Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers (Draft)

Transcription

1 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

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3 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

4 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 ). 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), 286 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) Fax: (202) Mail: Stop SSOP, Washington, DC

5 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 iii

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

8 Contributors to the Investigation Draft for Public Comment Jonathan Demarest Stuart Dols Michelle Donnelly Dat Duthinh David Evans Richard Fields Tim Foecke Glenn Forney William Fritz Anthony Hamins Edward Hnetkovsky Erik Johnsson Dave Kelley Mark Kile Erica Kuligowski Jack Lee William Luecke Alexander Maranghides David McColskey Chris McCowan Jay McElroy Kevin McGrattan Roy McLane George Mulholland Lakeshia Murray Kathy Notarianni Joshua Novosel Long Phan William Pitts Thomas Ohlemiller Victor Ontiveros Richard Peacock Max Peltz Lisa Petersen Rochelle Plummer Kuldeep Prasad Natalia Ramirez Ronald Rehm 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 NIST Experts and Consultants 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 vi NIST NCSTAR 1, WTC Investigation

9 Draft for Public Comment Contributors to the Investigation NIST Contractors Anter Laboratories, Inc. Renee Jacobs-Fedore Daniela Stroe Applied Research Associates, Inc. Steven Kirkpatrick* Robert T. Bocchieri Robert W. Cilke Marsh Hardy Samuel Holmes Robert A. MacNeill Claudia Navarro Brian D. Peterson Justin Y-T. Wu Computer Aided Engineering Associates Peter Barrett* Michael Bak Daniel Fridline James J. Kosloski DataSource John Wivaag GeoStats Marcello Oliveira Hughes Associates, Inc. Ed Budnick* Mike Ferreira Mark Hopkins Matt Hulcher Alwin Kelly Chris Mealy John Schoenrock Steven Strege Indepdendent Contractors Ajmal Abbasi Eduardo Kausel David Parks Daniele Veniziano Josef Van Dyck Kaspar Willam John Jay College Norman Groner Leslie E. Robertson Associates William J. Faschan* Richard B. Garlock* William C. Howell Raymond C. Lai NIST NCSTAR 1, WTC Investigation 7

10 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* Carlos Arce Heather Contrino Christopher Frye Nancy McGuckin Sandra Rodriguez Della Santos Robert Santos Rolf Jensen Associates Ray Grill* Ed Armm Tom Brown Duane Johnson Bob Keough Joseph Razz 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 Heather Duvall John Eichner* Mark Huffman Charlotte Johnson Michael Kalmar Jacquelyn Rhone Cheri Sawyer* Walter Soverow Paul Updike Yvonne Zagadou Simpson Gumpertz Heger Mehdi Zarghamee* Glenn Bell Said Bolourchi Daniel W. Eggers Omer O. Erbay Ron Hamburger Frank Kan Yasuo Kitane Atis Liepins Michael Mudlock Wassim I. Naguib Rasko P. Ojdrovic Andrew T. Sarawit Pedro Sifre 8 NIST NCSTAR 1, WTC Investigation

11 Draft for Public Comment Contributors to the Investigation S.K. Ghosh Associates, Inc. S.K. Ghosh* Analdo Derecho Dave Fanella Xumei Liang 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 Aldo Jimenez William Joy John Mammoser 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 NIST NCSTAR 1, WTC Investigation 9

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

13 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 xi

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15 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 xiii

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17 TABLE OF CONTENTS National Construction Safety Team for the Federal Building and Fire Safety Investigation of the World Trade Center Disaster...iii Contributors to the Investigation... v Dedication... xi Abstract...xiii List of Figures... xxi List of Tables... xxv List of Acronyms and Abbreviations...xxvii Units and Conversion Factors... xxix Preface... xxxi Executive Summary...xli Part I: September 11, 2001 Chapter 1 New York City s World Trade Center The Origination The World Trade Center Complex The Site The Towers... 5 Chapter 2 The Account of World Trade Center :46:30 a.m. EDT The Aircraft The Immediate Damage The Jet Fuel :47 a.m. to 9:02 a.m. EDT :02:59 a.m. EDT :03 a.m. to 9:57 a.m. EDT :58:59 a.m. EDT :59 a.m. to 10:28 a.m. EDT The Outcome NIST NCSTAR 1, WTC Investigation xv

18 Table of Contents Draft for Public Comment Chapter 3 The Account of World Trade Center :46:30 a.m. EDT :02:59 a.m. EDT The Immediate Damage The Jet Fuel :03 a.m. to 9:36 a.m. EDT :36 a.m. to 9:58 a.m. EDT The Outcome Chapter 4 The Toll Part II: Reconstructing the Disaster Chapter 5 The Design and Construction of the Towers Building and Fire Codes The Codes and the Towers The New York City Building Code Pertinent Construction Provisions Tenant Alteration Process Building Design Loads Aircraft Impact Construction Classification and Fire Resistance Rating Compartmentation Egress Provisions Active Fire Protection Building Innovations The Need for Innovations Framed Tube System Deep Spandrel Plates Uniform External Column Geometry Wind Tunnel Test Data to Establish Wind Loads Viscoelastic Dampers Long-Span Composite Floor Assemblies xvi NIST NCSTAR 1, WTC Investigation

