulation was stripped from a 50 ft 
wide path on the south side of the floor space. 
On the 98th and 99th floors, the outboard 30 ft of the starboard wing was sliced by the perimeter columns, 
of which five were severed. The debris cut a shallow path through the west and center array of trusses, 
damaging the insulation up to the north wall of the building core. 
This devastation took 0.7 s. The structural and insulation damage was considerable and was estimated 
to be: 
• 35 exterior columns severed, 2 heavily damaged. 
• 6 core columns severed, 3 heavily damaged. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
NIST NCSTAR 1, WTC Investigation 23 
• 43 of 47 core columns stripped of insulation on one or more floors. 
• Insulation stripped from trusses covering 60,000 ft2 of floor area. 
Figure 2–4. Simulation of aircraft impact damage to the 96th floor in WTC 1. 
Even with all this damage, the building still stood. The acceleration from the impact had been so severe 
that people even on lower floors were knocked down and furniture was thrown about. Some survivors 
reported fallen ceiling tiles throughout the building, all the way down to the Concourse Level. The pipes 
that fed the automatic fire sprinkler system were severed. At least 166 windows were broken. Damage to 
interior walls was reported from the Lobby to the 92nd floors. However, the building was designed with 
reserve capacity: it could support significantly more load than the weight of the structure and its people 
and contents. The building redistributed the load from the severed perimeter columns, mainly to their 
neighboring columns. The undamaged core columns assumed the remaining load, as well as the load 
from their damaged neighbors. WTC 1 still stood, and would have continued to do so, if not for the fires 
that followed. 
NIST could not determine how many occupants were in the path of the aircraft as it entered the tower. 
Those in the direct collision path were almost certainly killed instantly. Many more would have lost their 
lives from the burst of heat from the burning jet fuel. Fatal injuries were reported on floors as low as the 
Concourse Level, where a fireball swept through the lobby. 
In the impact region was further damage that would cost the lives of all the 1,355 people from the 92nd 
floor to the 110th floor. The crash and flying debris had collapsed the walls of all three stairwells and 
interrupted all elevator service to the upper 60 floors. All opportunity for escape had been eliminated. 
Column Damage 
Severed 
Heavy Damage 
Moderate Damage 
Light Damage 
Severe Floor Damage 
Insulation 
and partitions 
Floor system 
structural damage 
Floor system 
removed 

Chapter 2 Draft for Public Comment 
2.4 THE JET FUEL 
To the wings of the 767-200ER, the perimeter columns acted like knife blades, slashing the aluminum 
fuel tanks and atomizing much of the 10,000 gal of jet fuel liquid into a spray of fuel droplets. Atomized 
jet fuel is highly flammable (similar to kerosene), so both the hot debris and the numerous pieces of 
electrical and electronic gear in the offices were more than sufficient as ignition sources. A surge of 
combusting fuel rapidly filled the floors, mixing with dust from the pulverized walls and floor slabs. The 
pressure created by the heated gases forced the ignited mist out the entrance gash and blown-out windows 
on the east and south sides of the tower. The resulting fireballs could be seen for miles, precipitating 
many 9-1-1 calls. 
Less than 15 percent of the jet fuel burned in the spray cloud inside the building. A roughly comparable 
amount was consumed in the fireballs outside the building. Thus, well over half of the jet fuel remained 
in the building, unburned in the initial fires. Some splashed onto the office furnishings and combustibles 
from the aircraft that lodged on the impacted floors, there to ignite (immediately or later) the fires that 
would continue to burn for the remaining life of the building. Some of the burning fuel shot up and down 
the elevator shafts, blowing out doors and walls on other floors all the way down to the basement. Flash 
fires in the lobby blew out many of the plate glass windows. Fortunately, there were not enough 
combustibles near the elevators for major fires to start on the lower floors. 
2.5 8:47 A.M. TO 9:02 A.M. EDT 
The burning of the jet fuel cloud had consumed much of the oxygen within the 94th and 96th floors, 
although photographs showing survivors indicated there were some zones with breathable air. The 
oxygen-starved fires died down, but didn’t quite go out. Within the first 2 min after the impact, fires 
could be seen in the north side windows on the 93rd through 97th floors, the 96th floor of the south face, 
and the 94th floor of the east face. As fresh air entered the perforated facades, there began the steady 
burning of the office furnishings and the 13 tons of combustibles from the aircraft that would eventually 
overwhelm the already damaged building. By 9:00 a.m., these fires had grown and spread to the extent 
shown in Figure 2–5. In addition to burning around the aircraft entrance hole, there was intense burning 
on the north, east, and west faces of the 97th floor. Large fires burned on the south side of the 96th floor 
and the east side of the 94th floor. At 8:52 a.m., a stream of smoke emerged from the south side of the 
104th floor, although there was no evidence of a significant fire there yet. 
There was no way to fight the fires. The piping providing the water supply to the automatic sprinklers 
had been broken, and water was flowing down the stairwells. Even had this not happened, the system 
was designed to supply water to about 8 sprinkler heads at one time, enough to control the flames from as 
much as 1,500 ft2 of burning material. The water supply was likely sufficient to control fires up to triple 
that size. The fires, however, had already grown far larger than that. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
Note: Color coding—white, no fire; yellow, spot fire; red, fire visible inside; orange, external flaming. 
Figure 2–5. Representation of exterior views of the fires on the four faces of WTC 1 from 
8:47 a.m. to about 9:02 a.m. 
There was also no way to abate the opaque, hot, and toxic smoke that quickly began accumulating. The 
manually activated smoke purging system was intended for smoke removal during fire department 
operations following a fire. Thus, it was not turned on during the 102 min that the tower would remain 
standing. It would not have helped anyway. Neither the World Trade Center Safety Director nor the 
arriving firefighters knew where the fires were located, so they could not have known how to direct the 
intake and exhaust flows. Furthermore, the integrity of the vent shafts on the upper floors had been 
compromised by the aircraft impact, making it unlikely that the system could have functioned as intended. 
Most of the people in WTC 1 were aware of the possibility of an emergency. A quarter of them had been 
working in the building since before the 1993 bombing, and most of those had been in the building on 
that day. Half the people had been working in the building for at least two years; many had heard the 
stories and had participated in the emergency drills. 
The building occupants knew that something serious had happened. A third of the survivors had heard 
the roar of the plane. Nearly two thirds reported feeling the violent movement of the building. Half 
sensed that they were in a life-threatening situation. At the Concourse Level, a fatal fireball filled the 
space from the elevators to the exit toward WTC 3. Almost immediately, people began calling 9-1-1, 
both for help and to find out more about what was going on. 
Within 5 min to 8 min of the strike, most of the 7,545 people below the floors of impact began to 
evacuate. Their progress is tracked in Table 2–1. Water and debris were in the three stairwells. The air 
smelled of jet fuel and was becoming gray with smoke and pulverized gypsum, thermal insulation, and 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
concrete. Nonetheless, perhaps due to the guidance they had received since the 1993 bombing, for the 
most part the people moved in an orderly manner down the stairs, helping those who needed assistance. 
Within 15 min of the strike, nearly all of the people below the impact floors had descended about 
10 floors from their original location. 
Table 2–1. Locations of occupants of WTC 1. 
Time Evacuated Lobby to 91st Floor 92nd to 110th Floor 
8:46 0 7,545 1,355 
9:03 1,250 6,300 1,355 
9:59 6,700 850 1,355 
10:28 7,450 107 1,355 
Note: The numbers in the rows do not add to the estimated total of 8,900 due to rounding errors in the less certain values. 
At the time, there were some survivors from the 92nd through 99th floors. 
While the occupants Most of those who were able moved to the areas where the fires had not were not advised in 
yet spread. Some were seen looking out from the former window spaces advance that roof 
and even standing on the deformed structural steel. At 8:52 a.m., the first evacuation was not 
of at least 111 people was observed falling from the building. possible, there was, and 
is, no requirement in the 
Hundreds of people were on the floors above the impact zone. They soon NYC Building Code for 
the roof to be accessible 
realized that they were unable to go downward to get away from the smoke for emergency 
and heat that were building up around them. At 8:54 a.m., occupants evacuation or rescue. 
began breaking windows to provide access to fresh air. By 9:02 a.m., Even had the roof been 
26 calls, representing hundreds of people, had been made to 9-1-1, asking accessible, the 
for help and seeking more information about what was happening. Some helicopters could not 
have landed due to the 
of the people went toward the roof. However, there was no hope because severe heat and smoke. 
roof evacuation was neither planned nor practical, and the exit doors to the 
roof were locked. 
Outside the building, a flurry of activity was beginning. Personnel of the Fire Department of the City of 
New York (FDNY) were several blocks away, investigating a gas leak at street level, and observed the 
aircraft impact. Within a minute, FDNY had notified its communications center and requested additional 
alarms for the WTC. A Port Authority Police Department (PAPD) unit had reported to its Police Desk 
that there had been an explosion with major injuries. By 8:50 a.m., the first fire engines had arrived, and 
an Incident Command Post had been established in the WTC 1 lobby. An Emergency Medical Service 
(EMS) Command was established 3 min later. More and more reports of damage, injuries, and deaths 
flooded the communications channels, and knowledge of the extent of the catastrophe was emerging. At 
8:52 a.m., the first New York City Police Department (NYPD) aviation unit arrived to evaluate the 
possibility of roof rescue, but reported they were unable to land on the roof due to the heavy smoke. At 
8:55 a.m., the firefighters entering WTC 1 began climbing the stairs (Figure 2–6). Their objectives were 
to evacuate and rescue everyone below the fires, then to cut paths through the fires and rescue all those 
above the fires. 
At 8:59 a.m., a senior PAPD official called for evacuation of the entire WTC complex, although that call 
was not heard nor heeded by others. By 9:00 a.m., 66 FDNY units had been dispatched to the scene, and 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
the FDNY had called a fifth alarm for the dispatch of additional department personnel and equipment to 
the WTC. Spectators had begun converging on the complex, but were advised to stand clear. 
Figure 2–6. Firefighters on the scene at about 9:07 a.m. 
The aircraft impact also did damage to the communications in the tower. The capability for buildingwide 
broadcast from the Fire Command Desk was knocked out. Emergency responder radio traffic 
peaked at about five times its normal traffic volume during the 20 min period after to the attack. This 
peak gradually tapered off, but still continued at a sustained level three times the normal traffic volume. 
The radio systems were not adequate to handle the high flow of emergency communications required for 
this scale of operations. Many of the radio messages were unintelligible because many individuals were 
trying to talk on the same radio channel at the same time. 
2.6 9:02:59 A.M. EDT 
At 9:02:59 a.m., five hijackers flew United Airlines Flight 175 with 9 crew and 51 passengers into the 
east side of the south face of WTC 2. For the most part, there was little awareness of this among the 
people below the 92nd floor of WTC 1. Almost one-fifth of these had already left the building, and nearly 
all the 6,300 others were already in the stairwells. 
2.7 9:03 A.M. TO 9:57 A.M. EDT 
A fire needs a continuing supply of both gaseous fuel and oxygen if it is to keep burning, and the initially 
burning combustibles in WTC 1 were being consumed. The additional fuel came from the office 
furnishings next to those that were reaching the end of their burning life. The thermal radiation from the 
flames and from the hot gases heated the nearby combustibles, creating flammable vapors. These vapors 
needed a source of nearby air to continue the burning. The same flames and hot ceiling layer gases heated 
the windows and window frames in the vicinity. The hot gases pushed on the weakened aluminum 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
frames, sending some windows outward to fall to the Plaza below. Other windows were sucked into the 
building. The fires now had both new fuel and fresh air. 
And so the fires continued to spread, likely aided by as-yet unburned jet fuel that had soaked into some of 
the furnishings and flooring. The coating of (non-combustible) gypsum and concrete fragments slowed 
the burning rate by as much as half, but could not halt the fire from spreading. The overall movement of 
the fires was toward the south side of the tower. By 9:15 a.m., the fires on the 97th floor had intensified 
and filled most of the floor. Large fires had erupted on the east sides of the 92nd and 96th floors. 
Seventy-five minutes after the impact, approaching 10:00 a.m., the fire on the 97th floor had begun to burn 
itself out, but the fire on the 94th floor had intensified and filled much of the north half of the floor 
(Figure 2–7). Starting about 9:30 a.m., there were vigorous fires on nearly the full perimeter of the 
98th floor. There was still almost no burning on the 99th floor or above. 
Note: Color coding—white, no fire; yellow, spot fire; red, fire visible inside; orange, external flaming. 
Figure 2–7. Representation of exterior views of the fires on the four faces of WTC 1 from 
about 9:38 a.m. to 9:58 a.m. 
The hot smoke from the fires now filled nearly all the upper part of the tenant space on the impact floors. 
Aside from isolated areas, perhaps protected by surviving gypsum walls, the cooler parts of this upper 
layer were at about 500 °C, and in the vicinity of the active fires, the upper layer air temperatures reached 
1,000 °C. The aircraft fragments had broken through the core walls on the 94th through the 97th floors, 
and temperatures in the upper layers there were similar to those in the tenant spaces. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
The perimeter columns, floors, and core columns were immersed in these hot gases and began to weaken. 
Where the insulation was dislodged, the temperature of the steel rose rapidly, in contrast to steel members 
where insulation was intact (Figure 2–8). The heaviest core columns with damaged insulation heated 
slowly, as the absorbed heat was dissipated through their massive cross sections. The temperatures of the 
lighter columns and the floor slabs rose more quickly, and those of the stripped trusses even more so. 
Temperature ( oC) 
600 
500 
400 
with SFRM 
bare 
Figure 2–8. Steel 
surface temperatures on 
the bottom chords of 
fire-exposed trusses, 
uninsulated and 
300 
200 
insulated with ¾ in. of 
BLAZE-SHIELD DC/F. 
100 
0
0 1020304050
Time (min) 
As a steel column is heated, its ability to support gravity loads and resist 
Structural steels do not 
lateral loads decreases. At temperatures of about 300 °C, steel loses about 
need to melt to lose 
20 percent of its yield strength (Figure 2–9). Under modest loads, steel is strength. Their melting 
elastic, that is, it can compress, or shorten, but will recover when loads are points are about 
1,600 ºC, well above
removed. As the load increases, the steel becomes plastic, and the 
shortening is unrecoverable. At still higher loads, the column buckles. At the 1,100 ºC typical 
temperatures above 500 ºC, the steel weakens, the loss of strength and 
peak value reached by 
fires of common 
stiffness become significant, and the column's ability to carry its share of the building combustibles. 
building loads decreases. It shortens due to a combination of plastic 
deformation and an additional, time-dependent deformation called creep that can increase column 
shortening and hasten buckling. Figure 2–10 indicates the rates at which structural steel could have been 
heated by the WTC fires and the effect of the thermal insulation in slowing the heating process.6 
At this point, the core of WTC 1 could be imagined to be in three sections. There was a bottom section 
below the impact floors that could be thought of as a strong, rigid box, structurally undamaged and at 
almost normal temperature. There was a top section above the impact and fire floors that was also a 
heavy, rigid box. In the middle was the third section, partially damaged by the aircraft and weakened by 
heat from the fires. The core of the top section tried to move downward, but was held up by the hat truss. 
The hat truss, in turn redistributed the load to the perimeter columns. 
Chapter 6 contains an explanation of how these temperature profiles were developed. 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
Figure 2–9. Temperature dependence of 
fraction of the value at room temperature. 
yield strength of structural steel as a 
Note: The red data are for structural steel components without insulation; the blue data are for steel components that 
are still insulated. 
Figure 2–10. Simulated temperatures of two adjacent trusses (left) and two adjacent 
perimeter columns (right) exposed to the fires in WTC 1. 
Simultaneously, the fires were creating another problem for the tower. The floors of the 93rd through the 
97th stories were being heated both by the hot gases from below and by thermal radiation from the fires on 
the floor above (Figure 2–11). On the south side of the building, where the fires were heating the longspan 
trusses whose SFRM had been dislodged, the floors began to sag. In so doing, they began pulling 
inward on their connections to the south face and to the core columns. Pull-in forces due to the sagging 
floors did not fail the floor connections in most areas. 
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Draft for Public Comment The Account of WTC 1 
Top surface Bottom surface 
Figure 2–11. Temperature contours on the top and bottom faces of the concrete slab 
(96th floor, WTC 1) at 100 min after impact. A portion of the concrete slab on the north 
face (top) was damaged by the impact of the aircraft. 
Meanwhile, the occupants from below the impact floors were moving steadily down the stairs at roughly 
a floor per minute. Although they encountered firefighters climbing upward, this did not slow the 
downward progress much. Within 75 min of the impact, 90 percent of the people who would survive had 
left WTC 1. At 9:37 a.m., a Port Authority official instructed all units to direct the evacuees over the 
bridge on West Street to the Financial Center. However, this change in evacuation route actually began 
with the collapse of WTC 2. 
Conditions on floors 92 and above continued to deteriorate. The presence of the fires and the resulting 
high smoke and radiant heat levels made the 92nd floor through the 99th floor uninhabitable except in 
small areas. Above the impact zone, there were only seven calls to 9-1-1 between 9:03 a.m. and 
9:10 a.m.; and then, more than a half hour later, three last calls from floors 104 and 105 between 
9:43 a.m. and 9:57 a.m. More people jumped through windows they broke or that had been broken by the 
fires. 
By 9:15 a.m., 30 FDNY units had signaled their arrival, and by 9:59 a.m., the 
number had grown to 74. They had been told to stop short of the site Since the Command 
Boards were 
because of the large number of ambulances already there and the debris destroyed in thefalling from the buildings. Many of the firefighters proceeded into WTC 1. collapse, it is unknown 
Once inside, they found that only one of the 99 upward elevators was just how many 
working, one that went as far as the 16th floor. Most of the firefighters then firefighters went into 
proceeded to ascend the three stairways, intending to help evacuate the WTC 1, when they 
went in, or, in most 
occupants, cutting paths through the fires as necessary. Because the cases, what level they 
firefighters were carrying as much as a hundred pounds of bulky firefighting reached. 
gear, their progress was slow and was impeded by the flow of evacuees 
coming down the stairs. A few reached as high as floors in the 40s and 50s. 
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Chapter 2 Draft for Public Comment 
2.8 9:58:59 A.M. EDT 
With no warning that could be discerned in WTC 1, WTC 2 collapsed. The shudder as the more than 
250,000 tons of steel, concrete, and furnishings hit the ground was felt well beyond the site. Seismic 
sensors located 100 miles away recorded the time and intensity of the event. 
The gigantic concussion was felt by some of the nearly 800 people still in the stairwells in WTC 1. The 
evacuation rate slowed to half its prior level as a new cloud of dust, smoke, and debris filled the 
Concourse and the stairwells, and the lights went out. Higher up, no more calls to 9-1-1 originated from 
above the 91st floor. 
At 10 a.m., NYPD and FDNY ordered all emergency responders out of WTC 1 and away from the WTC 
site. 
2.9 9:59 A.M. TO 10:28 A.M. EDT 
For the next half hour, the last 690 of the eventual survivors worked their At 10:06 a.m., an NYPDway down the last flights of stairs, across West Street to the west and 
aviation unit advised that 
across Vesey Street to the north and to safety. By 10:28 a.m., all but 107 WTC 1 would come 
of the roughly 7,500 people who had been below the impact floors were down and that all 
able to escape. emergency vehicles 
should be moved away 
Having heard over their radios the orders that they should evacuate, some from it. At 10:20 a.m., 
observers in NYPD 
of the responders inside the tower headed down the stairwells and out of helicopters said that the 
the building, telling their comrades on the way. Others did not, having not top of the building was 
leaning; and at 
received the message, having climbed too high to now get out in time, or 
continuing on the missions to help others still in the building. 10:21 a.m., they said 
that WTC 1 was buckling 
on the southwest corner 
A pressure pulse generated by the collapse of WTC 2 appeared to intensify and leaning to the south. 
the fires in WTC 1. Within 4 seconds of the collapse of WTC 2, flames 
burst from the south side windows of the 98th floor. The fires on the north faces of the 92nd, 94th, and 
96th floors brightened noticeably. Flames near the south end of the east face of the 92nd and 96th floors 
also flared. The fires on the east and south faces of the 98th floor already extended out the windows. 
Those in the WTC 1 stairwells felt a gush of wind. 
At 10:01 a.m., flames began coming out of the south side of the west face of the 104th floor, three floors 
higher than any floor where fire had been previously observed and five floors above the highest floor with 
a major fire. After a rapid growth period, this fire burned intensely up to the time the tower collapsed. 
By 10:18 a.m., a substantial pressure pulse inside the building ejected jets of smoke from the 92nd and 94th 
through 98th floors of the north faces and the 94th and 98th floors of the west face. Fires raged on the south 
side of the 96th through 99th floors. 
The sagging of the floors had increased. Although the floors on the north side of the tower had sagged 
first, they contracted due to cooling when the fires moved toward the south. Now, the south side floors 
had sagged to the point where the south perimeter columns bowed inward (Figure 2–12). By 10:23 a.m., 
the south exterior wall had bowed inward as much as 55 in. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
The tower was being overwhelmed. Three of the four major structural systems—the core, the floors, and 
the perimeter walls—were weakening. The south wall became unstable and tried to transfer its remaining 
load to the weakened core via the hat truss and to adjacent perimeter columns via the spandrels. The 
entire section of the building above the impact zone began tilting as a rigid block toward the south. The 
upper section of the building then collapsed onto the floors below. Within 12 s, the collapse of WTC 1 
had left nothing but rubble. 
Note: Enhanced by NIST. 
Figure 2-12. South face of WTC 1 at 10:23 a.m., showing inward buckling (in inches) of 
perimeter columns. 
NIST NCSTAR 1, WTC Investigation 

Chapter 2 Draft for Public Comment 
2.10 THE OUTCOME 
Seven major factors led to the collapse of WTC 1: 
• Structural damage from the aircraft impact; 
• Large amount of jet fuel sprayed into the building interior, that ignited widespread fires over 
several floors; 
• Dislodging of SFRM from structural members due to the aircraft impact, that enabled rapid 
heating of the unprotected structural steel; 97 
• Open paths for fire spread resulting from the open plan of the impact floors and the breaking 
of partition walls by the impact debris; 95 
• Weakened core columns that increased the load on the perimeter walls; 
• Sagging of the south floors, that led to pull-in forces on the perimeter columns; and 
• Bowed south perimeter columns that had a reduced capacity to carry loads. 
After the building withstood the initial aircraft damage, the timing of the collapse was largely determined 
by the time it took for the fires to weaken the core and to reach the south side of the building and weaken 
the columns and floor assemblies there. 
There were no survivors among the 1,355 people who were on or above the 92nd floor. The aircraft had 
destroyed all egress paths downward, and roof rescue was impossible. 
Of the roughly 7,545 building occupants who started that morning below 
Had the building been 
the 92nd floor, all but 107 escaped the building. Those left behind were significantly more than 
trapped by debris, awaiting assistance, helping others, or were just too late one-third occupied, the 
in starting their egress. For the most part, the evacuation was steady and casualties would have 
orderly. been far higher, since 
the exiting population 
Six percent (almost 500) of the survivors from WTC 1 had a limitation that would have exceeded 
impaired their ability to evacuate. Many were able to evacuate, often with the capacity of the 
stairwells to evacuate 
assistance; others were less fortunate. About 40 to 60 mobility-impaired them in the time 
occupants were found on the 12th floor, where they had been placed in an available. 
attempt to clear the stairways. Just before the collapse of WTC 1, 
emergency responders were assisting about 20 of these people down the stairwell. It remains unclear how 
many of these people survived. 
Those emergency responders who entered the building and the emergency personnel who were already in 
the building were helpful in assisting the evacuation of those below the impact floors. However, there 
was insufficient time and no path to reach any survivors on the impact floors and above. Any attempts to 
mitigate the fires would have been fruitless due to the lack of water supply and the difficulty in reaching 
the fire floors within the time interval before the building collapse. It is not known precisely how many 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 1 
emergency responders entered the building nor how many of the 421 responder casualties occurred in 
WTC 1. NIST estimated that approximately 160 FDNY fatalities occurred outside the WTC towers. 
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Chapter 2 Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

Chapter 3 
THE ACCOUNT OF WORLD TRADE CENTER 2 
8:46:30 A.M. EDT 
The nature of the events leading to the collapse of WTC 2 had a number of important features distinct 
from those of WTC 1. Those contrasts led to a larger overall fraction of the occupants surviving despite 
the building collapsing in a shorter period. As was the case with WTC 1, what follows is the result of an 
extensive, state-of-the-art reconstruction of the events that accompanied and followed the aircraft impact. 
Numerous facts and data were obtained, then combined with validated computer modeling to produce an 
account that is believed to be close to what actually occurred. The reader should again keep in mind that 
the building and the records kept within it were destroyed and the remains of the towers were disposed of 
before this Investigation began. As a result, there are some facts that could not be discerned, and there are 
uncertainties in this accounting. Nonetheless, NIST was able to gather sufficient evidence and 
documentation to conduct a full investigation upon which to reach firm findings and recommendations. 
The reconstruction effort, the uncertainties, the assumptions made, and the testing of these assumptions 
are documented in Part II of this report. 
The ordeal for the occupants of WTC 2 began at the same time as it did for those in WTC 1, when 
American Airlines Flight 11 stuck WTC 1 at 8:46 a.m. Nearly all of the roughly 8,600 people in WTC 2 
were well aware that something serious had occurred in the other tower. Half the people heard the 
terrible sound of the aircraft hitting WTC 1, just a few hundred feet away. One-fifth of the people saw the 
flames, smoke, or the debris ejected from the south side of WTC 1, over 10 percent felt WTC 2 moving, 
and another fifth in WTC 2 were quickly alerted to the seriousness of what had happened by co-workers, 
phone calls, or the morning news. Over half believed they were personally at risk. 
Many began talking to each other, gathering personal items and helping others. Fortunately, they began 
to get out of the building. Within 5 min, half the people had left their floor, and that fraction grew 
rapidly. About one-sixth used the elevators, with more of these people starting on the higher floors. The 
remainder divided themselves evenly among the three stairways. NIST estimated that approximately 
3,000 people escaped because of the actions they took in the 16 min following the aircraft impact on 
WTC 1, especially their use of the elevators. 
At 9:00 a.m. came the first building-wide public address system announcement that there was a fire in 
WTC 1, that WTC 2 was secure, and that people should return to their offices. 
This added confusion to an already tense situation, a situation that became even more turbulent when at 
9:02 a.m., a contradictory announcement said that people may wish to start an orderly evacuation if 
conditions on their floor warranted. 
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Chapter 3 Draft for Public Comment 
9:02:59 A.M. EDT 
Sixteen and a half minutes after the first impact, five hijackers flew United Airlines (UA) Flight 175, with 
9 crew and 51 passengers, into WTC 2 at about 540 mph, about 100 mph faster than AA 11 (Figure 3–1). 
UA 175 was also a Boeing 767-200ER and had also left Boston, bound for Los Angeles. It flew into 
WTC 2 carrying about 9,100 gal (62,000 lb) of jet fuel, evenly distributed between the inboard portions of 
the left and right wing tanks. The cargo bay held about nine tons of luggage, mail, electrical equipment, 
and food. Combining this with the combustible cabin materials and luggage, the plane brought about 
14 tons of solid combustibles into the tower with it. 
Figure 3–1. Imminent impact of United Airlines Flight 175 with WTC 2. 
3.3 THE IMMEDIATE DAMAGE 
The aircraft completely disappeared into the building in a fifth of a second. In response to the force of the 
collision, the top of the tower swayed 27 in. to the north, taking 2.6 s to reach the maximum 
displacement. UA Flight 175 was heading approximately 15 degrees east of Plan North7 when it hit the 
south face of WTC 2 about 23 ft east of the center. The off-center impact twisted the upper part of the 
7 
Plan North was approximately 29 degrees clockwise from True North. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 2 
tower in a counterclockwise movement. The building vibrated in the north-south direction, along with a 
twisting motion, with the amplitude decreasing steadily with each oscillation. 
The center of the nose of the plane struck at the 81st floor slab. The plane was banked 38 degrees to the 
left (right wing upward) and was heading slightly (6 degrees) downward from the horizontal. Since the 
bank angle was steeper than that of AA Flight 11, this entry wound stretched over nine floors, from 77 to 
85, rather than eight in WTC 1 (Figure 3–2). The occupancy of those floors is shown in Table 3–1. 
Figure 3–2. South face damage of WTC 2 with key aircraft component locations marked. 
NIST NCSTAR 1, WTC Investigation 

Chapter 3 Draft for Public Comment 
40 NIST NCSTAR 1, WTC Investigation 
Table 3–1. Tenants on impact floors in WTC 2. 
Floors Tenant Business 
85 Harris Beach Legal 
84 Eurobrokers Brokerage 
83 Mitsui; IQ Banking; Financial Software 
79 through 82 Fuji Bank Banking 
77 and 78 Baseline Investment Services 
The bulk of the impact damage was confined to six floors. Figure 3–3 shows the combined damage. 
Floors 77, 84, and 85 were struck only by the outer extent of the wings. Empty of fuel, the light framing 
and aluminum sheet of the wing did little damage to the building structure or the SFRM on the columns 
and trusses on these floors. There were 433 broken windows on the north, east, and south facades. 
Figure 3–3. Simulation of aircraft impact damage to the 78th through 83rd floors in WTC 2. 
The middle of the left wing hit the 78th floor, severing nine perimeter columns and breaking 19 windows 
on the south face. The SFRM was stripped from the floor trusses over the same width as the building 
core. The stripping of insulation from the trusses continued inward across the tenant space and about two 
thirds of the way into the core. There was no direct core column damage from the debris on this floor. 
However, the southeast corner core column was so damaged on the 80th floor that it broke at its splices on 
the 77th and 83rd floors. 
There was heavier damage to the 79th floor. The left engine and the inboard section of the left wing 
shattered a 25 ft wide section of the center of the floor slab all the way to the core of the building and 
Column Damage 
Severed 
Heavy Damage 
Moderate Damage 
Light Damage 
Severe Floor Damage 
Insulation 
and partitions 
Floor system 
structural damage 
Floor system 

Draft for Public Comment The Account of WTC 2 
severed 15 perimeter columns. Reaching the building core, the debris severed nine columns, heavily 
damaged another, and abraded the SFRM from the eastern two thirds of the columns and trusses all the 
way to the north end of the core. 
The damage was most severe on the 80th and 81st floors, hit directly by the fuselage. On the lower floor, a 
chunk of the floor slab was broken, just above the affected piece of the 79th floor. In addition, a 70 ft 
deep strip along the east side of the core floor was crushed. The north side floor slab sagged along its 
eastern end. Ten of the perimeter columns severed on the 79th floor were displaced here also. Within the 
building core, ten columns were severed, including many that were severed on the 79th floor. The SFRM 
was stripped not only from the eastern two thirds of the core structural elements, nearly to the north wall, 
but also from most of the trusses on the east tenant space, all the way to the north façade. 
On the 81st floor, the fuselage pulverized a section of the floor 40 ft wide that extended into the southeast 
corner of the core. The SFRM and gypsum fire protection on the full depth of the east side of the core 
and in the entire east side of the tenant space was stripped. The structural damage to the core columns 
was limited to near the southeast corner, but as mentioned above, the impulses felt here caused damage to 
the key corner column all the way down to the 78th floor. The right engine passed all the way through the 
81st floor, exited from the northeast corner, and damaged the roof of a building on Church Street, before 
coming to rest some 1,500 ft northeast of WTC 2 near the corner of Murray and Church Streets. The right 
landing gear assembly passed through the 81st floor at the east side of the north face and landed near the 
engine on the roof of a building on Park Place. (See Figure 1–1 for the street locations relative to the 
towers.) 
The right engine hit the 82nd floor spandrels about 50 ft from the east edge of the building, crushing part 
of the 82nd floor slab. Along with the inboard section of the right wing, it severed eight to nine perimeter 
columns, including some to the east of those severed on the lower floors. The wing caused truss damage 
up to the southeast corner of the core and severed five columns. As on the 81st floor, the fire protection 
on the east side of the tenant space and the east side of the core was dislodged. 
The 83rd floor caught the middle of the starboard wing. The east side floor slab appeared to be dislodged 
and sagged at least half of the way into the building. 
The result of the core column damage was that the building core leaned slightly to the southeast above the 
impact zone. The tendency of the core to lean was resisted by the floors and the hat truss. 
The direct impact of the aircraft was over in about 0.6 s. The structural and insulation damage was 
estimated to be: 
• 33 exterior columns severed, 1 heavily damaged. 
• 10 core columns severed, 1 heavily damaged. 
• 39 of 47 core columns stripped of insulation on one or more floors. 
• Insulation stripped from trusses covering 80,000 ft2 of floor area . 
NIST NCSTAR 1, WTC Investigation 

Chapter 3 Draft for Public Comment 
The tower swayed more than one foot back and forth in each direction on the impact floors, about onethird 
the sway under the high winds for which the building was designed. Nonetheless, just like WTC 1 
across the Plaza, WTC 2 absorbed the aircraft strike and remained standing. 
By 9:03 a.m., most of the people in WTC 2 had already left their usual work floors. Nearly 40 percent of 
all the occupants had left the building, (Table 3–2), and 90 percent of those who would survive had begun 
their evacuation. Many of those still on the east side of the impact floors were likely killed or seriously 
injured by the impact. The same was true for many of those on the 78th floor skylobby, who were 
deciding on a course of action, waiting for the express elevators to transport them to the ground floor, or 
attempting to return to their offices. Those on the west side of the building were less seriously affected. 
In calls to 9-1-1, they reported fallen ceiling tiles, collapsed walls, jet fuel, heat, smoke, and fire. 
Table 3–2. Location of occupants of WTC 2. 
Time Escaped Lobby to 76th Floor 77th to 110th Floor 
8:46 0 5,700 2,900 
9:03 3,200 4,800 637 
9:36 6,950 1,050 619 
9:59 8,000 11 619 
Note: The numbers in the rows do not add to the estimated total of 8,600 occupants due to rounding in the less certain values. 
This aircraft had also severed the pipes that fed the automatic sprinklers and 
Stairwell A remained 
destroyed all elevator service to the impact floors. But, unlike AA Flight 11, passable because it 
the off-center strike of UA Flight 175 had left one of the three stairways 
was well west of the 
passable, Stairway A on the north side of the building core. aircraft strike center 
and partially 
When the aircraft struck WTC 2, emergency responders had already been protected by 
dispatched to the WTC site, and the initial surge of emergency responder radio elevator machinery 
had subsided to a level approximately three times that of normal operations. and the long 
dimension of the 
However, the radio traffic volume was still at a level where approximately building core. 
one-third to one-half of the radio communications was not understandable. 
THE JET FUEL 
Within about one half of a second, dust and debris flew out of windows on the east and north faces. 
Several small fireballs of atomized jet fuel burst from windows on the east face of the 81st and 82nd floors, 
coalescing into a single, large fireball that spanned the entire face. A tenth of a second later, fire appeared 
in the dust clouds ejected from the south face of the 79th, 81st, and 82nd floors. Almost simultaneously, 
three fireballs came from the east side of the north face. The largest came from the 80th through 82nd 
floors. A second, somewhat smaller one came from the same floors on the northeast corner of the 
building. The smallest emerged from the 79th floor. No dust or fireballs came from the west face. 
As in WTC 1, less than 15 percent of the jet fuel burned in the spray cloud inside the building. Roughly 
10 percent to 25 percent was consumed in the fireballs outside the building. Thus, well over half of the 
jet fuel remained after the initial fireballs. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 2 
The rapid burning of the jet fuel inside the building created an overpressure that was estimated at 2 psi to 
3 psi for 0.5 s to 2 s. For a window and frame of over 10 ft2, this amounts to over 3,000 pounds of force, 
more than enough to break windows. Photographs of the north and east faces appeared to show hanging 
floor slabs where the fireballs had been ejected from the building. Based on the failure of the truss seat 
connections, NIST estimated that the static capacity of an undamaged floor was 4.8 psi against uplift 
pressure and 4.4 psi against downward pressure over the entire floor. It is not unreasonable that a 
combination of physical damage from the impact and overpressure from the fireballs caused the partial 
collapse of these floor slabs. 
9:03 A.M. TO 9:36 A.M. EDT 
The fireballs burned for 10 s, extending almost 200 ft out from the north, east, and south faces. Having 
consumed the aerosol fuel, the flames then receded. 
For the next half hour, small fires were burning in and near the aircraft impact cavity on the south side of 
the building. There were vigorous fires on the east side of the 80th through 83rd floors (Figure 3–4), 
especially on the northeast end of the 81st and 82nd floors, where the aircraft had bulldozed the office 
desks and chairs and added its own combustibles. In addition to the ample supply of fuel, these fires had 
access to plenty of air, as numerous windows on the east face had been blown out by the impact or 
fireball. They would continue to burn as long as the building stood. 
Note: Color coding—white, no fire; yellow, spot fire; red, fire visible inside; orange, external flaming. 
Figure 3–4. Representation of exterior views of the fires on the four faces 
of WTC 2 at about 9:20 a.m. 
Between 9:30 a.m. and 9:34 a.m., there were several large bursts of smoke from the 79th and 80th floors of 
the north face, possibly resulting from the ignition of pools of jet fuel that had settled there, or from 
shifting of dislodged floor slabs elsewhere. 
Dire structural changes were occurring in the building interior. Core columns, including the massive 
southeast corner column, had been severed by the aircraft. The loads from these columns had been 
NIST NCSTAR 1, WTC Investigation 

Chapter 3 Draft for Public Comment 
redistributed to other, intact core columns and to the east exterior wall. The core leaned to the south and 
east, restrained from further movement by the east and south walls through the floors and the hat truss. 
The fires were weakening the structure in a manner different from WTC 1. First, the severed core 
columns in the southeast corner led to the failure of some column splices to the hat truss. Nonetheless, 
the hat truss continued to transfer loads from the core to the perimeter walls. Second, the overall load 
redistribution increased the loads on the east wall. Third, the increasing temperatures over time on the 
long-span floors on the east side had led to significant sagging on the 79th through 83rd floors, resulting in 
an inward pull force. Fourth, within 18 min of the aircraft impact, there was inward bowing of the east 
perimeter columns as a result of the floors sagging. As the exposure time to the high temperatures 
lengthened, these pull-in forces from the sagging floors increased the inward bowing of the east perimeter 
columns. 
Meanwhile, people continued their evacuation. By 9:36 a.m., almost 7,000 of the 8,600 occupants had 
left the building. From the impact floors and above, 18 occupants had discovered that the hot, smokefilled, 
debris-laden Stairway A was not fully blocked and had made their way to survival. It is not known 
how many more of the 619 other people who had been on or above the impact floors became aware of 
this, but none made it out of the building. There are no records of information regarding this escape route 
having been collected and transmitted to others who might have been able to use it. 
The PAPD, NYPD, and FDNY centers were now being inundated with calls from the two buildings. In 
the confusion, some of the callers did not identify which building they were in. At 9:12 a.m., PAPD was 
notified that the WTC 2 floor warden phones were not working. Other calls alerted them to trapped and 
injured people. At 9:18 a.m., FDNY reported that they had a single elevator working to floor 40. A 
simultaneous call indicated that FDNY was relocating its command post across West Street. At 
9:30 a.m., EMS set up a triage desk in the lobby of WTC 2. 
9:36 A.M. TO 9:58 A.M. EDT 
By 9:58 a.m., all but eleven of the occupants who had been below the impact floors had left the building 
and crossed the street to safety. 
The fires continued to burn in the east half of the building. 
At 9:55 a.m., firefighters communicated that they had reached floor 55 of WTC 2, one of the few calls for 
which a record survived indicating how high the responders had reached. Before WTC 2 collapsed, 
firefighters had reached the 78th floor by using the single functioning elevator to the 40th floor and then 
climbing the stairs. 
The physical condition of the tower had deteriorated seriously. The inward bowing of columns on the 
east wall spread along the east face. The east wall lost its ability to support gravity loads, and, 
consequently, redistributed the loads to the weakened core through the hat truss and to the adjacent north 
and south walls through the spandrels. But the loads could not be supported by the weakened structure, 
and the entire section of the building above the impact zone began tilting as a rigid block to the east and 
south (Figure 3-5). Column failure continued from the east wall around the corners to the north and south 
faces. The top of the building continued to tilt to the east and south, as, at 9:58:59 a.m., WTC 2 began to 
collapse. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment The Account of WTC 2 
THE OUTCOME 
Seven factors led to the collapse of WTC 2: 
• Direct structural damage from the aircraft impact, which included more severe damage to the 
core columns than in WTC 1; 
• Jet fuel sprayed into the building interior, that ignited widespread fires over several floors; 
• Dislodging of SFRM from structural members due to the aircraft impact and aircraft and 
building debris, which enabled rapid heating of the unprotected structural steel; 
• Sustained fires on the east side of the tower and an ample air supply; 
• Weakened core columns that increased the loads on the perimeter walls; and 
• Sagging of the east floors, that led to pull-in forces on the east perimeter columns; and 
• Bowed east perimeter columns that had a reduced capacity to carry loads. 
After the building withstood the initial aircraft damage, the timing of the collapse was largely determined 
by the time for the fires to weaken the perimeter columns and floors on the east and south sides of the 
building. That the aircraft impact damage to the core was more severe in WTC 2 than in WTC 1 
contributed to the shorter time to collapse. 
The loss of life in WTC 2 was significantly reduced by the prompt 
As with WTC 1, had the start of evacuation activity before the tower was hit by the aircraft. building been more than 
Only a quarter of those initially on or above the impact floors died one-third occupied, the 
when the building collapsed, as contrasted with 100 percent in WTC 1. casualties would have been 
Eighteen people on those upper floors found that one stairwell was far higher as the population 
passable in time to evacuate. Whether others found this escape route would have exceeded the 
is unknown. capacity of the stairwells to 
evacuate them in the time 
available.
Of the roughly 6,000 people who started the morning below the 
77th floor, all but 11 evacuated the building, indicating sufficiently 
efficient movement within the three stairwells within the time available. 
Even more than in WTC 1, those emergency responders who entered WTC 2 and the emergency 
personnel who were already in the building were helpful in assisting the evacuation of those below the 
impact floors. However, there was insufficient time to reach any survivors on the impact floors and 
above. Any attempts to mitigate the fires were fruitless due to the lack of water supply and the difficulty 
in reaching the fire floors within the time interval before the building collapse. It is not known precisely 
how many emergency responders entered the building nor how many of the 421 emergency responder 
casualties occurred in WTC 2. 
NIST NCSTAR 1, WTC Investigation 

Chapter 3 Draft for Public Comment 
Figure 3–5. Photograph of WTC 2 tilting to the southeast at the onset of collapse. 
NIST NCSTAR 1, WTC Investigation 

Chapter 4 
THE TOLL 
By sunset on September 11, 2001, all seven buildings on the World Trade Center site lay in ruins 
(Figure 4–1). Table 4–1 compiles the locations of the decedents. 
iSource: National Oceanographic and Atmospheric Adminstration. 
Figure 4–1. The WTC site on September 17, 2001. 
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Chapter 4 Draft for Public Comment 
Table 4–1. Likely locations of World Trade Center decedents at time of impact. 
Locationa Number 
World Trade Center 1 Occupants (Total) 1,462 
At or Above the Impact Floors 1,355 
Below the Impact Floors 107 
World Trade Center 2 Occupants (Total) 630 
At or Above the Impact Floors 619 
Below the Impact Floors 11 
Confirmed Below Impact Zone in WTC 1 or WTC 2 30b 
Unknown Location Inside WTC 1 or WTC 2 24c 
Emergency Responders (Total) 421d 
FDNY 343 
NYPD 23 
PAPD 37 
Hospital/Paramedic 7 
Federal 2 
Volunteer Responders 9 
Bystander/Nearby Building Occupant 18 
American Airlines Flight 11 87e 
United Air Lines Flight 175 60e 
No Information 17 
Total 2,749 
a. Where possible, NIST used eyewitness accounts to place individuals. Where no specific accounts existed, NIST used 
employer and floor information to place individuals. 
b. These individuals were typically security guards and fire safety staff who were observed performing activities below the 
floors of impact after the aircrafts struck. 
c. These 24 individuals were largely performing maintenance, janitorial, delivery, safety, or security functions. 
d. Emergency responders were defined to be people who arrived at the site from another location. Thus, security staff and Port 
Authority staff (different from PA Police Officers) were not defined as emergency responders. 
e. Does not include the five hijackers per aircraft. 
NIST NCSTAR 1, WTC Investigation 

PART II: RECONSTRUCTING THE DISASTER

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NIST NCSTAR 1, WTC Investigation 

Chapter 5 
THE DESIGN AND CONSTRUCTION OF THE TOWERS 
5.1 BUILDING AND FIRE CODES 
Codes for the design, construction, operation, and maintenance of buildings are the blueprints by which a 
society manifests its intent to provide public safety and welfare. They incorporate the knowledge, 
experience, procedures and practices of the applicable engineering disciplines, the values of the 
contemporary society, the experiences from prior successes and failures, and knowledge of the 
commercial products, services, and technologies available for the tasks at hand. 
In the United States, building and safety regulations of state and local jurisdictions are most frequently 
based on national "model" building codes (model codes) developed by private sector organizations. 
Model codes are developed by panels of experts, updated periodically, and generally adapted to local 
conditions by state and local governments. Proposals to modify the model codes, offered by individuals 
or organizations, are discussed in open forums before being accepted or rejected by a voting process. 
Localities adopting model codes update their versions periodically as well, but typically not on the same 
schedule. To a lesser and decreasing extent, some jurisdictions have generated their own building codes 
to reflect specialized local conditions and preferences. The Federal government's role in determining 
specific codes is minimal and not mandatory (except for federally owned, leased, regulated or financiallyassisted 
properties). 
The model codes adopt by reference voluntary consensus standards developed by a large number of 
private sector standards development organizations. These standards include measurement methods; 
calculation methods; data sets; and procedures for design, construction, and practice. 
There are also key stakeholder groups that are responsible for or influence the practices used in the 
design, construction, operation, and maintenance of buildings in the United States through the code 
development process. These include organizations representing building owners and managers, real 
estate developers, contractors, architects, engineers, suppliers, and insurers. (Infrequently, members of 
the general public and building occupants participate in this process.) These groups also provide training, 
especially as it affects the ability to implement code provisions in practice, since lack of adequate training 
programs can limit the application of improved code provisions. 
5.2 THE CODES AND THE TOWERS 
5.2.1 The New York City Building Code 
The New York City (NYC) Building Code was and is part of the Administrative Code of New York City. 
The Code has been amended from time to time by Local Laws to update safety requirements or to 
incorporate technological advances. These Local Laws were enacted by the New York City Council. To 
aid the implementation of and to clarify building code requirements, New York City issued mandatory 
NIST NCSTAR 1, WTC Investigation 

