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 ...................................................... 2
    1.2.1 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

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
    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
    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

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<97>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 Investigation

Chapter 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
    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