Final Report on the Collapse of World Trade Center Building

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Final Report on the Collapse of World Trade Center Building

Postby admin » Sat Jul 01, 2017 3:49 am

Final Report on the Collapse of World Trade Center Building 7
by NIST National Institute of Standards and Technology
U.S. Department of Commerce, Carlos M. Gutierrez, Secretary
National Institute of Standards and Technology, Patric D. Gallagher, Deputy Director
NIST NCSTAR 1A: Federal Building and Fire Safety Investigation of the World Trade Center Disaster
November, 2008

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Keywords: building evacuation, emergency response, fire safety, structural collapse, tall buildings, World Trade Center.

TABLE OF CONTENTS:

• National Construction Safety Team for the Federal Building and Fire Safety Investigation of the World Trade Center Disaster
• Contributors to the Investigation of WTC 7
• Dedication
• Abstract
• Table of Contents
• List of Figures
• List of Tables
• List of Acronyms and Abbreviations
• Preface
• Executive Summary
• Chapter 1: The New York City World Trade Center Building 7
• 1.1 The World Trade Center Complex
• 1.2 WTC 7
• 1.2.1 The Edifice
• 1.2.2 The Con Edison Substation
• 1.2.3 The Structure
• 1.2.4 Fire Protection
• 1.2.5 The Workplace
• 1.2.6 The Combustible Contents
• 1.3 References
• Chapter 2: The Account of WTC 7
• 2.1 Introduction
• 2.2 Activity at the WTC 7 Site
• 2.2.1 8:46 a.m. to 9:59 a.m. EDT
• 2.2.2 9:59 a.m. to 10:28 a.m. EDT
• 2.2.3 10:29 a.m. to 5:21 p.m. EDT.
• 2.3 Progress of the Fires in WTC 7
• 2.4 The Probable Collapse Sequence
• Chapter 3
• Deriving The Probable Collapse Sequence
• 3.1 Gathering of Evidence
• 3.2 The Leading Hypothesis
• 3.3 Hypothetical Blast Scenarios
• 3.4 The Four-step Simulation Process
• 3.4.1 Technical Approach
• 3.4.2 Fires Simulated
• 3.4.3 Fire Dynamics Simulator (FDS)
• 3.4.4 Fire Structure Interface (FSI)
• 3.4.5 Structural Analysis of the Initial Failure Event using ANSYS
• 3.4.6 Global Collapse Analysis using LS-DYNA
• 3.5 Accuracy of the Probable Collapse Sequence
• 3.5.1 Aspects prior to the Global Collapse
• 3.5.2 Aspects following the Global Collapse Initiation
• 3.5.3 Accuracy Appraisal
• 3.6 Timing of Collapse Initiation and Progression
• 3.7 References
• Chapter 4: Principal Findings
• 4. 1 Introduction
• 4.2 Summary
• 4.3 The Mechanisms of Building Collapse
• 4. 3.1 Debris Impact Damage from the Collapse of WTC 1
• 4.3.2 Reconstruction of the Fires
• 4.3.3 Fire-induced Thermal Effects
• 4.3.4 Structural Response and Collapse
• 4.4 Life Safety Factors
• 4.4.1 Evacuation of WTC 7
• 4.4.2 Emergency Response
• 4.5 Codes. Standards, and Practices
• 4.5.1 General
• 4.5.2 Building Design and Structural Safety
• 4.5.3 SFRM Requirements and Application
• 4.5.4 Fire Safety and Fire Protection Systems
• 4.6 Future Factors That Could Have Mitigated Structural Collapse
• 4.7 Human Performance Factors
• Chapter 5: Recommendations
• 5.1 General
• 5.2 NIST's Recommendations for Improving the Safety of Buildings, Occupants, and Emergency Responders
• 5.2.1 Group 1. Increased Structural Integrity
• 5.2.2 Group 2. Enhanced Fire Endurance of Structures
• 5.2.3 Group 3. New Methods for Fire Resistant Design of Structures
• 5.2.4 Group 4. Improved Active Fire Protection
• 5.2.5 Group 6. Improved Emergency Response 5.2.6 Group 7. Improved Procedures and Practices 5.2.7 Group 8. Education and Training
• Appendix A: National Construction Safety Team Act
• Appendix B: World Trade Center Investigation Publications