Chapter 5 Draft for Public Comment 
“rules” that were typically initiated by City Government offices and issued under authority of the 
Building Commissioner. 
At the time the WTC project began in the early 1960s, the 1938 NYC Building Code was in effect. In 
1960, reflecting growing dissatisfaction with the failure of the Code to keep pace with changes that had 
occurred in the building industry, the Building Commissioner requested the New York Building Congress 
to form a working committee to study the problem. On December 6, 1968, Local Law 76 repealed the 
1938 code and replaced it with the 1968 code. As is the general custom with changes to building codes, 
the new provisions did not apply to buildings approved under the prior code, provided they did not 
represent a danger to public safety and welfare. 
The 1968 NYC Building Code also included “Reference Standards.” These included standard test 
methods and design standards published by standards development organizations. Some of these 
Reference Standards included modifications to the published standards, as well as stand-alone standards 
developed by New York City. 
Through 2002, 79 Local Laws had been adopted that modified the 1968 Building Code. The major Local 
Law affecting the structural design of buildings dealt with seismic provisions. Five of the Local Laws 
had provisions that pertained to fire protection and life safety that were of interest to the WTC 
Investigation: 
• Local Law 5 (1973) added, among other specifications, requirements for: 
- Compartmentation (subdivision) within upper story, unsprinklered, large floor areas. Its 
provisions applied retroactively to existing office buildings. 
- Signs regarding the use of elevators and stairs, also retroactive. 
- A fire alarm system for buildings more than 100 ft in height. 
• Local Law 55 (1976) added a requirement for inspection of all sprayed fire protection, 
effective immediately but not retroactive. 
• Local Law 33 (1978) added a requirement for trained fire wardens on each floor. 
• Local Law 86 (1979), among other provisions, required full compliance with Local Law 5 by 
February 7, 1988, and exempted fully sprinklered buildings from compartmentation 
requirements. 
• Local Law 16 (1984) added requirements for sprinklers in new and existing buildings taller 
than 100 ft. Since Local Law 5 only required compartmentation of non-sprinklered spaces, 
this negated the compartmentation requirements from Local Law 5. 
The World Trade Center was located in Manhattan and would normally have been designed and 
constructed according to the NYC Building Code of 1938. However, the WTC was constructed by The 
Port Authority on land that it owned. As an interstate agency established under a clause of the United 
States Constitution permitting compacts between states, The Port Authority's construction projects were 
not required to comply with any building code. Nonetheless, The Port Authority instructed its consultants 
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Draft for Public Comment The Design and Construction of the Towers 
to design the two towers to comply with the 1938 NYC Code. In 1965, The Port Authority directed the 
architect and consulting engineers to revise their designs for the towers to comply with the second and 
third drafts of what would become the 1968 NYC Code. The rationale for this step was that the new 
Code, allowed the use of advanced techniques in the design of the World Trade Center, as well as the 
more lenient provisions regarding exit stairs and the reduced fire ratings. To reaffirm a "long standing 
policy" of The Port Authority that its facilities meet or exceed New York Building Code requirements, a 
formal memorandum of understanding between The Port Authority and the New York City Department of 
Buildings was established after the bombing in 1993. 
5.2.2 Pertinent Construction Provisions 
To gain perspective on the conditions under which the WTC towers were constructed, the rationale for the 
design, and the building structures themselves, the Investigation Team and its contractors reviewed tens 
of thousands of pages of documents provided by The Port Authority and its contractors and consultants, 
Silverstein Properties and its contractors and consultants, the Fire Department of the City of New York, 
the New York City (NYC) Police Department, the NYC Law Department, the NYC Department of 
Design and Construction, the NYC Department of Buildings, the NYC Office of Emergency 
Management, the manufacturers and fabricators of the building components, the companies that insured 
the WTC towers, and the building tenants. 
NIST deemed it important to understand how the provisions under which the WTC was constructed and 
maintained compared to equivalent provisions in place elsewhere in the United States at that time. The 
Investigation selected three codes for comparison: 
• The 1964 New York State (NYS) Building Code, which governed construction outside the 
New York City limits; 
• The 1965 BOCA Basic Building Code, a model building code typically adopted by local 
jurisdictions in the northeastern region of the United States; and 
• The 1967 Municipal Code of Chicago, under which the Sears Tower (110 stories) and the 
John Hancock Center (100 stories) were built. 
For the most part, the provisions in the various codes were similar, if not identical, indicating that there 
was a common understanding of the essentials of building safety and that the codes were at similar stages 
of evolution: 
• There were only modest differences among the codes in the provisions for gravity loads. 
• All three of the contemporaneous building codes had provisions for wind loads that were less 
stringent than those used for the tower design. 
• None of the codes had provisions for design against progressive collapse. 
• For alterations or additions to a building, there were criteria to determine whether the whole 
building or only the alterations needed to comply with the current code requirements. The 
"trigger" was either the fraction of the building cost involved in the renovation or the fraction 
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Chapter 5 Draft for Public Comment 
of the building dimensions. The NYC 1968 code was slightly less conservative than the 
Chicago Code and the BOCA Code. The NYS Code required that any addition or alteration 
conform to the contemporary code. 
• The 1968 NYC Code required inspection of sprayed fire protection, but did not specify if 
testing was required. 
• Only the NYC Code contained provisions for bracing (lateral support to prevent buckling of 
columns and walls) and stresses associated with transverse deflections of structural members. 
The investigators examined the 2001 edition of the NYC Building Code to determine the extent to which 
Local Laws had modified the code provisions between the times of construction and collapse of the 
towers. The 2001 NYC Building Code was essentially the same as the 1968 edition, as amended by the 
intervening Local Laws. 
5.2.3 Tenant Alteration Process 
With hundreds of tenants, The Port Authority realized that many would want extensive modifications to 
their space, both before they moved in and during the course of their occupancy. In anticipation, The Port 
Authority: 
• Set up a special office to review and approve plans, issue variances, and conduct inspections. 
• Developed a formal tenant alteration process for any modifications to leased spaces in WTC 
1 and WTC 2 to maintain structural integrity and fire safety. The Tenant Construction 
Review Manual, first issued in 1971, contained the technical criteria, standards, and review 
criteria for use in planning alterations (architectural, structural, mechanical, electrical, and 
fire protection). Alteration designs were to be completed by registered design professionals, 
and as-built drawings were to be submitted to The Port Authority. The 1968 NYC Building 
Code was referenced. The review manual was updated four times until, in 1998, the manual 
was replaced by the Architectural and Structural Design Guidelines, Specifications, and 
Standard Details. 
The interiors of the towers had been heavily modified over the years due to tenant turnover, same-tenant 
space usage changes, the addition of sprinklers, and asbestos abatement. 
5.3 BUILDING DESIGN 
5.3.1 Loads 
The NYC Building Code specified minimum design values for both dead and live gravity loads and for 
lateral (wind) loads. 
• Each tower was designed to support dead loads (its own weight) consistent with the 
provisions in the 1968 NYC Building Code. The dead loads included the weight of the 
structural system and loads associated with architectural, mechanical, plumbing, and 
electrical systems. 
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Draft for Public Comment The Design and Construction of the Towers 
• Each tower was designed to support live loads (the combined weights of the people and the 
building contents) exceeding those specified in the 1968 NYC Building Code. 
• The design wind loads used in the towers were higher than those required by the 1968 NYC 
Building Code and the three other codes identified earlier. 
5.3.2 Aircraft Impact 
The accidental 1945 collision of a B-25 bomber with the Empire State Building sensitized designers of 
high-rise buildings to the potential hazards of such an event. However, building codes did not then, and 
do not currently, require that a building withstand the impact of a fuel-laden commercial jetliner. A Port 
Authority document indicated that the impact of a Boeing 707 aircraft flying at 600 mph was analyzed 
during the design stage of the WTC towers. However, the investigators were unable to locate any 
documentation of the criteria and method used in the impact analysis and were thus unable to verify the 
assertion that “…such collision would result in only local damage which could not cause collapse or 
substantial damage to the building and would not endanger the lives and safety of occupants not in the 
immediate area of impact.”8 Since the ability for rigorous simulation of the aircraft impact and of the 
ensuing fires are recent developments and since the approach to structural modeling was developed for 
this Investigation, the technical capability available to The Port Authority and its consultants and 
contractors to perform such an analysis in the 1960s would have been quite limited. 
5.3.3 Construction Classification and Fire Resistance Rating 
The model building codes classify building constructions into different “Types” or "Classes." The Class 
pertinent to the WTC towers was Class 1 (fire resistive). The 1938 New York Building Code had no 
subdivisions of Class 1 construction, which required a 4 hour fire resistance rating for columns and a 
3 hour rating for floors. The 1968 version of the Code subdivided Class 1 for office occupancies into 1A, 
with requirements identical to the 1938 Class 1, and 1B. Class 1B specified a 3 hour rating for columns 
and girders supporting more than one floor and a 2 hour rating for floors including beams. There were no 
height or area requirements that differentiated between Class 1A and Class 1B, and the towers could have 
been classified as either one. The Port Authority elected to provide the fire protection in the WTC towers 
with Class 1B standards. 
Achieving a specified rating for a truss-supported floor using a spray-applied fire resistive material 
(SFRM) was an innovation at the time of the WTC design and construction. NIST was not able to find 
any evidence that there was a technical basis to relate SFRM thickness to a fire resistance rating, nor was 
there sufficient prior experience to establish such thickness requirements by analogy. NIST did find 
documentation that the Architect of Record and the Structural Engineer of Record had each written to the 
Port Authority, stating that the fire rating of the WTC floor system could not be determined without 
testing. NIST was unable to find any indication that such tests were performed nor any technical basis for 
the specification of the particular SFRM product selected or its application thickness. 
8 
Letter with an attachment dated November 13, 2003 from John R. Dragonette (Retired Project Administrator, Physical 
Facilities Division, World Trade Department) to Saroj Bhol (Design and Engineering Department, PANYNJ). 
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Chapter 5 Draft for Public Comment 
The NYC Building Code required inspection at the time of application of the There were no code 
SFRM, to be conducted under the supervision of a building inspector or a requirements nor 
licensed design professional who assumed responsibility for compliance. general practice by 
This inspection included verification of the thickness of the material, its which spray applied 
density, and its adhesion, each using a specific ASTM test method. The insulation was to be 
inspected over the life 
Code contained a requirement that SFRM installed in areas where it was of the building. 
subject to mechanical damage be protected and maintained in a serviceable 
condition. 
5.3.4 Compartmentation 
Both the 1968 NYC Building Code and The Port Authority practice required partitions to separate tenant 
spaces from each other and from common spaces, such as the corridors that served the elevators, stairs 
and other common spaces in the building core. These were intended to limit fire spread on a floor and to 
prevent the spread of a fire from one tenant space to that of another. 
• The Port Authority specified partitions separating tenant spaces from exit access corridors to 
have a 2 hour rating. This allowed dead end hallways to extend to 100 ft (rather than 50 ft 
with 1 hour partitions), which permitted more flexibility in tenant layouts. Above the ceiling, 
penetrations for ducts or to allow for return airflow were fitted with rated fire dampers to 
preserve the fire rating. This 2 hour rated construction was not used in the original design, 
but was specified later by The Port Authority as tenant spaces were altered. 
• For walls separating tenant spaces to achieve a 1 hour rating, they needed to continue through 
any concealed spaces below the floor and above the ceiling. The Port Authority chose to stop 
these demising walls at the bottom of the suspended ceiling and use 10 ft strips of 1 hour 
rated ceiling on either side of the partition. There was no precedent for this approach and, 
after a warning from the general contractor, the tenant alteration guidelines required that 
tenant partitions have a continuous fire barrier from top of floor to bottom of slab. 
• There were no requirements in the 1968 Building Code or in The Port Authority guidelines 
for partitions wholly within tenant spaces. As mentioned in Section 1.2.2, these gypsum 
board walls generally ran from the floor slab to just above the suspended ceiling, although 
some extended to the slab above when the tenant desired additional sound attenuation. For 
these partitions to be fire rated, the ceiling would have had to be rated as well but were not 
required to be so. 
• Enclosures for vertical shafts, including stairways and transfer corridors, elevator hoistways, 
and mechanical or utility shafts were required to be of 2 hour fire rated construction. These 
innovative walls are further described below. 
There was a conflict regarding the number of partitions within a tenant space. On the one hand, the 
design of the WTC towers was intended to provide about 30,000 ft2 per floor of nearly uninterrupted 
space and access to views of the Manhattan panorama. On the other hand, Local Law 5 dictated 
compartmentation into no more than 7,500 ft2 areas for unsprinklered spaces. These areas could be 
increased to 15,000 ft2 if protected by 2 hour fire resistive construction and smoke detectors. The 
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Draft for Public Comment The Design and Construction of the Towers 
compartmentation limit was removed when complete sprinkler protection was provided. Following a 
1975 fire, The Port Authority began installing sprinklers at the time a new tenant moved in. By 
September 11, 2001, the installations had been completed throughout the towers, and, in general, the 
tenants on the impact floors had few internal partitions except for those surrounding conference rooms 
and executive offices. 
Firestopping materials are used to fill gaps in walls and floors through which smoke and flames might 
pass. Such passage could negate the fire endurance value of the wall or floor. The 1968 NYC Building 
Code included comprehensive requirements identifying when and where firestopping was required. The 
1964 New York State Building Code addressed the issue in less detail, and the Chicago Building Code 
had no requirements. The NFPA Life Safety Code had firestopping requirements for exterior and interior 
partitions at floor levels, and did allow a trade-off for sprinklered concealed spaces. In the towers, unlike 
many buildings, the exterior wall was connected with the floors without gaps. 
5.3.5 Egress Provisions 
The primary egress system for the office spaces was the three stairways 
The NYC Building Code 
located in the building core. There were four main requirements for these used the “units of exit 
stairways: number, width (including separate width requirements for the 
width” method for 
doors), separation of the doors to the stairways, and travel distance to the specifying exit capacity, 
stairway doors. in which each 22 in. unit 
of exit width provided 
The number of stairways and the width of the doors resulted from the the capacity for 60 
people. Thus each
implementation of the 1968 edition of the NYC Building Code, whose 44 in. stairwell provided 
provisions were less restrictive than those in the 1938 edition. The for 120 people and the 
1968 code eliminated a fire tower (an enclosed staircase accessed through 56 in. stairwell provided 
a naturally ventilated vestibule) as a required means of egress and reduced 2½ units, or 150 people, 
for a total occupant load 
the number of required stairwells from six to three and the width of the per floor of 390. 
doors leading to the stairs from 44 in. to 36 in. 
Of the three staircases, two (designated A and C) were 44 in. wide; stairway B was 56 in. wide. The 
largest occupant load in the office spaces was 365 people per floor (36,500 ft2 on the largest floor, with 
100 ft2 per person). Neither the 1968 NYC Building Code nor any of the contemporaneous codes 
mandated consideration of the number of building stories in determining the number and widths of the 
stairwells. 
For the floors classified in the office use group (all floors except the observation deck and 
restaurant/meeting spaces), a minimum of two stairwells would have been required to serve the 
occupants, each equally sized. The three modern building codes considered in this report (International 
Building Code (IBC) (2000), NYC Building Code (2003), and NFPA 5000 (2003)), as well as the 
1968 NYCBC, were consistent in this requirement, each regardless of building height. However, the 
resulting width of these minimum requirements would differ. Two 44 in. stairwells would have satisfied 
IBC minimum requirements, two 65 in. stairwells would have satisfied NFPA 500 requirements, and two 
78 in. stairwells would have satisfied the 1968 and 2003 NYC Building Code requirements. Alternatively, 
as was built at WTC 1 and WTC 2, three stairwells of narrower construction, but equivalent or greater 
total required width, would also satisfy the egress requirements in the modern building codes. 
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Chapter 5 Draft for Public Comment 
The 1968 NYC Building Code contained a requirement that the stairwells be “as far apart as practicable.” 
Since the stairwells on the impact floors of WTC 1 were substantially closer together than those on the 
impact floors of WTC 2, it certainly was possible to have designed a greater separation in WTC 1. Local 
Law 16 (1984) added a quantitative requirement that the separation between exit door openings be at least 
one-third of the maximum travel distance of the floor. For the WTC towers, this maximum distance was 
180 ft, and the smallest separation of stairwell doors was 70 ft. The towers were consistent with this 
requirement. 
NFPA 5000 (2003) and IBC (2000) incorporate a requirement that the separation of the stairwells be no 
less than one-third the overall diagonal length of the building. For the towers this length was 294 ft, and 
one-third was 98 ft. Thus, the stairwell separations on some floors would have been inconsistent with the 
later codes (with which the buildings in New York City were not required to comply). 
At the top of the two towers were floors that were classified as public assembly floors: the Windows on 
the World restaurant complex in WTC 1 (Floors 106 and 107) and the Top of the World observation deck 
in WTC 2 (Floor 107). The design number of occupants on each of these floors was over 1,000. On 
September 11, 2001, there were about 188 people in the Windows on the World and few in the Top of the 
World since it was before the opening hour. Thus, had the stairwells remained passable through the 
impact region, the capacity would have been sufficient for the occupant load observed on that morning. 
Nonetheless, the egress requirements for assembly occupancy were more stringent than for business 
occupancy in both the NYC Building Code in 1968 and in 1996, when the Windows on the World 
re-opened after refurbishment following the 1993 bombing in the basement. NIST found documentation 
that The Port Authority had created areas of refuge consistent with the width provisions of the 1968 NYC 
Building Code but was unable to find evidence indicating that the requirements for the number of exits 
for the evacuation of over 1,000 people from each of these floors had been considered in the design or 
operation of the buildings. The New York City Department of Buildings, however, had reviewed the 
egress capacity from these floors and apparently concurred that the proposed remodel to these spaces 
would meet the intent of the NYC Building Code. 
Subsequently, NIST communications in 2005 with The Port Authority and the NYC Department of 
Buildings identified a difference of interpretation regarding the number of exits required to serve these 
floors. The Port Authority stated that a fourth exit was not required since the assembly use space in 
question constituted less than 20 percent of the area of principal use, with principal use area defined as the 
entire building. The Department of Buildings stated that the 20 percent rule did not apply to assembly 
spaces such as restaurants and observation decks that are open to the public, and therefore exit reduction 
cannot be applied and a fourth exit was required. 
The Department further clarified that areas of refuge and horizontal exits are not to be credited for 
required means of egress (unless the spaces are used non-simultaneously) and that for places of assembly, 
with occupant load in excess of 1,000, the floor shall have a minimum of four independent means of 
egress (stairs) to street. If the floor were divided into areas of refuge with rated walls, as was the case for 
the WTC towers, each area is to be considered an independent place of assembly that needs its own 
access to two means of egress (stairs) without going through another assembly space if they have an 
occupant load of less than 500 each (or three means of egress if the area of refuge had an occupant load 
between 500 and 999). Further, since the only means of egress from the roof-top deck was through the 
space on the observation floor, the Department clarified that occupant load from the deck would need to 
be added to the occupant load of the observation floor and that the travel distance from the roof deck 
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Draft for Public Comment The Design and Construction of the Towers 
along the connecting stairs to the required means of egress at the observation floor shall be within the 
maximum permitted by the NYC Building Code. 
Given the low occupancy level on September 11, 2001, NIST found that the issue of egress capacity from 
these places of assembly, or from elsewhere in the buildings, was not a significant factor on that day. It is 
conceivable that such a fourth stairwell, depending on its location and the effects of aircraft impact on its 
functional integrity, could have remained passable, allowing evacuation by an unknown number of 
additional occupants from above the floors of impact. If the buildings had been filled to their capacity 
with 25,000 occupants, however, the required fourth stairway would likely have mitigated the insufficient 
egress capacity for conducting a full building evacuation within the available time. 
The elevator system was described in Chapter 1. These were not to be used for emergency evacuation 
except under the control of the fire department. However, roughly 3,000 of the people who were initially 
at or above the impact floors in WTC 2 and were warned by the attack on WTC 1, survived, in large part 
by taking the elevators downward before the aircraft struck WTC 2. 
Following the 1993 bombing, The Port Authority instituted the following changes to reduce egress time, 
in addition to those stairwell improvements mentioned in Section 1.1.2: 
• Construction of new egress corridors, north (to Church St. and Vesey St.) and south (to 
Liberty St.) for faster evacuation from the Concourse (mall), and of two escalators from the 
Concourse (mall), one to the plaza at WTC 5 and one up to WTC 4 and onto Church St. 
• Semiannual fire drills in conjunction with the FDNY. 
• Appointment of Fire Wardens, specially trained and equipped with flashlights, whistles, and 
identifying hats. 
Building Communications 
WTC emergency procedures specified that all building-wide announcements A Fire Command Desk 
were to be broadcast from the Fire Command Desk (FCD), located in the (Figure 5–1) was 
lobby of each WTC tower, using prepared text (Figure 5–1). A situation located in the lobby of 
requiring evacuation for any reason, including fire or smoke, would have led each tower. The 
computer screen 
to the following announcement, enabling a phased evacuation: 
monitored the fire 
alarms, smoke 
“Your attention please. We are experiencing a smoke condition in sensors, sprinkler 
the vicinity of your floor. Building personnel have been dispatched water flow, elevator 
to the scene and the situation is being addressed. However, for lobby smoke 
detectors, fire signal 
precautionary reasons, we are conducting an orderly evacuation of 
floors _____. Please wait until we announce your floor number over activation, air handling 
fans, status of 
the public address system. Then follow the instructions of your fire 
elevators, and troubles 
safety team. We will continue to keep you advised. We apologize with the fire systems. 
for the inconvenience and we thank you for your cooperation.”9 
Port Authority of New York and New Jersey. World Trade Center Emergency Procedures Manual 2001, Confidential. 
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Chapter 5 Draft for Public Comment 
The announcement to be used when a particular floor required an evacuation was: 
“Your attention please. It is now time for your floor to be evacuated. In accordance with the 
directions from your fire safety team, please take the exit stairs nearest to your location. We 
remind you that communications, emergency lighting and other essential services are in service. 
We will continue to keep you advised. We apologize for the inconvenience and we thank you for 
your cooperation.”10 
At the discretion of the Fire Safety Director, the information and instructions broadcast to the building 
occupants could be modified to suit the nature of the emergency. 
Figure 5–1. Fire Command 
Desk in WTC 1, as seen from a 
mezzanine elevator, looking 
west. 
5.3.6 Active Fire Protection 
The provision of fire safety in the WTC towers revolved around a Fire Safety Plan that provided direction 
for fire emergency response and was organized around a hierarchy of staff trained in its implementation. 
In charge in each tower was the Fire Safety Director, who oversaw emergency response until the arrival 
of the Fire Department of the City of New York (FDNY), gathered necessary information, and relayed it 
to the Fire Chief upon arrival. In an emergency, the Fire Safety Director proceeded to the FCD or the fire 
scene. He/she had one or more Deputy Fire Safety Directors located at the FCD and at the sky lobbies. 
The front line was a set of Floor Wardens and Deputy Floor Wardens who were responsible for assessing 
conditions and assisting the evacuation of occupants on their respective floors. The Floor Wardens had 
their own communications system. 
Built into each tower were four resources to mitigate the effects of a fire: an alarm system to alert people 
to the presence of the fire, an automatic sprinkler system and a standpipe system for controlling the fire 
by the application of water, and a smoke venting system to improve visibility as people proceeded toward 
exits. The primary documentation of the design, installation, maintenance, and modification of these 
systems was stored on the 81st floor of WTC 1 and was lost when that building collapsed. Contractors to 
the Investigation Team were able to re-create descriptions of the physical systems and their capabilities 
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Draft for Public Comment The Design and Construction of the Towers 
from limited duplicate information provided by The Port Authority, Silverstein Properties, Inc, and 
contractors, consultants, and operators involved with the systems. 
The original fire alarm system used the technology current at the time and was engineered exclusively for 
the World Trade Center towers. The 1993 bomb explosion in WTC 1 destroyed the communications to 
the Operations Control Center, and the alarm system was revealed to be vulnerable to a single point of 
failure. Repair was problematic, since spare parts for the 25-year-old system were unavailable and the 
software was no longer supported. The Port Authority immediately commissioned a new state-of-the-art 
system for WTC 1, WTC 2, WTC 4, WTC 5, and the subterranean levels. This retrofit involved the 
installation of over 10,000 detectors, pull stations, and monitors; 30,000 notification devices (speakers 
and strobe lights); 150 miles of conduit; and 1,000 miles of wiring. Redundant Operations Control 
Centers were located in the basements of both towers. 
The primary monitoring and control of the fire alarm system was performed at the FCD located in the 
lobby of each building. The new system included: 
• Numerous interconnected microprocessors located in each of the four WTC buildings. 
• Smoke sensors located throughout the tenant spaces, at each elevator landing, in return air 
ducts, and in electrical and mechanical rooms. 
• At least one manual fire alarm station installed in each story in the evacuation path. 
• Emergency voice and alarm speakers for notification and communication in all areas within 
the buildings, designed to ensure system function in the event 50 percent of the system 
became inoperable. 
• Automatic notification of the fire department upon fire alarm activation. 
• Two-way communications stations at the remote fire panels, at the Floor Warden stations, 
and at the standpipes. 
• A two-way telephone system for the firefighters to make announcements. 
• Emergency voice and alarm communication capability, both under manual control at 
the FCD. 
• Strobe lights to provide alarm indications for the hearing impaired. 
• Water flow indicators for the fire sprinkler system, including indicators for disabled systems. 
No documentation of the status of the replacement system survived the 2001 attack. However, a 2002 
analysis estimated that over 80 percent of the towers had been retrofitted and that about 25 percent of the 
original system was still in use. 
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Chapter 5 Draft for Public Comment 
Although there were localized carbon dioxide and halon systems within the towers, the Safety Plan 
predominantly relied on water for containing and suppressing a fire (Figure 5–2). By 
September 11, 2001, automatic sprinklers had been installed throughout WTC 1 and WTC 2.10 The New 
York City water distribution system supplied water to the complex from two independent connections 
located under Liberty Street to the south and Vesey Street to the north. Within each tower were six 
5,000 gal water storage tanks, three located on the 110th floor and one each on the 20th, 41st, and 75th 
floors. These were filled from the city water supply through pipes that ran through the stairwell 
enclosures. In the event of a fire, the gravity-fed water would flow via two pipes down to as many of the 
thousands of installed sprinkler heads as had been activated. The WTC engineering staff could supply 
additional water upward from the city mains using manually started pumps located in the towers; the 
FDNY could augment the supply using fire department connections and truck-based pumps. While there 
were redundant vertical supply pipes, there was only a single connection to the array of sprinklers on any 
given floor. 
Figure 5–2. Schematic of sprinkler and standpipe systems. 
10 The exceptions to this were the computer rooms (protected with halon and carbon dioxide systems), kitchens (protected with 
dry chemical and steam smothering systems), mechanical spaces on the 108th through 110th floors, and the electrical rooms 
throughout the buildings, for which the application of water would have been inappropriate. 
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Draft for Public Comment The Design and Construction of the Towers 
In the aftermath of the 1993 bombing of WTC 1, dense smoke in the stairwells severely prolonged the 
evacuation of the tower. As a result, The Port Authority constructed a manually activated (by Port 
Authority staff at the direction of FDNY) smoke purge system and integrated the system's use into its 
WTC Fire Safety Plan. The system was designed to meet the 1968 NYC Building Code and was 
functional by September 11, 2001. The non-dedicated system used the existing building ventilation 
system, in contrast with an alternative dedicated system that would have been used only for smoke 
management. Each tower was divided into three zones, with the blowers located on the mechanical 
equipment floors (7, 41, 75 and 108). In the smoke purge mode, the mechanical system was aligned so 
that an entire zone was vented; there was no provision to vent an individual floor. The smoke from the 
impact floors in WTC 1 would have been drawn upward to the 108th floor, while the smoke from the 
impact floors in WTC 2 would have been drawn downward to the 75th floor. The system was designed to 
clear the zone of smoke after the fire was extinguished, perhaps during post-fire cleanup operations, lest 
the forced air increase the burning intensity. 
5.4 BUILDING INNOVATIONS 
5.4.1 The Need for Innovations 
Had the towers been built according to conventional design, they would have been heavier and would 
have had less usable space on each floor. Thus, a resourceful approach was taken in translating The Port 
Authority’s needs and Yamasaki’s design into practice. 
The Investigation Team identified six innovations incorporated in the lateral-load-resisting system and the 
gravity-load-carrying system of the towers. Their roles were discussed in Chapter 1. In addition, there 
were two innovations in achieving the required fire resistance ratings. The innovative, tiered elevator 
system was also discussed in Chapter 1. The following sections describe these new technologies. The 
use of spray-applied fire resistive materials is discussed in more detail in Section 5.6. 
5.4.2 Framed Tube System 
WTC 1 and WTC 2 were among the first steel-structure, high-rise buildings built using the framed-tube 
concept to provide resistance to lateral (wind) loads. The framed-tube system had previously been used 
in the concrete-framed, 43-story DeWitt-Chestnut and the 38-story Brunswick buildings, both in Chicago 
and both completed in 1965. 
In the framed-tube concept, the exterior frame system resists the force of the wind. The exterior columns 
carry a portion of the building gravity loads, and in the absence of wind, are all in compression, i.e., the 
loads push down on and shorten the columns. Under the effect of a strong wind alone, columns on the 
windward side are in tension, i.e., they elongate as the top of the building bends away from the wind. The 
columns on the leeward side are compressed. The columns on the walls parallel to the wind are half in 
tension (on the windward side) and half in compression (on the leeward side). The net effect of combined 
gravity and wind loads is larger compression on the leeward side and reduced compression, or in rare 
instances even tension, on the windward side. 
Prior to final design, tests had been performed at the University of Western Ontario to assess the stiffness 
of the wall panels, which consisted of three columns, each three stories high, and the associated spandrel 
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Chapter 5 Draft for Public Comment 
plates as shown in Figure 1–4. These tests used quarter-scale thermoplastic models of panels planned for 
the 20th, 47th, and 74th floors. (Recall that the structural members became lighter at the higher floors.) 
The tests also examined the effect of the spandrel thickness, the width of the box columns, and the 
presence and thickness of stiffeners. Forces were applied to the models, and the resulting deflections 
measured. The results of these tests guided the final design of the wall panels and provided support for 
The Port Authority’s acceptance of the resulting structural design. This included the innovations 
described in Sections 5.4.3 and 5.4.4. 
5.4.3 Deep Spandrel Plates 
The standard approach to construction of the framed tube would have used spandrel beams or girders to 
connect the columns. The towers used a band of deep plates as spandrel members to tie the perimeter 
columns together. 
5.4.4 Uniform External Column Geometry 
In a typical high-rise building, the columns would have been larger near the base of the building and 
would have become smaller toward the top as they bore less wind and gravity loads. However, the 
Yamasaki design called for the appearance of tall, uniform columns (Figure 1–2). This was achieved by 
varying both the strength of the steels and the thickness of the plates that made up the perimeter columns. 
5.4.5 Wind Tunnel Test Data to Establish Wind Loads 
To determine the extreme wind speeds that could be expected at the top of the towers, Worthington, 
Skilling, Helle & Jackson (WSHJ) collected data on the wind speeds and directions recorded in the New 
York area over the prior 50 years. From these data, a design wind speed for the buildings was determined 
for a 50 year wind event, defined as the wind speed, averaged over a 20 min duration at 1,500 ft above the 
ground. The estimated value was just under 100 mph in all directions. 
To estimate how the buildings would perform under wind loads, both during construction and upon 
completion, WSHJ conducted a then unique wind tunnel testing program at Colorado State University 
(CSU) and the National Physical Laboratory (NPL) in the United Kingdom. In each wind tunnel, a 
physical model of Lower Manhattan, including the towers, was subjected to steady and turbulent winds 
consistent with the estimated design wind speeds. The model scale was 1/500 for the CSU tests and 
1/400 for the NPL tests. The tower models were thus about 3 ft tall. Separate tests were conducted for 
the single tower and for the two towers at various spacings, with various values of the tower stiffness and 
damping, and for various wind directions. The two laboratories obtained similar results. Tests on the twotower 
models showed that the wind response of each tower was significantly affected by the presence of 
the other tower. 
WSHJ also conducted experiments to determine the wind-induced conditions that would be tolerated by 
the people who would work in and visit the towers. Breaking new ground in human perception testing, 
the investigators found that surprisingly low building accelerations caused discomfort. 
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Draft for Public Comment The Design and Construction of the Towers 
The test results led to changes in the building design, including stiffer perimeter columns, and the 
addition of viscoelastic dampers described in the next section. The dampers were used to reduce the 
building vibrations due to winds. 
5.4.6 Viscoelastic Dampers 
The tower design included the first application of damping units to supplement the framed-tube in 
limiting wind-induced oscillations in a tall building. Each tower had about 10,000 dampers. 
On most truss-framed floors (tenant floors), a damper connected the lower chord of a truss to a perimeter 
column. A depiction of the units is shown in Figure 5–3. On beam-framed floors (generally the 
mechanical floors with their heavier loads), a damper connected the lower flange of a wide-flange beam 
(that spanned between the core and the perimeter wall) to a spandrel. 
Figure 5–3. Diagram of floor truss showing viscoelastic damper. 
Two sets of experiments, conducted by the 3M Company (the manufacturer of the viscoelastic material) 
and by the Massachusetts Institute of Technology, examined the damping characteristics of the units. 
Both studies found that the units provided significant supplemental damping under design conditions. 
5.4.7 Long-Span Composite Floor Assemblies 
The floor system in the towers (Figure 1–6) was novel in two respects: 
• The use of open-web, lightweight steel trusses topped with a slab of lightweight concrete; and 
• The composite action of the steel and concrete that resulted from the “knuckles” of the truss 
diagonals extending above the top chord and into the poured concrete. 
Tests conducted in 1964 by Granco Steel Products and Laclede Steel Company (the manufacturer of the 
trusses for WTC 1 and WTC 2) determined the effectiveness of the knuckles in providing composite 
action. Another set of tests, performed by Laclede Steel Company, determined that any failure of the 
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Chapter 5 Draft for Public Comment 
knuckles occurred well beyond the design capacity. A third set of tests, performed at Washington 
University in 1968, confirmed the prior results and indicated that failure was due to crushing of the 
concrete near the knuckles. 
5.4.8 Vertical Shaft Wall Panels 
While similar to other gypsum shaft wall systems and firewalls, the compartmentation system used in the 
vertical shafts (e.g., for elevators, stairs, utilities and ventilation) was unique in that it eliminated the need 
for any framing. The walls consisted of gypsum planks placed into metal channels at the floor and ceiling 
slabs. The planks were 2 in. thick (2½ in. on floors with 16 ft ceiling heights) and 16 in. wide, with metal 
tongue and groove channels attached to the long sides that served as wall studs. An assembled wall was 
then covered with gypsum wallboard. The planks were likely custom fabricated for this job, as the 
investigators found no mention of similar products in gypsum industry literature of the time or since. 
5.5 STRUCTURAL STEELS 
5.5.1 Types and Sources 
Roughly 200,000 tons of steel were used in the construction of the two WTC towers. The building plans 
called for an unusually broad array of steel grades and multiple techniques for fabricating the structure 
from them. The NIST team obtained the information needed to characterize the steels from structural 
drawings provided by The Port Authority, copies of correspondence during the fabrication stages, steel 
mill test reports, interviews with fabrication company staff, search of the contemporaneous literature, and 
measurements of properties at NIST. Sorting through this immense amount of information was made 
difficult by the large number of fabricators and suppliers, the use of proprietary grades by some of the 
manufacturers; and the fact that the four fabricators of the impact and fire floor structural elements no 
longer existed at the time of this Investigation. 
Fortunately, the potential for confusion had led the building designers to a tracking system whereby the 
steel fabricators stamped and/or stenciled each structural element with a unique identifying number. The 
structural engineering drawings included these identifying numbers as well as the yield strengths of the 
individual steel components. Thus, when NIST found the identifying number on an element such as a 
perimeter column panel, the particular steel specified for each component of the element was known, as 
well as the intended location of the steel in the tower. 
In all, 14 grades of steel were specified in the structural engineering plans, having yield strengths from 
36 ksi to 100 ksi. Twelve were actually used, as the fabricators were permitted to substitute 100 ksi steel 
where yield strengths of 85 ksi and 90 ksi were specified. Table 5–1 indicates the elements for which the 
various grades were used. The higher yield strength steels were used to limit building weight while 
providing adequate load carrying capacity. 
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Draft for Public Comment The Design and Construction of the Towers 
Table 5–1. Specified steel grades for various applications. 
Yield Strength (ksi) 
Application 36 42 45 46 50 55 60 65 70 75 80 100 
Perimeter columns 9 9 9 9 9 9 9 9 9 9 9 9 
Spandrel plates 9 9 9 9 9 9 9 9 9 9 9 9 
Core columns 9 9 (a) (a) 
Floor trusses 9 9 
a. About 1 percent of the wide flange core columns were specified to be of these higher grades. 
5.5.2 Properties 
The Port Authority required a thorough and detailed quality assurance programs to ensure compliance 
with the specifications for the steel, welds and bolts. The steel data went beyond the minimum yield 
strength (the property of greatest importance) to include tensile strength and ductility. The quality 
assurance program included unannounced inspections and confirming tests. 
NIST performed confirmatory tests on samples of the 236 pieces of recovered steel to determine if the 
steel met the structural specifications. Making a definitive assessment was complicated by overlapping 
specifications from multiple suppliers, differences between the NIST test procedures and the test 
procedures that originally qualified the steel, the natural variability of steel properties, and damage to the 
steel from the collapse of the WTC towers. Nonetheless, the NIST investigators were able to determine 
the following: 
• There were 14 grades (strengths) of steel that were specified. However, a total of 32 steels in 
the impact and fire floors were sufficiently different (grade, supplier, and gage) to require 
distinct models of mechanical properties. 
• The steels in the perimeter columns met their intended specifications for chemistry, 
mechanical properties, yield strengths, and tensile properties. The steels in the core columns 
generally met their intended specifications for both chemical and mechanical properties. 
• Roughly 13 percent of the measured strength values for the perimeter and core columns were 
at or below the specified minimums (Figure 5–4). The strength variation was consistent with 
the historical variability of steel strength and with the effects from damage during the 
collapse of the towers. The measured values were within the typical design factor of safety. 
• The yield strengths of many of the steels in the floor trusses were above 50 ksi, even when 
they were specified to be 36 ksi. 
• Tests on a limited number of recovered bolts showed they were much stronger than expected 
based on reports from the contemporaneous literature. 
The mechanical properties of steel are reduced at elevated temperatures. Based on measurements and 
examination of published data, NIST determined that a single representation of the elevated temperature 
effects on steel mechanical properties could be used for all WTC steels. Separate values were used for 
the yield and tensile strength reduction factors for bolt steels. 
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Chapter 5 Draft for Public Comment 
Note: The ratio values less than 1 arose from natural variation in the steel and did not affect 
the safety of the towers on September 11, 2001. The bars represent maximum and 
minimum values from multiple measurements. 
Figure 5–4. Ratio of measured yield strength (Fy) to specified minimum 
yield strength for steels used in WTC perimeter columns. 
5.6 PASSIVE FIRE PROTECTION 
5.6.1 Thermal Insulation 
When steel is heated it loses both strength and stiffness. Thus, measures must be taken to protect the steel 
in a structure from temperature rise (and consequent loss of strength) in case of fire. 
Bare structural steel components can heat quickly when exposed to a fire of even moderate intensity. 
Therefore, some sort of thermal protection, or insulation, is necessary. This insulation can be in direct 
contact with the steel, such as a spray-applied fire resistive material (SFRM), or can be a fire resistant 
enclosure surrounding a structural element. 
5.6.2 Use of Insulation in the WTC Towers 
The thermal protection of the steel structures in the WTC towers included a combination of SFRM and 
enclosures of gypsum wallboard. The use of SFRM for floor truss protection was new in high-rise 
buildings, and the requirements evolved during the construction and life of the towers. By examining 
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Draft for Public Comment The Design and Construction of the Towers 
documents supplied by The Port Authority, LERA, and the SFRM manufacturers, NIST was able to 
document much of the sequence of these changing requirements and arrive at an estimation of the passive 
protection in place on September 11, 2001. 
Floor Systems 
At the time the WTC was designed, the ASTM E 119 test method had been used for nearly 50 years to 
determine the fire resistance of structural members and assemblies. However, The Port Authority 
confirmed to the Investigation Team that there was no record of fire endurance testing of the innovative 
assemblies representing the thermally protected floor system used in the towers. The floor assembly was 
not tested despite the fact that the Architect of Record and the Structural Engineer of Record stated that 
the fire rating of this novel floor system could not be determined without testing. 
Prior to construction, the Architect of Record had used information from (unidentified) manufacturers to 
recommend a 1 in. thickness of SFRM around the top and bottom chords of the trusses and a 2 in. 
thickness for the web members of the trusses. This was to achieve the fire endurance requirements for 
Class 1A construction (Section 5.3.3). 
In 1969, The Port Authority directed that a ½ in. thick coating of CAFCO BLAZE-SHIELD Type D 
(CAFCO D), a mixture of cement and asbestos fibers, be used to insulate the floor trusses. This was to 
achieve a Class 1A rating, even though the preponderance of evidence suggests that the towers were 
chosen to be Class 1B, the minimum required by the NYC Building Code. NIST found no evidence of a 
technical basis for selection of the ½ in. thickness. This coating had been installed as high as the 38th floor 
of WTC 1 when its use was discontinued due to recognition of adverse health effects from inhalation of 
asbestos fibers. The spraying then proceeded with CAFCO DC/F, a similar product in which the asbestos 
was replaced by a glassy mineral fiber and whose insulating value was reported by Underwriters 
Laboratories, Inc., to be slightly better than that of CAFCO D. On the lower floors, the CAFCO D was 
encapsulated with a sprayed material that provided a hard coat to mitigate the dispersion of asbestos fibers 
into the air. 
In 1994, The Port Authority measured the SFRM thickness on trusses on floors 23 and 24 of WTC 1. In 
all, average thicknesses were reported for 32 locations, and the overall average thickness was found to be 
0.74 in. NIST performed a further evaluation of the SFRM thickness using photographs taken in the 
1990s of floor trusses on (non-upgraded) floors 22, 23, and 27 of WTC 1 (Figure 5–5). By measuring 
dimensions on the photographs, NIST estimated the insulation thicknesses on the diagonal web members 
of trusses. (The thickness of chord member insulation could not be measured.) The average thickness 
and standard deviation of web members was 0.6 in. ± 0.3 in. on the main trusses, 0.4 in. ± 0.25 in. on the 
bridging trusses, and 0.4 in. ± 0.2 in. on the diagonal struts. These numbers indicated that there were 
areas where the coating thickness was less than the specified 0.5 in. 
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Chapter 5 Draft for Public Comment 
Missing 
Insulation 
Note: Enhancement by NIST. 
Figure 5–5. Irregularity of coating thickness and gaps in coverage on 
SFRM–coated bridging trusses. 
In 1995, The Port Authority performed a study to establish requirements for retrofit of sprayed insulation 
to the floor trusses during major alterations when tenants vacated spaces in the towers. Based on design 
information for fire ratings of a similar, but not identical, composite floor truss system contained in the 
Fire Resistance Directory published by Underwriters Laboratories, Inc., the study concluded that a 1½ in. 
thickness of sprayed mineral fiber material would provide a 2 hour fire rating, consistent with the 
Class 1B requirements. In 1999, the removal of existing SFRM and the application of new material to 
this thickness became Port Authority policy for full floors undergoing new construction and renovation. 
For tenant spaces in which only part of a floor was being modified, the SFRM needed only to be patched 
to ¾ in. thickness or to match the 1½ in. thickness, if it had previously been upgraded. In the years 
between 1995 and 2001, thermal protection was upgraded on 18 floors of WTC 1, including those on 
which the major fires occurred on September 11, 2001, and 13 floors of WTC 2 that did not include the 
fire floors. The Port Authority reported that the insulation used in the renovations was CAFCO 
BLAZE-SHIELD II. 
In July 2000, an engineering consultant to The Port Authority issued a report on the requirements of the 
fire resistance of the floor system of the towers. Based on calculations and risk assessment, the consultant 
concluded that the structural design had sufficient inherent fire performance to ensure that the fire 
condition was never the critical condition with respect to loading allowances. The report recommended 
that a 1.3 in. thickness be used for the floor trusses. 
In December 2000, another condition assessment concluded that the structural insulation in the towers 
had an adequate 1 hour rating, considering that all floors were now fitted with sprinklers. The report also 
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Draft for Public Comment The Design and Construction of the Towers 
noted the ongoing Port Authority program to upgrade the fire resistive material thickness to 1½ in. in 
order to achieve a 2 hour fire rating. 
The Port Authority provided NIST with the records of measurements of SFRM thickness on upgraded 
floors in both towers. The average thickness and standard deviation on the main trusses was 2.5 in. ± 
0.6 in. NIST analysis of several Port Authority photographs from the 1990s of the upgraded 31st floor of 
WTC 1 indicated an average thickness and standard deviation on the main trusses of 1.7 in. ± 0.4 in. 
NIST found no statistically significant difference in the average thickness of the upgraded insulation in 
the two towers. 
Perimeter Columns 
In 1966, the contractor responsible for insulating the perimeter columns proposed applying a 1 3/16 in. 
thick coating of CAFCO D to the three external faces (Figure 5-6) to achieve a 4 hour rating, which is a 
Class 1A rating requirement (1 hour more than Class 1B). NIST found evidence of a technical basis for 
this decision. In the construction drawings prepared by the exterior cladding contractor, the following 
SFRM thicknesses were specified: 
• 7/8 in. of vermiculite plaster on the interior face and 1 3/16 in. of CAFCO D on the other 
three faces. 
• ½ in. of vermiculite plaster on the interior surfaces of the spandrels and ½ in. of CAFCO D 
on the exterior surfaces. 
Figure 5-6. Thermal insulation for perimeter columns. 
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Chapter 5 Draft for Public Comment 
Vermiculite plaster had a higher thermal conductivity and thereby increased heat migration from the room 
air to the column steel and, thus, could keep the steel temperature at 70 °F when the temperature was 0 °F 
outside. 
In October 1969, The Port Authority provided the following instructions to the contractor applying the 
sprayed fire protection, in order to maintain the Class 1-A Fire Rating of the NYC Building Code: 
• 2 3/16 in. of CAFCO D for columns smaller than 14WF22811 and 1 3/16 in. for columns 
equal to or greater than 14WF228. 
• ½ in. covering of CAFCO D for beams, spandrels and bar joists. 
NIST’s review of available documents has not uncovered the reasons for selecting CAFCO fire resistive 
material or the technical basis for specifying ½ in. thickness of SFRM for the floor trusses. As with the 
trusses, CAFCO DC/F was applied to the perimeter columns above the 38th floor of WTC 1 and all the 
perimeter columns in WTC 2. 
Core Columns and Beams 
Multiple approaches were used to insulate structural elements in the core: 
• Those core columns located in rentable and public spaces, closets, and mechanical shafts 
were enclosed in boxes of gypsum wallboard (and thus were inaccessible for inspection). 
The amount of the gypsum enclosure in contact with the column varied depending on the 
location of the column within the core. SFRM (CAFCO D and DC/F) was applied on those 
faces that were not protected by the gypsum enclosure. The thicknesses specified in the 
construction documents were 1 3/16 in. for the heavier columns and 2 3/16 in. for the lighter 
columns. 
• Columns located at the elevator shafts were protected using the same SFRM thicknesses. 
They were not enclosed and thus were accessible for routine inspections. 
Inspection of the columns within the elevator shaft spaces in 1993 indicated some loss of SFRM 
coverage. As a result, new insulation was applied to selected columns within the elevator shaft space. 
Information provided to NIST indicated that a different SFRM, Monokote Type 2-106, was used. 
Thickness measurements for columns and beams below the 45th floor indicated average thicknesses of 
0.82 in. and 0.97 in., respectively. Information from The Port Authority indicated that the minimum 
required thickness of the re-applied SFRM was ½ in. for the columns and ¾ in. for the beams. 
NIST was unable to locate information from which to characterize the insulation of the core columns and 
beams that were not accessible. Except as noted above, once completed, the core was generally not 
inspected. NIST was not able to locate any post-collapse core beams or columns with sufficient 
insulation still attached to make pre-collapse thickness measurements. 
11 This designation indicates that the column is a 14 in. deep wide flange section and weighs 228 pounds per foot. 
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Draft for Public Comment The Design and Construction of the Towers 
Summary of SFRM on September 11, 2001 
Table 5–2 summarizes the types and thicknesses of the SFRMs used in the towers. According to Port 
Authority documents, in the upper part of the towers, trusses on floors 92 through 100 and 102 in WTC 1 
had upgraded insulation by September 11, 2001. In WTC 2, truss insulation had been upgraded on 
floors 77, 78, 85, 88, 89, 92, 96, 97, and 99. 
Table 5–2. Types and locations of SFRM on fire floors. 
Thickness (in.) 
Building Component Material Specifieda Installed 
Used in 
Analysisb 
FLOOR SYSTEM 
Original 
Main trusses and diagonal struts CAFCO DC/F 0.5 0.75 0.6 
Bridging trusses CAFCO DC/F 0.5c 0.38d 0.6a, 0.3b 
Upgraded 
Main trusses BLAZE-SHIELD II 1.5 2.5 2.2 
Main truss diagonal struts BLAZE-SHIELD II 1.5 2.5 2.2 
Bridging trusses BLAZE-SHIELD II 1.5 2.5 2.2 
EXTERIOR WALL PANEL 
Box columns 
Exterior face CAFCO DC/F 1 3/16 (e) 1.2 
Interior face Vermiculite plaster 7/8 (e) 0.8 
Spandrels 
Exterior face CAFCO DC/F 0.5 (e) 0.5 
Interior face Vermiculite plaster 0.5 (e) 0.5 
CORE COLUMNS 
Wide flange columns 
Light CAFCO DC/F 2 3/16 (e) 2.2 
Heavy CAFCO DC/F 1 3/16 (e) 1.2 
Box columns 
Light CAFCO DC/F (f) (e) 2.2(g) 
Heavy CAFCO DC/F (f) (e) 1.2(g) 
CORE BEAMS CAFCO DC/F 0.5 (e) 0.5 
a. “Specified” means material and thicknesses determined from correspondence among various parties. 
b. The analysis is described in Chapter 6. 
c. Not expressly specified. SFRM was required for the areas where the main trusses ran in both directions (a) and, while not 
required, was also applied in the areas where they ran in one direction only (b). 
d. Analysis of photographs indicated that the thickness was approximately one half that on the main trusses. 
e. Not able to determine. 
f. Not specified. 
g. Thickness assumed equal to wide flange columns of comparable weight per foot. 
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Chapter 5 Draft for Public Comment 
5.7 CONCRETE 
Two types of concrete were used for the floors of the WTC towers: lightweight concrete in the tenant 
office areas and normal weight concrete in the core area. Because of differences in composition and 
weight, the two types of concrete respond differently to elevated temperatures, as shown in Figure 5–7. 
While their tensile strengths degrade identically, lightweight concrete retains more of its compressive 
strength at higher temperatures. 
400
6000 
Compressive Strength (psi) 
5000 
4000 
3000 
Tensile Strength (psi)
300 
200 
100 
2000 
1000 
0
0 200 400 600 0 200 400 600 
Temperature (°C) Temperature (°C) 
Normal-weight (3000psi) Normal-weight (3000psi) 
Normal-weight (4000psi) 
Normal-weight (4000psi) 
Light-weight (3000psi) 
Light-weight (3000psi) 
Figure 5–7. Temperature–dependent concrete properties. 
At ordinary temperatures, the concrete in the WTC floors would have been in compression. As the fires 
raged, the floors would have heated and sagged. When the forces due to the sagging exceeded the tensile 
strength of the concrete, the concrete would have cracked. At the point the concrete cracked, only the 
reinforcing steel and trusses would have been carrying the gravity loads. 
5.8 THE TENANT SPACES 
5.8.1 General 
About 80 percent of the floors had a single tenant. Many of these floors were filled with arrays of 
modular office cubicles, their low partitions affording sightlines to the windows, with perhaps an 
occasional perimeter conference room or executive office in the way (Figure 1–11). Trading floors 
(Figure 1–12) had tables and computers throughout and food service areas to minimize time away from 
the non-stop transactions. The remaining 20 percent of the floors were each subdivided among as many 
as 25 tenants. Some of the approximately 25 tenants that occupied two or more contiguous floors 
installed convenience stairways within their own space. 
Certain floors were of special interest to the Investigation. These were the floors on which there was 
structural damage from the aircraft and/or on which extensive fires were observed. These floors, 
designated as focus floors, and the information the Investigation Team obtained regarding them are 
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Draft for Public Comment The Design and Construction of the Towers 
characterized in Table 5–3. Additional information, obtained from the tenant firms and The Port 
Authority, is summarized in the remainder of this chapter. 
5.8.2 Walls 
The plans for the tenant spaces in WTC 1 showed no interior walls whose sole function was to subdivide 
the floors. There were a number of partitioned offices and conference areas. Although NIST was not 
able to obtain layout drawings for the fire floors in WTC 2, the verbal descriptions of those floors 
indicated similarly open space. The types of interior walls were described in Section 5.3.4. 
5.8.3 Flooring 
The floors themselves were the tops of the 4 in. thick concrete slabs that were integrated with the ceiling 
trusses of the story below. Some tenants had installed slightly raised (6 in.) floors on top of the slab 
under which communication cables were run. This was especially true on trading floors. There was a 
wide range of floor coverings in use. Inlaid wood and marble were used in some reception areas. Most 
commonly, the expanse of the floor was covered with nylon carpet. 
5.8.4 Ceilings 
There were two different ceiling tile systems originally installed in the towers under Port Authority 
specification. The framing for each was hung from the bottom of the floor trusses, resulting in an 
apparent room height of 8.6 ft and an above-ceiling height of about 3.4 ft. The tiles in the tenant spaces 
were 20 in. square, ¾ in. thick, lay-in pieces on an exposed tee bar grid system. The tiles in the core area 
were 12 in. square, ¾ in. thick, mounted in a concealed suspension system. Neither system was specified 
to be fire-rated, and it was estimated that in a fire they might provide only 10 min to 15 min of thermal 
protection to the trusses before the ceiling frame distorted and the tiles fell. Chemically, the tiles were 
similar, and their combustible content, flame spread, and smoke production were all quite low. 
5.8.5 Furnishings 
The decorating styles of the tower tenants 
ranged from simple, modular trading floors to 
customized office spaces. The most common 
layout of the focus floors was a continuous 
open space populated by a large array of 
workstations or cubicles (Figure 1–11). The 
number of different types of workstations in 
the two towers was probably large. However, 
discussions with office furniture distributors 
and visits to showrooms indicated that, while 
there was a broad range of prices and 
appearances, the cubicles were fundamentally 
similar to that shown in Figure 5–8. Source: Reproduced with permission of The Port Authority 
of New York and New Jersey. 
Figure 5–8. A WTC workstation. 
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Chapter 5 Draft for Public Comment 
The workstations were typically 8 ft square, bounded on all four sides by privacy panels, with an entrance 
opening in one side only. Within the area defined by the panels was a self-contained workspace: desktop 
(almost always a wood product, generally with a laminated finish), file storage, bookshelves, carpeting, 
chair, etc. Presumably there were a variety of amounts and locations of paper, both exposed on the work 
surfaces and contained within the file cabinets and bookshelves. The cubicles were grouped in clusters or 
rows, with up to 215 units on a given floor. 
NIST estimated the fuel loading on these floors to have been about 4 lb/ft2 (20 kg/m2), or about 60 tons 
per floor. This was somewhat lower than found in prior surveys of office spaces. The small number of 
interior walls, and thus the minimal amount of combustible interior finish, and the limited bookshelf 
space account for much of the differences. 
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Draft for Public Comment The Design and Construction of the Towers 
Table 5–3. Floors of focus. 
Building Floor Tenant Damagea Firesb 
Material 
Obtainedc General Description of Tenant Layout 
92 Carr Futures, empty Y FP (Carr), V 
93 Marsh & McLennan 
(M&M), Fred Alger 
Mgmt. 
Y Y FP, F, V M&M occupied the south side. Filled with workstations. Demising walls 
for the south façade to the edges of the core. Offices along the east side of 
the south core wall. Stairwell to the 94th floor. 
94 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices and conference 
rooms around most of the perimeter. Stairwell to the 93rd floor. 
95 Marsh & McLennan Y Y FP, F, V Generally open space filed with workstations. Offices, conferences and 
work areas in exterior corners. Large walled data center along north and 
east sides. Two separate stairwells, one to 94th floor, the other to the 96th 
and 97th floors. 
96 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners 
and middle of north and south facades. Some conference rooms on north 
and south sides of core. Stairwell connection to 95th and 97th floors. 
WTC 1 
97 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners 
and in the middle of the north façade. Two separate stairwells: one 
connected to the 95th and 96th floors, the other connected to the 98th, 99th , 
and 100th floors. 
98 Marsh & McLennan Y Y FP, F, V Generally open space filled with workstations. Offices at exterior corners 
and middle of north and south facades. Some conference rooms on north 
and south sides of core. Stairwell connected to the 97th, 99th, and 100th 
floors. 
99 Marsh & McLennan Y Y FP, F, V Open space filled with workstations on the east side and east half of the 
north side. Offices at exterior corners and along south and west sides. 
Large walled area on west side of north façade. Stairwell connected to the 
97th, 98th, and 100th floors. 
100 Marsh & McLennan Y FP, F, V Considerable number of workstations, but more individual offices than the 
other floors. Partitioned offices extended the full length of the west wall 
and also at other locations along walls and at exterior corners. Stairway 
connected to the 97th, 98th, and 99th floors. 
104 Cantor Fitzgerald Y V Trading floor. Tables with many monitors. 
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Chapter 5 Draft for Public Comment 
Building Floor Tenant Damagea Firesb 
Material 
Obtainedc General Description of Tenant Layout 
77 Baseline Y Y FP, V Generally open space. Offices along east and west core walls. A few 
offices in each exterior corner of the floor. 
78 Baseline, 1st 
Commercial Bank 
Y Y FP,V West side open. Northeast quadrant walled. Offices along south side of 
east core wall. Offices along east side of south façade. 
79 Fuji Bank Y Y V 
80 Fuji Bank Y Y FP, V Generally open space filled with workstations. Offices or conference 
rooms at exterior corners and along south half of west façade. Large vault 
WTC 2 at southeast corner of core. 
81 Fuji Bank Y Y V 
82 Fuji Bank Y Y V 
83 Chuo Mitsui, IQ 
Financial 
Y V Chuo Mitsui had half the area. Wide open space. No information 
regarding IQ Financial. 
84 Eurobrokers Y V Open floor for trading. Tables rather than workstations. Perimeter offices. 
85 Harris Beach Y FP. V Offices around full perimeter. Offices along east, west and south walls of 
core. 
a. Floors on which the exterior photographs indicated direct damage from the aircraft. 
b. Floors on which the exterior photographs indicated extensive or sustained fires. 
c. Types of descriptive material obtained: FP, floor plan; F, documentation of furnishings; V, verbal description of interior. 
78 NIST NCSTAR 1, WTC Investigation 

