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an attempt to uncover the truth about September 11th 2001
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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
Abstract Draft for Public Comment
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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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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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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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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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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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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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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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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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• 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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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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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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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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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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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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• 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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.
NIST NCSTAR 1, WTC Investigation
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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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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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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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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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
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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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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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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.
NIST NCSTAR 1, WTC Investigation
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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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.
NIST NCSTAR 1, WTC Investigation
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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.
NIST NCSTAR 1, WTC Investigation
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
NIST NCSTAR 1, WTC Investigation
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.
NIST NCSTAR 1, WTC Investigation
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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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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.
NIST NCSTAR 1, WTC Investigation
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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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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138 NIST NCSTAR 1, WTC Investigation
(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.
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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.
NIST NCSTAR 1, WTC Investigation
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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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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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.
NIST NCSTAR 1, WTC Investigation
Draft for Public Comment Principal Findings
• 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 st