19 Draft for Public Comment Table of Contents Vertical Shaft Wall Panels Structural Steels Types and Sources Properties Passive Fire Protection Thermal Insulation Use of Insulation in the WTC Towers Concrete The Tenant Spaces General Walls Flooring Ceilings Furnishings Chapter 6 Reconstruction of the Collapses Approach Development of the Disaster Timeline Learning from the Visual Images Learning from the Recovered Steel Collection of Recovered Steel Mechanical and Physical Properties Damage Analysis Information Gained from Other WTC Fires The Building Structural Models Computer Simulation Software The Reference Models Building Structural Models for Aircraft Impact Analysis Building Structural Models for Structural Response to Impact Damage and Fire and Collapse Initiation Analysis The Aircraft Structural Model Aircraft Impact Modeling Component Level Analyses Subassembly Impact Analyses Aircraft Impact Conditions NIST NCSTAR 1, WTC Investigation xvii

20 Table of Contents Draft for Public Comment Global Impact Analysis Aircraft Impact Damage Estimates Structural and Contents Damage Validity of Impact Simulations Damage to Thermal Insulation Damage to Ceiling System Damage to Interior Walls and Furnishings Thermal Environment Modeling Need for Simulation Modeling Approach The Four Cases Characterization of the Fires Global Heat Release Rates Data Transfer Thermal Mapping Approach The Fire-Structure Interface Thermal Insulation Properties FSI Uncertainty Assessment The Four Cases Characterization of the Thermal Profiles Measurement of the Fire Resistance of the Floor System Collapse Analysis of the Towers Approach to Determining the Probable Collapse Sequences Results of Global Analysis of WTC Results of Global Analysis of WTC Structural Response of the WTC Towers to Fire Without Impact or Insulation Damage Probable WTC 1 Collapse Sequence Probable WTC 2 Collapse Sequence Accuracy of the Probable Collapse Sequences Factors that Affected Building Performance on September 11, xviii NIST NCSTAR 1, WTC Investigation

21 Draft for Public Comment Table of Contents Chapter 7 Reconstruction of Human Activity Building Occupants Background The Building Egress System The Evacuation Data Sources Occupant Demographics Evacuation of WTC Evacuation of WTC Emergency Responders Data Gathered Operations Changes Following the WTC 1 Bombing on February 26, Responder Organization Responder Access Communications The Overall Response Factors That Contributed to Enhanced Life Safety Aggregate Factors Individual Factors Part III: The Outcome of the Investigation Chapter 8 Principal Findings Introduction Summary Findings on the Mechanisms of Building Collapse Summary of Probable Collapse Sequences Structural Steels Aircraft Impact Damage Analysis Reconstruction of the Fires Structural Response and Collapse Analysis Findings on Factors Affecting Life Safety Active Fire Protection Evacuation NIST NCSTAR 1, WTC Investigation xix

22 Table of Contents Draft for Public Comment Emergency Response Findings on Operational Codes, Standards, and Practices General Structural Safety Fire Safety Future Factors That Could Have Improved Life Safety Building Performance Factors Human Performance Factors Chapter 9 Recommendations Building Standards and Codes: Who Is In Charge? NIST S Recommendations for Improving the Safety of Buildings, Occupants, and Emergency Responders Group 1. Increased Structural Integrity Group 2. Enhanced Fire Resistance of Structures Group 3. New Methods for Fire Resistance Design of Structures Group 4. Improved Active Fire Protection Group 5. Improved Building Evacuation Group 6. Improved Emergency Response Group 7. Improved Procedures and Practices Group 8. Education and Training Opportunity for Public Comment Beginning the Implementation Process Appendix A National Construction Safety Team Act Appendix B Subject Index of Supporting Investigation Reports xx NIST NCSTAR 1, WTC Investigation

23 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....7 Figure 1 4. Perimeter column/spandrel assembly and floor structure Figure 1 5. Plan of the 96 th floor of WTC 1 showing the core and tenant spaces Figure 1 6. Schematic of composite floor truss system Figure 1 7. Schematic of a hat truss Figure 1 8. Photograph of insulated WTC trusses Figure 1 9. Schematic of the three-tier elevator system Figure Orientation of the three stairwells Figure Views of typical WTC office floors Figure A WTC trading floor Figure 2 1. Simulated impact of American Airlines Flight 11 with WTC Figure 2 2. Aircraft entry hole on the north side of WTC 1, photographed 30 s after impact Figure 2 3. South face damage of WTC 1 with key aircraft component locations marked Figure 2 4. Simulation of aircraft impact damage to the 96 th floor in WTC 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 Figure 2 6. Firefighters on the scene at about 9:07 a.m Figure 2 7. Figure 2 8. Figure 2 9. 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 Steel surface temperatures on the bottom chords of fire-exposed trusses, uninsulated and insulated with ¾ in. of BLAZE-SHIELD DC/F Temperature dependence of yield strength of structural steel as a fraction of the value at room temperature Figure Simulated temperatures of two adjacent trusses (left) and two adjacent perimeter columns (right) exposed to the fires in WTC Figure Temperature contours on the top and bottom faces of the concrete slab (96 th 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 NIST NCSTAR 1, WTC Investigation xxi