Chapter 6 
RECONSTRUCTION OF THE COLLAPSES 
APPROACH 
The following presents an overview of the methods used to reach the accounts in Part I. The details may 
be found in the companion reports to this document, which are indexed in Appendix B. 
A substantial effort was directed at establishing the baseline performance of the WTC towers, i.e., 
estimating the expected performance of the towers under normal design loads and conditions. This 
enabled meeting the third objective of the Investigation, as listed in the Preface to this report. The 
baseline performance analysis also helped to estimate the ability of the towers to withstand the 
unexpected events of September 11, 2001. Establishing the baseline performance of the towers began 
with the compilation and analysis of the procedures and practices used in the design, construction, 
operation, and maintenance of the structural, fire protection, and egress systems of the WTC towers. The 
additional components of the performance analysis were: 
• The standard fire resistance of the WTC truss-framed floor system, 
• The quality and properties of the structural steels used in the towers, and 
• The response of the WTC towers to design gravity and wind loads. 
The second substantial effort was the simulation of the behavior of each tower on September 11, 2001, 
providing the basis for meeting the first and second objectives of the Investigation. This entailed four 
modeling steps, each stretching the state of the technology and testing the limits of software tools: 
1. The aircraft impact into the tower, the resulting distribution of jet fuel, and the damage to the 
structure, partitions, insulation materials, and building contents. 
2. The spread of the multifloor fires. 
3. The heating of the structural elements by the fires. 
4. The response of the damaged and heated building structure, and the progression of structural 
component failures leading to the initiation of the collapse of the towers. 
For the final analyses, four cases were used, each involving all four of the modeling steps. Case A and 
Case B were for WTC 1, with Case B generally involving more severe impact and fire conditions than 
Case A. For WTC 2, Case D involved more severe impact and fire conditions than Case C. The results 
of the two cases for each tower provided some understanding of the uncertainties in the predictions. 
There were substantial uncertainties in the as-built condition of the towers, the interior layout and 
furnishings, the aircraft impact, the internal damage to the towers (especially the insulation), the 
redistribution of the combustibles, and the response of the building structural components to the heat from 
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the fires. To increase confidence in the simulation results, NIST used information from an extensive 
collection of photographs and videos of the disaster, eyewitness accounts from inside and outside the 
buildings, and laboratory tests involving large fires and the heating of structural components. Further, 
NIST applied formal statistical methods to identify those parameters that had the greatest effect on the 
model output. These key inputs were then varied to determine whether the results were reasonably 
robust. 
The combined knowledge from all the gathered data and analyses led to the development of a probable 
collapse sequence for each tower,12 the identification of factors that contributed to the collapses, and a list 
of factors that could have improved building performance or otherwise mitigated the loss of life. 
DEVELOPMENT OF THE DISASTER TIMELINE 
Time was the unifying factor in combining photographic and video information, survivor accounts, 
emergency calls from within the towers, and communications among emergency responders. The visual 
evidence was the most abundant and the most detailed. 
The destruction of the WTC towers was the most heavily photographed disaster in history. The terrorist 
attacks occurred in an area that is the national home base of several news organizations and has several 
major newspapers. New York City is also a major tourist destination, and visitors often carry cameras to 
record their visits. Further, the very height that made the towers accessible to the approaching aircraft 
also made them visible to photographers. As a result there were hundreds of both professional and 
amateur photographers and videographers present, many equipped with excellent equipment and the 
knowledge to use it. These people were in the immediate area, as well as at other locations in New York 
and New Jersey. 
There was a surprisingly large amount of photographic material shot early, when only WTC 1 was 
damaged. By the time WTC 2 was struck, the number of cameras and the diversity of locations had 
increased. Following the collapse of WTC 2, the amount of visual material decreased markedly as people 
rushed to escape the area and the huge dust clouds generated by the collapse obscured the site. There is a 
substantial, but less complete, amount of material covering the period from the tower collapses to the 
collapse of WTC 7 late the same afternoon. 
There were multiple sources of visual material: 
• Recordings of newscasts from September 11 and afterward, documentaries, and other 
coverage provided information and also pointed toward other potential sources of material. 
• Web sites of the major photographic clearinghouses. 
• Local print media. 
12 The focus of the Investigation was on the sequence of events from the instant of aircraft impact to the initiation of collapse for 
each tower. For brevity in this report, this sequence is referred to as the "probable collapse sequence," although it does not 
actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse 
became inevitable. 
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• NYPD and FDNY. 
• Collections of visual material assembled for charitable or historical purposes. 
• Individuals’ photographs and videos that began appearing on the World Wide Web as early 
as September 11, 2001. 
• Responses to public appeals for visual material by the Investigation Team. 
Investigation staff contacted each of the sources, requested the material, made arrangements for its 
transfer, and addressed copyright and privacy issues. Emphasis was placed on obtaining material in a 
form as close as possible to the original in order to maintain as much spatial and timing information as 
possible: direct digital copies of digital photographs and videos, high resolution digitized copies of film or 
slide photographs, and direct copies from the original source of analog video. 
The assembled collection included: 
• 6,977 segments of video footage, totaling in excess of 300 hours. The media videos included 
both broadcast material and outtakes. Additionally, NIST received videotapes recorded by 
more than 20 individuals. 
• 6,899 photographs from at least 200 photographers. As with the videos, many of the 
photographs were unpublished. 
This vast amount of visual material was organized into a searchable database in which each frame was 
characterized by a set of attributes: photographer (name and location), time of shot/video, copyright 
status, content (including building, face(s), key events (plane strike, fireballs, collapse), the presence of 
FDNY or NYPD people or apparatus, and other details, such as falling debris, people, and building 
damage). 
The development of a time line for fire growth and structural changes in the 
The TV network clocks 
WTC buildings required the assignment of times of known accuracy to each were quite close to the 
video frame and photograph. Images were timed to a single well-defined actual time since they 
event. Due to the large number of different views available, the chosen event were regularly 
was the moment the second plane struck WTC 2, established from the time updated from highly 
stamps in the September 11 telecasts. Based on four such video recordings, accurate 
the time of the second plane impact was established as 9:02:59 a.m. geopositioning 
satellites or the 
precise atomic-clock-
Absolute times were then assigned to all frames of all videos that showed the based timing signals 
second plane strike. By matching photographs and other videos to specific provided by NIST as a 
public service. 
events in these initially assigned videos, the time assignments were extended 
to visual materials that did not include the primary event. Times were also 
cross-matched using additional characteristics, such as the appearance and locations of smoke and fire 
plumes, distinct shadows cast on the buildings by these plumes, the occurrence of well-defined events 
such as a falling object, and even a clock being recorded in an image. By such a process, it was possible 
to place photographs and videos extending over the entire day on a single time line. As the time was 
assigned to a particular photograph or video, the uncertainty in the assignment was also logged into the 
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database. In all, 3,032 of the catalogued photographs and 2,673 of the video clips in the databases were 
timed with accuracies of ± 3 s or better. 
This process enabled establishing the times of four major events of September 11, listed in Table 6–1. 
The building collapse times were defined to be the point in time when the entire building was first 
observed to start to collapse. 
Table 6–1. Times for major events on September 11, 2001. 
Event Time 
First Plane Strike 8:46:30 a.m. 
Second Plane Strike 9:02:59 a.m. 
Collapse of WTC 2 9:58:59 a.m. 
Collapse of WTC 1 10:28:22 a.m. 
There were additional sources of timed information. Phone calls from people within the building to 
relatives, friends, and 9-1-1 operators conveyed observations of the structural damage and developing 
hazards. Communications among the emergency responders and from the building fire command centers 
contributed further information about the areas where the external photographers had no access. 
LEARNING FROM THE VISUAL IMAGES 
The photographic and video images were rich sources of information on the condition of the buildings 
following the aircraft impact, the evolution of the fires, and the deterioration of the structure. To enable 
analysis of this information, a shorthand notation (based on the building design drawings) was used to 
label the exterior columns and windows of the buildings: 
• First, the faces of the towers were numbered in a manner identical to those used in the 
original plans: 
WTC 1: north: 1 east: 2 south: 3 west: 4 
WTC 2: west: 1 north: 2 east: 3 south: 4 
• The 59 columns across each tower face were assigned three-digit numbers. Following the 
floor number, the first digit was that of the face, and the remaining two digits were assigned 
consecutively from right to left as viewed from outside the building. Thus, the fourth column 
from the right on the east face of the 81st floor of WTC 1 was labeled 81-204. 
• Each of the 58 windows on each floor and tower face was assigned the number of the column 
to its right as viewed from the outside of the building and was also identified by its floor. 
Thus the rightmost window on the east face of the 94th floor of WTC 1 was labeled 94-201. 
As an example of information that was extracted, Figure 6–1 shows an enhanced image of the east face of 
WTC 2. Figure 6–2 expands a section of interest. The amount of detail available is evident. For 
instance, large piles of debris are present on the north side of the tower on the 80th and 81st floors, and 
locations where fires are visible or where missing windows are easily identified. Many details of each 
frame were important in tracking the evolution of the fires and the damage to the buildings. 
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Note: Enhancements by NIST. 
Figure 6–1. 9:26:20 a.m. showing the east face of WTC 2. 
In each photograph and each video frame, each window was also coded to indicate whether the window 
was still in place or not and the extent to which flames and smoke were visible. Color-coded graphics of 
the four façades of the two towers were then constructed. Examples of these graphics were shown in 
Chapters 2 and 3. 
The results of the visual analysis included: 
• The locations of the broken windows, providing information on the source of air to feed the 
fires within. 
• Observations of the spread of fires. 
• Documentation of the location of exterior damage from the aircraft impact and subsequent 
structural changes in the buildings. 
• Identification of the presence or absence of significant floor deterioration at the building 
perimeter. 
• Observations of certain actions by building occupants, such as breaking windows. 
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Note: Enhancements by NIST. 
Figure 6–2. Close-up of section of Figure 6–1. 
The near-continuous observations of the externally visible fires provided input to the computer 
simulations of fire growth and spread. The discrete observations of changes in the displacement of 
columns and, to a far lesser degree, floors became validation data for the modeling of the approach to 
structural collapse of the towers. Table 6–2 lists the most important observations. 
6.4 LEARNING FROM THE RECOVERED STEEL 
6.4.1 Collection of Recovered Steel 
The Investigation Team had two reasons for obtaining specimens of structural steel from the collapsed 
towers. The primary objective was characterizing the quality of the steel and determining its properties 
for use in the structural modeling and analysis of the collapse sequences. The second reason was 
obtaining information regarding the behavior of the steel in the aircraft impact zone and in areas which 
had major fires. 
Within weeks of the destruction of the World Trade Center, contractors of the City of New York had 
begun cutting up and removing the debris from the site. Members of the FEMA-sponsored and ASCE-led 
Building Performance Assessment Team, members of the Structural Engineers Association of New York, 
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Table 6–2. Indications of major structural changes up to collapse initiation. 
Tower Time (a.m.) Observation 
10:18 Smoke suddenly expelled on the north face (floors 92, 94, 95 to 98) and west face (92, 
94 to 98). 
WTC 1 
10:23 Inward bowing of perimeter columns on the east side of the south face from floors 94 
to 100; maximum extent: 55 in. ± 6 in. at floor 97. 
10:28:22 
First exterior sign of collapse (downward movement of building exterior). 
Tilting of the building section above the impact and fire area to due south as the 
structural collapse initiated. First exterior sign of downward movement of building at 
floor 98. 
9:02:59 
Exterior fireball from the east face of floor 82 and from the north face from floors 79 
to 82. The deflagration prior to the fireballs may have caused a significant pressure 
pulse to act on floors above and below. 
9:21 Inward bowing of exterior wall columns on most of the east face from floors 78 to 83; 
maximum extent: 7 in. to 9 in. at floor 80. 
WTC 2 
9:58:59 
First exterior sign of collapse (downward movement of building exterior). 
The northeast corner tilted counterclockwise around the base of floor 82. Column 
buckling was then seen progressing across the north face and nearly simultaneously on 
the east face. 
Tilting of the building section above the impact and fire area to the east and south 
prior to significant downward movement of the upper building section. The tilt to the 
south did not increase any further as the upper building section began to fall, but the 
tilt to the east did increase until dust clouds obscured the view. 
and Professor A. Astaneh-Asl of the University of California, Berkeley, CA, with support from the 
National Science Foundation, had begun work to identify and collect WTC structural steel from the 
various recycling yards where the steel was taken during the clean-up effort. The Port Authority also 
collected structural steel elements for future exhibits and memorials. 
Over a period of about 18 months, 236 pieces of steel were shipped to the NIST campus, starting about 
six months before NIST launched its Investigation. These samples ranged in size and complexity from a 
nearly complete three-column, three-floor perimeter assembly to bolts and small fragments. Figures 6–3 
through 6–5 show some of the recovered steel pieces. Seven of the pieces were from WTC 5. The 
remaining 229 samples represented roughly 0.25 percent to 0.5 percent of the 200,000 tons of structural 
steel used in the construction of the two towers. 
The collection at NIST included samples of all the steel strength levels specified for the construction of 
the towers. The locations of all structural steel pieces in WTC 1 and WTC 2 were uniquely identified by 
stampings (recessed letters and numbers) and/or painted stencils. NIST was successful in finding and 
deciphering these identification markings on many of the perimeter panel sections and core columns, in 
many cases using metallurgical characterization to complete missing identifiers. In all, 42 exterior panels 
were positively identified: 26 from WTC 1 and 16 from WTC 2. Twelve core columns were positively 
identified: eight from WTC 1 and four from WTC 2. Twenty-three pieces were identified as being parts 
of trusses, although it was not possible to identify their locations within the buildings. 
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NIST.Source:
Figure 6–3. Examples of a WTC 1 core column (left) and truss material (right). 
NIST.Source:
Figure 6–4. WTC 1 exterior panel hit by the fuselage of the aircraft. 
Overlaying the locations of the specimens with photographs of the building exteriors following the 
aircraft impact (for perimeter columns and spandrels) and the extent-of-damage estimates (Section 6.8) 
(for core columns) enabled the identification of steel pieces near the impact zones. These included five 
specimens of exterior panels from WTC 1 and two specimens of core columns from each of the towers. 
6.4.2 Mechanical and Physical Properties 
NIST determined the properties of many of the recovered pieces for comparison with the original 
purchase requirements, comparison with the quality of steel from the WTC construction era, and input to 
the structural models used in the Investigation. Structural steel literature and producers' documents were 
used to establish a statistical basis for the variability expected in steel properties. 
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The properties of the steel samples tested were 
consistent with the specifications called for in the 
steel contracts. In particular, the yield strengths of 
all samples of the floor trusses were higher than 
called for in the original specifications. This was in 
part because the truss steels were supplied as a 
higher grade than specified. Overall, approximately 
87 percent of all perimeter and core column steel 
tested exceeded the required minimum yield 
strengths specified in design documents. Test data 
for the remaining samples were below specifications, 
but were within the expected variability and did not 
affect the safety of the towers on 
September 11, 2001. Furthermore, lower strength 
values measured by NIST could be expected due to 
(a) differences in test procedures from those used in 
the qualifying mill tests and (b) the damaged state of 
the samples. The values of other steel properties 
were similar to typical construction steels of the 
WTC construction era. The limited tests on bolts 
indicated that their strengths were greater than the 
specified minimum, and they were stronger than 
contemporaneous literature would suggest as typical. 
The tested welds performed as expected. 
NIST measured the stress-strain behavior at room
temperature (for modeling baseline performance), 
high temperature strength (for modeling structural 
response to fire), and at high strain rates (for Source: NIST. 
modeling the aircraft impact). Based on data from Figure 6–5. WTC 1 exterior panel hit by
published sources, NIST estimated the thermal the nose of the aircraft. 
properties of the steels (specific heat, thermal 
conductivity, and coefficient of thermal expansion) for use in the structural modeling of the towers’ 
response to fire. 
6.4.3 Damage Analysis 
NIST performed extensive analyses of the recovered steel specimens to determine their damage 
characteristics, failure modes, and (for those near the fire zones) fire-related degradation. In some cases, 
assessment of enhanced photographic and video images of the towers enabled distinguishing between 
damage that occurred prior to the collapse and damage that occurred as a result of the collapse. Because 
the only visual evidence was from the outside of the buildings, this differentiation was only possible for 
the perimeter panel sections. The observations of fracture and failure behavior, confirmed by an 
Investigation contractor, were also used to guide the modeling of the towers' performance during impact 
and subsequent fires and to evaluate the model output. 
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For two of the five exterior panels from the impact zone of WTC 1, the general shape and appearance of 
the recovered pieces matched photographs taken just before the building collapse. Thus, NIST was able 
to attribute the observed damage to the aircraft impact. NIST also made determinations regarding the 
connections between structural steel elements: 
• There was no evidence to indicate that the joining method, weld materials, or welding 
procedures were inadequate. Fractures of the columns in areas away from a welded joint 
were the result of stretching and thinning. Perimeter columns hit by the plane tended to 
fracture along heat-affected zones adjacent to welds. 
• The failure mode of spandrel connections varied. At or above the impact zone, bolt hole tearout 
was more common. Below the impact zone, it was more common for the spandrels to be 
ripped from the panels. There was no evidence that fire exposure changed these failure 
modes. 
• The exterior column splices at the mechanical floors, which were welded in addition to being 
bolted, generally did not fail. The column splices at the other floors generally failed by bolt 
fracture. 
• The perimeter truss connectors (or seats) below the impact zone in WTC 1 were 
predominantly bent down or torn off completely. Above the impact zone, the seats were as 
likely to be bent upward as downward. Core seats could not be categorized since their asbuilt 
locations could not be determined. 
• Failure of core columns was a result of both splice connection failures and fracture of the 
columns themselves. 
Examination of photographs showed that 16 of the exterior panels recovered from WTC 1 were exposed 
to fire prior to the building collapse. None of the nine recovered panels from within the fire floors of 
WTC 2 were directly exposed to fire. NIST used two methods to estimate the maximum temperatures 
that the steel members had reached: 
• Observations of paint cracking due to thermal expansion. Of the more than 170 areas 
examined on 16 perimeter column panels, only three columns had evidence that the steel 
reached temperatures above 250 °C: east face, floor 98, inner web; east face, floor 92, inner 
web; and north face, floor 98, floor truss connector. Only two core column specimens had 
sufficient paint remaining to make such an analysis, and their temperatures did not reach 
250 °C. NIST did not generalize these results, since the examined columns represented only 
3 percent of the perimeter columns and 1 percent of the core columns from the fire floors. 
• Observations of the microstructure of the steel. High temperature excursions, such as due to 
a fire, can alter the basic structure of the steel and its mechanical properties. Using 
metallographic analysis, NIST determined that there was no evidence that any of the samples 
had reached temperatures above 600 ºC. 
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These results were for a very small fraction of the steel in the impact and fire zones. Nonetheless, these 
analyses indicated some zones within WTC 1 where the computer simulations should not, and did not, 
predict highly elevated steel temperatures. 
INFORMATION GAINED FROM OTHER WTC FIRES 
There had been numerous fires in the towers prior to September 11, 2001. From these, the Investigation 
Team learned what size fire WTC 1 and WTC 2 had withstood and how the tower occupants and the 
responders functioned in emergencies. While The Port Authority's records of prior fires were lost in the 
collapses, FDNY provided reports on 342 fires that had occurred between 1970 and 2001. 
Most of these fires were small, and occupants extinguished many of them before FDNY arrival. Fortyseven 
of these fires activated one to three sprinklers and/or required a standpipe hose for suppression. 
Only two of the fires required the evacuation of hundreds of people. There were no injuries or loss of life 
in any of these fires, and the interruptions to operations within the towers were local. 
A major fire occurred in WTC 1 on February 13, 1975, before the installation of the sprinkler system. A 
furniture fire started in an executive office in the north end of an 11th floor office suite in the southeast 
corner of the building. The fire spread south and west along corridors and entered a file room. The fire 
flashed over, broke seven windows, and spread to adjacent offices north and south. The air conditioning 
system turned on, pulling air into the return air ducts. Telephone cables in the vertical shafts were 
ignited, destroying the fire-retarded wood paneling on the closet doors. The fire emerged on the 12th and 
13th floors, but there was little nearby that was combustible. The fire also extended vertically from the 9th 
to the 19th floors within the telephone closet. Eventually the fire was confined to 9,000 ft2 of one floor, 
about one-fourth of the total floor area. The trusses and columns in this area had been sprayed with 
CAFCO D insulation to a specified ½ in. thickness. Four trusses were slightly distorted, but the structure 
was not threatened. 
Only one major fire incident resulted in a whole-building evacuation. At 12:18 p.m. on February 26, 
1993, terrorists exploded a bomb in the second basement underground parking garage in the WTC 
complex. The blast immediately killed six people and caused an estimated $300 million in damage. An 
intense fire followed and, although the flames were confined to the subterranean levels, the smoke spread 
into four of the seven buildings in the WTC complex. Most of the estimated 150,000 occupants 
evacuated the buildings, including approximately 50,000 from the affected towers. In all, 1,042 people 
were injured in the incident, including 15 who received blast-related injuries. The evacuation of the 
towers took over 4 hours. The incident response involved more than 700 firefighters (approximately 
45 percent of FDNY’s on-duty personnel at the time). 
In addition, there was a fire on the 104th floor of WTC 1 on September 11, 2001, that apparently did not 
contribute to the eventual collapse, yet was quite severe. At 10:01 a.m., flames were first observed on the 
west face, and by 10:07 a.m., intense flames were emanating from several windows in the southern third 
of that face. The fire raged until the building collapsed at 10:28 a.m. Thus, the tower structure was able 
to withstand a sizable fire for about 20 min, presumably with the ceiling tile system heavily damaged and 
the truss system exposed to the flames. The 104th floor was well above the aircraft impact zone, so there 
should have been little damage to the insulation, which was the same (Table 5–3) as on the floors where 
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the fires led to the onset of the collapse. The photographic evidence showed no signs of column bowing 
or a floor collapse. 
6.6 THE BUILDING STRUCTURAL MODELS 
6.6.1 Computer Simulation Software 
Structural modeling of each tower was required in order to: 
• Establish the capability of the building, as designed, to support the gravity loads and to resist 
wind forces; 
• Simulate the effects of the aircraft impacts; 
• Reconstruct the mechanics of the aircraft impact damage, fire-induced heating, and the 
progression of local failures that led to the building collapse. 
The varied demands made different models necessary, and different software packages were used for each 
of these three functions. The reason for the choice in each case is presented in the next three sections of 
the report. 
6.6.2 The Reference Models 
Under contract to NIST, Leslie E. Robertson Associates (LERA) constructed a global reference model of 
each tower using the SAP2000, version 8, software. SAP2000 is a software package for performing finite 
element calculations for the analysis and design of building structures. These global, three-dimensional 
models encompassed the 110 stories above grade and the 6 subterranean levels. The models included 
primary structural components in the towers, resulting in tens of thousands of computational elements. 
The data for these elements came from the original structural drawing books for the towers. These had 
been updated through the completion of the buildings and also included most of the subsequent, 
significant alterations by both tenants and The Port Authority. LERA also developed reference models of 
a truss-framed floor, typical of those in the tenant spaces of the impact and fire regions of the buildings, 
and of a beam-framed floor, typical of the mechanical floors. 
LERA's work was reviewed by independent experts in light of the firm's earlier involvement in the WTC 
design. It was that earlier work, in fact, that made LERA the only source that had the detailed knowledge 
of the design, construction, and intended behavior of the towers over their entire 38-year life span. The 
accuracy of the four models was checked in two ways: 
• The two global models were checked by Skidmore, Owings and Merrill (SOM), also under 
contract to NIST, and by NIST staff. This entailed ensuring consistency of the models with 
the design documents, and testing the models, e.g., to ensure that the response of the models 
to gravity and wind loads was as intended and that the calculated stresses and deformations 
under these loads were reasonable. 
• The global model of WTC 1 was used to calculate the natural vibration periods of the tower. 
These values were then compared to measurements from the tower on eight dates of winds 
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ranging from 11.5 mph to 41 mph blowing from at least four different directions. As shown 
in Table 6–3, the N-S and E-W values agreed within 5 percent and the torsion values agreed 
within 6 percent, both within the combined uncertainty in the measurements and calculations. 
• SOM and NIST staff also checked the two floor models for accuracy. These reviews 
involved comparison with simple hand calculations of estimated deflections and member 
stresses for a simply supported composite truss and beam under gravity loading. For the 
composite truss sections, the steel stress results were within 4 percent of those calculated by 
SAP2000 for the long-span truss and within 3 percent for the short-span truss. Deflections 
for the beams and trusses matched hand calculations to within 5 percent to 15 percent. These 
differences were within the combined uncertainty of the methods. 
Table 6–3. Measured and calculated natural vibration periods (s) for WTC 1. 
Direction of Motion 
N-S E-W Torsion 
Average of Measured Data 11.4 10.6 4.9 
Original Predicted Values 11.9 10.4 – 
Reference Global Model Predictions 11.4 10.7 5.2 
The few discrepancies between the developed models and the original design documents, as well as the 
areas identified by NIST and SOM as needing modification, were corrected by LERA and approved by 
NIST. The models then served as references for more detailed models for aircraft impact damage 
analysis and for thermal-structural response and collapse initiation analysis. 
NIST also used these global reference models to establish the baseline performance of the towers under 
gravity and wind loads. The two key performance measures calculated were the demand-to-capacity ratio 
(DCR) and the drift. 
• Demand is defined as the combined effects of the dead, live, and wind loads imposed on a 
structural component, e.g., a column. Capacity is the permissible strength for that 
component. Normal design aims at ensuring that DCR values for all components be 1.0 or 
lower. A value of DCR greater than 1.0 does not imply failure since designs inherently 
include a margin of safety. 
• Drift is the extent of sway of the building under a lateral wind. Excessive deflection can 
cause cracking of partitions and cladding, and, in severe cases, building instability that could 
affect safety. 
Using SAP2000, NIST found that, under original WTC design loads, a small fraction of the structural 
components had DCR values greater than 1.0. (Most DCR values of that small fraction were less than 
1.4, with a few as high as 1.6.) For the perimeter columns, DCR values greater than 1.0 were mainly near 
the corners, on floors near the hat truss, and below the 9th floor. For the core columns, these members 
were on the 600 line between floors 80 and 106 and at core perimeter columns 901 and 908 for much of 
their height. (See Figure 1–5 for the column numbers.) One possible explanation to the cause of DCRs in 
excess of 1.0 may lie in the computer-based structural analysis and software techniques employed for this 
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baseline performance study in comparison with the relatively rudimentary computational tools used in the 
original design nearly 40 years ago. 
As part of its wind analysis, NIST calculated the drift at the top of the towers to be about 5 ft in a nearly 
100 mph wind—the wind load used in the original design. Common practice was, and is, to design for 
substantially smaller deflections; but drift was not, and still is not, a design factor prescribed in building 
codes. 
The estimation of wind-induced loads on the towers emerged as a problem. Two sets of wind tunnel tests 
and analyses were conducted in 2002 by independent laboratories as part of insurance litigation unrelated 
to the NIST Investigation. The estimated loads differed by as much as 40 percent. NIST analysis found 
that the two studies used different approaches in their estimations. This difference highlighted limitations 
in the current state of practice in wind engineering for tall buildings and the need for standards in the field 
of wind tunnel testing and wind effects estimation. 
6.6.3 Building Structural Models for Aircraft Impact Analysis 
Ideally, the Investigation would have used the reference global models of the towers as the "targets" for 
the aircraft. However, this was not possible. The impact simulations required inclusion of both a far 
higher level of detail of the building components and also the highly nonlinear behavior of the tower and 
aircraft materials, and the larger model size could not be accommodated by the SAP2000 program. There 
were also physical phenomena for which algorithms were not available in this software. Another finite 
element package, LS-DYNA, satisfied these requirements and was used for the impact simulations. 
Early in the effort, it became clear to both NIST and to ARA, Inc., the NIST contractor that performed the 
aircraft impact simulations, that the model had to “fit” on a state-of-the-art computer cluster and to run 
within weeks rather than months. To minimize the model size while keeping sufficient fidelity in the 
impact zone to capture the building deformations and damage distributions, various tower components 
were depicted with different meshes (different levels of refinement). For example, tower components in 
the path of the impact and debris field were represented with a fine mesh (higher resolution) to capture the 
local impact damage and failure, while components outside the impact zone were depicted more coarsely, 
simply to capture their structural stiffness and inertial properties. The model of WTC 1 included floors 92 
through 100; the model of WTC 2 extended from floor 77 through floor 85. The combined tower and 
aircraft model of more than two million elements, at time steps of just under a microsecond, took 
approximately two weeks of computer time on a 12-noded computer cluster to capture the needed details 
of the fraction of a second it took for the aircraft and its fragments to come to rest inside the building. 
The structural models, partially shown in Figures 6–6 through 6–9, included: 
• Core columns and spliced column connections; 
• Floor slabs and beams within the core; 
• Exterior columns and spandrels, including the bolted connections between the exterior panels 
in the refined mesh areas; and 
• Tenant space floors, composed of the combined floor slab, metal decking, and steel trusses. 
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They also included representations of the interior partitions and workstations. The live load mass was 
distributed between the partitions and cubicle workstations. 
Figure 6–6. Structural model of the 96th floor of WTC 1. 
Figure 6–7. Model of the 96th floor of WTC 1, including interior contents and partitions. 
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Figure 6–8. Multifloor global model of WTC 1, viewed from the north. 
Figure 6–9. Multifloor global model of WTC 2, viewed from the south. 
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Within these models, it was critical that the structural and furnishing materials behaved correctly when 
impacted by the aircraft or debris. For each grade of steel, the stress-strain behavior and the yield strength 
were represented using data from tests conducted at NIST. The weakening and failure of the concrete 
floor slabs were simulated using material models embedded in LS-DYNA. The primary influence of the 
nonstructural components on the impact behavior was their inertial contribution. Values for the resistance 
to rupture of gypsum panels and the fracture of the wood products in the workstations were obtained from 
published studies. 
In order to complete the global models of the two towers, models of sections of the buildings were 
developed. As shown in Section 6.8.1, these submodels enabled efficient identification of the principal 
features of the interaction of the buildings with specific aircraft components. 
6.6.4 Building Structural Models for Structural Response to Impact Damage and Fire 
and Collapse Initiation Analysis 
The structural response modeling and collapse analysis of the towers was conducted in three phases by 
NIST and Simpson, Gumpertz,& Heger, Inc. (SGH), under contract to NIST. The first phase included 
component and detailed subsystem models of the floor and exterior wall panels. The objectives of 
Phase 1 were to gain understanding into the response of the structure under stress and elevated 
temperatures, identify dominant modes of failure, and develop reductions in modeling complexity that 
could be applied in Phase 2. The second phase analyzed major subsystem models (the core framing, a 
single exterior wall, and full tenant floors) to provide insight into their behavior within the WTC global 
system. The third phase was the analysis of global models of WTC 1 and WTC 2 that took advantage of 
the knowledge gained from the more detailed and subsystem models. A separate global analysis of each 
tower helped determine the relative roles of impact damage and fires with respect to structural stability 
and sequential failures of components and subsystems and was used to determine the probable collapse 
initiation sequence. 
Phase 1: Component and Detailed Subsystem Analyses 
Floor Subsystem Analysis 
The floors played an important role in the structural response of the WTC towers to the aircraft impact 
and ensuing fires. Prior to the development of a floor subsystem model, three component analyses were 
conducted, as follows: 
• Truss seats. Figure 6–10 shows how an exterior seat connection was represented in the finite 
element structural model. The component analysis determined that failure could occur at the 
bolted connection between the bearing angle and the seat angle, and the truss could slip off 
the seat. Truss seat connection failure from vertical loads was found to be unlikely, since the 
needed increase in vertical load was unreasonable for temperatures near 600 °C to 700 °C. 
• Knuckles. The “knuckle” was formed by the extension of the truss diagonals into the 
concrete slab and provided for composite action of the steel truss and concrete slab. A model 
was developed to predict the knuckle performance when the truss and concrete slab acted 
compositely. 
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• Single composite truss and concrete slab section. A floor section was modeled to investigate 
failure modes and sequences of failures under combined gravity and thermal loads. The floor 
section was heated to 700 °C (300 °C at the top surface of the slab) over a period of 30 min. 
Initially the thermal expansion of the floor pushed the columns outward, but with increased 
temperatures, the floor sagged and the columns were pulled inward. Knuckle failure was 
found to occur mainly at the ends of the trusses and had little effect on the deflection of the 
floor system. Figure 6–11 shows that the diagonals at the core (right) end of the truss 
buckled and caused an increase in the floor system deflection, ultimately reaching 
approximately 42 in. Two possible failure modes were identified for the floor-truss section: 
sagging of the floor and loss of truss seat support. 
Stand-off 
Plates 
Seat angle 
5/8 in. 
bolt 
Gusset plate 
Strut 
Diameter 
Truss top chord 
Bearing angle 
Figure 6–10. Finite element model of an exterior truss seat. 
MN 
MX 
-42.11 -32.603 -23.095 -13.588 -4.081 
-37.357 -27.849 -18.342 -8.834 .673211 
Figure 6–11. Vertical displacement at 700 oC. 
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A finite element model of the full 96th floor of WTC 1 was translated from the SAP2000 reference models 
into ANSYS 8.1 for detailed structural evaluation (Figure 6–12)13. The two models generated comparable 
predictions of the behavior under dead or gravity loads. 
Figure 6–12. 
of 96th floor of WTC 1. 
ANSYS model 
The model was used to evaluate structural response under dead and live loads and elevated structural 
temperatures, identify failure modes and associated temperatures and times to failure, and identify 
reductions in modeling complexity for global models and analyses. The deformation and failure modes 
identified were floor sagging between truss supports, floor sagging resulting from failure of a seat at 
either end of the truss, and failure of the floor subsystem truss supports. 
Exterior Wall Subsystem 
The exterior walls played an important role in each tower's reaction to the aircraft impact and the ensuing 
fires. Photographic and video evidence showed inward bowing of large sections of the exterior walls of 
both towers just prior to the time of collapse. 
A finite element model of a wall section was developed in ANSYS for evaluation of structural response 
under dead and live loads and elevated structural temperatures, determination of loads that would have 
caused buckling, and identification of reductions of modeling complexity for global models and analyses. 
The modeled unit consisted of seven full column/spandrel panels (described in Section 1.2.2) and portions 
of four other panels. The model was validated against the reference model developed by LERA 
(Section 6.6.2) by comparing the stiffness for a variety of loading conditions. 
The model was subjected to several gravity loads and heating conditions, several combinations of 
disconnected floors, and pull-in from sagging floors until the point of instability. In one case, the 
13 ANSYS allowed including the temperature-varying properties of the structural materials, a necessary feature not available in 
SAP 2000. 
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simulation assumed three disconnected floors, and the top of the wall subsystem was subjected to “pushdown” 
analysis, i.e., an increasing force to provide a measure of remaining capacity in the wall section. 
The model captured possible failure modes due large lateral deformations, column buckling from loss of 
support at floor truss seats and diagonal straps, failure of column splice bolts, and failure of spandrel 
splice bolts or tearing of spandrel or splice plates at bolt holes. The model also showed: 
• Large deformations and buckling of the spandrels could be expected at high temperatures, but 
they did not significantly affect the stability of the exterior columns and generally did not 
need to be precisely modeled in the tower models. 
• Partial separations of the spandrel splices could be expected at elevated temperatures, but 
they also did not significantly affect the stability of the exterior columns. 
• Exterior column splices could be expected to fail at elevated temperatures and needed to be 
accurately modeled. 
• Plastic buckling of columns, with an ensuing rapid reduction of load, was to be expected at 
extremely high loads and at low temperatures. 
• The sagging of trusses resulted in approximately 14 kip of inward pull per truss seat on the 
attached perimeter column. 
Phase 2: Major Subsystem Analyses 
Building on these results, ANSYS models were constructed of each of the three major structural 
subsystems (core framing, a single exterior wall, and full composite floors) for each of the towers. The 
models were subjected to the impact damage and elevated temperatures from the fire dynamics and 
thermal analyses to be described later in this chapter. 
Core Framing 
The two tower models included the core columns, the floor beams, and the concrete slabs from the impact 
and fire zones to the highest floor below the hat truss structure: from the 89th floor to the 106th floor for 
WTC 1 and from the 73rd floor to the 106th floor for WTC 2. Within these floors, aircraft-damaged 
structural components were removed. Below the lowest floors, springs were used to represent the 
stiffness of the columns. In the models, the properties of the steel varied with temperature, as described in 
Section 5.5.2. This allowed for realistic structural changes to occur, such as thermal expansion, buckling, 
and creep. 
The forces applied to the models included gravity loads applied at each floor, post-impact column forces 
applied at the top of the model at the 106th floor, and temperature histories applied at 10 min intervals 
with linear ramping between time intervals. 
Under these conditions, the investigators first determined the stability of the core under impact conditions 
and then its response under thermal loads: 
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• In WTC 1, the core was stable under Case A (base case) impact damage, but the model could 
not reach a stable solution under Case B (more severe) impact damage. 
• The WTC 1 core became unstable under Case A impact damage and Case B thermal loads as 
it leaned to the northwest (due to insulation dislodged from the northwest corner column); the 
core model was restrained in horizontal directions at floors above the impact zone half way 
through the thermal loads. 
• The WTC 2 core was stabilized for Case C (base case) by providing horizontal restraint at all 
floors representing the restraint provided by the perimeter wall to resist leaning to the 
southeast. A converged, stable solution was not found for Case D (more severe) impact 
damage. 
• The WTC 2 stabilized core model for Case C impact damage was subjected to Case D 
thermal loads. 
Following each simulation, a pushdown analysis was performed to determine the core’s reserve capacity. 
The analysis results showed that: 
• The WTC 1 isolated core structure was most weakened from thermal effects at the center of 
the south side of the core. (Smaller displacements occurred in the global model due to the 
constraints of the hat truss and floors.) 
• The WTC 2 isolated core was most weakened from thermal effects at the southeast corner 
and along the east side of the core. (Larger displacements occurred in the global model as the 
isolated core model had lateral restraints imposed that were somewhat stiffer than in the 
global model.) 
Composite Floor 
The composite floor model was used to determine the response of a full floor to Case A and B thermal 
loads for WTC 1 floors and Case C and D thermal loads for WTC 2 floors. It included: 
• A reduced complexity truss model, validated against the single truss model results. 
• Primary and bridging trusses, deck support angles, spandrels, core floor beams, and a 
concrete floor slab. 
• Fire-generated local temperature histories applied at 10 min intervals with linear ramping 
between time intervals. 
• Temperature-dependent concrete and steel properties. 
• Restraint provided by exterior and core columns, which extended one floor above and below 
the modeled floor.While the potential for large deflections and buckling were included, the 
potential for creep was not. 
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The results showed that: 
• Floor sagging was caused primarily by either buckling of truss web diagonals or 
disconnection of truss seats at the exterior wall or the core perimeter. Except for the truss 
seat failures near the southeast corner of the core in WTC 2 following the aircraft impact, 
web buckling or truss seat failure was caused primarily by elevated temperatures of the 
structural components. 
• Analysis results from both the detailed truss model and the full floor models found that the 
floors began to exert inward pull forces when floor sagging exceeded approximately 25 in. 
for the 60 ft floor span. 
• Sagging at the floor edge was due to loss of vertical support at the truss seats. The loss of 
vertical support was caused in most cases by the reduction in vertical shear capacity of the 
truss seats due to elevated steel temperatures. 
• Case B impact damage and thermal loads for WTC 1 floors resulted in floor sagging on the 
south side of the tower over floors that reasonably matched the location of inward bowing 
observed on the south face. Case A impact damage and thermal loads did not result in 
sagging on the south side of the floors. 
• Cases C and D impact damage and thermal loads for WTC 2 both resulted in floor sagging on 
the east side of the tower over floors that reasonably matched the location of inward bowing 
observed on the east face. However, Case D provided a better match. 
Exterior Wall 
Exterior wall models were developed for the south face of WTC 1 (floors 89 to 106) and the east face of 
WTC 2 (floors 73 to 90). These sections were selected based on photographic evidence of column 
bowing. 
Many of the simulation conditions were similar to those for the isolated core modeling: removal of 
aircraft-damaged structural components, representation of lower floors by springs, temperature-varying 
steel properties, gravity loads applied at each floor, post-impact column forces applied at the 106th floor, 
and temperature histories applied at 10 min intervals with linear ramping between time intervals. 
The analysis results showed that: 
• Inward pull forces were required to produce inward bowing consistent with the displacements 
measured from photographs. The inward pull was caused by sagging of the floors. Heating of 
the inside faces of the exterior columns also contributed to inward bowing. 
• The observed inward bowing of the exterior wall indicated that most of the floor connections 
must have been were intact to cause the observed bowing. 
• The extent of floor sagging observed at each floor was greater than that predicted by the full 
floor models. The estimates of the extent of sagging at each floor was governed by the 
combined effects of insulation damage and fire; insulation damage estimates were limited to 
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areas subject to direct debris impact. Other sources of floor and insulation damage from the 
aircraft impact and fires (e.g., insulation damage due to shock and subsequent vibrations as a 
result of aircraft impact or concrete slab cracking and spalling as a result of thermal effects) 
were not included in the floor models. 
• Case B impact damage and thermal loads for the WTC 1 south wall, combined with pull-in 
forces from floor sagging, resulted in an inward bowing of the south face that reasonably 
matched the observed bowing. The lack of floor sagging for the Case A impact damage and 
thermal loads resulted in no inward bowing for the south face. 
• Cases D impact damage and thermal loads for the WTC 2 east wall, combined with pull-in 
forces from floor sagging, resulted in an inward bowing of the east face that reasonably 
matched the observed bowing. 
Phase 3: Global Modeling 
The global models were used for the two final simulations and provided complete analysis of results and 
insight into the subsystem interactions leading to the probable collapse sequence. Based upon the results 
of the major subsystem analyses, impact damage and thermal loads for Cases B and D were used for 
WTC 1 and WTC 2, respectively. The models extended from floor 91 for WTC 1 and floor 77 for WTC 2 
to the roof level in both towers. Although the renditions of the structural components had been reduced in 
complexity while maintaining essential nonlinear behaviors, based on the findings from the component 
and subsystem modeling, the global models included many of the features of the subsystem models: 
• Removal of aircraft-damaged structural components. 
• Application of gravity loads following removal of aircraft damaged components and prior to 
thermal loading. 
• Temperature-dependent concrete and steel properties. 
• Creep strains for column components. 
• Representation of lower floors by springs. 
• Local temperature histories applied at 10 min intervals with linear ramping between time 
intervals. 
There were several adjustments to the models based on the findings from the subsystem modeling: 
• Removal of thermal expansion from the spandrels and equivalent slabs in the tenant area to 
avoid local buckling that affected convergence but had little influence on global collapse 
initiation 
• Representing the WTC 2 structure above the 86th floor as a single "super-element" to reduce 
model complexity. The floors above the impact zone had only exhibited linear behavior in 
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the previous analyses. This modification assumed linear behavior of the hat truss, which was 
checked as part of the review of analysis results. 
• Representation of the lower part of the tower (starting several floors below the impact 
damage) as a super-element. This prevented the use of construction sequence in applying 
gravity loads to the model (where loads are applied in stages to simulate the construction of 
the building). The lack of construction sequence increased the forces on the exterior columns 
slightly, and decreased those on the core columns slightly. 
The inclusions of creep for column components was necessary for the accuracy of the models, but its 
addition also greatly increased the computation time. As a result, the simulations of WTC 1 took 22 days 
and those of WTC 2 took 14 days on a high-end computer workstation. The results of these simulations 
are presented in Section 6.14. 
THE AIRCRAFT STRUCTURAL MODEL 
Due to their similarity, the two Boeing 767-200ER aircraft were represented by a single, finite element 
model, two views of which are shown in Figure 6–13. The model consisted of about 800,000 elements. 
The typical element dimensions were between 1 in. and 2 in. for small components, such as spar or rib 
flanges, and 3 in. to 4 in. for large parts such as the wing or fuselage skin. Structural data on which to 
base the model were collected from the open literature, electronic surface models and CAD drawings, an 
inspection of a 767-300ER, Pratt and Whitney Engine Reference Manuals, American Airlines and United 
Airlines, and the Boeing Company website. 
More detailed models of subsections of the aircraft were constructed for the component level analyses 
described below. Special emphasis was placed on modeling the aircraft engines, due to their potential to 
produce significant damage to the tower components. The element dimensions were generally between 
1 in. and 2 in., although even smaller dimensions were required to capture some details of the engine 
geometry. The various components of the resulting engine model are shown in Figure 6–14. Fuel was 
distributed in the wing as shown in Figure 6–15 based on a detailed analysis of the fuel distribution at the 
time of impact. 
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Figure 6–13. Finite element model of the Boeing 767-200ER. 
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Figure 6–14. Pratt & Whitney PW4000 
turbofan engine model. 
Figure 6–15. Boeing 767-200ER 
showing the jet fuel distribution at 
time of impact. 
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6.8 AIRCRAFT IMPACT MODELING 
6.8.1 Component Level Analyses 
Prior to conducting the full simulations of the aircraft impacting the towers, a series of smaller scale 
simulations was performed to develop understanding of how the aircraft and tower components 
fragmented and to develop the simulation techniques required for the final computations. These 
simulations began with finely meshed models of key components of the tower and aircraft structures and 
progressed to relatively coarsely meshed representations that could be used in the global models. 
Examples of these component-level analyses included impact of a segment of an aircraft wing with an 
exterior column, impact of an aircraft engine with exterior wall panels, and impact of a fuel-filled wing 
segment with exterior wall panels. 
Figure 6–16 shows two frames from the last of these analyses, with the wing segment entering from the 
left, being fragmented as it penetrates the exterior columns, and spraying jet fuel downstream. 
t = 0.0 s t = 0.04 s 
Figure 6–16. Calculated impact on an exterior wall by a fuel-laden wing section. 
The Investigation Team gained valuable knowledge from these component impact analyses, for example: 
• Moving at 500 mph, an engine broke any exterior column it hit. If the engine missed the 
floor slab, the majority of the engine core remained intact and had enough residual 
momentum to sever a core column upon direct impact. 
• The impact of the inner half of an empty wing significantly damaged exterior columns but did 
not result in their complete failure. Impact of the same wing section, but filled with fuel, did 
result in failure of the exterior columns. 
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6.8.2 Subassembly Impact Analyses 
Next, a series of simulations were performed for intermediate-sized sections of a tower. These 
subassembly analyses investigated different modeling techniques and associated model sizes, run times, 
numerical stability, and impact response. Six simulations were performed of an aircraft engine impacting 
a subassembly that included structural components from the impact zone on the north face of WTC 1, 
exterior panels, truss floor structures, core framing, and interior contents (workstations). One response of 
the structure to the engine impact is shown in Figure 6–17. 
Figure 6-17. Response of a tower subassembly model to engine impact. 
Typical knowledge gained from these simulations were: 
• The mass of the concrete floor slab and nonstructural contents had a greater effect on the 
engine deceleration and subsequent damage than did the concrete strength. 
• Variation of the failure criteria of the welds in the exterior columns did not result in any 
noticeable difference in the damage pattern or the energy absorbed by the exterior panels. 
6.8.3 Aircraft Impact Conditions 
From the NIST photographic and video collection, the speed and orientation of the aircraft (Table 6-4) 
were estimated at the time of impact. The geometry of the wings, different in flight from that at rest, was 
estimated from the impact pattern in the photographs and the damage documented on the exterior panels 
by NIST. United Airlines and American Airlines provided information on the contents of the aircraft, the 
mass of jet fuel, and the location of the fuel within the wing tanks. 
Table 6–4. Summary of aircraft impact conditions. 
Condition AA 11 (WTC 1) UAL 175 (WTC 2) 
Impact Speed (mph) 443 ± 30 542 ± 24 
Vertical Approach Angle 10.6° ± 3° below horizontal 
(heading downward) 
6° ± 2° below horizontal (heading 
downward) 
Lateral Approach Angle 180.3° ± 4° clockwise from Plan 
Northa 
13° ± 2° clockwise from Plan 
Northa 
Roll Angle (left wing downward) 25° ± 2° 38° ± 2° 
a. Plan North is approximately 29 degrees clockwise from True North. 
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6.8.4 Global Impact Analysis 
From the component and subassembly simulations, it became apparent that each computation of the full 
tower and aircraft would take weeks. Furthermore, the magnitude and location of damage to the tower 
structure were sensitive to a large number of initial conditions, to assumptions in the representation of the 
collision physics, and to any approximations in the numerical methods used to solve the physics 
equations. Thus, it was necessary to choose a manageable list of the factors that most influenced the 
outcome of a simulation. Careful screening was conducted at the component and subassembly levels, 
leading to identification of the following prime factors: 
• Impact speed, 
• Vertical approach angle of the aircraft, 
• Lateral approach angle of the aircraft, 
• Total aircraft weight, 
• Aircraft materials failure strain, 
• Tower materials failure strain, and 
• Building contents weight and strength. 
Guided by these results and several preliminary global simulations, two global simulations were selected 
for inclusion in the four-step simulation of the response of each tower, as described in Section 6.1. The 
conditions for these four runs are shown in Table 6–5. The computers simulate the aircraft flying into the 
tower, calculated the fragments that were formed from both the aircraft and the building itself, and then 
followed the fragments. The jet fuel, atomized upon impact into about 60,000 “blobs” averaging one 
pound, dispersed within and outside the building. Each simulation continued until the debris motion had 
reduced to a level that was not expected to produce any significant further impact damage. 
Table 6–5. Input parameters for global impact analyses. 
Analysis Parameters 
WTC 1 WTC 2 
Case A Case B Case C Case D 
Flight Parameters 
Impact Speed 443 mph 472 mph 542 mph 570 mph 
Vertical Approach 
Angle 10.6° 7.6° 6.0° 5.0° 
Lateral Approach 
Angle 
180.0° 180.0° 13.0° 13.0° 
Aircraft Parameters 
Weight 100 % 105 % 100 % 105 % 
Failure Strain 100 % 125 % 100 % 115 % 
Tower Parameters 
Failure Strain 100 % 80 % 100 % 90 % 
Live Load Weighta 25 % 20 % 25 % 20 % 
Contents Strength 100 % 100 % 100 % 80 % 
a. Live load weight expressed as a percentage of the design live load. 
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These simulations each took about 2 weeks on a 12-node computer cluster. Figure 6–18 shows six frames 
from the animation of one such simulation. 
(a) Time=0.00 s 
(b) Time=0.10 s 
(c) Time=0.20 s 
Figure 6–18. Side view of simulated aircraft impact into WTC 1, Case B. 
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(d) Time=0.30 s 
(e) Time=0.40 s 
(f) Time=0.50 s 
Figure 6–18. Side view of simulated aircraft impact into WTC 1, Case B (Cont.) 
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6.9 AIRCRAFT IMPACT DAMAGE ESTIMATES 
6.9.1 Structural and Contents Damage 
Each of the four global simulations generated information about the state of the structural components 
following the impact of the aircraft. The four degrees of column damage are defined as follows and 
shown graphically in Figure 6–19. The unstrained areas are blue and the highly strained areas are red. 
• Lightly damaged column: column impacted, but without significant structural deformation; 
• Moderately damaged column: visible local distortion, but no deformation of the column 
centerline; 
• Heavily damaged column: Permanent deflection of the column centerline; and 
• Failed column: Column severed. 
(a) Light (b) Moderate (c) Heavy (d) Severed 
Figure 6–19. Column damage levels. 
Figure 6–20 shows the calculated damage to a floor slab. Figure 6–21 shows the response of the 
furnishings and the jet fuel to the impact. Figures 6–22 through 6–25 show the combined damage for all 
floors for the four global simulations. The latter proved useful in visualizing the extent of aircraft impact 
in one graphic image. 
Figure 6–20. Case B damage to the slab of floor 96 
of WTC 1. 
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(a) Pre-impact configuration 
(b) Calculated impact response 
(c) Calculated impact response (fuel removed) 
Figure 6–21. Case B simulation of response of contents of 96th floor of WTC 1. 
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112 NIST NCSTAR 1, WTC Investigation 
Figure 6–22. Combined structural damage to the floors and columns of WTC 1, Case A. 
Figure 6–23. Combined 
structural damage to the 
floors and columns of WTC 1, 
Case B. 
Severe Floor Damage 
Insulation 
and partitions 
Floor system 
structural damage 
Floor system 
removed 
Column Damage 
Severed 
Heavy Damage 
Moderate Damage 
Light Damage 

