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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 supp