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

25 Draft for Public Comment List of Figures Figure Pratt & Whitney PW4000 turbofan engine model Figure Boeing ER showing the jet fuel distribution at time of impact Figure Calculated impact on an exterior wall by a fuel-laden wing section Figure Response of a tower subassembly model to engine impact Figure Side view of simulated aircraft impact into WTC 1, Case B Figure Column damage levels Figure Case B damage to the slab of floor 96 of WTC Figure Case B simulation of response of contents of 96 th floor of WTC Figure Combined structural damage to the floors and columns of WTC 1, Case A Figure Combined structural damage to the floors and columns of WTC 1, Case B Figure Combined structural damage to the floors and columns of WTC 2, Case C Figure Combined structural damage to the floors and columns of WTC 2, Case D Figure Observed and Case A calculated damage to the north face of WTC Figure Schematic of observed damage (top) and Case A calculated damage (lower) to the north face of WTC Figure Schematic of observed damage (above) and Case C calculated damage (right) to the south face of WTC Figure Ceiling tile system mounted on the shaking table Figure Eight floor model of WTC 1 prior to aircraft impact Figure Fire test of a single workstation Figure Interior view of a 3-workstation fire test Figure Rubblized workstations Figure Three-workstation fire test, 2 min after the start Figure Measured and predicted heat release rate from the burning of three office workstations Figure Upper layer temperatures on the 94 th floor of WTC 1, 15 min after impact Figure Direction of simulated fire movement on Floors 94 and 97 of WTC Figure Predicted heat release rates for fires in WTC 1 and WTC Figure Simple bar dimensions (in.) Figure Tubular column dimensions (in.) Figure Truss Dimensions (in.) Figure SFRM-coated steel components prior to a test Figure Finite element representation of the insulated steel truss (blue), the SFRM (violet), and the ceiling (red) Figure Comparison of numerical simulations with measurements for the steel surface temperature at four locations on the top chord of a bare truss NIST NCSTAR 1, WTC Investigation xxiii

26 List of Figures Draft for Public Comment Figure Comparison of numerical simulations with measurements for the temperature of the steel surface at four locations on the top chord of an insulated truss Figure Temperatures ( C) on the columns and trusses of the 96 th floor of WTC 1 at 6,000 s after aircraft impact, Case B Figure Temperatures ( C) on the columns and trusses of the 81 st floor of WTC 2 at 3,000 s after aircraft impact, Case D Figure Frames from animation of the thermal response of columns on the 96 th Floor of WTC 1, Case A Figure 7 1. Simulated impact damage to 95 th floor of WTC 1, including stairwells, 0.7 s after impact 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 evacuation in WTC Figure 7 4. Observations of building damage from tenant spaces in WTC Figure 7 5. Location of the radio repeater Figure 7 6. Timing of FDNY unit arrivals Figure 7 7. Fire Command Board located in the lobby of WTC xxiv NIST NCSTAR 1, WTC Investigation

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

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

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

31 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 m 3 GJ gigajoule GW gigawatt in. inch kg kilogram kip 1,000 lb ksi 1,000 lb/in. 2 lb pound 1 lb = 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 xxix

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33 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 ), 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 xxxi

34 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 ). 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 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 Document and analyze the code provisions, procedures, and Practices; Project Leaders: Dr. H. S. Lew practices used in the design, construction, operation, and and Mr. Richard W. Bukowski 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 Mechanical and Metallurgical Analysis of Structural Steel; Project Leader: Dr. Frank W. Gayle Investigation of Active Fire Protection Systems; Project Leader: Dr. David D. Evans Reconstruction of Thermal and Tenability Environment; Project Leader: Dr. Richard G. Gann Structural Fire Response and Collapse Analysis; Project Leaders: Dr. John L. Gross and Dr. Therese P. McAllister Occupant Behavior, Egress, and Emergency Communications; Project Leader: Mr. Jason D. Averill Emergency Response Technologies and Guidelines; Project Leader: Mr. J. Randall Lawson 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. Determine and analyze the mechanical and metallurgical properties and quality of steel, weldments, and connections from steel recovered from WTC 1, 2, and 7. 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. 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. 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. 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. 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. xxxii NIST NCSTAR 1, WTC Investigation

35 Draft for Public Comment Preface NIST WTC Investigation Projects WTC Building Performance Study Recommendations Government, Industry, Professional, Academic Inputs Public Inputs Video/ Photographic Records Oral History Data Emergency Response Records Analysis of Steel Analysis of Codes and Practices Baseline Performance & Impact Damage Active Fire Protection Structural Collapse Thermal and Tenability Environment Emergency Response Recovered Structural Steel Evacuation 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 xxxiii

36 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 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. xxxiv NIST NCSTAR 1, WTC Investigation

37 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 xxxv

38 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) 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) 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 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 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 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 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 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 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 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New xxxvi NIST NCSTAR 1, WTC Investigation