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NIST NCSTAR 1, WTC Investigation 113 
Figure 6–24. Combined structural damage to the floors and columns of WTC 2, Case C. 
Figure 6–25. Combined 
structural damage to the floors 
and columns of WTC 2, Case D. 
Column Damage 
Severed 
Heavy Damage 
Moderate DamageLight Damage 
Severe Floor Damage 
Insulation 
and partitions 
Floor system 
structural damage 
Floor system 
removed 

Chapter 6 Draft for Public Comment 
6.9.2 Validity of Impact Simulations 
Assessment of the aircraft impact simulations of exterior damage to the towers involved comparing the 
predicted perimeter wall damage near the impact zone with post-impact photographs of the walls. 
Figure 6–26 shows a photograph of the north face of WTC 1 after impact and the results of the Case A 
simulation. The calculated silhouettes capture both the position and shape of the actual damage. 
Figures 6–27 and 6–28 depict more detailed comparisons between the observed and calculated damage. 
The aircraft hole is shown in white. The colored dots characterize the mode in which the steel or 
connection failed (e.g., severed bolt, ripped weld) and the magnitude of the deformation of the steel: 
• Green: proper match of failure mode and magnitude 
• Yellow: proper match in the failure mode, but not the magnitude 
• Red: neither the failure mode nor the magnitude matched 
• Black: the observed damage was obscured by smoke, fire, or other factors 
The predominance of green dots and the scarcity of red dots indicate that the overall agreement with the 
observed damage was very good. The agreement for Cases B and D was slightly lower. 
Assessment of the accuracy of the predictions of damage inside the buildings was more difficult, as NIST 
could not locate any interior photographs near the impact zones. Three comparisons were made: 
• The Case A simulation for WTC 1 predicted that the walls of all three stairwells would have 
been collapsed. This agreed with the observations of the building occupants. The Case A 
simulation for WTC 2 showed that the walls of stairwell B would have been damaged, but 
that Stairwell A would have been unaffected. Stairwell C was not included in the WTC 2 
model, but is adjacent to where damage occurred. The building occupants reported that 
Stairwells B and C were impassable; Stairwell A was damaged but passable. 
• The two simulations of WTC 2 showed accumulations of furnishings and debris in the 
northeast corner of the 80th and 81st floors. These piles were observed in photographs and 
videos. 
• Two pieces of landing gear penetrated WTC 1 and landed to the south of the tower. The Case 
B prediction showed landing gear penetrating the building core, but stopping before reaching 
the south exterior wall. For WTC 2, a landing gear fragment and the starboard engine 
penetrated the building and landed to the south. The Case D prediction correctly showed the 
main landing gear emerging from the northeast corner of WTC 2. However, Case D showed 
that engine not quite penetrating the building. Minor modifications to the model (all within 
the uncertainty of the input data) would have resulted in the engine passing through the north 
exterior wall of the tower. 
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(a) Observed Damage 
= 
(b) Calculated damage 
Figure 6–26. Observed and Case A calculated damage to the north face of WTC 1. 
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Figure 6–27. 
Schematic of 
observed 
damage (top) 
and Case A 
calculated 
damage 
north face of 
WTC 1. 
(lower) to the 
Figure 6–28. 
Schematic of 
observed 
damage (above) 
and Case C 
calculated 
damage (right) 
to the south face 
of WTC 2. 
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Draft for Public Comment Reconstruction of the Collapses 
Not all of the observables were closely matched by the simulations due to the uncertainties in exact 
impact conditions, the imperfect knowledge of the interior tower contents, the chaotic behavior of the 
aircraft breakup and subsequent debris motion, and the limitations of the models. In general, however, 
the results of the simulations matched these observables sufficiently well that the Investigation Team 
could rely on the predicted trends. 
Simulations of the damage to the core columns had been performed previously by staff of Weidlinger 
Associates, Inc. (WAI) and the Massachusetts Institute of Technology (MIT). Each developed a range of 
numbers of failed and damaged columns, as did NIST. The range of the MIT results straddled the NIST 
results. WAI’s analysis resulted in more failed and damaged columns, with WTC 2 being unstable 
immediately following impact. 
6.9.3 Damage to Thermal Insulation 
The dislodgement of thermal insulation from structural members could have occurred as a result of direct 
impact by debris and could have occurred by inertial forces due to vibration of structural members as a 
result of the aircraft impact. In interpreting the output of the aircraft impact simulations, NIST assumed 
that the debris impact dislodged insulation if the debris force was strong enough to break a gypsum board 
partition immediately in front of the structural component. Experiments at NIST confirmed that an array 
of 0.3 in. diameter pellets traveling at 350 mph stripped the insulation from steel bars like those used in 
the WTC trusses. 
Determining the adherence of SFRM outside the debris zones was more difficult. There was 
photographic evidence that some fraction of the SFRM was dislodged from perimeter columns not 
directly impacted by debris. 
NIST developed a simple model to estimate the range of accelerations that might dislodge the SFRM 
from the structural steel components. As the SFRM in the towers was being upgraded with BLAZE-
SHIELD II (CAFCO II) in the 1990s, The Port Authority had measured the force required to pull the 
insulation from the steel. The model used these data as input to some basic physics equations. The 
resulting ranges of accelerations depended on the geometry of the coated steel component and the SFRM 
thickness, density and bond strength. For a flat surface (as on the surface of a column), the range was 
from 20g to 530g, where g is the gravitational acceleration. For an encased bar (such as used in the WTC 
trusses), the range was from 40g to 730g. NIST estimated accelerations from the aircraft impacts of 
approximately 100g. 
An intact ceiling tile system 
The analyses were not sufficient to establish justifiable, general could have provided the floor 
criteria for a coherent pattern of vibration-induced dislodging. Thus, trusses with approximately 
NIST made the conservative assumption that all other insulation 10 min to 15 min of thermal 
remained adhered to the structural components. protection from ceiling air 
temperatures near 1,000 °C. 
These temperatures would 
6.9.4 Damage to Ceiling System quickly heat steel without 
thermal insulation to 
The aircraft impact modeling did not include the ceiling tile systems. temperatures for reduction of 
the strength of structural 
To estimate whether the tiles would survive the aircraft impact, the 
University at Buffalo, under contract to NIST, conducted tests of steels. 
NIST NCSTAR 1, WTC Investigation 

Chapter 6 Draft for Public Comment 
WTC-like ceiling tile systems using their shake table (Figure 6–29) and impulses related to those induced 
by the aircraft impact on the towers. The data indicated that accelerations of approximately 5g would 
most likely result in substantial displacement of ceiling tiles. Given the estimated impact accelerations of 
approximately 100g, NIST assumed that the ceiling tiles in the impact and fire zones were fully 
dislodged. This was consistent with the multiple reports of severely damaged ceilings (Chapter 7). 
NISTSource:
Figure 6–29. Ceiling tile system mounted on the shaking table. 
6.9.5 Damage to Interior Walls and Furnishings 
As shown in Figure 6–18, the aircraft impact simulations explicitly included the fracture of walls in the 
debris path and the "bulldozing" of furnishings. Damage to the impacted furnishings was not modeled. 
Walls and furnishings outside the debris paths were undamaged in the simulations. 
6.10 THERMAL ENVIRONMENT MODELING 
6.10.1 Need for Simulation 
Following the impact of the aircraft, the jet-fuel-ignited fires created the sustained and elevated 
temperatures that heated the remaining building structure to the point of collapse initiation. The 
photographic evidence provided some information regarding the locations and spreading of the fires. 
However, the cameras could only see the periphery of the building interior. The steep viewing angles of 
nearly all of the photographs and videos further limited the depth of the building interior for which fire 
information could be obtained. NIST could not locate any photographic evidence regarding the fire 
exposure of the building core or the floor assemblies. 
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Draft for Public Comment Reconstruction of the Collapses 
The simulations of the fires were the second computational step in the identification of the probable 
sequences leading to the collapses of the towers. The required output of these simulations was a set of 
three-dimensional, time varying renditions of the thermal and radiative environment to which the 
structural members in the towers were subjected from the time of aircraft impact until the onset of 
building collapse. The rigor of the Investigation placed certain requirements on the computational tool 
(model) used to generate these renditions: 
• Resolution of the varying thermal environment across key dimensions, e.g., the truss space; 
• Representation of the complex combustibles; 
• Computation of flame spread across the large expanses of the WTC floors; and 
• Confidence in the accuracy of the predictions. 
6.10.2 Modeling Approach 
The time frame of the Investigation and the above requirements led to the use of the Fire Dynamics 
Simulator (FDS). Under development at NIST since 1978, FDS was first publicly released in February 
2000 and had been used worldwide on a wide variety of applications, ranging from sprinkler activation to 
residential and industrial fire reconstructions. However, it had never before been applied to spreading 
fires in a building with such large floor areas. 
Figure 6–30 shows how FDS represented the eight modeled floors (92 through 99) of the undamaged 
WTC 1. A similar rendition was prepared for floors 78 through 83 of WTC 2. The layout of each floor 
was developed from architectural drawings and from the information described in Section 5.8. There was 
a wide range of confidence in the accuracy of these floor plans, varying from high (for the floors occupied 
by Marsh & McLennan in WTC 1, for which recent and detailed plans were obtained) to low (for most of 
the space in WTC 2 occupied by Fuji Bank, for which floor plans were not available). 
The effects of the aircraft impact were derived from the simulations described in Section 6.8. The 
portions of walls and floors that were “broken” in those simulations were simply removed from the FDS 
models of the towers. The furnishings outside the aircraft-damaged regions were assumed to be unmoved 
and undamaged. The treatment of furnishings within the impact zone is discussed later in this section. 
FDS represented the spaces in which the fires and their effluent were to be modeled as a grid of 
rectangular cells. These grids included the walls, floors, ceilings, and any other obstructions to the 
movement of air and fire. In the final simulations, the grid size was 0.5 m x 0.5 m x 0.4 m high 
(1.6 ft x 1.6 ft x 1.3 ft.). Each floor contained about 125,000 grid cells, and the nature of each cell was 
updated every 10 ms (100 times every second). The computations were performed using parallel 
processing, in which the fires on each floor were simulated on a different computer. At the end of each 
10 ms update, the processors exchanged information and proceeded to the computations for the next time 
interval. Each simulation of 105 min of fires for WTC 1 took about a week on eight Xeon computers 
with a combined 16 GB of memory. The simulations for WTC 2, with fewer floors and 60 min of real 
time fires, took about half the time. 
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Chapter 6 Draft for Public Comment 
Figure 6–30. Eight floor model of WTC 1 prior to aircraft impact. 
The fires were started by ignition of the jet fuel, whose distribution was provided by the aircraft impact 
simulations. The radiant energy from these short-lived fires heated the nearby combustibles, creating 
flammable vapors. When these mixed with air in the right proportion within a grid cell, FDS burned the 
mixture. This generated more energy, which heated the combustibles further, and continued the burning. 
The floors of the tower on which the 
dominant burning occurred were 
characterized by large clusters of office 
workstations (Figure 1–11). NIST 
determined their combustion behavior 
from a series of single-workstation fire 
tests (Figure 6–31). In these highly 
instrumented tests, the effects of 
workstation type, the presence of jet fuel, 
and the presence of fallen inert material 
(such as pieces of ceiling tiles or gypsum 
board walls) on the burning surfaces were 
all assessed. While FDS properly captured 
the gross behavior of these fires, the state 
of modeling the combustion of real 
furnishings was still primitive. Thus, the Source: NIST. 
results of this test series were used to Figure 6–31. Fire test of a single workstation. 
refine the combustion module in FDS. 
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Draft for Public Comment Reconstruction of the Collapses 
The accuracy of FDS predictions was then assessed using two different The large fires 
types of fire tests. In each case, the model predictions were generated prior discussed in this report 
to conducting the test. are characterized by 
heat release rate, or 
The first series provided a measure of the ability of FDS to predict the burning intensity, (in 
thermal environment generated by a steady state fire. A spray burner MW), by total energy 
released (in GJ), and 
generating 1.9 MW or 3.4 MW of power was ignited in a 23 ft by 11.8 ft by by the heat flux, or
12.5 ft high compartment. The temperatures near the ceiling approached radiant intensity 
900 °C. FDS predicted: (in kW/m2). 
• Room temperature increases near the ceiling to within 4 percent. 
• Gas velocities at the air inlet to the compartment (and thus the air drawn into the 
compartment by the fire) within the uncertainty in the experimental measurements. 
• The leaning of the fire plume due to the asymmetry of the objects within the compartment. 
The extent of the leaning was underestimated. 
• Radiant heat flux near the ceiling to within 10 percent, within the uncertainty of the 
experimental measurements. 
The second series was a preamble to the modeling of the actual WTC fires. Arrays of three WTC 
workstations were burned in a 35.5 ft by 23 ft by 11 ft high compartment (Figure 6–32). The tests 
examined the effects of the type of workstation, the presence of jet fuel, and the presence of fallen inert 
material on the burning surfaces. In one of the tests, the workstations were rubblized (Figure 6–33). 
Figure 6–34 depicts the intensity of the test fires. Figure 6–35 shows the measured and predicted heat 
release rate data from one of the tests in which there was no jet fuel nor inert material present. 
Figure 6–32. Interior view of a three-workstation 
fire test. 
Source: NIST. 
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Chapter 6 Draft for Public Comment 
Figure 6–33. 
Rubblized 
workstations. 
Source: NIST. 
NISTSource:
Figure 6–34. Three-workstation fire test, 2 min after the start. 
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Draft for Public Comment Reconstruction of the Collapses 
Figure 6–35. Measured and predicted heat release rate from the burning of three 
office workstations. 
The differences in the fire behavior under the different experimental conditions were profound in these 
roughly hour-long tests. The jet fuel greatly accelerated the fire growth. Only about 60 percent of the 
combustible mass of the rubblized workstations was consumed. The near-ceiling temperatures varied 
between 800 °C and 1,100 °C. Nonetheless, FDS successfully replicated: 
• The general shape and magnitude of the time-dependent heat release rate. 
• The time at which one half of the combustion energy was released to within 3 min. 
• The value of the heat release rate at this time to within 9 percent. 
• The duration of the fires to within 6 min. 
• The peak near-ceiling temperature rise to within 10 percent. 
All these predictions were within the combined uncertainty in the model input data and the experimental 
measurements. 
Combined, these results led to the assessment that the uncertainty in the thermal environment predictions 
of the WTC fires would be dominated less by the FDS errors and more by the unknowns in such factors 
as the distribution of the combustibles, ventilation, and building damage. 
NIST NCSTAR 1, WTC Investigation 

Chapter 6 Draft for Public Comment 
6.10.3 The Four Cases 
Four fire scenarios (Case A and Case B for WTC 1 and Case C and Case D for WTC 2) were 
superimposed on the four cases of aircraft-driven damage of the same names (Section 6.9). 
A number of preliminary simulations had been performed to gain insight into the factors having the most 
influence on the severity of the fires. The most influential was the mass of combustibles per unit of floor 
area (fuel load); second was the extent of core wall damage, which affected the air supply for the fires. 
The aforementioned workstation fire tests had also indicated that the damage condition of the furnishings 
also played a key role. The scenario variables and their values are shown in Table 6–6. 
Table 6–6. Values of WTC fire simulation variables. 
Variable 
WTC 1 WTC 2 
Case A Case B Case C Case D 
Tenant fuel loada 20 kg/m2 (4 lb/ft2) 25 kg/m2 (5 lb/ft2) 20 kg/m2 (4 lb/ft2) 20 kg/m2 (5 lb/ft2) 
Distribution of Even Weighted toward Heavily Moderately 
disturbed the core concentrated in the concentrated in the 
combustibles northeast corner northeast corner 
Condition of Undamaged except Displaced furniture All rubblized Undamaged except 
combustibles in impact zone rubblized in impact zone 
Representation of 
impacted core wallsb 
Fully removed Soffit remained Fully removed Soffit remained 
a. In addition, approximately 12,000 kg (27,000 lb) of solid combustibles from the aircraft were distributed along the debris path. 
b. In Cases A and C, the walls impacted by the debris field were fully removed. This enabled rapid venting of the upper layer 
into the core shafts and reduced the burning rate of combustibles in the tenant spaces. In Cases B and D, a more severe 
representation of the damage was to leave a 1.2 m soffit that would maintain a hot upper layer on each fire floor. This 
produced a fire of longer duration near the core columns and the attached floor membranes. 
FDS contained no algorithm for breaking windows from the heat of the fires. Thus, during each 
simulation, windows were removed at times when photographs indicated they were first missing. 
Damage to the ventilation shafts was derived from the aircraft impact simulations. For undamaged floors, 
all the openings to the core area were assumed to total 5 m2 in area. 
6.10.4 Characterization of the Fires 
For each of the four scenarios, FDS was used to generate a time-dependent gas temperature and radiation 
environment on each of the floors. The results of the FDS simulations of the perimeter fire were 
compared with the fire duration and spread rate as seen in the photographs and videos. For ease of 
visualization, contour plots of the room gas temperature 0.4 m below the ceiling slab (in the “upper layer” 
of the compartment) were superimposed on profiles of the photographed fire activity. An example is 
shown in Figure 6–36. The stripes surrounding the image represent a summary of the visual observations 
of the windows, with the black stripes denoting broken windows, the orange stripes denoting external 
flaming, and the yellow stripes denoting fires that were seen inside the building. Fires deeper than a few 
meters inside the building could not be seen because of the smoke obscuration and the steep viewing 
angle of nearly all the photographs. 
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Draft for Public Comment Reconstruction of the Collapses 
Figure 6–36. Upper layer temperatures on the 94th floor of WTC 1, 15 min after impact. 
Given the uncertainties in some of the floor plans, the damage to the internal walls, and movement of the 
office furnishings, the intent of the simulations was to capture the magnitudes of the fires and the broad 
features of their locations and movement; and they did so. 
The following sections summarize the simulated behavior of the fires (which was used in the following 
stages of the disaster reconstruction) and their correlation with the analysis of the photographic evidence. 
WTC 1 
Much of the fire activity was initially in the vicinity of the impact area in the north part of the building. 
As a result of the orientation of the impacting aircraft and its fuel tanks, the early fires on the 92nd through 
94th floors tended toward the east side of the north face, while the early fires on the 97th through 
99th floors tended toward the west side of the north face. The fires on all the floors spread along the east 
and west sides and were concentrated in the south part of the building at the time of collapse, as depicted 
in Figure 6–37. 
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Chapter 6 Draft for Public Comment 
Figure 6–37. Direction of simulated fire movement on floors 94 and 97 of WTC 1. 
The fire simulation results for Case A and Case B were similar, indicating only a modest sensitivity to the 
fuel load and the degree of aircraft-generated damage. This was because, in general, the size and 
movement of the fires in WTC 1 were limited by the supply of air from the exterior windows. Since the 
window breakage pattern was not changed in Case B, the additional and re-distributed combustibles 
within the building did not contribute to a larger fire. The added fuel did slow the spread slightly because 
the fires were sustained longer in any given location. 
Although there was generally reasonable agreement between the simulated and observed fire spread rates, 
there were instances where the fires burned too quickly and too near the windows. This resulted from an 
artifact of the model: the combustible vapors burned immediately upon mixing with the incoming oxygen. 
Simulations performed with doubled fuel loads slowed the fire spread well below the observed rates. 
Combined with the above results, this suggested that the estimated overall combustible load of 4 lb/ft2 
was reasonable. 
The simulations showed high temperatures in some of the elevator shafts. The late fire observed on the 
west face of the 104th floor may have started from fuel gases in the core shafts that had accumulated over 
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Draft for Public Comment Reconstruction of the Collapses 
the course of the first hour of fires below. The presence of fire in the shafts on the 99th floor provided 
some support for this hypothesis, but no simulations were performed for floors higher than the 99th. 
The predictions of maximum temperatures (e.g., red zones in Figure 6–37) were consistent with those in 
the three-workstation fire tests. 
The use of an “average” gas temperature was not a satisfactory means of assessing the thermal 
environment on floors this large and would also have led to large errors in the subsequent thermal and 
structural analyses. The heat transferred to the structural components was largely by means of thermal 
radiation, whose intensity is proportional to the fourth power of the gas temperature. At any given 
location, the duration of temperatures near 1,000 °C was about 15 min to 20 min. The rest of the time, the 
calculated temperatures were near 500 °C or below. To put this in perspective, the radiative intensity onto 
a truss surrounded by smoke-laden gases at 1,000 °C was approximately 7 times the value for gases at 
500 °C. 
WTC 2 
Simulating the fires in WTC 2 posed challenges in addition to those encountered in simulating the fires in 
WTC 1. The aircraft, hitting the tower to the east of center, splintered much of the furnishings on the east 
side of the building and plowed them toward the northeast corner. Neither the impact study nor the 
validation experiments performed at NIST could be completely relied upon to predict the final 
distribution, condition, and burning behavior of the demolished furnishings. In addition, only the layouts 
of the 78th and 80th floors were available to the Investigation; the other floors were only roughly described 
by former occupants. As a result of these unknowns, the uncertainty in these calculations was distinctly 
greater than in those for WTC 1. To help mitigate gross differences between the simulations and the 
observables, NIST made floor-specific adjustments, based on the results of preliminary computations. In 
particular, the fuel load and volatility on the 80th floor were reduced, and the fuel load on the 81st and 
82nd floors was increased. 
In contrast with WTC 1, in WTC 2 there was less movement of the fires. The major burning occurred 
along the east side, with some spread to the north. There was no significant burning on the west side of 
the tower. 
Also unlike WTC 1, changing the combustible load in WTC 2 had a noticeable effect on the outcome of 
the simulations. Because so many windows on the impact floors in WTC 2 were broken out by the 
aircraft debris and the ensuing fireballs, there was an adequate supply of air for the fires. Thus, the 
burning rate of the fires was determined by the fuel supply. In the Case D simulation, the office 
furnishings and aircraft debris were spread out over a wider area, and the furnishings away from the 
impact area were undamaged. Both of these factors enabled a higher burning rate for the combustibles. 
In general, the Case D simulations more closely approximated the observations in the photographs and 
videos, although there was still some prediction of burning too close to the perimeter, especially on the 
east side of the 78th, 79th, 81st and 83rd floors. The burning in the northeast corner of the 81st and 
82nd floors was more intense in Case D than in Case C. The fire in the east side of the 79th floor burned 
more intensely and reached the south face sooner. 
NIST NCSTAR 1, WTC Investigation 

Chapter 6 Draft for Public Comment 
Nothing in the simulations explained the absence of fires in the “cold spot,” the 10-window expanse 
toward the east of the north face of the 80th, 81st, and 82nd floors. 
Heat Release Rate (GW) 
6.10.5 Global Heat Release Rates 
Much of the information needed to simulate the fires came from laboratory-scale tests. While some of 
these involved enclosures several meters in dimension and fires that reached heat release rates of 10 MW 
and 12 GJ in total heat output, they were still far smaller than the fires that burned in the WTC towers. 
Figure 6–38 shows the heat release rates from the FDS simulations of the WTC fires. The peak plateau 
heat release rates were about 2 GW for WTC 1 and 1 GW for WTC 2. Integrating the areas under these 
curves produced total heat outputs from the simulated fires of about 8,000 GJ from WTC 1 and 3,000 GJ 
from WTC 2. 
3.0 
2.5 
2.0 
1.5 
1.0 
0.5 
0.0 
WTC 1, Case A 
WTC 2, Case C 
WTC 1, Case B 
WTC 2, Case D 
0 20 40 60 80 100 
Time (min) 
Figure 6–38. Predicted heat release rates for fires in WTC 1 and WTC 2. 
6.11 DATA TRANSFER 
The following data from FDS were compiled for use as boundary conditions for the finite-element 
calculation of the structural temperatures: 
• The upper and lower layer gas temperatures, time-averaged over 100 s and spatially averaged 
over 1 m. The upper layer gas temperatures were taken 0.4 m (one grid cell) below the 
ceiling. The lower layer temperatures were taken 0.4 m above the floor. 
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• The depth of the smoke layer. 
• The absorption coefficient of the smoke layer 0.4 m below the ceiling. 
6.12 THERMAL MAPPING 
6.12.1 Approach 
Simulating the effect of a fire on the structural integrity of a building required a means for transferring the 
heat generated by the fire to the surfaces of the insulation on structural members and then conducting the 
heat through those members. In the Investigation, this meant mapping the time- and space-varying gas 
temperatures and radiation field generated by FDS onto and throughout the (insulated) columns, trusses 
and other elements that made up the tower structure. 
This process was made difficult for these large, geometrically complex buildings by the wide disparity in 
length and time scales that had to be accounted for in the simulations. FDS generated thermal maps with 
dimensional resolution of the order of a meter and temperatures fluctuating on a time scale of 
milliseconds. The finite element models for thermal analysis resolved length of the order of ½ in. on a 
time scale of seconds. Devising a computation scheme to accommodate the finest of these scales, while 
simulating the largest of these scales, presented a software challenge in order to avoid unacceptably long 
computation times. 
6.12.2 The Fire-Structure Interface 
NIST developed a computational scheme to overcome this difficulty, 
The transfer of radiant 
the Fire Structure Interface (FSI). energy from a hot mass to a 
cool mass is proportional to 
These computations began with the structural models of each WTC the absolute temperature 
tower as described in Section 6.6.4, damaged by the aircraft as (Kelvin) to the fourth power. 
described in Section 6.8.4 and exposed to fire-generated heat, as Thus, the contribution of the 
described in Section 6.10.4. For a particular tower and damage hot upper layer dominates 
scenario, FSI "bathed" each small section of each structural member in the overall radiative heat 
transfer. Convective heat 
an air environment that had been generated by FDS. For efficiency of transfer is linearlycomputation, two simplifications were made: proportional to the difference 
in temperature between the 
• The fluctuating environment was averaged over 30 s hot gas and the cool solid. 
intervals, and 
• The local environment was represented by a hot, soot-laden upper layer and a cooler, 
relatively clear lower layer. 
FSI then calculated the radiative and convective heat transfer to each of these small sections using 
conventional physics. Finally, the temperature data were read into the ANSYS 8.0 finite element 
program, which applied the temperature distribution to the structural elements. 
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Chapter 6 Draft for Public Comment 
6.12.3 Thermal Insulation Properties 
Equivalent Uniform Thickness of SFRM 
Preliminary simulations with FSI explored the extent to which bare steel structural elements would heat 
more rapidly than the same elements would if they were well insulated. In one such calculation 
experiment, one of the largest columns in the tower structure was immersed in a furnace at 1,100 °C. 
Uninsulated, it took just 13 min for the steel surface temperatures to reach 600 °C, in the range where 
substantial loss of strength occurs. When insulated with 1 1/8 in. of SFRM, the same column had not 
reached that temperature in 10 hours. This established that the fires in WTC 1 and WTC 2 would not be 
able to significantly weaken the insulated core or perimeter columns within the 102 min and 56 min, 
respectively, after impact and prior to collapse. Thus, it was important to know whether the insulation 
was present or removed and much less important to know the exact thickness of the SFRM. 
It was likely that the thinner steel bars and angles in the floor trusses would be more sensitive to the 
condition of the insulation. If the insulation were present, but too thin or imperfectly applied, these 
components might have been heated to failure in times on the order of an hour. 
NIST performed additional simulations to probe the effect of gaps in the truss insulation and of variations 
in the thickness, similar to those observed in real SFRM application (Figure 5–6). It was evident that 
incorporation of these small-scale variations into the description of the structural members would have 
lengthened the FSI computations to an extreme. Furthermore, there was insufficient information to 
determine how the thickness varied over the length of the structural members. NIST combined the 
measured variations in the SFRM thickness (as described in Section 5.6.2) with simulations of the heat 
transfer through the uneven material. This led to the identification of a uniform thickness that provided 
the same insulation value as did the measured coatings. These values, shown in Table 5–3, were used in 
the thermal calculations. They were found to be greater than the specified thicknesses but slightly smaller 
than the average measured thicknesses, as they should be. 
SFRM Thermophysical Properties 
When the Investigation began, there were few published data on the insulating properties of SFRMs, 
especially at elevated temperatures. It was expected, and soon confirmed, that the fires could generate 
temperatures up to 1,100 °C. Therefore, NIST contracted for measurement of the key SFRM 
thermophysical properties that, along with coating thickness, determine the insulating effect of the 
coatings. These properties included thermal conductivity, specific heat capacity, and density. These were 
measured for each SFRM at temperatures up to 1,200 °C. Since there were no ASTM test methods 
developed specifically for characterizing the thermophysical properties of SFRMs as a function of 
temperature, ASTM test methods developed for other materials were used. Samples were prepared by the 
manufacturers of the fire resistive materials, which included CAFCO BLAZE-SHIELD DC/F and 
CAFCO BLAZE-SHIELD II. 
• The thermal conductivity measurements were performed according to ASTM C 1113, 
Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum 
Resistance Thermometer Technique). The room temperature values were in general 
agreement with the manufacturer’s published values for both materials. The thermal 
conductivities increased with temperature. 
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• Specific heat capacity was measured in accordance with ASTM E 1269, Standard Test 
Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry (DSC). 
By including DSC measurement of a NIST Reference Material (sapphire), the measured 
SFRM quantities were directly traceable to NIST standards. 
• The densities of the SFRMs were calculated from measurements of changes in the mass and 
dimensions of samples as their temperatures were increased. The length-change 
measurements were performed according to ASTM E 228, Standard Test Method for Linear 
Thermal Expansion of Solid Materials. The mass loss measurements were performed 
according to ASTM E 1131, Standard Test Method for Compositional Analysis by 
Thermogravimetry. 
It was not known which type(s) of gypsum wallboard were used to enclose the core columns. Therefore, 
the thermophysical properties of four types of gypsum panels were examined. 
• Thermal conductivity was measured using the heated probe technique described in ASTM D 
5334, Standard Test Method for Determination of Thermal Conductivity of Soil and Soft 
Rock by Thermal Needle Probe Procedure. In general, the thermal conductivity initially 
decreased as the temperature increased to 200 °C and then increased with increasing 
temperature above 300 °C. 
• Specific heat capacities of the cores of the four gypsum panel samples were measured using a 
differential scanning calorimeter at NIST according to ASTM E 1269, Standard Test Method 
for Determining Specific Heat Capacity by Differential Scanning Calorimetry. The four 
panels had nearly identical specific heat capacities as a function of temperature. 
• The variation of density with temperature was determined from the change in volume of the 
gypsum material and the mass loss. The linear expansion was determined using a dilatometer 
and the mass loss from thermogravimetric analysis. All four materials showed the same trend 
as a function of temperature. 
6.12.4 FSI Uncertainty Assessment 
As was done for FDS, it was necessary to establish the quality of FSI’s predictions of temperature profiles 
within insulated and bare structural steel components. This was accomplished using data from a series of 
six tests in which assorted steel members were exposed to controlled fires of varying heat release rate and 
radiative intensity. The steel members, depicted in Figures 6–39 through 6–41, were either bare or coated 
with spray-applied BLAZE-SHIELD DC/F in two thicknesses. The fibrous insulation was applied by an 
experienced applicator, who took considerable care to apply an even coating of the specified thickness. 
As such, the insulated test subjects represent a best case in terms of thickness and uniformity. 
Figure 6–42 shows some of the coated components. 
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Chapter 6 Draft for Public Comment 
1 
118 
Figure 6–39. Simple bar dimensions (in.). 
132 
10 
14 
1/4 
1/4 
Figure 6–40. Tubular column dimensions (in.). 
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Draft for Public Comment Reconstruction of the Collapses 
31 
181 
Web Bar Top Chord Angles Bottom Chord Angles 
3 1/4 
1 
1/4 
1/4 
2.5 
3 1/4 
2.5 
Figure 6–41. Truss Dimensions (in.) 
NIST.Source:
Figure 6-42. SFRM-coated steel components prior to a test. 
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Table 6–7 shows the dimensions and variability of the insulation for the two successful tests involving 
coated steel. The thickness measurements were taken at numerous locations along the perimeter and 
length of each specimen using a pin thickness gauge specifically designed for this type of insulation. 
Table 6–7. Summary of insulation on steel components. 
Test Item Specified Thickness (mm) 
Applied Thickness (mm) 
Mean Std. Deviation 
5 Bar 19.1 23.0 5.5 
Column 38.1 41.0 3.0 
Truss A 19.1 26.9 7.3 
Truss B 38.1 40.5 8.2 
6 Bar 19.1 25.3 4.6 
Column 19.1 21.4 3.5 
Truss A 19.1 26.0 6.9 
Truss B 19.1 25.6 6.9 
Temperatures were recorded at multiple locations on the surfaces of the steel, the insulation, and the 
compartment. As an example, Figure 6-43 shows the finite element representation of the coated truss. 
Figure 6-43. Finite element representation of the insulated steel truss (blue), the 
SFRM (violet), and the ceiling (red). 
Figure 6-44 compares the measured and predicted temperatures on the steel surface of the top chord of a 
bare truss. Figure 6-45 is the analogous plot of the measured and predicted temperatures on the steel 
surface of the top chord of a truss insulated with 19 mm (3/4 in.) of BLAZE-SHIELD DC/F. Similar 
curves were generated for each of the steel pieces, bare and insulated. 
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Figure 6–44. Comparison of numerical simulations with measurements for the steel 
surface temperature at four locations on the top chord of a bare truss. 
Figure 6–45. Comparison of numerical simulations with measurements for the 
temperature of the steel surface at four locations on the top chord of an insulated truss. 
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Examination of the graphs for the insulated steel pieces indicated the following: 
• FSI captured the shape of the temperature rise at the steel surfaces and the significant 
decrease in the rate of temperature rise when the SFRM was present. 
• The times to the peak temperature (or a near-plateau) were predicted to within about a minute 
in all cases. 
• There was no consistent pattern of overprediction or underprediction of the surface 
temperatures. 
• On the average, the numerical predictions of the steel surface temperature were within 
7 percent of the experimental measurements for bare steel elements and within 17 percent for 
the insulated steel elements. The former was within the combined uncertainty in the 
temperature measurements and the heat release rate in the fire model. The increase in the 
latter was attributed to model sensitivity to the SFRM coating thickness and thermal 
conductivity. 
In general, the FSI added little to the overall uncertainty in the simulation of the temperatures at the outer 
surfaces of bare steel elements and, more importantly, at the SFRM-steel interface. 
An additional, important outcome of the experiments was the demonstration of the insulating effect of 
even 3/4 in. of SFRM. Trusses, made of relatively thin steel, were far more susceptible to heating than 
the perimeter and core columns. As shown in Figure 2–10, in 15 min, a bare truss reached a temperature 
at which significant loss of strength was imminent. An identical, but insulated truss had not reached that 
temperature in 50 min. 
6.12.5 The Four Cases 
FSI imposed the thermal environment from each of the four FDS fire scenarios (Cases A and B for 
WTC 1 and Cases C and D for WTC 2) on the four damaged structures from the aircraft simulations, 
which carried the same case letters. The FSI output files carried the same case letters as the input files. 
The FSI calculations were performed at time steps ranging from 1 ms to 50 ms. Use of the resulting data 
set for structural analysis would have required a prohibitive amount of computation time. Thus, for each 
case, the instantaneous temperature and temperature gradient for each grid volume was provided at 
10 min intervals after aircraft impact. For WTC 1, there were 10 such intervals, ending at 6,000 s; for 
WTC 2 there were 6 intervals, ending at 3,600 s. Comparison of these coarsely timed output files with 
files at 1 min resolution showed any differences to be within the combined uncertainty. 
Each floor in the FSI simulation provided thermal information for the floor assembly above. Thus, there 
was not sufficient information for FSI to model the lowest floor in the FDS simulations. For WTC 1, the 
global thermal response generated by FSI included floors 93 through 99; for WTC 2, the included floors 
were 79 through 83. 
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For ease of visualization, two graphic representations were developed. Figure 6–46 shows an example of 
the temperature map for the 96th floor of WTC 1. Severed columns and broken floor segments are not 
shown. Figure 6–47 shows a similar map for the 81st floor of WTC 2. 
A third visualization tool was animation of the evolving temperatures of the columns. Frames from an 
example, again of the 96th floor of WTC 1, Case A, are shown in Figure 6-48. The size of the square 
representing a column represents its yield strength. Columns may have been heated when the fire was 
nearby and then cooled after the local combustibles were consumed. 
Figure 6–46. Temperatures (°C) on the columns and trusses of the 96th floor of WTC 1 at 
6,000 s after aircraft impact, Case B. 
Figure 6-47. Temperatures (°C) on the columns and trusses of the 81st floor of WTC 2 at 
3,000 s after aircraft impact, Case D. 
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(a) Time = 1000 s (b) Time = 2000 s (c) Time = 3000 s 
(d) Time = 4000 s (e) Time = 5000 s (f) Time = 6000 s 
Figure 6–48. Frames from animation of the thermal response of columns on the 96th floor of WTC 1, Case A. 