LIST OF FIGURES

• Figure P-l. Technical components of the Federal Building and Fire Safety Investigation of the WTC Disaster
• Figure 1-1. The World Trade Center in Lower Manhattan
• Figure 1-2. Photograph of the World Trade Center Complex, showing WTC 7
• Figure 1-3. Footprints of the Con Edison substation and WTC 7
• Figure 1-4. Aerial view from the north side of WTC 7, showing the Con Edison substation
• Figure 1-5. Typical WTC 7 floor showing locations of the columns, girders, and beams
• Figure 1-6. 3D schematic view of transfer trusses and girders between Floors 5 and 7
• Figure 1-7. Schematic drawing of the elevators in WTC 7
• Figure 1-8. Schematic of Floor 8
• Figure 1-9. Schematic of Floor 11
• Figure 1-10. Section view of diesel firel distribution components in WTC 7
• Figure 2-1. Observed damage to WTC 7 following the collapse of WTC 1
• Figure 2- 2. Eastward buckling of Column 79, viewed from the southeast
• Figure 3- 1. Peak overpressure and broken window locations
• Figure 3- 2. WTC 7 analysis sequence and interdependencies
• Figure 3- 3. Schematic of the layout of the 6th floor of WTC 7
• Figure 3-4. Schematic layout of the 5th floor of WTC 7 showing the locations of the emergency power system components
• Figure 3- 5. Progression of simulated fire on Floor 8 of WTC 7 showing gas temperatures near the ceiling
• Figure 3-6. Progression of simulated fire on Floor 12 of WTC 7 showing gas temperatures near the ceiling
• Figure 3- 7. Computed temperature distribution (DC) of Floor 13 steel framing at five different instants in time
• Figure 3- 8. Computed temperature distribution (DC) in the top layer of the concrete slab of Floor 12 at five different instants in time
• Figure 3- 9. Damage state of connections in Floor 13 for Case B temperatures
• Figure 3- 10. Vertical progression of failures on the east side of the building at 0.5 s following the initiation of the collapse
• Figure 3-11. Failure of Columns 77 and 78 due to failure of Truss 2 fails from debris impact at 2.5 s following the initiation of the collapse
• Figure 3-12. Failure of Columns 73 to 75 from the load redistribution and debris impact at 4.5 s following the initiation of the collapse
• Figure 3-13. Buckling of all interior columns at 6.5 s following the initiation of the collapse
• Figure 3-14. Buckling of the lower exterior columns within 1 s of Figure 3-13
• Figure 3-15. Downward velocity of north face roofline as WTC 7 began to collapse

LIST OF TABLES

• Table P- l. Federal Building and Fire Safety Investigation of the WTC Disaster
• Table P- 2. Public meetings and briefings of the WTC Investigation
• Table 1-1. Use of floors in WTC 7
• Table 1-2. Emergency power systems in WTC 7
• Table 3- l. Comparison of global structural model predictions and observations for WTC 7, Case B

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 2008

For sale by the Superintendent of Documents, U.S. Government Printing Office Internet: bookstore.gpo.gov - Phone: (202) 512-1 BOO - Fax: (202) 512-2250 Mail: Stop SSOP, Washington, DC 20402-0001
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 3:49 am

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 2B1a; as amended by P.L. 107-231 ).

National Institute of Standards and Technology National Construction Safety Team Act Report 1A Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1A, 130 pages (November 2008) CODEN: NSPUE2
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 3:59 am

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): Final Report Editor; Project Leader, Project 5: ReCollstmctioll ofThennai and Tenability Environment

William L. Grosshandler, Ph.D. (NIST): Associate Lead Investigator; Project Leader, Project 4: Investigation of Active Fire Protection Systems

H.S. Lew, Ph.D., P.E. (NIST): Co-Project Leader, Project I: Analysis of Building and Fire Codes and Practices

Richard W. Bukowski, P.E. (NIST): Co-Project Leader, Project 1: Analysis of Building and Fire Codes and Practices

Fahim Sadek, Ph.D. (NIST): Project Leader, Project 2: Baseline Structural Perf0l1n3nCe and Aircraft Impact Damage Analysis

Frank W. Gayle, Ph.D. (NIST): Project Leader, Project 3: Mechanical and Metallurgical Analysis of Structural Steel

John L. Gross, Ph.D., P.E. (NIST): Co-Project Leader, Project 6: Structural Fire Response and Collapse Analysis