Draft for Public Comment Reconstruction of the Collapses 
6.12.6 Characterization of the Thermal Profiles 
Tables 6–8 and 6–9 summarize the regions of the floors in which the structural steel reached temperatures 
at which their yield strengths would have been significantly diminished. Instances of brief heating of one 
or two columns early in the fires were not included. 
Even in the vicinity of the fires, the columns and trusses for which the insulation was intact did not heat to 
temperatures where significant loss of strength occurred. 
Unlike the simulations of the aircraft impact and the fires, there was no evidence, photographic or other, 
for direct comparison with the FSI results. 
Table 6–8. Regions in WTC 1 in which temperatures of structural steel exceeded 600 °C. 
Floor 
Number 
Trusses Perimeter Columns Core Columns 
Case A Case B Case A Case B Case A Case B 
93 – – – – – – 
94 – – – – N, S NE, S 
95 N N, S – – S NW, S 
96 N N, S – S S W, S 
97 N, S N, S – S N W, S 
98 N N, S – – – – 
99 – – – – – – 
Key: N, north; S, south; W, west; NE, northeast; NW, northwest. 
Table 6–9. Regions in WTC 2 in which temperatures of structural steel exceeded 600 °C. 
Floor 
Number 
Trusses Perimeter Columns Core Columns 
Case C Case D Case C Case D Case C Case D 
79 – – – – – – 
80 – – – – – – 
81 NE NE NE NE – NE 
82 E E E E E E 
83 E E – E – E 
Key: E, east; N, north; S, south; W, west; NE, northeast; NW, northwest. 
6.13 MEASUREMENT OF THE FIRE RESISTANCE OF THE FLOOR SYSTEM 
As described in Section 5.4.7, the composite floor system, composed of open-web, lightweight steel 
trusses topped with a slab of lightweight concrete, was an innovative feature. As further noted in 
Section 5.6.2, the approach to achieving the specified fire resistance for these floors was the use of a 
SFRM. Documents indicated that the fire performance of the composite floor system of the WTC towers 
was an issue of concern to the building owners and designers. However, NIST found no evidence 
regarding the technical basis for the selection of insulation material for the floor trusses or for the 
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insulation thickness to achieve a 2 hour rating. Further, NIST has found no evidence that fire resistance 
tests of the WTC floor system were conducted. 
Most of the possible building collapse sequences included some contribution from the floors, ranging 
from their ability to transfer load to their initiating the collapse by their failure. Thus, it became central to 
the Investigation to obtain data regarding the limits of the insulated floors in withstanding the heat from 
the fires. The standard test for determining the fire endurance of floor assemblies is ASTM E 119, 
“Standard Test Methods for Fire Tests of Building Construction and Materials.” The conduct of the test 
is described in Section 1.2.2 under "Fire Protection Systems." 
Accordingly, NIST contracted with Underwriters Laboratories, Inc. to conduct tests to obtain information 
on the fire endurance of trusses like those in the WTC towers. The objective was to understand the 
effects of three factors: 
• Scale of the test. There were no established facilities capable of testing the 60 ft lengths of 
the long spans that were used in the towers, but there is a history of testing reduced-scale 
assemblies and scaling them to practical dimensions. In the Investigation's tests, the fullscale 
test specimens were 35 ft long, equal to the shorter span between the core and the 
perimeter of the WTC towers. Their construction replicated, as closely as possible, the 
original short-span floors. The reduced-scale specimens were half that length and height. All 
assemblies were 14 ft wide. The simulation of a “maximum load condition,” as required by 
ASTM E 119, involved placing a combination of concrete blocks and containers filled with 
water on the top surface of the floor. The load on the shorter truss was double that of the 
longer truss. Traditionally, relatively small-scale assemblies have been tested and results 
have been scaled to practical floor system spans. 
• SFRM thickness. The Port Authority originally specified BLAZE-SHIELD Type D as the 
SFRM, applied to a ½ in. covering. The average measured thicknesses were found to be 
approximately 0.75 in. These two thicknesses of BLAZE-SHIELD DC/F were used in the 
Investigation tests. 
• Test restraint conditions. In 1971, well after the design of the towers was completed, the 
ASTM E 119 Standard began differentiating between thermally restrained and unrestrained 
floor assemblies. An unrestrained assembly is free to expand and rotate at its supports; a 
restrained assembly is not. It is customary in the United States to conduct standard fire tests 
of floor assemblies in the restrained condition. The current standard describes a means to 
establish unrestrained ratings for floor assemblies from restrained test samples. In practice, a 
floor assembly such as that used in the WTC towers is neither restrained nor unrestrained but 
is likely somewhere in between. Testing under both restraint conditions, then, is thought to 
bound performance under the standard fire exposure. In addition, it provides a comparison of 
unrestrained ratings developed from both restrained and unrestrained test conditions. 
The test plan included 4 tests, which varied the 3 factors: 
Test 1: 35 ft floor, ¾ in. insulation, restrained 
Test 2: 35 ft floor, ¾ in. insulation, unrestrained 
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Test 3: 17 ft floor, ¾ in. insulation, restrained 
Test 4: 17 ft floor, ½ in. insulation, restrained 
The results of the four tests are summarized as follows: 
• All four test assemblies were able to withstand standard fire conditions for between ¾ hour 
and 2 hours without exceeding the limits prescribed by ASTM E 119. 
• All four test specimens sustained the maximum design load for approximately 2 hours 
without collapsing. 
• The restrained full-scale floor system obtained a fire resistance rating of 1½ hours, while the 
unrestrained floor system achieved a 2 hour rating. Past experience with the ASTM E 119 
test method led investigators to expect the unrestrained floor assembly to receive a lower 
rating than the restrained assembly. 
• For assemblies with a ¾ in. SFRM thickness, the 17 ft assembly’s fire rating was 2 hours; the 
35 ft assembly’s rating was 1½ hours. This result raised the question of whether or not a fire 
rating of a 17 ft floor assembly is scalable to the longer spans in the WTC towers. 
• The specimen in Test 4, with a fire rating of ¾ hour, would not have met the 2 hour 
requirement of the NYC Building Code. 
The Investigation Team was cautious about using these results directly in the formulation of collapse 
hypotheses. In addition to the scaling issues raised by the test results, the fires in the towers on 
September 11, and the resulting exposure of the floor systems, were substantially different from the 
conditions in the test furnaces. Nonetheless, the results established that this type of assembly was capable 
of sustaining a large gravity load, without collapsing, for a substantial period of time relative to the 
duration of the fires in any given location on September 11. 
6.14 COLLAPSE ANALYSIS OF THE TOWERS 
6.14.1 Approach to Determining the Probable Collapse Sequences 
At the core of NIST’s reconstruction of the events of September 11, 2001, were the archive of 
photographic and video evidence, the observations of people who were on the scene, the assembled 
documents describing the towers and the aircraft, and Investigation-generated experimental data on the 
properties of construction and furnishing materials and the behavior of the fires. Information from all of 
these sources fed the computer simulations of the towers, the aircraft impacts, the ensuing fires and their 
heating of the structural elements, and the structural changes that led to the collapses of the towers. To 
the extent that the input information was complete and accurate, the output of the simulations would have 
provided definitive responses to the first three objectives of the Investigation. However, the available 
information, as extensive as it was, was neither complete nor of assured precision. As a result, the 
Investigation Team took steps to ensure that the conclusions of the effort were credible explanations for 
how the buildings collapsed and the extent to which the casualties occurred. 
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One principal step was the determination of those variables that most affected the outcome of the various 
computer simulations. Sensitivity studies and examination of components and subsystems were carried 
out for the modeling of the aircraft impact, the fires, and the structural response to impact damage and 
fires. For each of the most influential variables, a central or middle value and reasonable high and low 
values were identified. Further computations refined the selection of these values. The computations also 
were improved to include physical processes that could play a significant role in the structural 
degradation of the towers. 
The Investigation Team then defined three cases for each building by combining the middle, less severe, 
and more severe values of the influential variables. Upon a preliminary examination of the middle cases, 
it became clear that the towers would likely remain standing. The less severe cases were discarded after 
the aircraft impact results were compared to observed events. The middle cases (which became Case A 
for WTC 1 and Case C for WTC 2) were discarded after the structural response analysis of major 
subsystems were compared to observed events. The more severe case (which became Case B for WTC 1 
and Case D for WTC 2) was used for the global analysis of each tower. 
Complete sets of simulations were then performed for Cases B and D. To the extent that the simulations 
deviated from the photographic evidence or eyewitness reports, the investigators adjusted the input, but 
only within the range of physical reality. Thus, for instance, the observed window breakage was an input 
to the fire simulations and the pulling forces on the perimeter columns by the sagging floors were 
adjusted within the range of values derived from the subsystem computations. 
The results were a simulation of the structural deterioration of each tower from the time of aircraft impact 
to the time at which the building became unstable, i.e., was poised for collapse. Cases B and D 
accomplished this in a manner that was consistent with the principal observables and the governing 
physics. 
6.14.2 Results of Global Analysis of WTC 1 
After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed 
to other columns. The north wall lost about 7 percent of its loads after impact. Most of the load was 
transferred by the hat truss, and the rest was redistributed to the adjacent exterior walls by spandrels. Due 
to the impact damage and the tilting of the building to the north after impact, the south wall also lost 
gravity load, and about 7 percent was transferred by the hat truss. As a result, the east and west walls and 
the core gained the redistributed loads through the hat truss. 
Structural steel expands when heated. In the early stages of the fire, structural temperatures in the core 
rose, and the resulting thermal expansion of the core was greater than the thermal expansion of the 
(cooler) exterior walls. About 20 min. after the aircraft impact, the difference in the thermal expansion 
between the core and exterior walls, which was resisted by the hat truss, caused the core column loads to 
increase. As the fires continued to heat the core areas without insulation, the columns were thermally 
weakened and shortened and began to transfer their loads to the exterior walls through the hat truss until 
the south wall started to bow inward. At about 100 min, approximately 20 percent of the core loads were 
transferred by the hat truss to the exterior walls due to thermal weakening of the core; the north and south 
walls each gained about 10 percent more loads, and the east and west walls each gained about 25 percent 
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higher loads. Since the hat truss outriggers to the east and west walls were stiffer than the outriggers to 
the north and south walls, they transferred more loads to the east and west exterior walls. 
The inward bowing of the south wall caused failure of exterior column splices and spandrels, and these 
columns became unstable. The instability spread horizontally across the entire south face. The south 
wall, now unable to bear its gravity loads, redistributed these loads to the thermally weakened core 
through the hat truss and to the east and west walls through the spandrels. The building section above the 
impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to 
carry the increased loads. This further increased the gravity loads on the core columns. Once the upper 
building section began to move downwards, the weakened structure in the impact and fire zone was not 
able to absorb the tremendous energy of the falling building section and global collapse ensued. 
6.14.3 Results of Global Analysis of WTC 2 
Before aircraft impact, the load distribution across the exterior walls and core was symmetric with respect 
to the centerline of each exterior wall. After aircraft impact, the exterior column loads on the south side 
of the east and west walls and on the east side of south wall increased. This was due to the leaning of the 
building towards the southeast. After aircraft impact, the core carried 6 percent less load. The north wall 
load reduced by 6 percent and the east face load increased by 24 percent. The south and west walls 
carried 2 percent to 3 percent more load. 
In contrast to the fires in WTC 1, which generally progressed from the north side to the south side over 
approximately an hour, the fires in WTC 2 were located on the east side of the core and floors the entire 
time, with the fires spreading somewhat from south to north. With insulation dislodged over much of the 
same area, the structural temperatures became elevated in the core, floors, and exterior walls at similar 
times. During the early stages of the fires, columns with dislodged insulation elongated due to thermal 
expansion. As the structural temperatures continued to rise, the columns thermally weakened and 
consequently shortened. 
The south exterior wall displaced downward following the aircraft impact, but did not displace further 
until the east wall became unstable 43 min later. The inward bowing of the east wall caused failure of 
exterior column splices and spandrels and resulted in the east wall columns becoming unstable. The 
instability progressed horizontally across the entire east face. The east wall, now unable to bear its 
gravity loads, redistributed them to the thermally weakened core through the hat truss and to the east and 
west walls through the spandrels. 
The building section above the impact zone began tilting to the east and south as column instability 
progressed rapidly from the east wall along the adjacent north and south walls, and increased the gravity 
load on the weakened east core columns. As with WTC 1, once the upper building section began to move 
downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous 
energy of the falling building section and global collapse ensued. 
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6.14.4 Structural Response of the WTC Towers to Fire Without Impact or Insulation 
Damage 
To complete the assessment of the relative roles of aircraft impact and ensuing fires, NIST examined 
whether an intense, but conventional, fire, occurring without the aircraft impact, could have led to the 
collapse of a WTC tower, were it in the same condition as it was on September 10, 2001. The 
characteristics of such an intense, conventional fire could have been: 
• Ignition on a single floor by a small bomb or other explosion. If arson were involved, there 
might have been multiple small fires ignited on a few floors. 
• Air supply determined by the building ventilation system. 
• Moderate fire growth rate. In the case of arson, several gallons of an accelerant might have 
been applied to the building combustibles, igniting the equivalent of several workstations. 
• Water supply to the sprinklers and standpipes maliciously compromised. 
• Intact structural insulation and interior walls. 
The four cases described in this chapter represented fires that were far more severe than this: 
• About 10,000 gallons of jet fuel were sprayed into multiple stories, quickly and 
simultaneously igniting hundreds of workstations. 
• The aircraft and subsequent fireballs created large open areas in the building exterior through 
which air could flow to support the fires. 
• The impact and debris removed the insulation from a large number of structural elements that 
were then subjected to the heat from the fires. 
Additional findings from the Investigation showed that: 
• Both the results of the multiple workstation experiments and the simulations of the WTC fires 
showed that the combustibles in a given location, if undisturbed by the aircraft impact, would 
have been almost fully burned out in about 20 min. 
• In the simulations of Cases A through D, none of the columns and trusses for which the 
insulation was intact reached temperatures at which significant loss of strength occurred. 
• Both WTC 1 and WTC 2 were stable after the aircraft impact, standing for 102 min and 
56 min, respectively. The global analyses with structural impact damage showed that both 
towers had considerable reserve capacity. This was confirmed by analysis of the post-impact 
vibration of WTC 2, the more severely damaged building, where the damaged tower 
oscillated at a period nearly equal to the first mode period calculated for the undamaged 
structure. 
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• Computer simulations, supported by the results of large-scale fire tests and furnace testing of 
floor subsystems, showed that insulated structural steel, when coated with the average 
installed insulation thickness of ¾ in., would have withstood the heat from nearby fires for a 
longer time than the burnout time of the combustibles. Simulations also showed that 
variations in thickness resulting from normal application, even with occasional gaps in 
coverage, would not have changed this result. 
• Cases A and C, which combined aircraft damage and multifloor fires (more severe than the 
intense, conventional fire) did not predict either building becoming unstable. 
From these, NIST concluded: 
• An intense, conventional fire, in the absence of structural and insulation damage, would not 
have led to the collapse of a WTC tower. 
• The existing condition of the insulation prior to aircraft impact, which was found to be mostly 
intact, and the insulation thickness on the WTC floor system did not play a significant role in 
initiating collapse of the towers. 
• The towers would not have collapsed under the combined effects of aircraft impact and the 
subsequent multifloor fires if the insulation had not been widely dislodged or had been only 
minimally dislodged by aircraft impact. 
6.14.5 Probable WTC 1 Collapse Sequence 
Aircraft Impact Damage 
• The aircraft impact severed a number of exterior columns on the north wall from the 93rd to 
the 98th floors, and the wall section above the impact zone moved downward. 
• After breaching the building’s perimeter, the aircraft continued to penetrate into the building, 
severing floor framing and core columns at the north side of the core. Core columns were 
also damaged toward the center of the core. Insulation was damaged from the impact area to 
the south perimeter wall, primarily through the middle one-third to one-half of the core width. 
Finally, the aircraft debris removed a single exterior panel at the center of the south wall 
between the 94th and 95th floors. 
• The impact damage to the exterior walls and to the core resulted in redistribution of severed 
column loads, mostly to the columns adjacent to the impact zones. The hat truss resisted the 
downward movement of the north wall. 
• Loads on the damaged core columns were redistributed mostly to adjacent intact core 
columns and to a lesser extent to the north perimeter columns through the core floor systems 
and the hat truss. 
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• As a result of the aircraft impact damage, the north and south walls each carried about 
7 percent less gravity load after impact, and the east and west walls each carried about 
7 percent more load. The core carried about 1 percent more gravity load after impact. 
Thermal Weakening of the Structure 
• Under the high temperatures and stresses in the core area, the remaining core columns with 
damaged insulation were thermally weakened and shortened, causing the columns on the 
floors above to move downward. The hat truss resisted the core column shortening and 
redistributed loads to the perimeter walls. The north and south walls' loads increased by 
about 10 percent, and the east and west walls' loads increased by about 25 percent, while the 
core's loads decreased by about 20 percent. 
• The long-span sections of the 95th to 99th floors on the south side weakened with increasing 
temperatures and began to sag. Early on, the floors on the north side had sagged and then 
contracted as the fires moved to the south and the floors cooled. As the fires intensified on 
the south side, the floors there sagged, and the floor connections weakened. About 
20 percent of the connections on the south side of the 97th and 98th floors failed. 
• The sagging floors with intact floor connections pulled inward on the south perimeter 
columns, causing them to bow inward. 
Collapse Initiation 
• The bowed south wall columns buckled and were unable to carry the gravity loads. Those 
loads shifted to the adjacent columns via the spandrels, but those columns quickly became 
overloaded as well. In rapid sequence, this instability spread all the way to the east and west 
walls. 
• The section of the building above the impact zone (near the 98th floor), acting as a rigid block, 
tilted at least 8 degrees to the south. 
• The downward movement of this structural block was more than the damaged structure could 
resist, and global collapse began. 
6.14.6 Probable WTC 2 Collapse Sequence 
Aircraft Impact Damage 
• The aircraft impact severed a number of exterior columns on the south wall from the 
78th floor to the 84th floor, and the wall section above the impact zone moved downward. 
• After breaching the building’s perimeter, the aircraft continued to penetrate into the building, 
severing floor framing and core columns at the southeast corner of the core. Insulation was 
damaged from the impact area through the east half of the core to the north and east perimeter 
walls. The floor truss seat connections over about one-fourth to one-half of the east side of 
the core were severed on the 80th and 81st floors and over about one-third of the east 
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perimeter wall on the 83rd floor. The debris severed four columns near the east corner of the 
north wall between the 80th and 82nd floors. 
• The impact damage to the perimeter walls and to the core resulted in redistribution of severed 
column loads, mostly to the columns adjacent to the impact zones. The impact damage to the 
core columns resulted in redistribution of severed column loads, mainly to other intact core 
columns and the east exterior wall. The hat truss resisted the downward movement of the 
south wall. 
• As a result of the aircraft impact damage, the core carried about 6 percent less gravity load. 
The north wall carried about 10 percent less, the east face carried about 24 percent more, and 
the west and south faces carried about 3 percent and two percent more, respectively. 
• The core was then leaning slightly toward the south and east perimeter walls. The perimeter 
walls restrained the tendency of the core to lean via the hat truss and the intact floors. 
Thermal Weakening of the Structure 
• Under the high temperatures and stresses in the core area, the remaining core columns with 
damaged insulation were thermally weakened and shortened, causing the columns on the 
floors above to move downward. 
• At this point, the east wall carried about 5 percent more of the gravity loads, and the core 
carried about 2 percent less. The other three walls carried between 0 percent and 3 percent 
less. 
• The long-span floors on the east side of the 79th to 83rd floors weakened with increasing 
temperatures and began to sag. About one-third of the remaining floor connections to the 
east perimeter wall on the 83rd floor failed. 
• Those sagging floors whose seats were still intact pulled inward on the east perimeter 
columns, causing them to bow inward. The inward bowing increased with time. 
Collapse Initiation 
• As in WTC 1, the bowed columns buckled and became unable to carry the gravity loads. 
Those loads shifted to the adjacent columns via the spandrels, but those columns quickly 
became overloaded. In rapid sequence, this instability spread all across the east wall. 
• Loads were transferred from the failing east wall to the weakened core through the hat truss 
and to the north and south walls through the spandrels. The instability of the east face spread 
rapidly along the north and south walls. 
• The building section above the impact zone (near the 82nd floor) tilted 7 to 8 degrees to the 
east and 3 to 4 degrees to the south prior to significant downward movement of the upper 
building section. The tilt to the south did not increase any further as the upper building 
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section began to fall, but the tilt to the east was seen to increase to 20 degrees until dust 
clouds obscured the view. 
• The downward movement of this structural block was more than the damaged structure could 
resist, and the global collapse began. 
6.14.7 Accuracy of the Probable Collapse Sequences 
Independent assessment of the validity of the key steps in the collapses of the towers was a challenging 
task. Some of the photographic information had been used to direct the simulations. For example, the 
timing of the appearance of broken windows was an input to the fire growth modeling. However, there 
were significant observables that were usable as corroborating evidence, as shown in Tables 6–10 and 
6–11. Some of these were used to establish the quality of the individual simulations of the aircraft impact 
and the fire growth, as described in Sections 6.9 and 6.10. While the agreement between observations 
and simulation was not exact, the differences were within the uncertainties in the input information. The 
generally successful comparisons lent credibility to the overall reconstruction of the disaster. 
There remained a small, but important number of observations against which the structural collapse 
sequences could be judged. The comparisons are for Cases B and D impact damage and temperature 
histories, for which the better agreement was obtained. 
The agreement between the observations and the simulations is reasonably good, supporting the validity 
of the probable collapse sequences. The exact times to collapse initiation were sensitive to the factors that 
controlled the inward bowing of the exterior columns. The sequence of events leading to collapse 
initiation was not sensitive to these factors. 
Table 6–10. Comparison of global structural model predictions and 
observations for WTC 1, Case B. 
Observation Simulation 
Following the aircraft impact, the tower still stood. The tower remained upright with significant reserve 
capacity. 
The south perimeter wall was first observed to have 
bowed inward at 10:23 a.m. The bowing appeared over 
nearly the entire south face of the 94th to 100th floors. 
The maximum bowing was 55 in. on the 97th floor. (The 
central area in available images was obscured by 
smoke.) 
The inward bowing of the south wall at 10:28 a.m. It 
extended from the 94th to the 100th floor, with a 
maximum of about 43 in. 
As the structural collapse began, the building section 
above the impact and fire zone tilted at least 8 degrees to 
the south with no discernable east or west component in 
the tilt. Dust clouds obscured the view as the building 
section began to fall downward. 
The south side bowed and weakened. The analysis 
stopped as the initiation of global instability was 
imminent. 
The time to collapse initiation was 102 min from the 
aircraft impact. 
There was significant weakening of the south wall and 
the core columns. Instability was imminent at 100 min. 
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Draft for Public Comment Reconstruction of the Collapses 
Table 6–11. Comparison of global structural model predictions and 
observations for WTC 2, Case D. 
Observation Simulation 
Following the aircraft impact, the tower still stood. The tower remained upright with significant reserve 
capacity. 
The east perimeter wall was first observed to have 
bowed inward approximately 10 in. at floor 80 at 
9:21 a.m. The bowing extended across most of the east 
face between the 78th and 83rd floors. 
The inward bowing of the east wall had a maximum 
value of about 9.5 in. at 9:23 a.m. The bowing extended 
from the 78th floor to the 83rd floor. 
The building section above the impact and fire area tilted 
to the east and south as the structural collapse initiated. 
The angle was approximately 3 to 4 degrees to the south 
and 7 to 8 degrees to the east prior to significant 
downward movement of the upper building section. The 
tilt to the south did not increase as the upper building 
section began to fall, but the tilt to the east rose to 
approximately 25 degrees before dust clouds obscured 
the view. 
At point of instability, there was tilting to the south and 
east. 
The time to collapse initiation was 56 min after the 
aircraft impact. 
The analysis predicted global instability after 43 min. 
6.14.8 Factors that Affected Building Performance on September 11, 2001 
• The unusually dense spacing of perimeter columns, coupled with deep spandrels, resulted in a 
robust building that was able to fragment the aircraft upon impact and redistribute loads from 
severed perimeter columns to adjacent, intact columns. 
• The wind loads used for the WTC towers, which governed the design of the framed-tube 
system, significantly exceeded the requirements of the building codes of the era and were 
consistent with the independent NIST estimates that were based on current state-of-the-art 
considerations. 
• The robustness of the perimeter framed-tube system and the large lateral dimension of the 
towers helped the buildings withstand the impact of the aircraft. 
• The composite floor system enabled the floors to redistribute loads from places of aircraft 
impact damage to other locations, avoiding larger scale collapse upon impact. 
• The hat truss resisted the significant weakening of the core by redistributing loads form the 
damaged columns to intact columns. 
As a result of these factors, the buildings would likely not have collapsed under the combined effects of 
the aircraft damage and subsequent fires, if the insulation had not been widely dislodged. The thickness 
and the condition of the insulation prior to aircraft impact did not play a significant role in the initiation of 
building collapse. 
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Chapter 7 
RECONSTRUCTION OF HUMAN ACTIVITY 
7.1 BUILDING OCCUPANTS 
7.1.1 Background 
While much attention has properly focused on the nearly three thousand people who lost their lives at the 
World Trade Center (WTC) site that day, the circumstances and efforts that led to the successful 
evacuation of five times that many people from the WTC towers also have been given attention. 
Understanding why the loss of life was high or low was one of the four objectives of the Investigation. 
Success in clearing a building in an emergency can be characterized by two quantities: the time people 
need to evacuate and the time available to them to do so. For the World Trade Center towers, the times 
available for escape were set by the collapses of the buildings. Neither the building occupants nor the 
emergency responders knew those times in advance. Moreover, the times were also three or four times 
shorter than the time needed to clear the tenant spaces of WTC 1 following the 1993 bombing. 
The investigators examined the design of the buildings, the behavior of the people, and the evacuation 
process in detail to ascertain the factors that figured prominently in the time needed for evacuation. In 
analyzing these factors, NIST recognized that there were inherent uncertainties in constructing a valid 
portrayal of human behavior on that day. These included limitations in the recollections of the people, the 
need to derive findings from a statistical sampling of the building population, the lack of information 
from the decedents on the factors that prevented their escape, and the limited knowledge of the damage to 
the interior of the towers. NIST carefully considered these uncertainties in developing its findings and is 
confident in those findings and related recommendations. 
7.1.2 The Building Egress System 
Examination of drawings, memoranda, and calculations showed that the standard emergency evacuation 
procedures required using the three stairwells. The elevators were not to be used, and the doors to the 
roof were to be kept locked. Under most circumstances, a local evacuation would be ordered. The people 
on the floors near the threat would move to three floors below the incident. Under more severe 
circumstances, a full building evacuation would be ordered, requiring all occupants to leave the building 
by way of the stairwells. 
As noted in Section 1.2.2, the locations of the stairwells differed at various heights in the buildings. This, 
combined with the aircraft impacting different floors in the two towers, the different aircraft impact 
location relative to the center of the building, and the different orientation of the core (Section 1.2.2), led 
to different damage to the stairwells. As shown in Figure 7-1, a frame from an ARA simulation 
(Section 6.9), the stairwell separation in this region of WTC 1 was the smallest in the building—clustered 
together well within the building core—and American Airlines Flight 11 destroyed all three stairwells 
from the 92nd floor upward. By contrast, the separation of the stairwells in the impacted region of WTC 2 
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Chapter 7 Draft for Public Comment 
was the largest in the building, i.e., they were located along different boundaries of the building core. 
United Airlines Flight 175 destroyed Stairwells B and C, but not Stairwell A (Figure 7-2). 
Figure 7–1. Simulated impact damage to 95th floor of WTC 1, including stairwells, 0.7 s 
after impact. 
152 
Figure 7–2. Simulated impact 
damage to WTC 2 on floor 78, 
0.62 s after impact. 
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7.1.3 The Evacuation—Data Sources 
To document the egress from the two towers as completely as possible, NIST: 
• Contracted with the National Fire Protection Association and the National Research Council 
of Canada to index a collection of over 700 previously published interviews with WTC 
survivors. 
• Listened to and analyzed 9-1-1 emergency phone calls made during the morning of 
September 11. 
• Analyzed transcripts of emergency communication among building personnel and emergency 
responders. 
• Examined complaints filed with the Occupational Safety and Health Administration by 
surviving occupants and families of victims regarding emergency preparedness and 
evacuation system performance. 
In addition NIST, in conjunction with NuStats, Partners, LLP, conducted an extensive set of interviews 
with survivors of the disaster and family members of occupants of the buildings. First, telephone 
interviews were conducted with 803 survivors, randomly selected from the list of approximately 100,000 
people who had badges to enter the towers on that morning. The results enabled a scientific projection of 
the population and distribution of occupants in WTC 1 and WTC 2, as well as exploration of factors that 
affected evacuation. Second, 225 face-to-face interviews, averaging 2 hours each, gathered detailed, firsthand 
accounts and observations of the activities and events inside the buildings on the morning of 
September 11. These people included occupants near the floors of impact, witnesses to fireballs, 
mobility-challenged occupants, floor wardens, building personnel with emergency response 
responsibilities, family members who spoke to an occupant after 8:46 a.m., and occupants from regions of 
the building not addressed by other groups. Third, six complementary focus groups, a total of 28 people, 
were convened, consisting of: 
1. Occupants located near the floors of impact, to explore the extent of the building damage and 
how the damage influenced the evacuation process. 
2. Floor wardens, to explore the implementation of the floor warden procedures and the effect 
those actions had on the evacuation of the occupants on a floor and the evacuation of the floor 
warden. 
3. Mobility challenged occupants, to explore the effect of a disability on the evacuation of the 
occupant and any other individuals who may have assisted or otherwise been affected by the 
evacuee. 
4. Persons with building responsibilities, to capture the unique perspective of custodians, 
security, maintenance, or other building staff. 
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5. Randomly selected evacuees in WTC 1, to explore further the variables that best explained 
evacuation delay and normalized stairwell evacuation time, including environmental cues, 
floor, and activities. 
6. Randomly selected evacuees in WTC 2, for the same purpose as the preceding group. 
The following sections describe the key findings from this large data set. 
7.1.4 Occupant Demographics 
The following were estimated from statistical analysis of the telephone interview data: 
• There were 17,400 ± 1,180 occupants inside WTC 1 and WTC 2 at 8:46 a.m. Of these, 
8,900 ± 750 were inside WTC 1 and 8,540 ± 920 were inside WTC 2. 
• Men outnumbered women roughly two to one. 
• The mean and median ages were both about 45, with the distribution ranging from the early 
20s to the late 80s. 
• The mean length of employment at the WTC was almost six years, but the median was only 
two years tenure within WTC 1 and three years within WTC 2. 
• Sixteen percent of the evacuees were present during the 1993 bombing, although many others 
knew of the evacuation. 
• Two-thirds had participated in at least one fire drill in the 12 months prior to the 2001 
disaster. Eighteen percent did not recall whether they had participated or not; 18 percent 
reported that they had not. New York City law prohibited requiring full evacuation using the 
stairs during fire drills. 
• Six percent reported having a limitation that constrained their ability to escape. (This 
extrapolated to roughly 1,000 of the WTC 1 and WTC 2 survivors.) The most common of 
these limitations, in decreasing order, were recent injury, chronic illness, and use of 
medications. 
Estimates based on the layouts of the tenant spaces indicated that approximately 20,000 people worked in 
each tower. A typical number of visitors and tourists was estimated to increase the total occupancy to as 
high as 25,000. Relatively few visitors would have been present at 8:46 a.m. Thus, the towers were 
about one-third full at the time of the attack. 
7.1.5 Evacuation of WTC 1 
The number of survivors evacuated from WTC 1 was large, given the severity of the building damage and 
the unexpectedly short available time. Of those who were below the impact floors when the aircraft 
struck, 99 percent survived. About 84 percent of all the occupants of the tower at the time survived. The 
aircraft impact damage left no exit path for those who were above the 91st floor. It is not known how 
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Draft for Public Comment Reconstruction of Human Activity 
many of those could have been saved had the building not collapsed. While it is possible that a delayed 
or avoided collapse could have improved the outcome, it would have taken many hours for the FDNY to 
reach the 92nd floor and higher and then to conduct rescue and fire suppression activity there. 
The general pattern of the evacuation was described in Chapter 2. The following are specific facts 
derived from the interviews: 
• The median time to initiate evacuation was 3 min for occupants from the ground floor to floor 
76, and 5 min for occupants near the impact region (floors 77 through 91). The factors that 
best explained the evacuation initiation delays were the floor the respondent was on when 
WTC 1 was attacked, whether the occupant encountered smoke, damage or fire, and whether 
he or she sought additional information about what was happening. 
• Occupants throughout the building observed various types of impact indicators throughout 
the building, including wall, partition, and ceiling damage and fire and smoke conditions. 
The filled-in squares in Figure 7–3 indicate the floors on which the different observations 
were reported. 
• Damage to critical communications hardware likely prevented announcement transmission, 
and thus occupants did not hear announcements to evacuate, despite repeated attempts from 
the lobby fire command station. 
• Evacuation rates reached a maximum in approximately 5 min, and remained roughly constant 
until the collapse of WTC 2, when the rate in WTC 1 slowed to about 20 percent of the 
maximum. 
• The maximum downward travel rate was just over one floor per minute, slower than the 
slowest speed measured for non-emergency evacuations. This was in part because: 
- Occupants encountered smoke and/or damage during evacuation. 
- Occupants were often unprepared for the physical challenge of full building evacuation. 
- Occupants were not prepared to encounter transfer hallways during the descent. 
- Mobility impaired occupants were not universally identified or prepared for full building 
evacuation. 
- Occupants interrupted their evacuation. 
• The mobility impaired occupants did not evacuate as evenly as the general population. 
- Those who were ambulatory generally walked down the stairs with one hand on each 
handrail, taking one step at a time. They were typically accompanied by another 
occupant or an emergency responder. Combined, they blocked others behind them from 
moving more rapidly. 
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- On the 12th floor, FDNY personnel found 40 to 60 people, some of whom were mobilityimpaired. 
The emergency responders were assisting about 20 of these mobility-impaired 
people down the stairs just prior to the collapse of the building. It is unknown how many 
of this group survived. 
- Some mobility impaired occupants requiring assistance to evacuate were left by 
coworkers, thereby imposing on strangers for assistance. 
7.1.6 Evacuation of WTC 2 
The evacuation from WTC 2 was markedly different from that from WTC 1. Over 90 percent of the 
occupants had started to self-evacuate before the second aircraft struck, and three-quarters of those from 
above the 78th floor had descended below the impact region prior to the second attack. (Nearly 3,000 
occupants were able to survive due to self-evacuation and the use of the still-functioning elevators.) As a 
result, 91 percent of all the occupants survived. Eleven people from below the impact floors perished, 
about 0.1 percent. Eighteen people in or above the impact zone when the plane struck are known to have 
found the one passable stairway and escaped. It is not known how many others from the impact floors or 
above found their way to the passable stairway and did not make it out or how many could have been 
saved had the building not collapsed. A delayed or avoided collapse could have provided the additional 
time for more people to learn about and use the passable stairway. 
The general pattern of the evacuation was described in Chapter 3. The following are specific facts 
derived from the interviews: 
• The median time to initiate evacuation was 6 min, somewhat longer than in WTC 1. 
• As in WTC 1, occupants observed various types of impact indicators throughout the building, 
including wall, partition, and ceiling damage and fire and smoke conditions (Figure 7–4). 
• Building announcements were cited by many as a constraint to their evacuation, principally 
due to the 9:00 a.m. announcement instructing occupants to return to their work spaces. 
Crowdedness in the stairways, lack of instructions and information, as well as injured or 
disabled evacuees in the stairwells were the most frequently reported obstacles to evacuation. 
• Evacuation rates from WTC 2 showed three distinct phases: 
(1) Before WTC 2 was attacked, occupants used elevators, as well as stairs, to evacuate, 
resulting in approximately 40 percent of the eventual survivors leaving the building during 
that 16 min window. 
(2) After WTC 2 was attacked and the elevators were no longer operational, the evacuation 
rate slowed down to a steady rate equivalent to the rate observed in WTC 1, which also had 
only stairs available to occupants. 
(3) About 20 min prior to building collapse, the rate in WTC 2 slowed to approximately 
20 percent of the stairwell-only evacuation rate. 
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Figure 7–3. Observations of building damage after initial awareness but before beginning evacuation in WTC 1 
NIST NCSTAR 1, WTC Investigation 

Chapter 7 Draft for Public Comment 
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Figure 7–4. Observations of building damage from tenant spaces in WTC 2. 
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7.2 EMERGENCY RESPONDERS 
7.2.1 Data Gathered 
The attack on the World Trade Center produced a massive response from the emergency services within 
New York City. As a result, copious information was produced concerning the attack and the emergency 
response. Although some key information was lost when the buildings collapsed, an extensive amount 
was obtained from three organizations that contributed to the emergency response: the Port Authority, 
The Fire Department of the City of New York (FDNY), and the New York City Police Department 
(NYPD). There also was a significant amount of information available through the various media 
services. Some of the items were transferred to NIST; the Investigation Team examined others at 
locations in the New York City area. The data fell into four categories. 
Documentary Data 
This included procedures for conducting operations at the WTC, records generated during the WTC 
operations, and records generated following the event. The last group of documents included detailed 
investigative reports of the FDNY and NYPD operations by McKinsey and Company, documents of 
investigative first-person interviews, and lists of decedents. 
Electronic Data 
These were recordings of radio and telephone communications. Some were already in digital format; 
those on tape were digitized and/or transcribed. Some recordings required sound enhancement to 
improve comprehension. 
First-Person Interviews 
In October 2003, NIST entered into a three-party agreement between NIST, New York City (NYC), and 
the National Commission on Terrorist Acts Upon the United States (the 9/11 Commission). The 
agreement provided procedures under which NIST and the 9/11 Commission would interview a 
maximum of 125 NYC emergency responders, 100 from FDNY and 25 from NYPD. In December 2003, 
NIST officially requested and the Port Authority agreed to interviews with 15 Port Authority personnel, 
including emergency responders, safety, security, and management personnel. In addition to the 
interviews conducted under the agreements described above, NIST interviewed eight people who 
contacted NIST directly and volunteered. The first-person interviews were conducted beginning in 
October 2003 and were completed in December 2004. 
The organizations and the number of interviews conducted were: 
• FDNY (68 interviews): Senior management and officers, mid-level officers, company 
officers, firefighters, emergency medical personnel, and dispatchers 
• NYPD (25 interviews): Senior management and officers, mid-level officers, Emergency 
Service Unit personnel, aviation personnel, and dispatchers 
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• PANYNJ/PAPD (15 interviews): Senior management personnel, facility safety personnel, 
building security personnel, facility communications personnel, building vertical 
transportation personnel, senior PAPD officers, mid-level PAPD officers, and line PAPD 
officers 
• Other (8 interviews): A building security guard, dispatcher, firefighters, WTC building 
engineer, and a fire safety director 
Each interview generally took from 1 to 4 hours to complete, depending on the person’s job and the 
complexity of their involvement in emergency operations. 
An interview included a self-narrative regarding the emergency responder's experience at the WTC and 
follow-up questions by staff from NIST and the 9/11 Commission. 
Visual Data 
These still photographs and video footage became part of the collection described in Section 6.3. 
7.2.2 Operations Changes Following the WTC 1 Bombing on February 26, 1993 
This unprecedented act had provided insight into the complex nature of responding to a large incident at 
the WTC towers. As a result, numerous issues were raised concerning the WTC buildings in relationship 
to the emergency response. A multiagency study identified issues of security, occupant safety, and 
emergency responder operations and safety. The following changes made by The Port Authority and the 
FDNY had a direct impact on emergency responder operations on September 11, 2001. 
The Port Authority 
• Improved egress from the towers at the Concourse Level. 
• Made improvements to the stairwells: battery operated emergency lighting, photoluminescent 
floor strips indicating the path to be followed, and explicit signs on each doorway to indicate 
where it led. 
• Established a PAPD Command Center inside of WTC 5. 
• Installed Fire Command Desks in the lobbies of WTC 1 and WTC 2. 
• Installed in WTC 5 a radio repeater that operated on the FDNY city-wide high-rise frequency. 
(The radio repeater 's function was to receive FDNY radio communication on a specified 
radio frequency, amplify the signal power, and retransmit the radio communications on 
another specified radio frequency that the FDNY radios could receive. This could enhance 
communications in buildings made of steel and reinforced concrete that pose challenges to 
radio-frequency communication.) The antenna was located on the top of WTC 5 and was 
directed at WTC 1 and WTC 2 (Figure 7-5). On September 11, 2001, the controls for 
operating the repeater were located at the Fire Command Desks in the tower lobbies. 
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• Upgraded the elevator intercom system to be 
monitored at the lobby Fire Command Desks. 
• Constructed an Operations Control Center on 
the B1 level of WTC 2 with the capability to 
monitor all HVAC systems and elevators. 
• Installed a decentralized fire alarm system, 
with three separate data risers to transponders 
located every three floors, redundant control 
panels and electronics, and multiple control 
station announcement capability.
• Conducted fire drills in conjunction with 
FDNY. 
Source: Original artwork by Marco Crupi. 
Enhanced by NIST. 
FDNY Figure 7–5. Location of the radio 
repeater. 
• Published a new Incident Command System
manual in May 1997.
• Purchased eighty 800 MHz radios for use by deputy fire commissioners, each staff chief, and 
the Field Communications Unit. Twenty of the radios were to be distributed by the Field 
Communications unit at an incident, if needed. 
• Issued Port Authority radios to those FDNY companies located near the WTC that often 
responded to the WTC, allowing them to communicate with the building’s Deputy Fire 
Safety Directors and with PAPD. 
In addition, The Port Authority and New York City signed two agreements applying to the fire safety of 
Port Authority facilities located in New York City. The first agreement was for the implementation of 
fire safety recommendations that would be made by FDNY after they had inspected Port Authority 
facilities located in New York City. The second recognized the agreement that FDNY could conduct fire 
safety inspections of Port Authority properties in New York City. It provided guidelines for FDNY to 
communicate needed corrective actions to The Port Authority, and it assured that new or modified fire 
safety systems were to be in compliance with local codes and regulations. It also required a third party 
review of the systems by a New York State licensed architect or engineer. 
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7.2.3 Responder Organization 
The emergency response to the attack was 
immediate. Within 3 min of the aircraft impact 
on WTC 1, PAPD was providing information 
on the attack to the police desk, FDNY had 
dispatched 26 units to the scene, and NYPD 
had called a department mobilization that 
included dispatching aviation units to the WTC 
for visual assessment. Within 10 min, PAPD 
had called a chemical mobilization; NYPD had 
dispatched five Emergency Service Unit (ESU) 
teams and had two aviation units at the scene 
providing observations. Within 30 min, 
121 FDNY units had been dispatched to the 
scene and 30 units had signaled their arrival at Figure 7–6. Timing of FDNY unit arrivals. 
the scene by pushing the “10-84” button on the 
vehicle's communications console (Figure 7–6). 
3 
26 35 
66 
121 
171 
214 
0 0 6 
23 30 
74 
103 
0 
50 
100 
150 
200 
250 
Time 
8:46 8:48 8:50 9:00 9:15 9:59 10:29 
Number of Units 
Dispatched Signal Arrival 10-84 
FDNY 
Under New York City policy, since this was identified as a fire incident, FDNY was to be in control of 
the site. By 8:50, FDNY was operating from the Fire Command Desk in the lobby of WTC 1. Within 
minutes, the Incident Command Post was moved outside to West Street. The FDNY also maintained the 
lobby Command Post inside WTC 1 and established one in WTC 2. Additional command posts were 
established in the lobby of the Marriott Hotel (WTC 3) and on the corner of West and Liberty Streets. 
Some of the first FDNY personnel on the scene had actually seen the aircraft hit the building and knew 
that the upper floors were badly damaged, including the building safety systems. They also saw the 
victims burned by the fireball that came into the building lobby. Upon meeting with Port Authority 
personnel and the WTC 1 Deputy Fire Safety Director to learn more about building conditions, FDNY 
personnel quickly made judgments related to building conditions and emergency response operations that 
were, in retrospect, highly accurate, e.g.: 
• There were large fires burning on multiple floors at and above the impact zone. 
• Smoke, fire, and structural damage in the buildings prevented many building occupants from 
evacuating floors above the impact zones. 
• Many of the people trapped above the impact zones were already dead or would likely die 
before emergency responders could reach them. 
• Localized collapses within and above the impact zones were possible due to the structural 
damage and fires. 
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• The elevators, some with people trapped inside, were generally not working and/or were not 
safe for use during the WTC operations. 
• Firefighters would have to gain access to the injured and trapped occupants by climbing the 
stairs and carrying the equipment needed up the stairs. 
• It would take hours to accumulate sufficient people and equipment to access the impact 
zones. 
• The sprinkler and standpipe systems were compromised at the impact zone and firefighting 
would not be an option until a reliable water supply was established and equipment was 
carried up. 
• Jet fuel had flowed into the elevator shafts and into other parts of the buildings and presented 
a danger to building occupants and emergency responder personnel. 
Those in command decided that the response strategy was to enable the evacuation of those below the 
impact and fire zones. However, those directing initial operations inside the buildings followed an 
additional strategy: get sufficient people and equipment upstairs to cut a path through the fire and debris 
to rescue occupants above the fires. The strategy of company-level personnel, who were trained to fight 
fires and perceived this as a conventional, large high-rise fire, was to get to the fire floors and extinguish 
the fires. 
Overlaying this trinity of operational strategies was the fact that this was the largest emergency response 
in FDNY history, with roughly 1,000 firefighters on the scene. Even the singularly large response to the 
1993 bombing involved about 700 emergency personnel. A typical two-alarm fire might have involved 
about 100 personnel. 
Thus, keeping track of what all these people were doing, where there were located, where they were 
going, and what they would do when they got there was a task without precedent. The principal tools for 
this were three 18 in. x 28 in. magnetic boards known as Fire Command Boards (Figure 7–7). They were 
located in the lobbies of WTC 1 and WTC 2 and at the Incident command Post on West Street. On each 
Board, magnetic identifiers of different colors identified engines, ladder and tower ladders, battalions, 
special units, and sectors. Unit numbers were written on the identifiers with marking pens. These Boards 
became overwhelmed after about 30 min due to the large number of people and units arriving at the 
scene. Some emergency personnel that arrived at the site did not report to the Command Posts or were 
not logged in on the Command Board. A formal analysis of arrivals and missions of the various units was 
compromised by the loss of the Boards in the collapses of the towers; there were no backup records. 
NYPD 
The roles of the NYPD were to establish traffic control and perimeter security at the site, provide security 
for the command posts, and conduct evacuation and rescue operations within the towers. Their aviation 
units supplied observation capability and assessed the potential for roof rescue. 
The primary mobilization point for the NYPD Special Operations Division (SOD) that sent Emergency 
Service Unit (ESU) rescue teams into the WTC was at the corner of Church and Vesey Streets at the 
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Figure 7–7. Fire Command Board located in the lobby of WTC 1. 
northeast corner of the WTC tract. The post was managed by a SOD detective who had just gone off of 
duty and was still at his office when the attack occurred. He dispatched six ESU teams, each consisting 
of about five people. Records for each team were written on paper attached to a clipboard. 
A second SOD mobilization point was established at the corner of West and Vesey Streets at the 
northwest corner of the WTC tract. The armed NYPD officers and ESU teams provided security for the 
FDNY Incident Command Post. 
Since there were few NYPD units and since they typically arrived with all members, keeping track of the 
units was less problematic than for the FDNY. However, with the collapse of WTC 2, all written records 
were lost as the high winds and debris blew through the mobilization points. Since NYPD had only about 
50 personnel operating in or near the towers, the managers of the mobilization points were able to easily 
reconstruct the lost data on their personnel. 
Although The Port Authority had not endorsed a plan for roof rescue from the towers, it appeared to be 
one of the few options available for occupants trapped above the fires. NYPD helicopters reached the 
scene by 8:52 to assess the possibility of roof rescue. They were unable to land on the roof due to heavy 
smoke conditions. During the first hour, FDNY did not consider the option of roof rescue. When the 
aircraft struck WTC 2, it was clear that this was criminal activity, and the decision regarding roof top 
operations became the responsibility of NYPD. The NYPD First Deputy Commissioner ordered that no 
roof rescues were to be attempted, and at 9:43 a.m., this directive was passed to all units. 
Roof rescue was not intended to be an option, and The Port Authority reported that it never advised 
tenants to evacuate upward. The Port Authority's standard full-building occupant evacuation procedures 
and drills required the use of stairways to exit at the bottom of the WTC towers. The standard procedures 
were to keep the doors to the roof locked. Roof access required use of an electronic swipe card to get 
through the first two doors and a security officer watching a closed-circuit camera on the 22nd floor of 
WTC 1 to open the third door via a buzzer. (The 1968 NYC Building Code required access to roofs like 
these, most likely to provide FDNY access. The 2003 code does not intend roof access to be used for 
evacuation and has no prohibition on locking this access.) 
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The NYPD and FDNY did not consider roof rescue a viable strategy for general evacuation. First, the 
NYPD and FDNY policies for roof operations were focused mainly on providing emergency responders 
with access into the building above the fire floors for firefighting, conventional rescue, and comforting 
occupants. Roof rescue was considered a measure of last resort to be used, for example, to assist 
occupants with medical emergencies. Second, although on September 11, an NYPD aviation unit was 
early on the scene to consider the possibility, smoke and heat conditions at the top of the towers prevented 
the conduct of safe roof operations, despite repeated attempts. Even if it had been possible for a 
helicopter to gain access to the roof, only a very small fraction of the large number of people trapped 
above the impact zone could have been rescued before the towers collapsed. Nonetheless, perhaps as an 
indication of the dire situation in the top floors of the towers, at least two decedents tried to get to the roof 
and found the roof access locked in both the WTC towers. 
PAPD 
The roles of the PAPD were to establish security at the WTC and to conduct evacuation operations. 
PAPD officers were performing their normal law enforcement duties at the WTC site when the attack on 
WTC 1 occurred. Several additional PAPD teams were dispatched from various locations from around 
the city and from Jersey City, with some arriving before the collapse of WTC 2 and reporting to PAPD 
personnel at the WTC 1 lobby Fire Command Desk. There were dozens of PAPD officers on site and on 
orders to report to the site. With the collapse of WTC 2, the PAPD Police Desk (in WTC 5) and the 
Command Center were evacuated. Many of the emergency response records were lost initially, but were 
recovered some days later. 
Interdepartmental Interactions 
The coordination of communications and operations between the responding authorities at the WTC site 
was a challenge for all emergency responders working that morning. The short time duration between the 
initial attack and the collapse of the towers, coupled with the large number of responders and their 
staggered arrivals, compounded the difficulty of establishing a unified operation. 
FDNY (and the Emergency Medical Services), NYPD, PAPD, The Port Authority, and OEM were 
attempting to work together. These efforts were stymied by a lack of existing protocols that clearly 
defined authorities and responsibilities, communications systems problems, and multiple major attacks 
and threats. Although there was merit to having the FDNY and NYPD Command Posts separated, there 
was no uniform means for communicating between the two Command Posts at the time when WTC 2 
collapsed. FDNY and NYPD were primarily operating as independent organizations based on their 
operational responsibilities. 
7.2.4 Responder Access 
Fighting fires in the upper levels of tall buildings is not the same as fighting fires in buildings that are less 
than 100 ft high. In the case of the WTC towers, the people needing assistance were mostly many stories 
above the ground, and climbing tens of flights of stairs was the only way upward for the emergency 
responders. In the time available, they were not able to get very far. For example, emergency responders 
wearing police uniforms, not wearing Self-contained Breathing Apparatus (SCBA), and not carrying extra 
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equipment, were able to climb the stairs at a rate of approximately 1.4 min per floor while climbing to 
floors in the 40s inside of WTC 1. The climbing rate for firefighters wearing protective clothing and 
SCBA and carrying extra firefighting and rescue equipment was about 2 min per floor. The downward 
flow of evacuees, especially those who had physical disabilities or were obese, also slowed the 
responders' progress, especially in the 44 in. wide stairwells. The flow of the evacuees caused teams of 
emergency responders to become separated, further disrupting team operations. 
Neither the number of responders who entered the towers nor the floors they reached are known, due to 
the incompleteness of the Command Boards and their eventual destruction. From radio communications 
and first-person interviews, it appears that there were responders as high as floors in the 50s in WTC 1 
and the 78th floor in WTC 2. 
7.2.5 Communications 
There were multiple equipment systems for command-to-field communications, for responders to 
communicate among themselves, and for contact to and from building occupants: 
• Landline telephone system (including access to the 9-1-1 system), 
• Emergency announcement systems within WTC 1 and WTC 2, 
• Cellular systems (including access to the 9-1-1 system), 
• Warden phones (tower stair landings to Command Post), 
• Firefighter phones, called standpipe phones, in the WTC towers, and 
• FDNY handie-talkies, with booster support from a repeaters on WTC 5 and a Battalion car 
repeater located inside WTC 2. 
Within WTC 1, the system used to make the emergency announcements was disabled by the first aircraft 
impact, communications to the elevators in the upper third of the buildings were lost, the Warden phones 
did not work, and attempts to use the landline phones to contact people upstairs were unsuccessful due to 
the failure of some phones in the building. 
Little is known about the function of the internal communications inside WTC 2 after the aircraft struck 
the building. This is because all of the key emergency responders working inside WTC 2 died when the 
building collapsed. However, interviews with some occupants who evacuated from the building and 
interviews with emergency responders who communicated with counterparts inside WTC 2 indicated the 
following: some of the building’s public address systems were working, some of the elevator phone 
systems were working, and some of the landline telephones were working. It is not known if the Warden 
Phone system was fully operational or if the standpipe phones were operational. Emergency responder 
communications inside WTC 2 primarily depended on radio and face-to-face communications. 
The collapse of WTC 2 caused the cellular phone system in Lower Manhattan to fail. However, there 
were still landlines working in the city blocks adjacent to the WTC site, and calls were still emanating 
from inside WTC 1. 
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All of the radio systems analyzed were working well just before the attack on the WTC. However, 
PAPD, FDNY, and NYPD were aware that radio communications had not fared well in high-rise 
buildings, including WTC 1 following the 1993 bombing. The vast amount of metal and steel-reinforced 
concrete in high-rise buildings was known to attenuate and block radio signals, especially the low output 
power emergency responder handie-talkies. This was again a problem on September 11, 2001, when all 
three agencies encountered difficulties with their hand-held units. 
Thus, there was a heavy burden placed on the FDNY repeater to boost the weak signals to a discernable 
level. The repeater was functional during operations at the WTC; apparently the antenna was not 
damaged by debris from the aircraft impacts. However, within WTC 1, the system did not function 
correctly. The cause of this malfunction could not be determined since the unit was destroyed in the 
collapse of WTC 1. Repeater recording communications suggest that it was used within WTC 2. The 
radio recordings showed that communications readability using the repeater channel was generally good 
to excellent. Where readability levels were poor, it was generally caused by multiple people attempting to 
communicate over the radio at one time. The heavy traffic continued until the repeater failed with the 
collapse of WTC 2. 
Had communications using the repeater been adequate in WTC 1, there would have been opposing effects 
on the quality of operations and life safety. On the positive side, the emergency personnel in the tower 
would have been in at least some contact with the Command Posts. However, two serious counterpoints 
would have occurred. First, if the responders in both towers were using the same repeater at the same 
time, the traffic would have been heavier, and more of the calls would have been indecipherable. Second, 
a firefighter in either tower would have had difficulty discerning which communications related to 
operations in his tower. Given the poor markings within the towers and the unfamiliarity of some 
emergency responders with the site, there was already a high degree of confusion as to which tower a 
responder was in. 
The poor radio communications at the WTC had a serious impact on the FDNY Command Post’s 
attempts to maintain command and control in general. All emergency responders struggled with the high 
volume and low quality of radio communications traffic at the WTC, described as “radio gridlock.” NIST 
estimates that one-third to one-half of the emergency responder radio communications were 
undecipherable or incomplete. 
The poor communications had a critical effect on the conveyance of evacuation instructions. As early as 
8:48, there was an order to WTC personnel to clear WTC 1. At 8:59 a.m., a senior PAPD officer called 
for the evacuation of the two towers. At 9:01 a.m., this was extended to the entire complex. This was 
before the second aircraft struck. At 9:04 a.m., WTC Operations told people to evacuate an unidentified 
building. At 10:06 a.m., an NYPD aviation unit reported that it wouldn’t be much longer before WTC 1 
would come down. Some survivors reported not having received any of these messages. It is not known 
how many others did not, nor whether their locations were such that they could have made it out of the 
buildings in time. 
7.2.6 The Overall Response 
It was difficult to quantify the responders' degree of success. There were multiple reports of FDNY, 
NYPD, and PAPD efforts making the difference between death and survival. There were reports of 
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assistance where the survival of the occupants was not determined. There were reports of firefighters 
quenching small fires on the lower floors of the towers and at the impact point in WTC 2. However, it 
would have been impossible for them to have had any significant effect on the fires that eventually led to 
the collapse of the structures. 
7.3 FACTORS THAT CONTRIBUTED TO ENHANCED LIFE SAFETY 
7.3.1 Aggregate Factors 
• Reduced number of people in the buildings at the times of aircraft impact. 
• Functioning elevators in WTC 2 for the 16 min prior to 9:02:59 a.m. 
• Remoteness of Stairwell A from the impact zone and debris field. 
• Participation of two-thirds of surviving occupants in recent fire drills. 
• Upgrades to the life safety system components after the 1993 bombing. 
• Evacuation assistance provided by emergency responders to evacuees. 
7.3.2 Individual Factors 
• Location below the floor of impact. 
• Shortness of delay in starting to evacuate. 
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PART III: THE OUTCOME OF THE INVESTIGATION

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Chapter 8 
PRINCIPAL FINDINGS 
8.1 INTRODUCTION 
This chapter presents the findings of the Investigation, organized according to the first three of the 
Investigation objectives for the WTC towers. The fourth objective is the subject of Chapter 9. WTC 7 is 
the subject of a companion report. The findings were derived from the extensive documentation 
summarized in the preceding chapters and described in detail in the accompanying reports. While NIST 
was not able to compile a complete documentation of the history of the towers, due to the loss of records 
over time and due to the collapses, the investigators were able to acquire information adequate to support 
the findings and recommendations compiled in this chapter and the next. The chapter begins with 
summary statements and continues with the listing of the full set of principal findings. 
8.2 SUMMARY 
Objective 1: Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of 
the aircraft. 
• The two aircraft hit the towers at high speed and did considerable damage to principal 
structural components: core columns, perimeter columns, and floors. However, the towers 
withstood the impacts and would have remained standing were it not for the dislodged 
insulation and the subsequent multifloor fires. The robustness of the perimeter frame-tube 
system and the large size of the buildings helped the towers withstand the impact. The 
structural system redistributed loads without collapsing in places of aircraft impact, avoiding 
larger scale damage upon impact. The hat truss, which was intended to support a television 
antenna atop each tower, prevented earlier collapse of the building core. In each tower, a 
different combination of impact damage and heat-weakened structural components 
contributed to the abrupt structural collapse. 
• In WTC 1, the fires weakened the core columns and caused the floors on the south side of the 
building to sag. The floors pulled the heated south perimeter columns inward, reducing their 
capacity to support the building above. Their neighboring columns quickly became 
overloaded as the south wall buckled. The top section of the building tilted to the south and 
began its descent. The time from aircraft impact to collapse initiation was largely determined 
by how long it took for the fires to weaken the building core and to reach the south side of the 
building and weaken the perimeter columns and floors. 
• In WTC 2, the core was damaged severely at the southeast corner and was restrained by the 
east and south walls via the hat truss and the floors. The steady burning fires on the east side 
of the building caused the floors there to sag. The floors pulled the heated east perimeter 
columns inward, reducing their capacity to support the building above. Their neighboring 
columns quickly became overloaded as the east wall buckled. The top section of the building 
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tilted to the east and to the south and began its descent. The time from aircraft impact to 
collapse initiation was largely determined by the time for the fires to weaken the perimeter 
columns and floor assemblies on the east and south sides of the building. WTC 2 collapsed 
more quickly than WTC 1 because there was more aircraft damage to the building core and 
there were early and persistent fires on the east side of the building, where the aircraft had 
extensively dislodged insulation from the structural steel. 
• The WTC towers would likely not have collapsed under the combined effects of aircraft 
impact damage and the extensive, multifloor fires if the thermal insulation had not been 
widely dislodged or had been only minimally dislodged by aircraft impact. 
Objective 2: Determine why the injuries and fatalities were so high or low depending on location, 
including all technical aspects of fire protection, occupant behavior, evacuation, and emergency 
response. 
• Approximately 87 percent of the estimated 17,400 occupants of the towers, and 99 percent of 
those located below the impact floors, evacuated successfully. In WTC 1, where the aircraft 
destroyed all escape routes, 1,355 people were trapped in the upper floors when the building 
collapsed. One hundred seven people who were below the impact floors did not survive. 
Because the flow of people from the building had slowed considerably 20 min before the 
tower collapsed, the stairwell capacity was adequate to evacuate the occupants on that 
morning. 
• In WTC 2, before the second aircraft strike, about 3,000 people got low enough in the 
building to escape by a combination of self-evacuation and use of elevators. The aircraft 
destroyed the operation of the elevators and the use of two of the three stairways. Eighteen 
people from above the impact zone found a passage through the damaged third stairway and 
escaped. The other 619 people in or above the impact zone perished. Eleven people who 
were below the impact floors did not survive. As in WTC 1, shortly before collapse, the flow 
of people from the building had slowed considerably, indicating that the stairwell capacity 
was adequate that morning. It is presumed that the 11 people did not escape for the same 
reasons as the victims in WTC 1. 
• About 6 percent of the survivors described themselves as mobility impaired, with recent 
injury and chronic illness being the most common causes; few, however, required a 
wheelchair. Among the 118 decedents below the aircraft impact floors, investigators 
identified seven who were mobility challenged, but were unable to determine the mobility 
capability of the remaining 111. 
• A principal factor limiting the loss of life was that the buildings were only one-third occupied 
at the time of the attacks. NIST estimated that if the towers had been fully occupied with 
25,000 occupants each, it would have taken about 4 hours to evacuate the buildings using the 
stairs and over 14,000 people might have perished since the stairwell capacity would not have 
been sufficient to evacuate that many people in the available time. Egress capacity required 
by current building codes is determined by single floor calculations that are independent of 
building height and does not consider the time for full building evacuation. 
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• Due to the presence of assembly use spaces at the top of each tower that were designed to 
accommodate over 1,000 occupants per floor for the Windows on the World restaurant 
complex and the Top of the Deck observation deck, the NYC Building Code would have 
required a minimum of four independent means of egress (stairs), one more than the three 
that were available in the buildings. Given the low occupancy level on September 11, 2001, 
NIST found that the issue of egress capacity from these places of assembly, or from 
elsewhere in the buildings, was not a significant factor on that day. It is conceivable that such 
a fourth stairwell, depending on its location and the effects of aircraft impact on its functional 
integrity, could have remained passable, allowing evacuation by an unknown number of 
additional occupants from above the floors of impact. Moreover, if the buildings had been 
filled to their capacity with 25,000 occupants, the required fourth stairway would likely have 
mitigated the insufficient egress capacity for conducting a full building evacuation within the 
available time. 
• Evacuation was assisted by participation in fire drills within the previous year by two-thirds 
of survivors and perhaps hindered by a Local Law that prevented employers from requiring 
occupants to practice using the stairways. The stairways were not easily navigated in some 
locations due to their design, which included “transfer hallways” where evacuees had to 
traverse from one stairway to another location where the stairs continued. Additionally, 
many occupants were unprepared for the physical challenge of full building evacuation. 
• The functional integrity and survivability of the stairwells was affected by the separation of 
the stairwells and the structural integrity of stairwell enclosures. In the impact region of 
WTC 1, the stairwell separation was the smallest over the building height—clustered well 
within the building core—and all stairwells were destroyed by the aircraft impact. By 
contrast, the separation of stairwells in the impact region of WTC 2 was the largest over the 
building height—located along different boundaries of the building core—and one of three 
stairwells remained passable after the aircraft impact. The shaft enclosures were fire rated 
but were not required to have structural integrity under typical accidental loads—there were 
numerous reports of stairwells obstructed by fallen debris from damaged enclosures. 
• The fire safety systems (sprinklers, smoke purge, and fire alarms,) were designed to meet or 
exceed current practice. However, they played no role in the safety of life on September 11 
because the water supplies to the sprinklers were fed by a single supply pipe that was 
damaged by the aircraft impact. The smoke purge systems were designed for use by the fire 
department after the fires and were not turned on; they also would have been ineffective due 
to aircraft damage. The violence of the aircraft impact served as its own alarm. In WTC 2, 
contradictory public address announcements contributed to occupant confusion and some 
delay in occupants beginning to evacuate. 
• For the approximately 1,000 emergency responders on the scene, this was the largest disaster 
they had even seen. Despite attempts by the responding agencies to work together and 
perform their own tasks, the extent of the incident was well beyond their capabilities. 
Communications were erratic due to the high number of calls and the inadequate performance 
of some of the gear. Even so, there was no way to digest, test for accuracy, and disseminate 
the vast amount of information being received. In the opinion of some first responders, 
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communications and information sharing cost the lives of some emergency responders. Their 
jobs were complicated by the loss of command centers in WTC 7 and then in the towers after 
WTC 2 collapsed. With nearly all elevator service disrupted and progress up the stairs taking 
about 2 min per floor, it would have taken hours for the responders to reach their destinations, 
assist survivors, and escape before the towers collapsed. 
Objective 3: Determine what procedures and practices were used in the design, construction, 
operation, and maintenance of WTC 1 and WTC 2. 
• Because of The Port Authority of New York and New Jersey’s (Port Authority’s) 
establishment under a clause of the United States Constitution, its buildings were not subject 
to any external building code. The buildings were unlike any others previously built, both in 
their height and in their innovative structural features. Nevertheless, the actual design and 
approval process produced two buildings that generally were consistent with nearly all of the 
provisions of the NYC Building Code and other building codes of that time. The loads for 
which the buildings were designed exceeded the code requirements. The quality of the 
structural steels was consistent with the building specifications. The departures from the 
building codes and standards did not have a significant effect on the outcome of 
September 11. 
• For the floor systems, the fire rating and insulation thickness used on the floor trusses were of 
concern from the time of initial construction. NIST found no technical basis or test data on 
which the thermal protection of the steel was based. However, on September 11, 2001, the 
minimum specified thickness of the insulation was adequate to delay heating of the trusses 
and the amount of insulation dislodged by the aircraft impact was sufficient to enable the 
critical heating of the structural steel. 
• Based on four standard fire resistance tests that were conducted under a range of insulation 
and test conditions, NIST found the fire rating of the floor system to vary between 3/4 hours 
and 2 hours; in all cases the floors continued to support the full design load without collapse 
for over 2 hours. 
• The wind loads used for the WTC towers, which governed the design of the external 
columns, significantly exceeded the requirements of the NYC Building Code and selected 
other building codes of the day. Two sets of wind load estimates for the towers obtained by 
independent commercial consultants in 2002, however, differed by as much as 40 percent. 
These estimates were based on wind tunnel tests conducted as part of insurance litigation 
unrelated to the Investigation. 
The tragic consequences of the September 11, 2001 attacks were largely a result of the fact that terrorists 
flew large jet-fuel laden commercial airliners into the WTC towers. Buildings for use by the general 
population are not designed to withstand attacks of such severity; building codes do not require building 
designs to consider aircraft impact. In our cities, there has been no experience with a disaster of such 
magnitude, nor has there been any in which the total collapse of a high-rise building occurred so rapidly 
and with little warning. 
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While there were unique aspects to the design of the WTC towers and the terrorist attacks of September 
11, 2001, there are several possibilities to improve the safety of tall buildings, occupants, and emergency 
responders that result from this investigation of commonly used procedures and practices that were used 
in the design, construction, operation, and maintenance of the WTC towers. There also are possible 
improvements for selected buildings that owners may determine to be at higher risk due to their iconic 
status, critical function, or design. The recommendations in Chapter 9 suggest a variety of ways in which 
to achieve these safety improvements. 
8.3 FINDINGS ON THE MECHANISMS OF BUILDING COLLAPSE 
8.3.1 Summary of Probable Collapse Sequences 
WTC 1 was struck by a hijacked aircraft at 8:46:30 a.m. and began to collapse at 10:28:22 a.m. WTC 2 
was struck by a hijacked aircraft at 9:02:59 a.m. and began to collapse at 9:58:59 a.m. The specific 
factors in the collapse sequences relevant to both towers (the sequences vary in detail for WTC 1 and 
WTC 2) are: 
• Each aircraft severed exterior columns, damaged interior core columns and knocked off 
insulation from steel as the planes penetrated the buildings. The weight carried by the severed 
columns was distributed to other columns. 
• Subsequently, fires began to grow and spread. They were initiated by the aircraft’s jet fuel, 
but were fed for the most part by the building contents and the air supply resulting from 
breached walls and fire-induced window breakage. 
• These fires, in combination with the dislodged insulation, were responsible for a chain of 
events in which the building core weakened and began losing its ability to carry loads. 
• The floors weakened and sagged from the fires, pulling inward on the exterior columns. 
• Floor sagging and exposure to high temperatures caused the exterior columns to bow inward 
and buckle—a process that spread across the faces of the buildings. 
• Collapse then ensued. 
Seven major factors led to the collapses of WTC 1 and WTC 2: 
• Structural damage from the aircraft impact; 
• Large amount of jet fuel sprayed into the building interior, that ignited widespread fires over 
several floors; 
• Dislodging of SFRM from structural members due to the aircraft impact, that enabled rapid 
heating of the unprotected structural steel; 
• Open plan of the impact floors and the breaking of the partition walls by the impact debris 
that resulted in increased ventilation; 
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• Weakened core columns that increased the load on the perimeter walls; 
• Sagging of the floors, that led to pull-in forces on the perimeter columns; and 
• Bowed perimeter columns that had a reduced capacity to carry loads. 
8.3.2 Structural Steels 
• Fourteen different strengths of steel were specified in the structural engineering plans, but 
only 12 steels of different strength were actually used in construction due to an upgrade of 
two steels. Ten different steel companies fabricated structural elements for the towers, using 
steel supplied from at least eight different suppliers. Four fabricators supplied the major 
structural elements of the 9th to the 107th floors. Material substitutions of higher strength 
steels were not uncommon in the perimeter columns and floor trusses. 
• About 87 percent of the tested steel specimens (columns, trusses and bolts) met or exceeded 
the required yield strengths specified in design documents. About 13 percent had NISTmeasured 
strengths that were slightly lower than the design values, but this may have arisen 
from mechanical damage during the collapse, the natural variability of structural steel, and 
slight differences between the NIST and original mill test report testing protocols. 
• The safety of the WTC towers on September 11, 2001 was most likely not affected by the 
fraction of steel that, according to NIST testing, was modestly below the required minimum 
yield strength. The typical factors of safety in allowable stress design were capable of 
accommodating the measured property variations below the minimum. 
• The pre-collapse photographic analysis showed that 16 recovered exterior panels were 
exposed to fire prior to collapse of WTC 1. None of the nine recovered panels from within 
the fire floors of WTC 2 were observed to have been directly exposed to fire. 
• None of the recovered steel samples showed evidence of exposure to temperatures above 
600 ºC for as long as 15 min. This was based on NIST annealing studies that established the 
set of time and temperature conditions necessary to alter the steel microstructure. These 
results provide some confirmation of the thermal modeling of the structures, since none of the 
samples were from zones where such heating was predicted. 
• Only three of the recovered samples of exterior panels reached temperatures in excess of 
250 °C during the fires or after the collapse. This was based on a method developed by NIST 
to characterize maximum temperatures experienced by steel members through observations 
of paint cracking. 
• Perimeter columns exposed to fire had a great tendency for local buckling of the inner web; a 
similar correlation did not exist for weld failure. 
• Observations of the recovered steel provided significant guidance for modeling the damage 
from the aircraft impact with the towers. 
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• For the perimeter columns struck by the aircraft, fractures of the plates in areas away from a 
welded joint exhibited ductile behavior (necking and thinning away from the fracture) under 
very high strain rates. Conversely, fractures occurring next to a welded joint exhibited little 
or no ductile characteristics. 
• There was no evidence to indicate that the type of joining method, materials, or welding 
procedures were improper. The welds appeared to perform as intended. 
• The failure mode of spandrel connections on perimeter panels differed above and below the 
impact zone. Spandrel connections on exterior panels at or above the impact zone were more 
likely to fail by bolt tear out. For those exterior panels below the impact zone, there was a 
higher propensity for the spandrels to be ripped off from the panels. This may be due to shear 
failures as the weight of the building came down on these lower panels. There was no 
difference in failure mode for the spandrel connections whether the exterior panels were 
exposed to fire or not. 
• With the exception of the mechanical floors, the perimeter panel column splices failed by 
fracture of the bolts. At mechanical floors, where splices were welded in addition to being 
bolted, the majority of the splices did not fail. 
• Core columns failed at both splice connection and by fracture of the columns themselves. 
• The damage to truss seats on perimeter panels differed above and below the impact zone in 
both towers. The majority of recovered perimeter panel floor truss connectors (perimeter 
seats) below the impact floors were either missing or bent downward. Above this level, the 
failure modes were more randomly distributed. 
• In the floor trusses, a large majority of the electric resistance welds at the web-to-chord 
connections failed. The floor truss and the perimeter panel floor truss connectors typically 
failed at welds and bolts. 
• The NIST-measured properties of the steels (strain rate, impact toughness, high-temperature 
yield and tensile strengths) were similar to literature values for other construction steels of the 
WTC era. 
• The creep behavior of the steels could be modeled by scaling WTC-era literature data using 
room temperature tensile strength ratios. 
8.3.3 Aircraft Impact Damage Analysis 
• Both towers withstood the significant structural damage to the exterior walls, core columns, 
and floor systems due to the aircraft impact. WTC 2 was the more severely damaged 
building and the first to collapse. WTC 2 displayed significant reserve capacity, as evidenced 
by a post-impact rooftop sway that was more than one-third of that under the hurricane force 
winds for which the building was designed. The oscillation period of this swaying was nearly 
equal to that calculated for the undamaged structure. (Such an analysis was not possible for 
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the less severely damaged WTC 1 due to the absence of equivalent video footage for the 
analysis.) 
• American Airlines Flight 11 impacted the north wall of WTC 1 at a speed of 443 mph ± 
30 mph, banked 25 degrees ± 2 degrees to the left (left wing downward) and with the nose 
tilted slightly downward. United Airlines Flight 175 impacted the south wall of WTC 2 at a 
speed of 542 mph ± 24 mph, banked 38 degrees ± 2 degrees to the left (left wing downward) 
and with the nose pointed slightly downward and to the right. 
• The aircraft impact on WTC 1 caused extensive damage to the north wall of the tower, 
principally in the regions impacted by the fuselage, engine, and fuel-filled wing sections. 
Photographic evidence showed that 34 perimeter columns were completely severed, while 
four columns were heavily damaged, and two columns were moderately damaged. 
• The impact simulations of WTC 1 indicated that three to six core columns were severed, and 
three to four columns were heavily damaged. The floor trusses, core beams, and floor slabs 
experienced significant impact-induced damage on floors 94 through 96, particularly in the 
path of the fuselage. The wing structures were fragmented at the exterior wall, and aircraft 
fuel was dispersed on multiple floors. Aircraft debris substantially damaged the nonstructural 
interior partitions and the workstations and dislodged insulation in its path. The bulk of the 
fuel and aircraft debris was deposited in floors 93 through 97 with the largest concentration 
on floor 94. 
• The aircraft impact on WTC 2 caused extensive damage to the south wall of the tower and to 
the regions impacted by the fuselage, engine, and fuel-filled wing sections. Photographic 
evidence showed that 29 perimeter columns were completely severed, one was heavily 
damaged, and three were moderately damaged. Four perimeter columns on the north wall 
also were severed. 
• The impact simulations of WTC 2 indicated that five to ten core columns were severed and 
up to four columns were heavily damaged. The rupture of some column splices on floors 77, 
80, and 83 contributed significantly to the failure of the core columns. The floor trusses, core 
beams, and floor slabs experienced significant impact-induced damage on floors 79 to 81, 
particularly in the path of the fuselage. The analyses indicated that the wing structures were 
fragmented due to the interaction with the exterior wall and, as a result, aircraft fuel was 
dispersed on multiple floors. The aircraft debris substantially damaged the building's 
contents and also dislodged insulation in its path. The bulk of the fuel was concentrated on 
floors 79, 81, and 82, while the bulk of the aircraft debris was deposited in floors 78 through 
80, with the largest concentration on floor 80. 
• Other effects of the aircraft impacts included (a) severing of the sprinkler and fire hose water 
supply systems, negating any possible fire suppression efforts; (b) dispersing of jet fuel and 
ignition of building contents over large areas; (c) increasing the air supply into the damaged 
buildings that permitted very large fires; and (d) damaging ceilings, enabling unabated heat 
transport to the floor structure above and over the floor-to-ceiling partition walls to the next 
compartment. These effects were consistent with photographic evidence and with the 
accounts of building occupants and emergency responders. 
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• The simulations fairly closely matched the exterior wall damage patterns from each of the 
aircraft impacts and correctly predicted the collapse of five of the six stairwell walls and the 
lesser damage to the sixth, the trajectories of the engine and wheels that penetrated the 
buildings, and the accumulation of furnishings and debris in the northeast corner of the 80th 
and 81st floors of WTC 2. 
8.3.4 Reconstruction of the Fires 
• In each tower, the fires were initiated simultaneously on multiple floors by ignition of some 
of the jet fuel from the aircraft. The initial jet fuel fires themselves lasted at most a few 
minutes. 
• The principal combustibles on the fire floors were workstations. The total fuel load on the 
WTC floors was low, about 4 lb/ft2. 
• The aircrafts added significant combustible material to their paths (and the paths of their 
breakup fragments) through the buildings. 
• It is possible to reconstruct a complex fire in a large building, even if the building is no longer 
standing. However, this requires extraordinary information to replace what might have been 
gleaned from an inspection of the post-fire premises. In the case of the WTC tower, this 
information included floor plans of the fire zones, burning behavior of the combustibles, 
simulations of damage to the building interior, and frequent photographic observations of the 
fire progress from the building exterior. 
• The fires in WTC 1 were generally ventilation limited, i.e., they burned and spread only as 
fast as windows broke. Where the combustibles were not significantly relocated by the 
aircraft debris, they tended to burn out in about 20 min. This was consistent with the results 
of workstation fire tests conducted by NIST, in which the fuel load was 4 lb/ft2. Although 
there were multiple fires on some of the impact floors, the general trend was for the fires to 
move toward the south side of the tenant spaces. 
• The fires in WTC 2 had sufficient air to burn at a rate determined by the properties of the 
combustibles. This was in large part due to the extensive breakage of windows in the fire 
zone by the aircraft impact. In contrast with WTC 1, there was little spread in WTC 2. The 
early fires persisted on the east side of the tower and particularly in the northeast corner of the 
80th and 81st floors, where the aircraft debris had pushed a lot of fractured combustibles 
• The Fire Dynamics Simulator can predict the room temperatures and heat release rate values 
for complex fires to within 20 percent, when the building geometry, fire ventilation, and 
combustibles are properly described. 
• The Fire Structure Interface, developed for this Investigation, mapped the fire-generated 
temperature and thermal radiation fields onto and through layered structural materials to 
within the accuracy of the fire-generated fields and the thermophysical data for the structural 
components. 
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• Conventional office workstations reached a peak burning rate in about 10 min and continued 
burning for a total of about a half hour. Partial covering of surfaces with inert material 
reduced the peak burning rate proportional to the fraction covered, but did not affect the total 
amount of heat release during the entire burning. 
• Jet fuel sprayed onto the surfaces of typical office workstations burned away within a few 
minutes. The jet fuel accelerated the burning of the workstation, but did not significantly 
affect the overall heat released. 
8.3.5 Structural Response and Collapse Analysis 
• The core columns were weakened significantly by the aircraft impact damage and thermal 
effects. Thermal effects dominated the weakening of WTC 1. As the fires moved from the 
north to the south side of the core, the core was weakened over time by significant creep 
strains on the south side of the core. Aircraft impact damage dominated the weakening of 
WTC 2. With the impact damage, the core subsystem leaned to the southeast and was 
supported by the south and east perimeter walls via the hat truss and floors. As the core 
weakened, it redistributed loads to the perimeter walls through the hat truss and floors. 
Additional axial loads redistributed to the exterior columns from the core were not significant 
(only about 20 percent to 25 percent on average) as the exterior columns were loaded to 
approximately 20 percent of their capacity before the aircraft impact. 
• The primary role of the floors in the collapse of the towers was to provide inward pull forces 
that induced inward bowing of perimeter columns (south face of WTC 1; east face of 
WTC 2). Sagging floors continued to support floor loads as they pulled inward on the 
perimeter columns. There would have been no inward pull forces if the floors connections 
had failed and disconnected. 
• Column buckling over an extended region of the perimeter face ultimately triggered the 
global system collapse as the loads could not be redistributed through the hat truss to the 
already weakened building core. As the exterior wall buckled (south face for WTC 1 and east 
face for WTC 2), the column instability propagated to adjacent faces and caused the initiation 
of the building collapse. Perimeter wall buckling was induced by a combination of thermal 
weakening of the columns, inward pull forces from sagging floors, and to a much lesser 
degree, additional axial loads redistributed from the core. 
• The insulation damage estimates were conservative as they ignored possibly damaged and 
dislodged insulation in a much larger region that was not in the direct path of the debris but 
was subject to strong vibrations during and after the aircraft impact. A robust criterion to 
generate a coherent pattern of vibration-induced dislodging could not be established to 
estimate the larger region of damaged insulation. 
• For WTC 1, partitions were damaged and insulation was dislodged by direct debris impact 
over five floors (floors 94, 95, 96, 97, and 98) and included most of the north floor areas in 
front of the core, the core, and central regions of the south floor areas, and on some floors, 
extended to the south wall. 
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• For WTC 2, partitions were damaged and insulation was dislodged by direct debris impact 
over six floors (floors 79, 80, 81, 82, and 83) and included the south floor area in front of the 
core, the central and east regions of the core, and most of the east floor area, and extended to 
the north wall 
• The adhesive strength of CAFCO BLAZE-SHIELD DC/F to steel coated with primer paint 
was found to be one-third to one-half of the adhesive strength to steel that had not been 
coated with primer paint. The SFRM products used in the WTC towers were applied to steel 
components with primer paint. 
• The average thickness of the original thermal insulation on the floor trusses was estimated to 
be 0.75 in. with a standard deviation of 0.3 in. The average thickness of the upgraded thermal 
insulation was estimated to be 2.5 in. with a standard deviation of 0.6 in. Based on finiteelement 
simulations, the thermal analyses for determining temperature histories of structural 
components used a thermally equivalent thickness of 0.6 in. and 2.2 in. for the original and 
upgraded insulation, respectively. For thermal analyses of the perimeter columns, spandrel 
beams, core beams, and core columns, the insulation on these elements was set to the 
specified thickness, due to a lack of field measurements. 
• Based on four Standard Fire Tests conducted for various length scales, insulation thickness, 
and end restraints, the floor assemblies were shown to be capable of sagging without 
collapsing and supported their full design load under standard fire conditions for 2 hours or 
more without failure. 
• For assemblies with a ¾ in. SFRM thickness, the 17 ft assembly’s fire rating was 2 hours; the 
35 ft assembly’s rating was 1½ hours. This result raised the question of whether or not a fire 
rating of a 17 ft floor assembly is scalable to the longer spans in the WTC towers. 
• The specimen with ½ in. SFRM thickness and a 17 ft span would not have met the 2 hour 
requirement of the NYC Building Code. 
• There is far greater knowledge of how fires influence structures in 2005 than there was in the 
1960s. The analysis tools available to calculate the response of structures to fires are also far 
better now than they were when the WTC towers were designed and built. 
8.4 FINDINGS ON FACTORS AFFECTING LIFE SAFETY 
8.4.1 Active Fire Protection 
• Active fire protection systems for many buildings are designed to the same performance 
specifications, regardless of height, size, and threat profile. 
• The active fire protection systems (alarms, suppression, and smoke purging) in the WTC 
towers were designed to meet or exceed then-current practice. However, the successful 
operation of these systems depended upon the fire threat being consistent with what had been 
anticipated based upon previous experience and best engineering practices of the day. 
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• The fire alarm systems in the towers provided for automatic fire detection, but required 
manual activation of notification devices. On September 11, 2001, the impact of the aircraft 
itself alerted occupants in the WTC buildings to the unfolding danger when the first aircraft 
hit. 
• Soon after the first aircraft impact, an overwhelming number of alarms were displayed at the 
Fire Command Station in WTC 1. The alarm systems were only capable of determining and 
displaying (a) areas that had at some time reached alarm point conditions and (b) areas that 
had not. The quality and reliability of information available to emergency responders at the 
Fire Command Station was not sufficient to understand the fire conditions. 
• Although the fire alarm systems used multiple communication path risers, the systems 
experienced performance degradation, especially in WTC 1 where all fire alarm notification 
and communication functions appear to have been lost above the floors of impact. 
• There was no means at the Fire Command Stations to determine whether or not 
announcements reached and could be heard on the intended floors. 
• Alarm systems store information that is valuable for understanding the fire and smoke 
development in a building, but no information from the fire alarm systems was located, and 
there was no indication that anyone looked for it during the cleanup of the WTC site. 
Survivability of alarm systems data on computer hard drives, memory modules, or printouts 
in building fires and collapse environments is not addressed in present installation standards. 
• Transmission of critical data outside the building to a monitoring station would provide 
means to preserve event data. 
• Except for specific areas that were exempted from required sprinkler coverage, sprinkler 
systems were installed throughout the towers. As designed, the water supplies (storage tanks 
and pumped city water), automatic sprinklers, and standpipe/pre-connected hose systems met 
or exceeded the applicable installation requirements in the NYC Building Code. There were 
other design features that were considered inconsistent with engineering best practices, but no 
evidence was found to indicate that these features affected the events that occurred on 
September 11. 
• All the fires that occurred in sprinklered spaces in the towers prior to September 11, 2001, 
were controlled with three or fewer sprinklers, in some cases supplemented by manual fire 
fighting. 
• On the floors where the major fires occurred on September 11, 2001, the sprinkler system 
played no part since their water supply was damaged by the aircraft impact. The typical 
sprinkler system was installed with one connection to the infrastructure riser, providing a 
single point of failure of the water supply to the floor level sprinklers. 
• The sprinkler systems could have provided fire control at coverage areas up to two or three 
times the specified design area of 1,500 ft2. However, 4,500 ft2 constituted less than 
15 percent of the area of a single floor in these buildings, and estimates of the extent of the 
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initial fires in WTC 1 and WTC 2 in 2001 were considerably greater than three times the 
specified design areas. 
• There were redundant and sufficient supplies of water for the standpipe and sprinkler systems 
for control of normally expected fires on the floors where the September 11 fires occurred. 
Activating the secondary water supplies required manual operation of fire pumps by a sizable 
number of people at various locations. 
• No information was available at the Fire Command Desk about the water supply in areas that 
were burning, leading to a Port Authority employee being sent up to assess the status. 
• There was no information available regarding the performance of the non-aqueous fire 
suppression systems on September 11, 2001.he manually operated smoke purge systems were 
not initiated on September 11, 2001. It is unlikely the systems would have functioned as 
designed, due to loss of electrical power and damage to the HVAC shafts and other structural 
elements in the impact zone that were integral parts of the systems. 
• Analysis indicated that the aircraft impact rupture of large return air shafts and related 
ductwork created a major path for vertical smoke spread in the towers. 
8.4.2 Evacuation 
• Approximately 87 percent of WTC occupants, and over 99 percent of those below the floors 
of impact, were able to evacuate successfully. 
• At the time of the aircraft impacts, the towers were only about one-third occupied. Had they 
been at the full capacity of 25,000 workers and visitors per tower, computer egress modeling 
indicated that a full evacuation would have required about 4 hours. Under those 
circumstances, over 14,000 occupants might have perished in the building collapses. 
• There were 8,900 ± 750 people in WTC 1 at 8:46 a.m. on September 11, 2001. Of those, 
7,470 (or 84 percent) survived, while 1,462 to 1,533 occupants died.14 At least 107 occupants 
were killed below the aircraft impact zone. No one who was above the 91st floor in WTC 1 
after the aircraft impact survived. This was due to the fact that the stairwells and elevators 
were destroyed and helicopter rescue was impossible. 
• There were 8,540 ± 920 people in WTC 2 at 8:46 a.m. on September 11, 2001. Of those, 
7,940 (or 93 percent) survived, while 630 to 701 occupants were killed.14 Eleven occupants 
died below the aircraft impact zone. Approximately 75 percent of the occupants above the 
78th floor at 8:46 a.m. had successfully descended below the 78th floor prior to the aircraft 
impact at 9:03 a.m. The use of elevators and self-initiated evacuation during this period 
saved roughly 3,000 lives. 
14 As shown in Table 4-1, there were a total of 71 decedents whose initial locations in the towers was not certain: 30 below the 
impact zone in either WTC 1 or WTC 2, 24 at an unknown location in WTC 1 or WTC 2, and 17 people for whom no location 
information was available. 
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• The evacuation from WTC 2 occurred in spite of conflicting announcements, first urging 
people to return to their offices around 9:00 a.m., and then informing them around 9:02 a.m. 
that they may initiate an evacuation if conditions warranted. A subsequent announcement at 
9:20 a.m., after the second aircraft strike, informed occupants that they could use the 
Concourse if they wished to leave the building. An announcement at 9:37 a.m. instructed 
occupants to go down the stairs. 
• Stairwell A in WTC 2 remained passable for at least some period of time after the aircraft 
impact because (1) only the end of the left wing, empty of jet fuel, was in line with the 
stairwell; (2) Stairwell A was behind the structural/architectural core in the area of impact; 
and (3) the aircraft debris had to travel through the longer dimension of the core and thus was 
slowed by a greater number of columns, shafts, walls, and mechanical equipment, and (4) 
Stairwell A was widely separated from Stairwells B and C. 
• Eighteen people successfully used the debris-cluttered Stairwell A in WTC 2 to leave the 
building after being on or above the 78th floor when United Airlines Flight 175 hit the 
building. It is possible that additional occupants from above the impact floors were making 
their way down the stairwell some minutes before building collapse. 
• 67 percent of WTC 1 occupants and 51 percent of WTC 2 occupants had started working at 
the WTC in the previous 4 years. 
• Two-thirds of WTC 1 and WTC 2 occupants participated in at least one fire drill in the twelve 
months prior to September 11, 2001. Nearly all (93 percent) of these occupants were 
instructed about the location of the nearest stairwell. However, only half of the survivors had 
previously used a stairwell, in part since NYC Local Law 5 prohibited requiring occupants to 
practice stairwell evacuation. 
• The NIST Investigation found no evidence that the occupants of WTC 1 heard public address 
system announcements, although the fire command station was attempting to make such 
announcements. 
• The delays of about 5 min in starting evacuation were largely spent trying to obtain additional 
information, trying to make sense of the situation, and generally preparing to evacuate. 
• People who started their evacuation on higher floors took longer to start leaving and 
substantially increased their odds of encountering smoke, damage or fire. These encounters, 
along with interruption for any reason, had a significant effect on increasing the amount of 
time that people spent to traverse their evacuation stairwell. 
• World Trade Center occupants were inadequately prepared to encounter horizontal transfers 
during the evacuation process and were occasionally delayed by the confusion as to whether a 
hallway led to a stairwell and confusion about whether the transfer hallway doors would open 
or be locked. 
• The WTC occupants were often unprepared for the physical challenge of full building 
evacuation. Numerous occupants required one or more rest periods during stairwell descent. 
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• In WTC 1, the average surviving occupant spent approximately 48 seconds per floor in the 
stairwell, about twice that observed in non-emergency evacuation drills. The 48 s does not 
include the time prior to entering the stairwell, which was often substantial. Some occupants 
delayed or interrupted their evacuation, either by choice or instruction. 
• Downward traveling evacuees reported slowing of their travel due to ascending emergency 
responders, but this counterflow was not a major factor in determining the length of their 
evacuation time. Emergency responders reported trouble ascending the stairs because of the 
volume of evacuees in the stairwells. 
• During the last 20 min before each building collapsed, the evacuation rate in each building 
had slowed to about one-fifth the immediately prior evacuation rate. This suggests that for 
those seeking and able to reach and use the undamaged exits and stairways, the egress 
capacity was adequate to accommodate survivors. 
• Many opportunities to communicate important information in a timely manner were missed, 
such as the general location of the impact region or whether to evacuate or not. As a result, 
building occupants, 9-1-1 operators, fire department dispatch, WTC building officials, and 
Port Authority personnel lacked necessary information about the situation. 
• Faced with an uncertain situation, occupants of both buildings received conflicting feedback / 
advice from a variety of sources (including 9-1-1 operators, FDNY, family and friends, and 
The Port Authority) regarding whether to evacuate, whether to break windows, and the nature 
of their situation. It is likely that, in many instances, the people giving advice had as little 
accurate information as those seeking it. 
• The decision to establish the primary evacuation route underground through the Concourse 
and then up to street level near WTC 5 prevented a significant number of injuries and/or 
deaths. 
• Approximately 1,000 surviving occupants had a limitation that impacted their ability to 
evacuate, including recent surgery or injury, obesity, heart condition, asthma, advanced age, 
and pregnancy. The most frequently reported disabilities were recent injuries and chronic 
illnesses. The fraction of occupants requiring use of a wheelchair was very small. 
• Mobility challenged occupants were not universally accounted for by existing evacuation 
procedures, as some were left by colleagues (later rescued by strangers), some in WTC 1 
were temporarily removed from the stairwells in order to allow more able occupants to 
evacuate the building, and others chose not to identify their mobility challenge to any 
colleagues. 
• Most mobility challenged individuals were able to evacuate successfully, often with 
assistance from co-workers or emergency responders, and it is not clear how many were 
among the 118 from below the impact floors who did not survive. It does not appear that 
mobility challenged individuals were significantly over-represented amongst the decedents. 
As many as 40 to 60 mobility impaired occupants and their companions were found on the 
12th floor of WTC 1 by emergency responders. About 20 of these were making their way 
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down the stairs shortly before the building collapsed. It is not known how many from this 
group survived. 
• Due to the presence of assembly use spaces at the top of each tower that were designed to 
accommodate over 1,000 occupants per floor for the Windows on the World restaurant 
complex and the Top of the Deck observation deck, the NYC Building Code would have 
required a minimum of four independent means of egress (stairs), one more than the three 
that were available in the buildings. Given the low occupancy level on September 11, 2001, 
NIST found that the issue of egress capacity from these places of assembly, or from 
elsewhere in the buildings, was not a significant factor on that day. It is conceivable that such 
a fourth stairwell, depending on its location and the effects of aircraft impact on its functional 
integrity, could have remained passable, allowing evacuation by an unknown number of 
additional occupants form above the floors of impact. Moreover, if the buildings had been 
filled to their capacity with 25,000 occupants, the required fourth stairway would likely have 
mitigated the insufficient egress capacity for conducting a full building evacuation within the 
available time. 
8.4.3 Emergency Response 
• New York City’s emergency responders had never experienced an operation of the size 
presented by the attack on the WTC. They typically followed their department policies and 
procedures for the operations they were required to carry out. Under these procedures, 
almost all emergency responder departments established their command posts within the 
potential collapse zone of the buildings. 
• In general, all departments attempted to work together to save as many lives as possible. This 
was done with no formal structure of unified command between departments below the 
Commissioner level of operations. 
• Unified operations were hindered by the FDNY and NYPD command posts being separated. 
Department Chiefs could not directly communicate with each other using their handie-talkies 
and did not formulate unified orders and directions for their departments. Neither FDNY nor 
NYPD had liaison officers working with the other department’s command posts until after 
WTC 1 collapsed. 
• The first emergency responders were colleagues and regular building occupants. Acts of 
individual heroism saved many people whom traditional emergency responders would have 
been unable to reach in time. 
• The initial fire department assessment of the situation was correct regarding the general 
magnitude of damage, the status of the water supply, and the further limitations imposed on 
firefighting by the height of the impact. FDNY command personnel learned from 9-1-1 
dispatch operators that smoke, fire, and structural damage in the buildings prevented many 
building occupants from evacuating floors above the impact zones. The decision was quickly 
made that fire department efforts should be directed toward evacuation and rescue of building 
occupants and should not focus on firefighting. 
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• Only one elevator in each building was of use to the responders. To gain access to the injured 
and trapped occupants, firefighters had to climb the stairs, carrying the equipment with them. 
• NIST estimated that emergency responder climbing rates varied between approximately 
1.4 min per floor for personnel not carrying extra equipment to approximately 2.0 min per 
floor for personnel wearing protective clothing and carrying extra equipment. 
• Intense smoke and heat conditions on the top of the two WTC buildings prevented the NYPD 
helicopters from conducting roof evacuations. 
• NYPD aviation unit personnel reported critical information about the impending collapse of 
the WTC towers several minutes prior to their collapse. No evidence has been found to 
suggest that the information was further communicated to all emergency responders at the 
scene. 
• There were roughly 1,000 emergency responders on the site. The Command Board approach 
to managing operations became overwhelmed with this large number of personnel and units 
reporting in for operations. The responding units generally followed good practices as related 
to accountability of staff. However, there were cases where individuals and ambulances did 
not report to the Command Posts. There was no way to locate or track units and individuals 
once they had departed to accomplish their tasks. 
• Generally, the equipment used by the emergency responders was adequate for the operations 
being carried out. Flashlights were valuable, since the stairwells were generally dark and 
many areas were opaque with smoke and dust, especially in WTC 1 after the collapse of 
WTC 2. Self-contained breathing apparatus enabled responders to breathe when they were in 
the zones where the air was contaminated. 
• For PAPD and NYPD, radio equipment did not appear to be a major problem during the 
operations. The PAPD's new radio antenna system provided it with a reasonable quality of 
radio communications until the collapse of WTC 2, at which time personnel were forced to 
switch to point-to-point communications. NYPD experienced successful operations with its 
radio equipment, mainly since only a few entered the WTC towers and the location of the 
mobilization point (a city block or more away from the towers) provided an unobstructed line 
of sight route for radio signals to enter and exit the building’s windows. 
• The overall emergency response was hampered by the loss of the Office of Emergency 
Management Command Center. 
• The FDNY Incident Command Desk was hampered by the lack of a fully functional Field 
Communications unit, poor radio communications, and limited access to shared information 
critical to operations. 
• The FDNY radio system was inadequate for locating and tracking the large number of 
personnel at the site. The FDNY relied heavily on hand-held units and knew that even the 
new handie-talkie radios did not work well in high-rise buildings where the signals were 
attenuated by the large amounts of metal and steel-reinforced concrete. Thus, the location of 
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Command Posts inside the buildings made calls to them subject to attenuation. Nonetheless, 
there were reports by emergency responders that their radios played a part in saving their 
lives. 
• Information overload quickly became a problem. After the first aircraft strike, there was a 
factor-of-five increase in emergency responder radio communications. This resulted in 
situations where the base station radio operators were unable to relay important information. 
Approximately 1/3 to 1/2 of the emergency radio communications were not complete 
messages or were not understandable. 
• Communications within, from, and to WTC 1 were problematic. The building emergency 
communications system used to make the emergency announcements inside was inoperable 
as a result of the aircraft impact. The warden and standpipe phone systems were also not 
operating. The radio repeater in WTC 5, though found to be operational, was not effective in 
WTC 1. Eventually, only about half of the responders located in WTC 1 heard radio 
messages calling for the immediate evacuation of the building. Emergency responders who 
had the evacuation information told others. 
• The WTC 5 repeater appeared to be effective in WTC 2. The Battalion Car Cross-band 
Repeater, recently developed by the FDNY and taken to the lobby of WTC 2, was used as a 
backup. 
• Information overload led to an inability to pool and analyze information in real time and to 
distribute it to the emergency responders and the WTC occupants in a timely fashion. As a 
result, many emergency responders did not get the critical information they needed to 
maintain good situational awareness. Some occupants did not get information that potentially 
could have saved their lives, such as notification that Stairwell A was passable from above 
the impact zone. 
• A preponderance of evidence indicted that lack of timely information sharing and inadequate 
communication capabilities likely contributed to the loss of emergency responder lives. 
• The collapse of WTC 2 totally disrupted the ongoing Incident Command System Operations 
being carried out by FDNY, NYPD, and PAPD. 
• The private ambulances and Emergency Medical Service teams that responded to the WTC 
had limitations to their effectiveness. They had no protective clothing. They did not have the 
same radios, so the other agencies could not communicate with them. Only paper records 
were kept of patients being treated by official and self-dispatched emergency medical units. 
These records were lost when the buildings collapsed. 
• Communications between the emergency response agencies and the media were problematic. 
Critical life safety and evacuation information from the WTC towers was not communicated 
to the news media so that it could be broadcast to people trapped inside the WTC towers 
above the building fires. By bypassing the appropriate emergency response agency contact 
points, some media firms interfered with the on-site operations. 
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8.5 FINDINGS ON OPERATIONAL CODES, STANDARDS, AND PRACTICES 
8.5.1 General 
• Although not required to conform to NYC codes, The Port Authority adopted the provisions 
of the proposed 1968 edition of the NYC Building Code, more than three years before it went 
into effect. The proposed 1968 edition allowed Port Authority to take advantage of less 
restrictive provisions and of technological advances compared with the 1938 edition, which 
was in effect when design for the WTC towers began in 1962. 
• The NYC Department of Buildings reviewed the WTC tower drawings in 1968 and provided 
comments to The Port Authority concerning the plans in relation to the 1938 NYC Building 
Code. The architect-of-record submitted to The Port Authority responses to those comments, 
noting how the plans conformed to the 1968 NYC Building Code 
• In 1993, The Port Authority and the NYC Department of Buildings entered into a 
memorandum of understanding that restated The Port Authority’s longstanding policy to 
ensure that its facilities in the City of New York meet and, where appropriate, exceed the 
requirements of the NYC Building Code. 
• The Port Authority was not required to yield, and appears not to have yielded, jurisdictional 
authority for regulatory and enforcement oversight to the New York City Department of 
Buildings. The Port Authority was created as an interstate entity, under a clause of the U.S. 
Constitution permitting compacts between states, and is not bound by the authority of any 
local or state jurisdiction. 
• It was remarkable that the Investigation Team was able to obtain the large quantity of 
documentation of the construction and subsequent modification of the WTC towers. Such 
documents are normally not archived for more than about 6 years to 7 years, with no 
requirements for storage remote from the building. In the case of the WTC towers, The Port 
Authority and its contractors and consultants maintained an unusually comprehensive set of 
documents, a dominant portion of which had not been destroyed in the collapse of the 
buildings but was assembled and provided to the Investigation Team. 
• The Architect of Record was responsible for specifying the fire protection and designing the 
evacuation system. There was not, and still is not, a requirement for a fire protection 
engineer to be part of the process. In the case of the WTC towers, the building owner played 
a significant role in specifying the fire protection and evacuation systems. 
• The current state-of-practice is not sufficiently advanced for engineers to routinely analyze 
the performance of a whole structural system under a prescribed design-basis fire scenario. 
• Buildings were not (and still are not) specifically designed to withstand the impact of fuelladen 
commercial aircraft, and building codes in the United States do not require building 
designs to consider aircraft impact. 
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• While two documents from The Port Authority indicated that the safety of the WTC towers 
and their occupants in an aircraft collision was a consideration in the original design, it 
appears that the effect of the subsequent fires was not considered. NIST was unable to locate 
any documentary evidence on the aircraft impact analysis considered by The Port Authority. 
8.5.2 Structural Safety 
• At the time of the design and construction of the WTC towers, there were no explicit 
structural integrity provisions to mitigate progressive collapse. U.S. Federal agencies and the 
United Kingdom have since developed and implemented such guidelines. New York City 
adopted by rule in 1973 a requirement for buildings to resist progressive collapse under 
extreme local loads. The rules apply specifically to buildings that used precast concrete wall 
panels and not to other types of buildings. 
• At the time of the design and construction of the WTC towers, there were no explicit 
minimum structural integrity provisions for the means of egress (stairwells and elevator 
shafts) in the building core that were critical to life safety. The building core, generally 
designed to be part of the vertical gravity load carrying system of the structure, need not be 
part of the lateral load carrying system of the structure. In this case, the structural designer 
may have preferred the use of partition walls rather than structural walls in the core area to 
reduce building weight. In the case of the WTC towers, the core had 2 hour fire-rated, 
gypsum partition walls with little structural integrity, and the core framing was required to 
carry only gravity loads. Had there been a minimum structural integrity requirement to 
satisfy normal building and fire safety considerations, it is conceivable that the damage to 
stairways, especially at the floors of impact, may have been less extensive. 
• Wind loads were a major factor in the design of structural components that made up the 
frame-tube steel framing system. Building codes allow the determination of wind forces 
from wind tunnel tests for use in design, but there was not (and still aren't) standards for 
conducting wind tunnel tests and for the methods used in practice to estimate design wind 
loads from test results. Results of two sets of wind tunnel tests conducted for the WTC 
towers in 2002 by independent commercial laboratories as part of insurance litigation, and 
voluntarily provided to NIST by the parties to the litigation, show up to 40 percent 
differences in resultant forces on the structures. There were also significant differences 
among various specified design wind speeds. Such disparities are indicative of the 
limitations associated with the current state of practice in wind engineering for tall buildings. 
• The original design wind loads on the towers exceeded those established in the prescriptive 
provisions of the NYC Building Code from 1968 through 2001. These wind loads were also 
higher than those required by other selected building codes and the relevant model building 
code of the time. Note, however, that the approach in these codes was oversimplified, and as 
a result, these codes may not be applicable for super-tall building design. 
• In the original design of the towers, the calculated drift (the maximum sway of the building) 
was significantly larger than what is currently used in practice. However, drift was not, and 
is not, a design factor prescribed in building codes. 
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• The demand/capacity ratios (DCRs) of the columns, estimated from the original WTC design 
loads, were in general close to those obtained under current wind design practice. The DCRs 
for over 99 percent of the floor trusses and beams were less than unity as they should have 
been. The safety of the WTC towers on September 11, 2001 was most likely not affected by 
the fraction of structural members for which the demand exceeded allowable capacity due to: 
(1) the factor of safety in the allowable stress design method, (2) the load redistribution 
capability of the steel structures, and (3) the towers having been subjected to lighter than 
routine live loads and minimal wind loads at the time of the attacks. 
• Under a combination of the original WTC design dead and wind loads, tension forces were 
developed in the exterior walls of both towers. The forces were largest at the base of the 
building and at the corners. The Investigation showed that the DCRs for the exterior wall 
splice connections were less than 1.0. 
• For the towers’ resistance to shear sliding under wind loads, the factor of safety was between 
10 and 11.5, while the factor of safety against overturning ranged from 1.9 to 2.7 for both 
towers. 
• The period of natural building oscillations calculated from the reference global model of the 
WTC 1 matched well those determined from accelerometers located atop the tower. This lent 
credence to the global models of the towers. 
8.5.3 Fire Safety 
• By being consistent with the proposed 1968 edition of the NYC Building Code, rather than 
the requirements of the 1938 Code, the tower design: 
- Eliminated a fire tower15 (also called a smoke-proof stairway) as a required means of 
egress; 
- Reduced the number of required stairwells from 6 to 3 and the size of doors leading to the 
stairs from 44 in. to 36 in.; 
- Reduced the fire rating of the shaft walls in the building core from 3 hours to 2 hours; and 
- Permitted a 1 hour reduction in fire rating for all structural components (columns from 
4 hours to 3 hours and floor framing members from 3 hours to 2 hours) by allowing the 
owner/architect to select Class 1B construction for business occupancy and unlimited 
building height. 
Many of these reductions are contained in current codes. 
15 A fire tower (also called a smoke-proof stair) is a stairway that is accessed through an enclosed vestibule that is open to the 
outside or to an open ventilation shaft providing natural ventilation that prevents any accumulation of smoke without the need 
for mechanical pressurization. 
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• In 1993, The Port Authority adopted a policy providing for implementation of fire safety 
recommendations made by local government fire departments after a fire safety inspection of 
a Port Authority facility and for the prior review by local fire safety agencies of fire safety 
systems to be introduced or added to a facility. Later that year, The Port Authority entered 
into an agreement with FDNY that reiterated the policy adopted by The Port Authority, 
recognized the right of FDNY to conduct fire safety inspections of Port Authority properties 
in the City of New York, provided guidelines for FDNY to communicate needed corrective 
actions to The Port Authority, ensured that new or modified fire safety systems are in 
compliance with local codes and regulations, and required third-party review of such systems 
by a New York State licensed architect or engineer. 
• Compartmentation of spaces is a key building fire safety requirement to limit fire spread. 
The WTC towers initially had 1 hour fire-rated partitions separating tenants (demising walls) 
that extended from the floor to the suspended ceiling, not the floor above (the ceiling tiles 
were not fire rated). Over the years, these partitions were replaced with partitions that were 
continuous from floor to floor (separation wall), consistent with the 1968 NYC Building 
Code. Some partitions had not been upgraded by 1997, and a consultant recommended to 
The Port Authority that it develop and implement a survey program to ensure that the 
remediation process occurred as quickly as possible. It appears that with few exceptions, 
nearly all of the floors not upgraded were occupied by a single tenant. The Port Authority 
adopted guidelines in 1998 that required such partitions to provide a continuous fire barrier 
from top of floor to underside of slab. 
• No technical basis was found for selecting the spray-applied fire resistive material (SFRM) 
used or its thickness for the large-span open-web floor trusses of the WTC towers. The 
assessment of the insulation thickness needed to meet the 2 hour fire rating requirement for 
the untested WTC floor system evolved over time: 
- In October 1969, The Port Authority directed the insulation contractor to apply 1/2 in. of 
insulation to the floor trusses. 
- In 1999, The Port Authority issued guidelines requiring that insulation be upgraded to 1 
1/2 in. for full floors undergoing alterations. 
- Unrelated to the WTC buildings, an International Conference of Building Officials 
(ICBO) Evaluation Service report (ER-1244), re-issued June 1, 2001, using the same 
SFRM recommends a minimum thickness of 2 in. for “unrestrained steel joists” with 
“lightweight concrete” slab. 
• There was no code provision that required the conduct of a fire resistance test if adequate data 
did not exist from other building components and assemblies to qualify an untested building 
element. Instead, several alternate methods were permitted, with limited guidance on detailed 
procedures to be followed. Both the Architect of Record (in 1966) and the Structural 
Engineer of Record (in 1975) stated that the fire rating of the floor system of the WTC towers 
could not be determined without testing. NIST did not find evidence indicating that such a 
test was conducted to determine the fire rating of the WTC floor system. The Port Authority 
informed NIST that there are no such test records in its files. 
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Draft for Public Comment Principal Findings 
• Neither the 1968 edition of the NYC Building Code, which was used in the design of the 
WTC towers, nor the 2001 edition of the code, adopted the “structural frame” requirement. 
Use of the “structural frame” approach, in conjunction with the prescriptive fire rating, would 
have required the floor trusses, the core floor framing, and perimeter spandrels in the WTC 
towers to be 3 hour fire-rated, like the columns for Class 1B construction in the 1968 NYC 
Building Code. This approach, which appeared in the Uniform Building Code (a model 
building code) as early as 1953, was carried into the 2000 International Building Code (one 
of two current model codes). The WTC floor system was essential to the stability of the 
building as a whole since it provided lateral stability to the columns and diaphragm action to 
distribute wind loads to the columns of the frame-tube system. 
• There was, and is, no technical basis to establish whether the construction classification and 
fire rating requirements were risk-consistent with respect to the design-basis hazard and the 
consequences of that hazard. For tall buildings, the likely consequences of a given threat to 
an occupant on the upper floors are more severe than the consequences to an occupant on the 
first floor, especially considering more difficult access by firefighters and increased time 
required for stairwell evacuation. There were, and are, no additional categories for tall 
buildings, where different building classification and fire ratings requirements may be 
appropriate. 
• There were, and are, no field application and inspection requirements to ensure that the 
as-built condition of the passive fire protection, such as SFRM, conformed to conditions 
found in fire resistance tests of building components and assemblies. This includes 
determination of whether the as-applied average insulation thickness and variability was 
thermally equivalent to the specified minimum fire proofing thickness. In addition, 
requirements were not available for in-service inspections of passive fire protection during 
the life of the building. The adequacy of the insulation of the WTC towers posed an issue of 
some concern to The Port Authority over the life of the buildings. 
• Structural design did not, and does not, consider fire as a design condition, as it does the 
effects of dead loads, live loads, wind loads, and earthquake loads. Current prescriptive code 
provisions are based on tests that provide relative ratings of fire resistance. These may be 
adequate for simple structures and for comparing the relative performance of structural 
components in more complex structures. The state-of-the-art did not enable evaluation of the 
actual performance (i.e., load carrying capacity) in a real fire of the structural components, or 
the structure as a whole system, including the connections between components. 
• The provisions that were used for the WTC towers did not require specification of a firerating 
requirement for connections separate from those for the connected elements. The 
Investigation Team was unable to determine the fire rating of a connection where the 
connected elements had different fire ratings, and whether the applied insulation achieved 
that rating. 
• There was, and is, no technical basis to establish whether the minimum mechanical and 
durability related properties of SFRM were sufficient to ensure acceptable in-service 
performance in buildings. This includes the ability of such materials to withstand typical 
shock, impact, vibration, or abrasion effects over the life of a building. There are now 
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measurement methods for many of these properties, but the relationship of the results to 
serviceability requirements is in need of technical support. 
• There were no validated tools to analyze the dynamics of building fires and their effects on 
the structural system that would have allowed engineers to evaluate structural performance 
under alternative fire scenarios and fire protection strategies. While considerable progress 
has been made in recent years, significant work remains to be done before adequate tools are 
available for use in routine practice. NIST had to further develop and validate tools to 
investigate the fire performance of the WTC towers. 
• Building code provisions for sprinkler installation were an option in lieu of 
compartmentation. NYC has since promulgated local laws to encourage installation of 
sprinklers in new buildings, and is now considering a law to require sprinklers in existing 
buildings. The WTC towers were fully sprinklered by 2001, about 30 years after their 
construction. 
• Active smoke management systems and/or combination fire/smoke dampers were not 
required in fully sprinklered buildings by the 1968 NYC Building Code or any subsequent, 
retroactive provisions. 
• With a few special exceptions, building codes in the United States did not, and do not, require 
the use of fire-protected elevators for routine emergency access by first responders or as a 
secondary method (after stairways) for full building evacuation of occupants in emergencies. 
• Firefighters moving up the stairs did not significantly lengthen the average time evacuees 
spent in the stairways. However, the climbing rate of the firefighter was hampered by the 
presence and movement of the evacuees. 
• The separation of the three stairwells in each tower exceeded the requirements of the 1968 
NYC Building Code. On some floors, the separation distances were not as large as the 
current model building codes require, while on other floors, the separation distances 
significantly exceeded the provisions in those codes. 
8.6 FUTURE FACTORS THAT COULD HAVE IMPROVED LIFE SAFETY 
In the course of the Investigation, NIST and its contractors were aware that there were modern, emerging, 
or even imagined capabilities that could have increased the survival rate of those in the WTC towers, had 
they been in place on September 11, 2001. These are listed here, not posed as recommendations for 
implementation, but presented for completeness in the portrayal of the findings of the Investigation. 
NIST has not conducted studies to evaluate the degree to which building performance and human factors 
could have been improved on September 11, 2001, had the capabilities been available. 
8.6.1 Building Performance Factors 
• Thermal insulation that bonds more firmly to structural steel. 
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• Perimeter column and floor framing with greater mass to enhance thermal and buckling 
resistance. 
• Improved compartmentation and stairwell enclosures. 
• Thermally resistant window assemblies to limit the air supply and retard fire growth. 
• Steels with improved high temperature properties, especially with regard to creep. 
• Fire protected and structurally hardened elevators for use in occupant evacuation and 
responder access. 
8.6.2 Human Performance Factors 
• More accurate and reliable communications among emergency responders and building 
occupants. 
• Better management of large-scale emergency incidents. 
• Better evacuation training. 
• Self-evacuation capability for the mobility impaired. 
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NIST NCSTAR 1, WTC Investigation 