Therese P. McAllister, Ph.D., P.E. (NIST): Co-Project Leader, Project 6: Structural Fire Response and Collapse Analysis

Jason D. Averill (NIST): Project Leader, Project 7: Occupant Behavior, Egress, and Emergency Communications

J. Randall Lawson (NIST): Project Leader, Project 8: Fire Service Technologies and Guidelines

Harold E. Nelson, PE: Fire Protection Engineering Expert

Stephen A. Cauffman (NIST): Program Manager

CONTRIBUTORS TO THE INVESTIGATION OF WTC 7

NIST TECHNICAL STAFF


Mohsell Altafi
Elisa Baker
Dilip Banarjee
Stephen Banovic
Howard Baum
Carlos Beauchamp
Dale Bentz
Charles Bouldin
Paul Brand
Lori Brassell
Kathryn Butler
Sandy Clagett
Ishmael Conteh
Matthew Covin
David Dayan
Stuatt Dols
Michelle Donnelly
Dat Duthinh
David Evans
Richard Fields
Tim Foecke
Glenn Forney
William Fritz
Anthony Hamins
Dave Kelley
Erica Kuligowski
William Luecke
Joseph Main
David McColskey
Chris McCowan
Kevin McGrattan
George Mulholland
Lakeshia Murray
Joshua Novosel
Thomas Ohlemiller
Victor Ontiveros
Richard Peacock
Lisa Petersen
Long Phan
William Pitts
Rochelle Plummer
Kuldeep Prasad
Natalia Ramirez
Ronald Rehm
Paul Reneke
Lonn Rodine
Schuyler Ruitberg
Jose Sanchez
Raymond Santoyo
Steven Sekellick
Michael Selepak
Thomas Siewert
Emil Simiu
Laura Sugden
Robert Vettori
Brendan Williams
Maureen Williams
Jiann Yang
Robert Zarr

NIST EXPERTS AlND CONSULTANTS

Najib Abboud
William Baker
Gene Corley
Vincent Dunn
Jim Harris
Steven Hill
John Hodgens
Valentine Junker
Kevin Malley
Shankar Nair
J. Keith Nelson

Contributors to the Investigation

DEPARTMENT OF COMMERCE AND NIST INSTITUTIONAL SUPPORT


Tomara Alrington
Kellie Beall
Tara Brown
Craig Burkhardt
Deborah Cramer
Gail Crum
Jane Dana
Matthew Heyman
James Hill
Nuala O'Connor Kelly
Fred Kopatich
Kenneth Lechter
Melissa Lieberman
Darren Lowe
Romena Moy
Michael E. Newman
Karen Perry
Gail Porter
Sharon Rinehart
Michael Rubin
John Sanderson
Joan Smith
Jack Snell
Nancy Snyder
Ben Stein
Kelly Talbott
Michael R. Rubin

NIST CONTRACTORS

Applied Research Associates. Inc.
Michael Anderson
Robert T. Bocchieri
Joseph Crepeau
Steven Kirkpatrick
Charles Needham
Robert A. MacNeill
Brian D. Peterson
Lee Ann Young

Gilsanz Murray Steflcek, LLP
Ramon Gilsanz

HeiTech Services. Inc.
Sonya D. Wilson

Koffel Associates, Inc.
William Kotfel

Loizeaux Group International
Mark Loizeaux

Rolf Jensen & Associates. Inc.
Ed Anmm
Tom Brown
Ray Grill
Duane Johnson
Bob Keough
Joseph Razz

Simpson Gumpertz & Heger Inc
Omer O. Erbay
Andrew T. Sarawit
Mehdi Zarghamee

Thornton Engineering Associates, LLC
William Thornton

COOPERATING ORGANIZATIONS

ABC

Tony Brackett
Joel Kanoff
Sanja Karabegovic
Vladimir Tokarev
Ana Villanueva

American Express
Peter Kane
Carol Schwaltz

AP
Brad Barkett
David Beatrice
Mike LeTorneau
Kevin O'Sullivan
Sean Thompson
James Wood