Chapter 9 
RECOMMENDATIONS 
BUILDING STANDARDS AND CODES: WHO IS IN CHARGE? 
Codes and standards for the design, construction, operation, and maintenance of buildings are the 
documents by which a society states its intent to provide public safety and functionality. They 
incorporate the knowledge, experience, procedures, and practices of the applicable engineering 
disciplines, the values of the contemporary society, the experiences from prior successes and failures, and 
knowledge of the commercial products, services, and technologies available for the tasks at hand. 
The United States has a unique approach to such codes and standards. In virtually all other developed 
countries the national government has a primary role in the development of national model codes. In the 
United States, the private sector develops such codes and standards. Further, with few exceptions, state 
and local governments are responsible for promulgating and enforcing building and fire safety regulations 
in the United States. These regulations provide minimum requirements for public welfare and safety. 
While a single, uniform set of statewide regulations is increasingly being adopted by the states, a similar 
pattern is developing in major cities and counties. The National Conference of States on Building Codes 
and Standards (NCSBCS)—a body of the National Governors Association and the Council of State 
Governments—includes members representing chief building regulatory officials of the states and local 
code officials from across the nation. While NCSBCS does not develop or implement building codes, it 
provides a national forum to discuss issues related to codes, standards, and practices that cut across 
jurisdictional boundaries. 
With some exceptions, building and fire safety regulations of state and local jurisdictions are based on 
national model codes developed by private sector organizations—the International Code Council (ICC) 
and the National Fire Protection Association (NFPA). At present (June 2005), 45 states plus the District 
of Columbia use the ICC’s International Building Code, while 36 states plus the District of Columbia use 
the ICC’s International Fire Code. Similarly, NFPA’s National Electrical Code is used in virtually all 
jurisdictions. Model codes are developed using committees of experts, generally adapted to reflect local 
climate and geological conditions by state and local governments, and updated every three years. 
Proposals to modify the model codes are offered by individuals or organizations. These are discussed in 
open fora before being accepted or rejected by vote. Localities adopting model codes update their 
versions periodically as well, to follow roughly the same schedule as the model codes. With the 
exception of standards for manufactured housing, the federal government’s role in determining specific 
codes is mandatory only for federally owned, leased, regulated, or financed facilities. 
The model codes adopt by reference voluntary consensus standards that are developed by a large number 
of private sector standards development organizations (SDOs). The SDOs include NFPA, ASTM 
International, the American Society of Civil Engineers (ASCE), the American Institute of Steel 
Construction (AISC), the American Concrete Institute (ACI), and the American Forest & Paper 
Association (AF&PA). The processes used by these organizations are accredited by the American 
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National Standards Institute (ANSI), which administers and coordinates the U.S. voluntary 
standardization and conformity assessment system. 
In addition to standards and codes organizations, there are other key stakeholder groups that either are 
responsible for or influence the practices used in the design, construction, operation, and maintenance of 
buildings in the United States. These typically include organizations representing building owners and 
managers (e.g., Building Owners and Managers Association, Construction Industry Institute), real estate 
developers (e.g., Real Estate Board of New York), contractors (e.g., Associated General Contractors, 
Associated Builders and Contractors), architects (e.g., American Institute of Architects), engineers (e.g., 
National Society of Professional Engineers, Society of Fire Protection Engineers, Structural Engineering 
Institute, National Council of Structural Engineering Associations), suppliers, and insurers. These groups 
also provide training, especially as it affects the ability to implement code provisions in practice. Lack of 
adequate training programs can limit the usefulness or widespread acceptance of improved code 
provisions. Very few members of the general public and building occupants participate in this process. 
The National Institute of Standards and Technology (NIST) is a non-regulatory agency of the U.S. 
Department of Commerce. NIST does not set building codes or standards, but provides technical support 
to the private sector and to other government agencies in the development of U.S. building and fire 
practice, standards and codes. NIST provides this support by: conducting research which helps to form 
the technical basis for such practice, standards, and codes; disseminating research results to practicing 
professionals; having its staff participate on technical and standards committees; and, providing technical 
assistance to the building and fire safety communities. Due to limited participation of the general public 
and building occupants, NIST has a responsibility to represent the public’s interest. As an objective and 
impartial technical entity, NIST recommendations are given serious consideration by private sector 
organizations that develop national standards and model codes, which provide minimum requirements for 
public welfare and safety. 
Rigorous enforcement of building codes and standards by state and local agencies, well trained and 
managed, is critical in order for standards and codes to ensure the expected level of safety. Unless 
they are complied with, the best codes and standards cannot protect occupants, emergency 
responders, or buildings. 
NIST’S RECOMMENDATIONS FOR IMPROVING THE SAFETY OF 
BUILDINGS, OCCUPANTS, AND EMERGENCY RESPONDERS 
NIST is conducting its building and fire safety investigation of the WTC disaster of September 11, 2001, 
under the authority of the National Construction Safety Team Act (15 USC 7301 et seq.). The National 
Construction Safety Team’s final report is required by the Act to include recommendations that address 
(1) specific improvements to building standards, codes, and practices, (2) changes to, or the establishment 
of, evacuation and emergency response procedures, and (3) research and other appropriate actions needed 
to help prevent future building failures. 
As part of its WTC Investigation, NIST is issuing draft recommendations for public comment that 
identify specific improvements in the way buildings are designed, constructed, maintained, and used and 
in evacuation and emergency response procedures. NIST believes that these recommendations are both 
realistic and achievable within a reasonable period of time and that their implementation would make 
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buildings safer for occupants and emergency responders in future emergencies. NIST strongly urges 
that immediate and serious consideration be given to these recommendations by the building and 
fire safety communities—especially designers, owners, developers, codes and standards 
development organizations, regulators, fire safety professionals, and emergency responders. NIST 
also strongly urges building owners and public officials to (1) evaluate the safety implications of 
these recommendations to their existing inventory of buildings, and (2) take the steps necessary to 
mitigate any unwarranted risks without waiting for changes to occur in codes, standards, and 
practices. NIST is assigning top priority to work vigorously with these communities to ensure that there 
is a complete understanding of the recommendations and their technical basis and to provide needed 
technical assistance. As part of this effort, NIST will develop and maintain a web-based system with 
information on the status of NIST’s recommendations that will be available to the public so that progress 
in implementing them can be tracked. 
In formulating its recommendations from the WTC Investigation, NIST considered: 
• Findings related to building performance, evacuation and emergency response, and to 
procedures and practices used in the design, construction, operation, and maintenance of the 
buildings; 
• Whether these findings relate to the unique circumstances surrounding the terrorist attacks of 
September 11, 2001, or to normal building and fire safety considerations (including 
evacuation and emergency response); 
• Technical solutions that are needed to address potential risks to buildings, occupants, and 
emergency responders, considering both identifiable hazards and the consequences of those 
hazards; and 
• Whether the risks apply to all buildings or are limited to certain building types (e.g., buildings 
that exceed a certain height and floor area or that employ a specific type of structural system), 
buildings that contain specific design features, iconic/signature buildings, or buildings that 
house critical functions. 
While there were unique aspects to the design of the WTC towers and the terrorist attacks of 
September 11, 2001, the design, construction, operation, and maintenance of the WTC towers—and the 
emergency response to the WTC towers—were based on procedures and practices that are commonly 
used for normal conditions. These include procedures and practices used for construction classification, 
establishing and determining fire resistance ratings, estimating wind loads, designing structural 
components and connections, designing egress systems, designing sprinkler systems, evacuation, and 
emergency response. 
As an integral part of its Investigation, NIST reviewed the relevant commonly used procedures and 
practices and established a baseline performance for the buildings, evacuation, and emergency response. 
The performance on September 11, 2001 was then compared to the baseline performance. NIST is 
making several recommendations based on findings from its review of these procedures and practices. 
NIST is also making recommendations for selected buildings that are at greater risk, e.g., due to their 
iconic status, critical function, or design. 
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In its recommendations, NIST does not prescribe specific systems, materials, or technologies; NIST 
encourages competition among different systems, materials, and technologies that can meet performance 
requirements. NIST also does not prescribe specific threshold levels; NIST believes that the 
responsibility for the establishment of threshold levels properly belongs in the public policy setting 
process, in which the standards and codes development process plays a key role. 
Based on its Investigation findings, NIST identified a broad set of issues related to practices, standards, 
and codes that provided the basis for the recommendations. The 30 draft recommendations resulting from 
the NIST Investigation were prepared by the Investigation Team with benefit of review by the National 
Construction Safety Team Advisory Committee. Table 9–1 (which follows the recommendations) shows 
a crosswalk between the recommendations in each of eight groups and three categories (responsible 
community, affected building population, and relation to outcome on September 11, 2001). The topics 
addressed in each group of recommendations are summarized below: 
1. Increased structural integrity, including methods for preventing conditions that could result in 
progressive collapse (when a building or a significant portion of a building collapses due to 
disproportionate spread of an initial local failure), standardizing the estimation of wind loads 
that frequently govern the design of tall buildings, and enhancing the stability of tall 
buildings. 
2. Enhanced fire resistance of structures, including the technical basis for determining 
construction classification and fire resistance ratings, improvements to the technical basis for 
standard fire resistance testing methods, adoption of the “structural frame” approach to fire 
resistance ratings, and in-service performance requirements and conformance assessment 
criteria for spray-applied fire resistive materials. 
3. New methods for designing structures to resist fires, including the objective of burnout 
without collapse, the development of performance-based methods as an alternative to current 
prescriptive design methods, the development and evaluation of new fire resistive coating 
materials and technologies, evaluation of the fire performance of conventional and highperformance 
structural materials, and elimination of technical and standards barriers to the 
introduction of new materials and technologies. 
4. Improved active fire protection, including the design, performance, reliability, and 
redundancy of sprinklers, standpipes/hoses, fire alarms, and smoke management systems. 
5. Improved building evacuation, including system designs that facilitate safe and rapid egress, 
methods for ensuring clear and timely emergency communications to occupants, better 
occupant preparedness for evacuation during emergencies, and incorporation of appropriate 
egress technologies. 
6. Improved emergency response, including better access to the buildings and better operations, 
emergency communications, and command and control in large-scale emergencies. 
7. Improved procedures and practices, including encouraging code compliance by 
nongovernmental and quasi-governmental entities, adoption and application of egress 
requirements in available code provisions for existing buildings, and retention and 
availability of building documents over the life of a building. 
8. Education and training programs for fire protection engineers, structural engineers, and 
architects. 
These improvements are to be achieved both by complying with existing codes and through provisions 
that address new requirements. Each recommendation was further noted for the (1) community 
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Draft for Public Comment Recommendations 
(stakeholders) responsible for addressing it, (2) the building population affected, and (3) whether the 
recommendation is related to the outcome on September 11, 2001. These three categories are further 
divided as follows (also included in Table 9–1): 
• Responsible Community: 
- Professional practices 
- Provisions in standards, codes, and regulations 
- Adoption and enforcement of the provisions 
- Research and development or requiring further study
- Education and training 
• Affected Population of Buildings: 
- All tall buildings (buildings over 20 stories in height16; building owners and public 
officials will need to determine appropriate performance requirements for buildings that 
are at risk due to types of structural, fire safety, or egress systems used, location, use, 
historic/iconic status, nature of occupancy, etc.) 
- Selected other buildings (buildings less than 20 stories in height that are at risk due to 
types of systems used, location, use, historic/iconic status, nature of occupancy, etc.). 
• Relation to the outcome on September 11, 2001: 
- If in place, could have changed the outcome on September 11, 2001 
- Would not have changed the outcome, yet is an important building and fire safety issue 
that was identified during the course of the Investigation 
The recommendations are listed below in eight groups, with each recommendation assigned a number 
(1, 2, 3, etc.) for easy reference. The numerical ordering does not reflect any priority. 
9.2.1 Group 1. Increased Structural Integrity 
The standards for estimating the load effects of potential hazards (e.g., progressive collapse, wind) 
and the design of structural systems to mitigate the effects of those hazards should be improved to 
enhance structural integrity. 
Recommendation 1. NIST recommends that: (1) progressive collapse should be prevented in 
buildings through the development and nationwide adoption of consensus standards and code 
provisions, along with the tools and guidelines needed for their use in practice; and (2) a 
16 NIST has found that the physiological impacts on emergency responders of climbing 20 or more stories makes it difficult to 
conduct effective and timely firefighting and rescue operations in building emergencies without functioning elevators. Better 
knowledge of the physiological impacts through research could refine the definition of tall buildings used here. 
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standard methodology should be developed—supported by analytical design tools and practical 
design guidance—to reliably predict the potential for complex failures in structural systems 
subjected to multiple hazards. 
a. Progressive collapse17 should be prevented in buildings. The primary structural systems should 
provide alternate paths for carrying loads in case certain components fail (e.g., transfer girders, 
columns supporting only gravity loads). This is especially important in buildings where structural 
components (e.g., columns, girders) support unusually large floor areas. 18 Progressive collapse is 
addressed only in a very limited way. While the initiating event in design to prevent progressive 
collapse typically is considered to be the failure of one or two columns at the bottom of the 
structure, possible initiating events at other locations within the structure, or involving other key 
components and subsystems, should be analyzed commensurate with the risks considered in the 
design. The effectiveness of mitigation approaches involving new system and subsystem design 
concepts should be evaluated with conventional approaches based on indirect design (continuity, 
strength, and ductility of connections), direct design (local hardening), and redundant (alternate) 
load paths. The capability to prevent progressive collapse due to abnormal loads should include: 
(i) comprehensive design rules and practice guides; (ii) evaluation criteria, methodology, and 
tools for assessing the vulnerability of structures to progressive collapse; (iii) performance-based 
criteria for abnormal loads and load combinations; (iv) analytical tools to predict potential 
collapse mechanisms; and (v) computer models and analysis procedures for use in routine design 
practice. The federal government should coordinate the existing programs that address this need: 
those in the Department of Defense; the General Services Administration; the Defense Threat 
Reduction Agency; and NIST. Affected National Standards19: ASCE-7, AISC Specifications, 
and ACI 318. These standards and other relevant committees should draw on expertise from 
ASCE-29 for issues concerning progressive collapse under fire conditions. National Model 
Building Codes: The standards should be adopted in national model building codes (i.e., the 
International Building Code and NFPA 5000) by mandatory reference to, or incorporation of, the 
latest edition of the standard. State and local jurisdictions should adopt and enforce the improved 
national model building codes and national standards based on all 30 WTC recommendations. 
The codes and standards may vary from the WTC recommendations, but satisfy their intent. 
b. A robust, integrated predictive capability should be developed, validated, and maintained to 
routinely assess the vulnerability of whole structures to the effects of potential hazards. This 
capability to evaluate the performance and reserve capacity of structures does not exist and is a 
significant cause for concern. This capability also would assist in investigations of building 
failure—as demonstrated by the analyses of the WTC building collapses carried out in this 
Investigation. The failure analysis capability should include proper identification of the complex 
failure phenomena to be analyzed under multiple hazards (e.g., bomb blasts, fires, impacts, gas 
explosions, earthquakes, and hurricane winds), experimentally-validated models that are capable 
of capturing the essential physical phenomena, and robust tools for routine analysis to predict 
such failures and their consequences. This capability should be developed via a coordinated 
effort involving federal, private sector, and academic research organizations in close partnership 
with practicing engineers. 
17 Progressive collapse (or disproportionate collapse) occurs when an initial local failure spreads from structural element to 
structural element resulting in the collapse of an entire structure or a disproportionately large part of it. 
18 While the WTC towers eventually collapsed, they had the capacity to redistribute loads from impact and fire damaged 
structural components and subsystems to undamaged components and subsystems. However, the core columns in the WTC 
towers lacked sufficient redundant (alternate) paths for carrying gravity loads. 
19 A full listing of the affected standards, including the complete names of these standards, is provided in Table 2, which is 
located following the recommendations. 
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Draft for Public Comment Recommendations 
Recommendation 2. NIST recommends that nationally accepted performance standards be 
developed for: (1) conducting wind tunnel testing of prototype structures based on sound 
technical methods that result in repeatable and reproducible results among testing laboratories; 
and (2) estimating wind loads and their effects on tall buildings for use in design, based on wind 
tunnel testing data and directional wind speed data. Wind loads specified in current prescriptive 
codes may not be appropriate for the design of very tall buildings since they do not account for 
building-specific aerodynamic effects. Further, a review of wind load estimates for the WTC towers 
indicated differences by as much as 40 percent from wind tunnel studies conducted in 2002 by two 
independent commercial laboratories. Major sources of differences in estimation methods currently 
used in practice occur in the selection of design wind speeds and directionality, the nature of 
hurricane wind profiles, the estimation of “component” wind effects by integrating wind tunnel data 
with wind speed and direction information, and the estimation of “resultant” wind effects using load 
combination methods. Wind loads were a major factor in the design of the WTC tower structures and 
were relevant to evaluating the baseline capacity of the structures to withstand abnormal events such 
as major fires or impact damage. Yet, there is lack of consensus on how to evaluate and estimate 
winds and their load effects on buildings. 
a. Nationally accepted standards should be developed and implemented for conducting wind tunnel 
tests, estimating site-specific wind speed and directionality based on available data, and 
estimating wind loads associated with specified design probabilities from wind tunnel test results 
and directional wind speed data. 
b. Nationally accepted standards should be developed for estimating wind loads in the design of tall 
buildings. The development of performance standards for estimating wind loads should consider 
(i) appropriate load combinations and load factors, including performance criteria for static and 
dynamic behavior, based on both ultimate and serviceability limit states, and (ii) validation of 
wind load provisions in prescriptive design standards for tall buildings, given the universally 
acknowledged use of wind-tunnel testing and associated performance criteria. Limitations to the 
use of prescriptive wind load provisions should be clearly identified in codes and standards. 
The standards development work can begin immediately to address many of the above needs. The 
results of those efforts should be adopted in practice as soon as they become available. The research 
that will be required to address the remaining needs also should begin immediately and results should 
be made available for standards development and use in practice. Affected National Standard: 
ASCE-7. National Model Building Codes: The standard should be adopted in national model 
building codes by mandatory reference to, or incorporation of, the latest edition of the standard. 
Recommendation 3. NIST recommends that an appropriate criterion should be developed and 
implemented to enhance the performance of tall buildings by limiting how much they sway 
under lateral load design conditions (e.g., winds and earthquakes). The stability and safety of tall 
buildings depend upon, among other factors, the magnitude of building sway or deflection, which 
tends to increase with building height. Conventional strength-based design methods, such as those 
used in the design of the WTC towers, do not limit deflections. The deflection limit state criterion, 
which is proposed here is in addition to the stress limit and serviceability requirement; it should be 
adopted either to complement the safety provided by conventional strength-based design or 
independently as an alternate deflection-based approach to the design of tall buildings for life safety. 
The recommended deflection limit state criterion is independent of the criterion used to ensure 
occupant comfort, which is met in current practice by limiting accelerations (e.g., in the 15 to 
20 milli-g range). Lateral deflections, which already are limited in the design of tall buildings to 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
control damage in earthquake-prone regions, should also be limited in non-seismic areas.20 Affected 
National Standards: ASCE-7, AISC Specifications, and ACI 318. National Model Building Codes: 
The standards should be adopted in national model building codes by mandatory reference to, or 
incorporation of, the latest edition of the standard. 
9.2.2 Group 2. Enhanced Fire Resistance of Structures 
The procedures and practices used to ensure the fire resistance of structures should be enhanced by 
improving the technical basis for construction classifications and fire resistance ratings, improving 
the technical basis for standard fire resistance testing methods, use of the “structural frame” 
approach to fire resistance ratings, and developing in-service performance requirements and 
conformance criteria for spray-applied fire resistive materials. 
Recommendation 4. NIST recommends evaluating, and where needed improving, the technical 
basis for determining appropriate construction classification and fire rating requirements 
(especially for tall buildings greater than 20 stories in height)—and making related code 
changes now as much as possible—by explicitly considering factors including21: 
• timely access by emergency responders and full evacuation of occupants, or the time 
required for burnout without local collapse; 
• the extent to which redundancy in active fire protection (sprinkler and standpipe, fire 
alarm, and smoke management) systems should be credited for occupant life safety22; 
• the need for redundancy in fire protection systems that are critical to structural integrity23; 
• the ability of the structure and local floor systems to withstand a maximum credible fire 
scenario without collapse, recognizing that sprinklers could be compromised, not 
operational, or non-existent; 
• compartmentation requirements (e.g., 12,000 ft2 (24)) to protect the structure, including fire 
rated doors and automatic enclosures, and limiting air supply (e.g., thermally resistant 
window assemblies) to retard fire spread in buildings with large, open floor plans; 
• the impact of spaces containing unusually large fuel concentrations for the expected 
occupancy of the building; and 
20 Analysis of baseline performance under the original design wind loads indicated that the WTC towers would need to have been 
between 50 and 90 percent stiffer to achieve a typical drift ratio used in current practice for non-seismic regions, though not 
required by building codes. Limiting drift would have required increasing exterior column areas in lower stories and/or 
significant additional damping. 
21 The construction classification and fire rating requirements should be risk-consistent with respect to the design-basis hazards 
and the consequences of those hazards. The fire rating requirements, which were originally developed based on experience 
with buildings less than 20 stories in height, have generally decreased over the past 80 years since historical fire data for 
buildings suggests considerable conservatism in those requirements. For tall buildings, the likely consequences of a given 
threat to an occupant on the upper floors are more severe than the consequences to an occupant on the first floor or the lower 
floors. For example, with non-functioning elevators, both the time requirements are much greater for full building evacuation 
from upper floors and emergency responder access to those floors. It is not clear how the current height and areas tables in 
building codes consider the technical basis for the progressively increasing risk to an occupant on the upper floors of tall 
buildings that are much greater than 20 stories in height. 
22 Occupant life safety, prevention of fire spread, and structural integrity are considered separate safety objectives. 
23 The passive fire protection system (includes fireproofing insulation, compartmentation, and firestopping) and the active 
sprinkler system each provide redundancy for maintaining structural integrity in a building fire, should one of the systems fail 
to perform its intended function. 
24 Or a more appropriate limit, which represents a reasonable area for active firefighting operations. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
• the extent to which fire control systems, including suppression by automatic or manual 
means, should be credited as part of the prevention of fire spread. 
Adoption of this recommendation will allow building codes to distinguish the risks associated with 
different building heights, fuel concentrations, and fire protection systems. Research is needed to 
develop the data and evaluate alternative proposals for construction classifications and fire ratings. 
National Model Building Codes: The national model building code committees should undertake a 
comprehensive review of current construction classification and fire rating requirements and establish 
a uniform set of revised thresholds with a firm technical basis that considers the factors identified 
above. 25 
Recommendation 5. NIST recommends that the technical basis for the century-old standard 
for fire resistance testing of components, assemblies, and systems should be improved through a 
national effort. Necessary guidance also should be developed for extrapolating the results of 
tested assemblies to prototypical building systems. This effort should address the technical issues 
listed below26: 
a. Criteria and test methods for determining: 
o structural limit states, including failure, and means for measurement; 
o effect of scale of test assembly versus prototype application, especially for long-span 
structures that significantly exceed the size of testing furnaces; 
o effect of end-restraint conditions (restrained and unrestrained) on test results, especially for 
long-span structures that have greater flexibility; 
o fire resistance of structural connections, especially the fire protection required for a loaded 
connection to achieve a specified rating27; 
o effect of the combination of loading and exposure (time-temperature profile) required to 
adequately represent expected conditions; 
o the repeatability and reproducibility of test results (typically results from a single test are used 
to determine rating for a component or assembly); and 
o realistic ratings for structural assemblies made with materials that have improved elevated 
temperature properties (strength, modulus, creep behavior). 
b. Improved procedures and guidance to analyze and evaluate existing data from fire resistance tests 
of building components and assemblies for use in qualifying an untested building element. 
c. Relationships between prescriptive ratings and performance of the assembly in real fires. 
25 The National Fire Protection Association (NFPA) 5000 model code and the International Building Code (IBC) both recognize 
the risks associated with different building heights and accepted changes in 2001 and 2004, respectively. Both model codes 
now require that buildings 420 feet and higher have a minimum 4 hour structural fire-resistance rating. The previous 
requirement was 2 hours. The change provides increased fire resistance for the structural system leading to enhanced 
tenability of the structure and gives firefighters additional protection while fighting a fire. While NIST supports these changes 
as an interim step, NIST believes that it is essential to complete a comprehensive review that will establish a firm technical 
basis for construction classification and fire rating requirements. 
26 The technical issues were identified from the series of four fire resistance tests of the WTC floor system and the review and 
analysis of relevant documents that were conducted as part of this Investigation. 
27 There is a lack of test data on the fire resistance ratings of loaded connections. The fire resistance of structural connections is 
not rated in current practice. Also, standards and codes do not provide guidance on fireproofing requirements for structural 
connections when the connected members have different fire resistance ratings. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
Affected National (and International) Standards 28: ASTM E 119, NFPA 251, UL 263, and ISO 834. 
National Model Building Codes: The standards should be adopted in national model building codes 
by mandatory reference to, or incorporation of, the latest edition of the standard. 
Recommendation 6. NIST recommends the development of criteria, test methods, and 
standards: (1) for the in-service performance of spray-applied fire resistive materials (SFRM, 
also commonly referred to as fireproofing or insulation) used to protect structural components; 
and (2) to ensure that these materials, as-installed, conform to conditions in tests used to 
establish the fire resistance rating of components, assemblies, and systems. This should include: 
• Improved criteria and testing methodology for the performance and durability of SFRM (e.g., 
adhesion, cohesion, abrasion and impact resistance) under in-service exposure conditions (e.g., 
temperature, humidity, vibration, impact, with/without primer paint on steel29) for use in 
acceptance and quality control. The current test method to measure the bond strength, for 
example, does not distinguish the cohesive strength from the tensile and shear adhesive strengths. 
Nor does it consider the effect of primer paint on the steel surface. Further, no test requirements 
explicitly consider the effects of abrasion, vibration, shock, and impact under normal service 
conditions. Also, the effects of elevated temperatures on thermal properties and bond strength are 
not considered in evaluating the performance and durability of SFRM. 
• Inspection procedures, including practical conformance criteria, for SFRM in both the building 
codes and fire codes for use after installation, renovation, or modification of all mechanical and 
electrical systems and by fire inspectors over the life of the building. While there are existing 
standards of practice (AIA MasterSpec and AWCI Standard 12), they are not required by codes 
nor are they enforced. Further, these standards require improvements to address the issues 
identified in this recommendation. 
• Criteria for determining the effective uniform SFRM thickness—thermally equivalent to the 
variable thickness of the product as it actually is applied—that can be used to ensure that the 
product in the field conforms to the near uniform thickness conditions in the tests used to 
establish the fire resistance rating of the component, assembly, or system. Such criteria are 
needed to ensure that the as-installed SFRM will provide the intended performance. 
• Methods for predicting the effectiveness of SFRM insulation as a function of its properties, the 
application characteristics, and the duration and intensity of the fire. 
• Methods for predicting service life performance of SFRM under in-service conditions. 
Affected National Standards: AIA MasterSpec and AWCI Standard 12 for field inspection and 
conformance criteria; ASTM for SFRM performance criteria and test methods. National Model 
Building Codes: The standards should be adopted in national model building codes by mandatory 
28 While the WTC recommendations are focused mainly on U.S. national standards, each U.S. standard has counterpart 
international standards. In a recent report (ISO/TMB AGS N 46), the International Organization for Standardization (ISO), 
through its Advisory Group for Security (AGS), has recommended that since many of the ISO standards for the design of 
buildings date to the 1980s, they should be reviewed and updated to make use of the studies done by NIST on the World Trade 
Center disaster, the applicability of new technology for rescue from high buildings, natural disasters, etc. ISO’s Technical 
Advisory Group 8 coordinates standards work for buildings. 
29 NIST tests showed that the adhesive strength of SFRM on steel coated with primer paint was a third to half of the adhesive 
strength on steel that had not been coated with primer paint. The SFRM products used in the WTC towers were applied to 
steel components coated with primer paint. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
reference to, or incorporation of, the latest edition of the standard. [See Recommendation 10 for more 
on this issue]. 
Recommendation 7. NIST recommends the nationwide adoption and use of the “structural 
frame” approach to fire resistance ratings. This approach requires that structural members—such 
as girders, beams, trusses and spandrels having direct connection to the columns, and bracing 
members designed to carry gravity loads—be fire protected to the same fire resistance rating as 
columns. This approach is currently required by the International Building Code (IBC), one of the 
national model codes, and is under consideration by NFPA 5000, the other national model code. This 
requirement ensures consistency in the fire protection provided to all of the structural elements that 
contribute to overall structural stability.30 National Model Building Codes: Both national model 
building codes should incorporate the structural frame requirement. State and local jurisdictions 
should adopt and enforce this requirement, which already exists in one of the national model building 
codes. 
9.2.3 Group 3. New Methods for Fire Resistance Design of Structures 
The procedures and practices used in the fire resistance design of structures should be enhanced by 
requiring an objective that uncontrolled fires result in burnout without local or global collapse. 
Performance-based methods are an alternative to prescriptive design methods. This effort should 
include the development and evaluation of new fire resistive coating materials and technologies and 
evaluation of the fire performance of conventional and high-performance structural materials. 
Technical and standards barriers to the introduction of new materials and technologies should be 
eliminated. 
Recommendation 8. NIST recommends that the fire resistance of structures should be 
enhanced by requiring a performance objective that uncontrolled building fires result in 
burnout without local or global collapse. Such a provision should apply to all tall buildings, 
recognizing that sprinklers could be compromised, non-operational, or non-existent. Current methods 
for determining the fire resistance rating of structural assemblies do not explicitly specify a 
performance objective. The rating resulting from current test methods indicates that the assembly 
(component or subsystem) continued to support its superimposed load (simulating a maximum load 
condition) during the test exposure without collapse. National Model Building Codes: This 
recommendation should be included into the national model codes as an objective and adopted as an 
integral part of fire resistance design for structures. The issue of non-operational sprinklers could be 
addressed using the existing concept of Design Scenario 8 of NFPA 5000, where such compromise is 
assumed and the result is required to be acceptable to the Authority Having Jurisdiction. Affected 
National Standards: ASCE-7, AISC Specifications, ACI 318, and ASCE 29. 
Recommendation 9. NIST recommends the development of: (1) performance-based standards 
and code provisions, as an alternative to current prescriptive design methods, to enable the 
design and retrofit of structures to resist real building fire conditions, including their ability to 
achieve the performance objective of burnout without structural or local floor collapse: and 
(2) the tools, guidelines, and test methods necessary to evaluate the fire performance of the 
structure as a whole system. Standards development organizations, including the American 
Institute of Steel Construction, have already begun developing performance-based provisions to 
consider the effects of fire in structural design. 
30 Had this requirement been adopted by the 1968 New York City building code, the WTC floor system, including its 
connections, would have had the 3 hour rating required for the columns since the floors braced the columns. 
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Chapter 9 Draft for Public Comment 
This performance-based capability should include, but not be limited to: 
a. Standard methodology, supported by performance criteria, analytical design tools, and practical 
design guidance; related building standards and codes for fire resistance design and retrofit of 
structures, working through the consensus process for nationwide adoption; comprehensive 
design rules and guidelines; methodology for evaluating thermostructural performance of 
structures; and computational models and analysis procedures for use in routine design practice. 
b. Standard methodology for specifying multicompartment, multifloor fire scenarios for use in the 
design and analysis of structures to resist fires, accounting for building-specific conditions such 
as geometry, compartmentation, fuel load (e.g., building contents and any flammable fuels such 
as oil and gas), fire spread, and ventilation; and methodology for rating the fire resistance of 
structural systems and barriers under realistic design-basis fire scenarios. 
c. Publicly available computational software to predict the effects of fires in tall buildings— 
developed, validated, and maintained through a national effort—for use in the design of fire 
protection systems and the analysis of building response to fires. Improvements should include 
the fire behavior and contribution of real combustibles; the performance of openings, including 
door openings and window breakage, that controls the amount of oxygen available to support the 
growth and spread of fires and whether the fire is fuel-controlled or ventilation-controlled; the 
floor-to-floor flame spread; the temperature rise in both insulated and uninsulated structural 
members and fire barriers; and the structural response of components, subsystems, and the total 
building system due to the fire. 
d. New test methods, together with associated conformance assessment criteria, to support the 
performance-based methods for fire resistance design and retrofit of structures. The performance 
objective of burnout without collapse will require the development of standard fire exposures that 
differ from those currently used. 
Affected National (and International) Standards: ASCE-7, AISC Specifications, ACI 318, and 
ASCE-29 for fire resistance design and retrofit of structures; NFPA, SFPE, ASCE, and 
ISO TC92SC4 for building-specific multicompartment, multifloor design basis fire scenarios; and 
ASTM, NFPA, UL, and ISO for new test methods. National Model Building Codes: The 
performance standards should be adopted as an alternate method in national model building codes by 
mandatory reference to, or incorporation of, the latest edition of the standard. 
Recommendation 10. NIST recommends the development and evaluation of new fire resistive 
coating materials, systems, and technologies with significantly enhanced performance and 
durability to provide protection following major events. This could include, for example, 
technologies with improved adhesion, double-layered materials, intumescent coatings, and more 
energy absorbing SFRMs.31 Consideration should be given to pre-treatment of structural steel 
members with some type of mill-applied fire protection to minimize the uncertainties associated with 
field application and in-use damage. If such an approach was feasible, only connections and any fire 
protection damaged during construction and fit-out would need to be field-treated. Affected National 
Standards: Technical barriers, if any, to the introduction of new structural fire resistance materials, 
systems, and technologies should be eliminated in the AIA MasterSpec and AWCI Standard 12 for 
field inspection and conformance criteria and in ASTM standards for SFRM performance criteria and 
test methods. National Model Building Codes: Technical barriers, if any, to the introduction of new 
31 Other possibilities include encapsulation of SFRM by highly elastic energy absorbing membranes or commodity grade carbon 
fiber or other wraps. The membrane would remain intact under shock, vibration, and impact but may be compromised in a 
fire, yet allowing the SFRM to perform its thermal insulation function. The carbon wrap would remain intact under shock, 
vibration, and impact and, possibly, under fire conditions as well. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
structural fire resistance materials, systems, and technologies should be eliminated from the national 
model building codes. 
Recommendation 11. NIST recommends that the performance and suitability of advanced 
structural steel, reinforced and pre-stressed concrete, and other high-performance material 
systems should be evaluated for use under conditions expected in building fires. This evaluation 
should consider both presently available and new types of steels, concrete, and high-performance 
materials to establish the properties (e.g., yield and ultimate strength, modulus, creep behavior, 
failure) that are important for fire resistance, establish needed test protocols and acceptance criteria 
for such materials and systems, compare the performance of newer systems to conventional systems, 
and the cost-effectiveness of alternate approaches. Technical and standards barriers to the 
introduction and use of such advanced steel, concrete, and other high-performance material systems 
should be identified and eliminated, or at least minimized, if they are found to exist. Affected 
National Standards: AISC Specifications and ACI 318. Technical barriers, if any, to the 
introduction of these advanced systems should be eliminated in ASTM E 119, NFPA 251, UL 263, 
ISO 834. National Model Building Codes: Technical barriers, if any, to the introduction of these 
advanced systems should be eliminated from the national model building codes. 
9.2.4 Group 4. Improved Active Fire Protection 
Active fire protection systems (i.e., sprinklers, standpipes/hoses, fire alarms, and smoke 
management systems) should be enhanced through improvements to design, performance, 
reliability, and redundancy of such systems. 
Recommendation 12. NIST recommends that the performance and redundancy of active fire 
protection systems (sprinklers, standpipes/hoses, fire alarms, and smoke management systems) 
in buildings should be enhanced to accommodate the greater risks associated with increasing 
building height and population, increased use of open spaces, available compartmentation, 
high-risk building activities, fire department response limits, transient fuel loads, and higher 
threat profile. The performance attributes should deal realistically with the system design basis, 
reliability of automatic/manual operations, redundancy, and reduction of vulnerabilities due to single 
point failures. Affected National Standards: NFPA 1, NFPA 13, NFPA 72, NFPA 90A, and 
NFPA 101. National Model Building Codes: The performance standards should be adopted in 
national model building codes by mandatory reference to, or incorporation of, the latest edition of the 
standard. 
Recommendation 13. NIST recommends that fire alarm and communications systems in 
buildings should be developed to provide continuous, reliable, and accurate information on the 
status of life safety conditions at a level of detail sufficient to manage the evacuation process in 
building fire emergencies, and that standards for their performance be developed. This should 
include means to maintain communications with evacuating occupants that can both reassure them 
and redirect them if conditions change. While pre-installed fire warden telephone systems in 
buildings can serve a useful purpose and may be installed in buildings, they should be made available 
for use by emergency responders. Pre-installed dedicated firefighter telephone systems in buildings 
are of limited use and effectiveness, and their installation is not encouraged. Affected National 
Standards: NFPA 1, NFPA 72, and NFPA 101. National Model Building and Fire Codes: The 
performance standards should be adopted in national model building and fire codes by mandatory 
reference to, or incorporation of, the latest edition of the standard. 
Recommendation 14. NIST recommends that control panels at fire/emergency command 
stations in buildings should be adapted to accept and interpret a larger quantity of more 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
reliable information from the active fire protection systems that provide tactical decision aids to 
fireground commanders, including water flow rates from pressure and flow measurement 
devices, and that standards for their performance be developed. Affected National Standards: 
NFPA 1, NFPA 72, and NFPA 101. National Model Building and Fire Codes: The performance 
standards should be adopted in national model building and fire codes by mandatory reference to, or 
incorporation of, the latest edition of the standard. 
Recommendation 15. NIST recommends that systems should be developed and implemented 
for: (1) real-time off-site secure transmission of valuable information from fire alarm and other 
monitored building systems for use by emergency responders, at any location, to enhance 
situational awareness and response decisions and maintain safe and efficient operations32; and 
(2) preservation of that information either off-site or in a black box that will survive a fire or 
other building failure for purposes of subsequent investigations and analysis. Standards for the 
performance of such systems should be developed, and their use should be required. Affected 
National Standards: NFPA 1, NFPA 72, and NFPA 101. National Model Building and Fire Codes: 
The performance standards should be adopted in national model building and fire codes by mandatory 
reference to, or incorporation of, the latest edition of the standard. 
9.2.5 Group 5. Improved Building Evacuation 
Building evacuation should be improved to include system designs that facilitate safe and rapid 
egress, methods for ensuring clear and timely emergency communications to occupants, better 
occupant preparedness for evacuation during emergencies, and incorporation of appropriate egress 
technologies.33 
Recommendation 16. NIST recommends that public agencies, non-profit organizations 
concerned with building and fire safety, and building owners and managers should develop 
and carry out public education campaigns, jointly and on a nationwide scale, to improve 
building occupants’ preparedness for evacuation in case of building emergencies. This effort 
should include better training and self-preparation of occupants, an effectively implemented system of 
floor wardens and building safety personnel, and needed improvements to standards. Occupant 
preparedness should include: 
a. Improved training and drills for building occupants to ensure that they know evacuation 
procedures, are familiar with the egress route, and are sufficiently aware of what is necessary if 
evacuation is required with minimal notice (e.g., footwear consistent with the distance to be 
traveled, a flashlight/glow stick for pathway illumination, and dust masks). 
32 The alarm systems in the WTC towers were only capable of determining and displaying: (a) areas that had at some time 
reached alarm point conditions; and (b) areas that had not. The quality and reliability of information available to emergency 
responders at the Fire Command Station was not sufficient to understand the fire conditions. The only information transmitted 
outside the building was the fact that the building had gone into alarm. Further, the fire alarm system in WTC 7, which was 
transmitted to a monitoring service, was on “test” on the morning of September 11, 2001 since routine maintenance was being 
performed. Under test conditions (1) the system is typically disabled for the entire building, not just for the area where work is 
being performed, and (2) alarm signals typically do not show up on an operator console. 
33 This effort should include standards and guidelines for the development and evaluation of emergency evacuation plans, 
including best practices for both partial and full evacuation, and the development of contingency plans that account for 
expected conditions that may require adaptation, including the compromise of all or part of an egress path before or during 
evacuation, or conditions such as widespread power failure, earthquake, or security threat that restrict egress from the building. 
Evacuation planning should include the process from initial notification of the need to evacuate to the point the occupants 
arrive at a place where their safety is ensured. These standards and guidelines should be suitable for assessing the adequacy of 
evacuation plans submitted for approval and should require occupant training through the conduct of regular drills. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
b. Improved training and drills that routinely inform building occupants that roof rescue is not (or is) 
presently feasible as a standard evacuation option, that they should evacuate down the stairs in 
any full-building evacuation unless explicitly instructed otherwise by on-site incident 
commanders, and that elevators can be used if they are still in service and haven’t been recalled 
or stopped. 
c. Improved codes, laws, and regulations that do not restrict or impede building occupants during 
evacuation drills from familiarizing themselves with the detailed layout of alternate egress routes 
for a full building evacuation34. 
Affected National Standard: NFPA 101. National Model Building and Fire Codes: The standard 
should be adopted in national model building and fire codes by mandatory reference to, or 
incorporation of, the latest edition of the standard. Affected National Organizations: NIBS, 
NCSBCS, BOMA, and CTBUH. 
Recommendation 17. NIST recommends that tall buildings should be designed to accommodate 
timely full building evacuation of occupants due to building-specific or large-scale emergencies 
such as widespread power outages, major earthquakes, tornadoes, hurricanes without sufficient 
advanced warning, fires, accidental explosions, and terrorist attack. Building size, population, 
function, and iconic status should be taken into account in designing the egress system. 
Stairwell and exit capacity35 should be adequate to accommodate counterflow due to emergency 
access by responders. 
a. Improved egress analysis models, design methodology, and supporting data should be developed 
to achieve a target evacuation performance (e.g., time for full building evacuation36) for the 
design building population by considering the building and egress system designs and human 
factors such as occupant size, mobility status, stairwell tenability conditions, visibility, and 
congestion. 
b. Mobility challenged occupants should be provided a means for self-evacuation in the event of a 
building emergency. Current strategies (and law) generally require the mobility challenged to 
shelter-in-place and await assistance. New procedures, which provide redundancy in the event 
that the floor warden system or co-worker assistance fails, should consider full building 
evacuation, and may include use of fire-protected and structurally hardened elevators37, motorized 
evacuation technology, and/or dedicated communication technologies for the mobility 
challenged. 
c. If protected/hardened elevators are provided for emergency responders but become unusable 
during an emergency, due to a malfunction or a conventional threat whose magnitude exceeds the 
magnitude considered in design, sufficient stairwell capacity should be provided to ensure timely 
emergency responder access to buildings that are undergoing full evacuation. Such capacity 
could be provided either via dedicated stairways for fire service use or by building sufficient 
34 New York City Local Law 5 prohibits requiring occupants to practice stairwell evacuation during drills. 
35 Egress capacity should be based on an all-hazards approach that considers the number and width of stairs (and doors) as well 
as the possible use of scissor stairs credited as a single stair. 
36 Use of egress models is required to estimate the egress capacity for a range of different evacuation strategies, including full 
building evacuation. NIST found that the average surviving occupant in the WTC towers descended stairwells at about half 
the slowest speed previously measured for non-emergency evacuations. 
37 Elevators should be explicitly designed to provide protection against large, but conventional, building fires. Fire-protected 
elevators also should be structurally hardened to withstand the range of foreseeable building-specific or large-scale 
emergencies. While progress has been made in developing the requirements and technologies for fire-protected elevators, 
similar criteria and designs for structurally hardened elevators remain to be developed. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
stairway capacity (i.e., number and width of stairways and/or use of scissor stairs credited as a 
single stair) to accommodate the evacuation of building occupants while allowing access to 
emergency responders with minimal hindrance from occupant counterflow. 
d. The egress allowance in assembly use spaces should be limited in state and local laws and 
regulations to no more than a doubling of the stairway capacity for the provision of a horizontal 
exit on a floor, as is the case now in the national model codes.38 The use of a horizontal exit 
creates an area of refuge with a 2 hour fire rated separation, at least one stair on each side, and 
sufficient space for the expected occupant load. 
Affected National Standard: NFPA 101, ASME A 17. National Model Building and Fire Codes: 
The standards should be adopted in national model building and fire codes by mandatory reference to, 
or incorporation of, the latest edition of the standard. 
Recommendation 18. NIST recommends that egress systems should be designed: (1) to 
maximize remoteness of egress components (i.e., stairs, elevators, exits) without negatively 
impacting the average travel distance; (2) to maintain their functional integrity and 
survivability under foreseeable building-specific or large-scale emergencies; and (3) with 
consistent layouts, standard signage, and guidance so that systems become intuitive and obvious 
to building occupants during evacuations. 
a. Within a safety-based design hierarchy that should be developed, highest priority should be 
assigned to maintain the functional integrity, survivability, and remoteness of egress components 
and active fire protection systems (sprinklers, standpipes, associated water supply, fire alarms, 
and smoke management systems). The design hierarchy should consider the many systems (e.g., 
stairs, elevators, active fire protection, mechanical, electrical, plumbing, and structural) and 
system components, as well as functional integrity, tenant access, emergency responder access, 
building configuration, security, and structural design. 
b. The design, functional integrity, and survivability of the egress and other life safety systems (e.g., 
stairwell and elevator shafts and active fire protection systems) should be enhanced by 
considering accidental structural loads such as those induced by overpressures (e.g., gas 
explosions), impacts, or major hurricanes and earthquakes, in addition to fire separation 
requirements. In selected buildings, structural loads due to other risks such as those due to 
terrorism may need to be considered. While NIST does not believe that buildings should be 
designed for aircraft impact, as the last line of defense for life safety, the stairwells and elevator 
shafts individually, or the core if these egress components are contained within the core, should 
have adequate structural integrity to withstand accidental structural loads and anticipated risks. 
c. Stairwell remoteness requirements should be met by a physical separation of the stairwells that 
provide a barrier to both fire and accidental structural loads. Maximizing stairwell remoteness, 
without negatively impacting the average travel distance, would allow a stairwell to maintain its 
structural integrity independent of any other stairwell that is subject to accidental loads, even if 
the stairwells are located within the same structural barrier such as the core. The current “walking 
path” measurement allows stairwells to be physically next to each other, separated only by a fire 
barrier. Reducing the clustering of stairways that also contain standpipe water systems provide 
the fire service with increased options for formulating firefighting strategies. This should not 
38 The New York City Building Code permits a doubling of allowed stair capacity when one area of refuge is provided on a floor 
and a tripling of stair capacity for two or more areas of refuge on a floor. In the world of post-September 11, 2001, it is 
difficult to predict (1) if, and for how long, occupants will be willing to wait in a refuge area before entering an egress 
stairway, and (2) what the impact would be of such a large group of people moving down the stairs on the orderly evacuation 
of lower floors. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
preclude the use of scissor stairs39 as a means of increasing stair capacity—provided the scissor 
stair is only credited as a single stair. 
d. Egress systems should have consistent layouts with standard signage and guidance so that the 
systems become intuitive and obvious to all building occupants, including visitors, during 
evacuations. Particular consideration should be given to unexpected deviations in the stairwells 
(e.g., floors with transfer hallways). 
Affected National Standard: NFPA 101. National Model Building and Fire Codes: The standard 
should be adopted in national model building and fire codes by mandatory reference to, or 
incorporation of, the latest edition of the standard. 
Recommendation 19. NIST recommends that building owners, managers, and emergency 
responders develop a joint plan and take steps to ensure that accurate emergency information is 
communicated in a timely manner to enhance the situational awareness of building occupants 
and emergency responders affected by an event. This should be accomplished through better 
coordination of information among different emergency responder groups, efficient sharing of 
that information among building occupants and emergency responders, more robust design of 
emergency public address systems, improved emergency responder communication systems, 
and use of the Emergency Broadcast System (now known as the Integrated Public Alert and 
Warning System) and Community Emergency Alert Networks. 
a. Situational awareness of building occupants and emergency responders in the form of information 
and event knowledge should be improved through better coordination of such information among 
emergency responder groups (9-1-1 dispatch, fire department or police department dispatch, 
emergency management dispatch, site security, and appropriate federal agencies), efficient 
sharing and communication of information between building occupants and emergency 
responders, and improved emergency responder communication systems (i.e., including effective 
communication within steel and reinforced concrete buildings, capacity commensurate with the 
scale of operations, and interoperability among different communication systems). 
b. The emergency communications systems in buildings should be designed with sufficient 
robustness and redundancy to continue providing public address announcements or instructions in 
foreseeable building-specific or large-scale emergencies, including widespread power outage, 
major earthquakes, tornadoes, hurricanes, fires, and accidental explosions. Consideration should 
be given to placement of building announcement speakers in stairways in addition to other 
standard locations. 
c. The Integrated Public Alert and Warning System (IPAWS) should be activated and used, 
especially during large-scale emergencies, as a means to rapidly and widely communicate 
information to building occupants and emergency responders to enhance their situational 
awareness and assist with evacuation. 
d. Local jurisdictions (cities and counties or boroughs) should seriously consider establishing a 
Community Emergency Alert Network (CEAN), within the framework of IPAWS, and make it 
available to the citizens and emergency responders of their jurisdiction to enhance situational 
awareness in emergencies40. The network should deliver important emergency alerts, information 
39 Two separate stairways within the same enclosure and separated by a fire rated partition. 
40 Types of emergency communications could include life safety information, severe weather warnings, disaster notifications 
(including information on terrorist attacks), directions for self-protection, locations of nearest available shelters, precautionary 
evacuation information, identification of available evacuation routes, and accidents or obstructions associated with roadways 
and utilities. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
and real-time updates to all electronic communications systems or devices registered with the 
CEAN. These devices may include e-mail accounts, cell phones, text pagers, satellite phones, 
and wireless PDAs. 
Affected National Standard: NFPA 101 and/or a new standard. National Model Building and Fire 
Codes: The standard should be adopted in national model codes by mandatory reference to, or 
incorporation of, the latest edition of the standard to the extent it is within the scope of building and 
fire codes. 
Recommendation 20. NIST recommends that the full range of current and next generation 
evacuation technologies should be evaluated for future use, including protected/hardened 
elevators, exterior escape devices, and stairwell navigation devices, which may allow all 
occupants an equal opportunity for evacuation and facilitate emergency response access. 
Affected National Standard: NFPA 101, ASME A 17, ASTM E 06. National Model Building and 
Fire Codes: The standards should be adopted in national model building and fire codes by mandatory 
reference to, or incorporation of, the latest edition of the standard. 
9.2.6 Group 6. Improved Emergency Response 
Technologies and procedures for emergency response should be improved to enable better access to 
buildings, response operations, emergency communications, and command and control in largescale 
emergencies. 
Recommendation 21. NIST recommends the installation of fire-protected and structurally 
hardened elevators to improve emergency response activities in tall buildings by providing 
timely emergency access to responders and allowing evacuation of mobility-impaired building 
occupants. Such elevators should be installed for exclusive use by emergency responders 
during emergencies. In tall buildings, consideration also should be given to installing such 
elevators for use by all occupants. The use of elevators for these purposes will require additional 
operating procedures and protocols.41 
a. The requirement for remote release of elevator cabs by emergency response personnel should be 
included in the ASME A 17.1 Safety Code for Elevators and Escalators. 
Affected National Standards: ASME A 17, NFPA 70, NFPA 101, NFPA 1221, NFPA 1500, 
NFPA 1561, NFPA 1620, and NFPA 1710. National Model Building and Fire Codes: The standards 
should be adopted in national model building and fire codes by mandatory reference to, or 
incorporation of, the latest edition of the standard. 
Recommendation 22. NIST recommends the installation, inspection, and testing of emergency 
communications systems, radio communications, and associated operating protocols to ensure 
that the systems and protocols: (1) are effective for large-scale emergencies in buildings with 
challenging radio frequency propagation environments; and (2) can be used to identify, locate, 
and track emergency responders within indoor building environments and in the field. The 
41 The access time for emergency responders, in tall building emergencies where elevators are not functioning and only stairways 
can be used, averages between 1 min and 2 min per floor, which, for example, corresponds to between 1 1/2 hour and 2 hours 
(depending on the amount of gear and equipment carried) to reach the 60th floor of a tall building. Further, the physiological 
impact on the emergency responders of climbing more than 10 to 12 floors in a tall building makes it difficult for them to 
immediately begin aggressive firefighting and rescue operations. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
federal government should coordinate its efforts that address this need within the framework provided 
by the SAFECOM program of the Department of Homeland Security. 
a. Rigorous procedures, including pre-emergency inspection and testing, should be developed and 
implemented for ensuring the operation of emergency communications systems and radio 
communications in tall buildings and other large structures (including tunnels and subways), or at 
locations where communications are difficult. 
b. Performance requirements should be developed for emergency communications systems and 
radio communications that are used within buildings or in built-up urban environments, including 
standards for design, testing, certification, maintenance, and inspection of such systems. 
c. An interoperable architecture for emergency communications networks—and associated 
operating protocols—should be developed for unit operations within and across agencies in largescale 
emergencies. The overall network architecture should cover local networking at incident 
sites, dispatching, and area-wide networks, considering: (a) the scale of needed communications 
in terms of the number of emergency responders using the system in a large-scale emergency and 
the organizational hierarchy; (b) challenges associated with radio frequency propagation 
especially in buildings; (c) interoperability with existing legacy emergency communications 
systems (i.e., between conventional two-way systems and newer wireless network systems); and 
(d) the need to identify, locate, and track emergency responders at an incident site. 
Affected National Standards: FCC, SAFECOM, NFPA Standards on Electronic Safety Equipment, 
NFPA 70, NFPA 297, and NFPA 1221. National Model Building Codes: The standards should be 
adopted in national model building codes by mandatory reference to, or incorporation of, the latest 
edition of the standard. 
Recommendation 23. NIST recommends the establishment and implementation of detailed 
procedures and methods for gathering, processing, and delivering critical information through 
integration of relevant voice, video, graphical, and written data to enhance the situational 
awareness of all emergency responders. An information intelligence sector42 should be 
established to coordinate the effort for each incident. Affected National Standards: NIMS, NRP, 
SAFECOM, FCC, NFPA Standards on Electronic Safety Equipment, NFPA 1500, NFPA 1561, 
NFPA 1620, NFPA 1710, and NFPA 1221. National Model Building Codes: The standards should 
be adopted in national model building codes by mandatory reference to, or incorporation of, the latest 
edition of the standard. 
Recommendation 24. NIST recommends the establishment and implementation of codes and 
protocols for ensuring effective and uninterrupted operation of the command and control 
system for large-scale building emergencies. 
a. State, local, and federal jurisdictions should implement the National Incident Management 
System (NIMS). The jurisdictions should work with the Department of Homeland Security to 
review, test, evaluate, and implement an effective unified command and control system. The 
NIMS addresses interagency coordination and establishes a response matrix – assigning lead 
agency responsibilities for different types of emergencies and functions. At a minimum, each 
supporting agency should assign an individual to provide coordination with the lead agency at 
each incident command post. 
42 A group of individuals that is knowledgeable, experienced, and specifically trained in gathering, processing, and delivering 
information critical for emergency response operations and is ready for activation in large and/or dangerous events. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
b. State, local, and federal emergency operations centers (EOCs) should be located, designed, built, 
and operated with security and operational integrity as a key consideration. 
c. Command posts should be established outside the potential collapse footprint of any building 
which shows evidence of large multifloor fires or has serious structural damage. A continual 
assessment of building stability and safety should be made in such emergencies to guide ongoing 
operations and enhance emergency responder safety. The information necessary to make these 
assessments should be made available to those assigned responsibility (see related 
Recommendations 15 and 23). 
d. An effective command system should be established and operating before a large number of 
emergency responders and apparatus are dispatched and deployed. Through training and drills, 
emergency responders and ambulances should be required to await dispatch requests from the 
incident command system and not to self-dispatch in large-scale emergencies. 
e. Actions should be taken via training and drills to ensure a coordinated and effective emergency 
response at all levels of the incident command chain by requiring all emergency responders that 
are given an assignment to immediately adopt and execute the assignment objectives. 
f. Command post information and incident operations data should be managed and broadcast to 
command and control centers at remote locations so that information is secure and accessible by 
all personnel needing the information. Methods should be developed and implemented so that 
any information that is available at an interior information center is transmitted to a emergency 
responder vehicle or command post outside the building. 
Affected National Standards: NIMS, NRP, SAFECOM, FCC, NFPA Standards on Electronic Safety 
Equipment, NFPA 1221, NFPA 1500, NFPA 1561, NFPA 1620, and NFPA 1710. National Model 
Building Codes: The standards should be adopted in national model building codes by mandatory 
reference to, or incorporation of, the latest edition of the standard. 
9.2.7 Group 7. Improved Procedures and Practices 
The procedures and practices used in the design, construction, maintenance, and operation of 
buildings should be improved to include encouraging code compliance by nongovernmental and 
quasi-governmental entities, adoption and application of egress and sprinkler requirements in 
codes for existing buildings, and retention and availability of building documents over the life of a 
building. 
Recommendation 25. Nongovernmental and quasi-governmental entities that own or lease 
buildings and are not subject to building and fire safety code requirements of any governmental 
jurisdiction are nevertheless concerned about the safety of the building occupants and the 
responding emergency personnel. NIST recommends that such entities should be encouraged 
to provide a level of safety that equals or exceeds the level of safety that would be provided by 
strict compliance with the code requirements of an appropriate governmental jurisdiction. To 
gain broad public confidence in the safety of such buildings, NIST further recommends that it is 
important that as-designed and as-built safety be certified by a qualified third party, 
independent of the building owner(s). The process should not use self-approval for code 
enforcement in areas including interpretation of code provisions, design approval, product 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
acceptance, certification of the final construction, and post-occupancy inspections over the life 
of the buildings.43 
Recommendation 26. NIST recommends that state and local jurisdictions should adopt and 
aggressively enforce available provisions in building codes to ensure that egress and sprinkler 
requirements are met by existing buildings44. Further, occupancy requirements should be 
modified where needed (such as when there are assembly use spaces within an office building) 
to meet the requirements in model building codes. Provisions related to egress and sprinkler 
requirements in existing buildings are available in such codes as the International Existing Building 
Code (IEBC), International Fire Code, NFPA 1, NFPA 101, and ASME A 17.3. For example, the 
IEBC defines three levels of building alteration (removal and replacement or covering of existing 
materials and equipment, reconfiguration of space or system or installation of new equipment, and 
work area in excess of 50 percent of the aggregate area of the building). At the lowest level there are 
no upgrade implications for sprinklers and the egress system. At the next level, sprinklers are 
required in work areas serving greater than 30 persons if certain other conditions related to building 
height and use such as shared exists also are met. There are numerous requirements for means of 
egress, including number of exits, specification of doors, dead-end corridors and travel distances, 
lighting, signage, and handrails. At the highest level, the sprinkler and egress requirements are 
identical to the second level without the minimum 30 person restriction and the other conditions 
related to building height and use. The Life Safety Code (NFPA 101) applies retroactively to all 
buildings, independent of whether any work is currently being done on the building, and ASME 17.3 
applies retroactively to all elevators as a minimum set of requirements. 
Recommendation 27. NIST recommends that building codes should incorporate a provision 
that requires building owners to retain documents, including supporting calculations and test 
data, related to building design, construction, maintenance and modifications over the entire 
life of the building45. Means should be developed for offsite storage and maintenance of the 
documents. In addition, NIST recommends that relevant building information should be made 
available in suitably designed hard copy or electronic format for use by emergency responders. 
Such information should be easily accessible by responders during emergencies. National Model 
Building Codes: Both national model codes should incorporate this recommendation. State and local 
jurisdictions should adopt and enforce these requirements. 
43 The long-standing stated policy of The Port Authority of New York and New Jersey (PANYNJ) was to meet and, where 
appropriate, exceed the requirements of local building and fire codes, and it entered into agreements with the New York City 
Department of Buildings and The Fire Department of the City of New York in accordance with that policy. Although the 
PANYNJ sought review and concurrence from New York City in the areas listed in the recommendation, the PANYNJ was not 
required to yield, and appears not to have yielded, approval authority to New York City. The PANYNJ was created as an 
interstate entity, a “body corporate and politic,” under its charter, pursuant to Article 1, Section 10 of the U.S. Constitution 
permitting compacts between states. Further, there are many other similar nongovernmental and quasi-governmental entities 
in the United States. A comprehensive review of documents conducted as part of this Investigation suggests that the WTC 
towers generally were designed and maintained consistent with the requirements of the 1968 New York City Building Code. 
Areas of concern included fireproofing of WTC floor system, height of tenant separation walls, and egress requirements for the 
assembly use space for the Windows of the World in WTC 1 and Top of the World observation deck in WTC 2. These areas 
of concern did not play a significant role in determining the outcomes related to the events of September 11, 2001. 
44 The WTC towers were unsprinklered when built. It took nearly 28 years after passage of New York City Local Law 5 in 1973, 
which required either compartmentation or sprinklering, for the buildings to be fully sprinklered (the Port Authority chose not 
to use the compartmentation option in Local Law 5). This was about 13 years more than the 15-year period for full compliance 
with Local Law 5 that was set by Local Law 84 of 1979. 
45 The availability of inexpensive electronic storage media and tools for creating large searchable databases make this feasible. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
Recommendation 28. NIST recommend that the role of the “Design Professional in Responsible 
Charge”46 should be clarified to ensure that: (1) all appropriate design professionals (including, 
e.g., the fire protection engineer) are part of the design team providing the standard of care 
when designing buildings employing innovative or unusual fire safety systems47, and (2) all 
appropriate design professionals (including, e.g., the structural engineer and the fire protection 
engineer) are part of the design team providing the standard of care when designing the 
structure to resist fires, in buildings that employ innovative or unusual structural and fire 
safety systems. Affected National Standards: AIA Practice Guidelines. National Model Building 
Codes: The IBC, which already defines the “Design Professional in Responsible Charge,” should be 
clarified to address this recommendation. The NFPA 5000 should incorporate the “Design 
Professional in Responsible Charge” concept and address this recommendation. 
9.2.8 Group 8. Education and Training 
The professional skills of building and fire safety professionals should be upgraded though a 
national education and training effort for fire protection engineers, structural engineers, and 
architects. 
Recommendation 29. NIST recommends that continuing education curricula should be 
developed and programs should be implemented for training fire protection engineers and 
architects in structural engineering principles and design, and training structural engineers, 
architects, and fire protection engineers in modern fire protection principles and technologies, 
including fire-resistance design of structures. The outcome would further the integration of the 
disciplines in effective fire-safe design of buildings. Affected National Organizations: AIA, SFPE, 
ASCE, ASME, AISC, ACI, and state licensing boards. National Model Building Codes: Detailed 
criteria and requirements should be incorporated into the national model building codes under the 
topic “Design Professional in Responsible Charge.” 
Recommendation 30. NIST recommends that academic, professional short-course, and webbased 
training materials in the use of computational fire dynamics and thermostructural 
analysis tools should be developed and delivered to strengthen the base of available technical 
capabilities and human resources. Affected National Organizations: AIA, SFPE, ASCE, ASME, 
AISC, and ACI. 
9.3 OPPORTUNITY FOR PUBLIC COMMENT 
NIST urges organizations responsible for building and fire safety at all levels to carefully consider the 
draft findings, issues, and recommendations contained in this report. Table 1 shows a crosswalk between 
the recommendations and the three categories (responsible community, application, and relation to 
outcome on September 11, 2001). NIST welcomes comments from technical experts and the public on 
this draft report—including specific improvements to the language in the recommendations to ease their 
speedy adoption—as soon as possible but no later than August 4, 2005. Comments can be sent by e-mail 
46 In projects involving a design team, the “Design Professional in Responsible Charge”—usually the lead architect—ensures that 
the team members use consistent design data and assumptions, coordinates overlapping specifications, and serves as the liaison 
to the enforcement and reviewing officials and to the owner. The term is defined in the International Building Code and in the 
ICC Performance Code for Buildings and Facilities (where it is the Principal Design Professional). 
47 If the fire safety concepts in tall buildings had been sufficiently mature in the 1960s, it is possible that the risks associated with 
jet-fuel ignited multifloor fires might have been recognized and taken into account when the impact of a Boeing 707 aircraft 
was considered by the structural engineer during the design of the WTC towers. 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
to wtc@nist.gov, facsimile to 301-975-6122, or regular mail to WTC Technical Information Repository, 
Stop 8610, 100 Bureau Drive, Gaithersburg, MD 20899-8610. 
BEGINNING THE IMPLEMENTATION PROCESS 
In its final report, which is expected to be released by September 2005, NIST will finalize these draft 
recommendations for specific and appropriate improvements to the way buildings are designed, 
constructed, maintained, and used. It will be important for these recommendations to be thoroughly and 
promptly considered by the many organizations responsible for building and fire safety. For example, 
several of the recommendations (e.g., 7, 10, 21, 25, 26, 27, and 28) can be considered immediately. 
After issuance of the final report, the National Construction Safety Team Act requires NIST to: 
• Conduct, or enable or encourage the conduct of, appropriate research recommended by the Team; 
• Promote (consistent with existing procedures for the establishment of building standards, codes, and 
practices) the appropriate adoption by the Federal Government, and encourage the appropriate 
adoption by other agencies and organizations, of the recommendations of the Team with respect to— 
o Technical aspects of evacuation and emergency response procedures; 
o Specific improvements to building standards, codes, and practices; and 
o Other actions needed to help present future building failures. 
As a part of NIST’s overall WTC response plan, the Institute has begun to reach out to the organizations 
responsible for building and fire safety (especially those listed in Table 9–2) to pave the way for a timely, 
expedited consideration of recommendations stemming from this Investigation. 
NIST will develop a web-based system that will be available to the public so that progress in 
implementing the recommendations can be tracked. The Web site will list each of the recommendations, 
the specific organization or organizations (e.g., standards and codes developers, professional groups, state 
and local authorities) responsible for its implementation, the status of its implementation by organization, 
and the plans or work in progress to implement the recommendation. 
In addition, NIST will hold a conference September 13–15, 2005, to reinforce the importance of its 
findings and recommendations from the Investigation and encourage their implementation. NIST already 
has expanded its research in areas of high-priority need. 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
Table 9–1. Crosswalk of Recommendations to Categories. 
Recommendation Area Recommendation 
Group 
Recommendation 
Number 
Responsible Community Application 
Relation to 
9/11 Outcome 
PracticesStandards, 
Codes, 
RegulationsAdoption &
Enforcement 
R&D/Further 
StudyEducation & 
Training 
All Tall Buildings 
Selected Other 
Buildings 
Related 
Unrelated 
Increased Structural 
Integrity 
1 1 
9 
9 
9 
9 
9 
9 
2 
9 
9 
9 
9 
3 
9 
9 
9 
9 
Enhanced Fire 
Resistance of 
Structures 
2 4 
9 
9 
9 
9 
5 
9 
9 
9 
6 
9 
9 
9 
9 
9 
7 
9 
9 
9 
9 
New Methods for Fire 
Resistance Design of 
Structures 
3 8 
9 
9 
9 
9 
9 
9 
9 
9 
9 
9 
9 
9 
10 
9 
9 
9 
9 
11 
9 
9 
9 
Improved Active Fire 
Protection 
4 12 
9 
9 
9 
9 
13 
9 
9 
9 
14 
9 
9 
9 
15 
9 
9 
9 
9 
Improved Building 
Evacuation 
5 16 
9 
9 
9 
9 
9 
17 
9 
9 
9 
9 
9 
18 
9 
9 
9 
19 
9 
9 
9 
9 
20 
9 
9 
9 
9 
220 NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
Recommendation Area Recommendation 
Group 
Recommendation 
Number 
Responsible Community Application 
Relation to 
9/11 Outcome 
PracticesStandards, 
Codes, 
RegulationsAdoption &
Enforcement 
R&D/Further 
StudyEducation & 
Training 
All Tall Buildings 
Selected Other 
Buildings 
Related 
Unrelated 
Improved Emergency 
Response 
6 21 
9 
9 
9 
9 
22 
9 
9 
9 
9 
9 
9 
23 
9 
9 
9 
9 
9 
9 
24 
9 
9 
9 
9 
9 
9 
Improved Procedures 
and Practices 
7 25 
9 
9 
9 
9 
26 
9 
9 
9 
27 
9 
9 
9 
9 
28 
9 
9 
9 
9 
Continuing Education 
and Training 
8 29 
9 
9 
9 
9 
30 
9 
9 
9 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
Table 9–2a. Standards Affected by the Recommendations. 
Affected Standard Group Number Recommendation 
American Concrete Institute, 
ACI 318 - Building Code Requirements for 
Structural Concrete 
1. Increased Structural Integrity 
3. New Methods for Fire Resistance 
Design of Structures 
1, 3, 8, 9, 11 
American Institute of Architects, 
AIA MASTERSPEC – Master Specification 
System for Design Professionals and the 
Building/Construction Industry 
2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
6, 10 
American Institute of Architects 
Practice Guidelines 
7. Improved Procedures and Practices 28 
American Institute of Steel 
Construction Specification for Structural 
Steel Buildings 
1. Increased Structural Integrity 
3. New Methods for Fire Resistance 
Design of Structures 
1, 3, 8, 9, 11 
American Society of Civil Engineers, 
ASCE 7 – Minimum Design Loads for 
Buildings and Other Structures 
1. Increased Structural Integrity 
3. New Methods for Fire Resistance 
Design of Structures 
1, 2, 3, 8, 9 
American Society of Civil Engineers, 
ASCE 29 – Standard Calculation Methods 
for Structural Fire Protection 
1. Increased Structural Integrity 
3. New Methods for Fire Resistance 
Design of Structures 
1, 8, 9 
American Society of Mechanical Engineers, 
ASME A 17 – Elevators and Escalators, and 
A 17.1 – Safety Code for Elevators and 
Escalators 
5. Improved Building Evacuation 
6. Improved Emergency Response 
17, 20, 21 
American Society of Mechanical Engineers, 
ASME A 17.3 – Safety Code for Existing 
Elevators and Escalators 
7. Improved Procedures and Practices 26 
Association of the Wall and Ceiling Industry 
AWCI 12 – Design Selection Utilizing 
Spray-Applied Fire-Resistive Materials 
AWCI 12-A – Standard Practice for the 
Testing and Inspection of Field Applied 
Fire-Resistive Materials 
AWCI 12-B – Standard Practice for the 
Testing and Inspection of Field Applied 
Itumescent Fire-Resistive Materials 
2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
6, 10 
ASTM International Committee E 06, 
Performance of Buildings; Subcommittee 
E 06.77, High-Rise Building External 
Evacuation Devices 
5. Improved Building Evacuation 20 
ASTM International, 
ASTM E 119 – Standard Test Methods for 
Fire Tests of Building Construction and 
Materials 
2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
5, 11 
Department of Homeland Security, 
National Incident Management System 
(NIMS) 
6. Improved Emergency Response 23, 24 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
Affected Standard Group Number Recommendation 
Department of Homeland Security, 6. Improved Emergency Response 23, 24 
National Response Plan (NRP) 
Department of Homeland Security, 6. Improved Emergency Response 22, 23, 24 
SAFECOM 
Federal Communications Commission, 6. Improved Emergency Response 22, 23, 24 
Emergency Responder Radio 
Communications Regulations 
International Organization for 
Standardization, 
ISO 834 – Fire Resistance Tests 
2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
5, 11 
National Fire Protection Association, 
NFPA 1 – Fire Prevention Code 
4. Enhanced Active Fire Protection 
7. Improved Procedures and Practices 
12, 13, 14, 15, 26 
National Fire Protection Association, 4. Enhanced Active Fire Protection 12 
NFPA 13 – Installation of Sprinkler Systems 
National Fire Protection Association, 6. Improved Emergency Response 21, 22 
NFPA 70 – National Electrical Code 
National Fire Protection Association, 4. Enhanced Active Fire Protection 12, 13, 14, 15 
NFPA 72 – National Fire Alarm Code 
National Fire Protection Association, 4. Enhanced Active Fire Protection 12 
NFPA 90A – Standard for Installation of Air-
Conditioning and Ventilating Systems 
National Fire Protection Association, 
NFPA 101 – Life Safety Code 
4. Enhanced Active Fire Protection 
5. Improved Building Evacuation 
7. Improved Procedures and Practices 
12, 13, 14, 15, 16, 17, 
18, 19, 20, 21, 26 
National Fire Protection Association, 
NFPA 251 – Standard Methods of Tests of 
Fire Endurance of Building Construction and 
Materials 
2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
5, 11 
National Fire Protection Association, 6. Improved Emergency Response 22 
NFPA 297 – Guide on Principles and 
Practices for Communications Systems 
National Fire Protection Association, 6. Improved Emergency Response 21, 22, 23, 24 
NFPA 1221 – Standard for the Installation, 
Maintenance, and Use of Emergency Service 
Communications Systems 
National Fire Protection Association, 6. Improved Emergency Response 21, 23, 24 
NFPA 1500 – Standard on Fire Department 
Occupational Safety and Health 
National Fire Protection Association, 6. Improved Emergency Response 21, 23, 24 
NFPA 1561 – Standard on Emergency 
Services Incident Management System 
National Fire Protection Association, 6. Improved Emergency Response 21, 23, 24 
NFPA 1620 – Recommended Practice for 
Pre-Incident Planning 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
Affected Standard Group Number Recommendation 
National Fire Protection Association, 6. Improved Emergency Response 21, 23, 24 
NFPA 1710 – Standard for the Organization 
and Deployment of Fire Suppression 
Operations, Emergency Medical Operations, 
and Special Operations to the Public by 
Career Fire Departments 
Underwriters Laboratories, UL 263 – Fire 2. Enhanced Fire Resistance of Structures 5, 9, 11 
Tests of Building Construction and Materials 3. New Methods for Fire Resistance 
Design of Structures 
Table 9–2b. Model Codes Affected by the Recommendations. 
Affected Model Code Group Recommendation 
International Building Code 2. Enhanced Fire Resistance of Structures 
7. Improved Procedures and Practices 
1–24, 26–29 
International Existing Building Code 7. Improved Procedures and Practices 26 
International Fire Code 7. Improved Procedures and Practices 26 
National Fire Protection Association, 
NFPA 5000 – Building Construction and 
Safety Code 
2. Enhanced Fire Resistance of Structures 
7. Improved Procedures and Practices 
1–24, 26–29 
Table 9–2c. Organizations Affected by the Recommendations. 
Affected Organization Group Recommendation 
American Concrete Institute 8. Education and Training 29, 30 
American Institute of Architects 8. Education and Training 29, 30 
American Institute of Steel Construction 8. Education and Training 29, 30 
American Society of Civil Engineers 3. New Methods for Fire Resistance 
Design of Structures 
8. Education and Training 
9, 29, 30 
American Society of Mechanical Engineers 8. Education and Training 29, 30 
ASTM International 2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
6, 9, 10 
Building Owners & Managers Association 5. Improved Building Evacuation 16 
Council on Tall Buildings and Urban Habitat 5. Improved Building Evacuation 16 
International Code Council 2. Enhanced Fire Resistance of Structures 4 
International Organization for 
Standardization, Technical Committee 
TC92SC4 – Fire Safety Engineering 
3. New Methods for Fire Resistance 
Design of Structures 
9 
National Conference of States on Building 
Codes & Standards, Inc. 
5. Improved Building Evacuation 16 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment Recommendations 
Affected Organization Group Recommendation 
National Fire Protection Association 2. Enhanced Fire Resistance of Structures 
3. New Methods for Fire Resistance 
Design of Structures 
4, 9 
National Fire Protection Association 
Technical Committee on Electronic Safety 
Equipment 
6. Improved Emergency Response 22, 23, 24 
National Institute of Building Sciences 5. Improved Building Evacuation 16 
Society of Fire Protection Engineers 3. New Methods for Fire Resistance 
Design of Structures 
8. Education and Training 
9, 29, 30 
State licensing boards 8. Education and Training 29 
NIST NCSTAR 1, WTC Investigation 

Chapter 9 Draft for Public Comment 
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NIST NCSTAR 1, WTC Investigation 

Appendix A 
NATIONAL CONSTRUCTION SAFETY TEAM ACT 
NIST NCSTAR 1, WTC Investigation 

Appendix A Draft for Public Comment 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment National Construction Safety Team Act 
NIST NCSTAR 1, WTC Investigation 

Appendix A Draft for Public Comment 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment National Construction Safety Team Act 
NIST NCSTAR 1, WTC Investigation 

Appendix A Draft for Public Comment 
NIST NCSTAR 1, WTC Investigation 

Draft for Public Comment National Construction Safety Team Act 
NIST NCSTAR 1, WTC Investigation 

Appendix A Draft for Public Comment 
NIST NCSTAR 1, WTC Investigation 

Appendix B 
SUBJECT INDEX OF SUPPORTING INVESTIGATION REPORTS 
The purpose of this index is to direct readers to the Investigation support reports in which more detailed 
descriptions of the topics covered in this report can be found. The citations refer to the NIST NCSTAR 
report numbers whose complete citations can be found in the Preface to this report. If a reference is 
provided to a report with a number and a letter, a summary description can be found in the report with just 
the number. 
Aircraft Impact 
Damage 1-2, 1-2B, 1-6 
Modeling 1-2, 1-2A, 1-2B 
Photographs 1-5A 
Building and Fire Codes 
Egress 
Fire 
Post-construction modifications 
Structural 
Used in WTC design 
1-1, 1-7 
1-1D, 1-1E, 1-1F, 1-1H 
1-1G 
1-1A, 1-1B, 1-1C 
1-1 
Ceiling Tile System 
Composition 1-5D 
NIST NCSTAR 1, WTC Investigation 

Appendix B Draft for Public Comment 
Shock tests 1-5D 
Condition on 9/11/01 1-7 
Collapse Mechanisms 1-6, 1-6D 
Emergency Response 1-8 
Evacuation 1-7, 1-8 
Fire Alarm System 
Design 1-4C 
Operation on 9/11/01 1-8 
Firefighting 1-8 
Fire Simulations 1-5F 
Fire Suppression System 1-4B 
Design 1-4B 
Operation on 9/11/01 1-8 
Fire Tests 
Exposure of structural steel 1-5B 
Floor systems 1-6B 
Single office workstations 1-5C 
NIST NCSTAR 1, WTC Investigation

Draft for Public Comment Subject Index of Supporting Investigation Reports 
Multiple workstations 1-5E 
Insulation 
Damage 1-2, 1-5G, 1-6A 
Properties 1-6A 
Specifications 1-6A 
Photographs 1-5A 
Port Authority of New York and New Jersey 
Smoke Control System 
Design 1-4, 1-4D 
Operation on 9/11/1/01 1-4D 
Structural Steel 
Damage 1-3C 
Mechanical properties 1-3D 
Recovered pieces 1-3B 
Specifications 1-3A 
Physical properties 1-3E 
Structure Modeling 
For aircraft impact 1-2, 1-2B 
Reference models 1-2, 1-2A 
NIST NCSTAR 1, WTC Investigation 

Appendix B Draft for Public Comment 
For structural modeling 1-2, 1-5F, 1-6C, 1-6D 
Time Lines 
Aircraft impact 1-2, 1-2B, 1-5A 
Evacuation 1-7, 1-8 
Fires 1-5A 
Videos 1-5A 
Walls 
Damage 1-2, 1-2B 
Specifications 1-1 
NIST NCSTAR 1, WTC Investigation