AP
Mike LeTourneau

APTN
Fulvia Cassarino
Tom Giovan Larry Mendillo
Roger Raiford
James Turner

Bernstein Associates Photographers
Neal Lehrer

CBS
Joseph Alessi
Margery Baker
Steve Bikofsky
Roy Carubia
Barbara Casey
Jessica Cooper
Hillary Dann
Daniel Di Pierro
William Felling
Ann Fotiades
Laura Galli
Mary Gera
Michael Hernandez
Andrew Heyward
Mark Laganga
Linda Mason
Kathy Mosolino
Tony St. Pierre
Jean Stevenson
Cheryl Williams

CNN
Kathy Christensen
Moira Danehy
Tiffany Dumas
Felicia Dunston
Dina Gunderson
Eason Jordan
Bill Schneider
James Seward
David Sheehan
James Tzetzo
David Vigilante

Citigroup, Inc.
James Goddard

Cleary Gottlieb Steen & Hamilton LLP
Karen Bekker
Adam Itzkowitz
Thomas Moloney
Christopher Moore
Rahul Mukhi
Kimberly Spiering
John VanSickle

Clifford Law Offices
Timothy S. Tomasik

Con Edison
Martin Heslin

Corbis
Amy Bizjak
Ted Ciuzio
Candice Luz
Rachel Wright

FEMA
Audrey Massa
Bruce Swiren

Fire Department of the City of New York
Amy Adelman
Alexandra Fischer
John Tsanas

Flemming, Zulack, WilIiamson, Zauderer, LLP
Gregg Kanter

Fox News
Christopher J. Silvestri

Friedman Kaplan Seiler and Adelman
Kent Anker

G&S Technologies
Jay Spector

General Services Administration
Mary Guida

Gogick, Byrne & O'Neill, LLP
Kevin J. O'Neill
Stephen P. Schreckinger

Here is New York
Paul Constantine
Ruth Sergel

Keegan Werlin LLP
Richard B. Kirby
Ryan T. Parsons (paralegal)

Lower Manhattan Development Corporation
Irene Chang
David Ridley

Magnum
Michael Shulman

National Commission on Terrorist Attacks upon the United States
Madeline Blot
Sam M. W. Casperson
George L. Delgrosso
Daniel Marcus
James Miller
Catherine S. Taylor

NBC
Nancy Cole Billy Ray

New York City Department of Design and Construction (DDC)
David J. Varoli

New York City Law Department
Ken Becker
Jay L. Cohen
Lawrence S. Kahn
Jessie Levine
Faye Lubinof
Gary Shaffer
Katherine Winningham

New York City Police Department
Edward Alexander
Michael Healey
Sophia Sifneos

New York City Transit Authority
Kavita Bhatt
Paul Fleuranges
Veronica Hakim
George Miller
Leonard Wiggin

The Hartford Insurance Co.
Mary Chepovsky

The Port Authority of New York and New Jersey
James Begley
Saroj Bhol
Jeffrey Gertier
Frank Lombardi
Alan Reiss
Timothy Stickelman

Robins Kaplan Miller & Ciresi
Margo Brownell

Schiff Harden
Beth Jacob

Siemens
John Farrington
Robert Salamone
Steven Shamash

Silverstein Properties
Dara McQuillan
Steven Nathan
Larry Silverstein
Walter Weems

Simpson, Thacher & Bartlett LLP
Jamie Gamble

SOM
Sharyn Fitter
Jeffrey Goldsmith

Testwell Craig
Richard Bridglal

Tribeca Towers
Chris Bricker
Brendan Farrell
Edwin Ginario
Raine Philliops

Turner Construction Company
Bernadette Forte

UHY Advisors FLVS, Inc
Jonathan L. Newcomb

U.S. Securities and Exchange Commission
Al Basile
Robert DeLeonardus
Ray Ferrari
Richard Lee

WABC
Kenny Plotnik
Hector Montalvo

Wachtel, Lipton, Rosen & Katz
Ian Botchko
Andrew Cheun
Gina Iannello
Mark Wolinskiy

WCBS
Richard Bamberger
Jennifer Bennett
Vince Dementri
Nadine Kerr
Stephen Sanchez
Marc Smith
Erika Vasconcellos

Weidlinger Associates Inc.
Najib N. Abboud

WNBC
Daniel Forman
Burtonn Kravitz
Thomas O'Brien
Dennis Swanson

WNJU
Hugo Balta
Bill Mierisch
Migdalia Perez

WNYW
Jim Clayton
Deborah Daft
Ron Magocsi
Scott Matthews
Kai Simonsen

WPIX
Melinda Murphy
Edith Rivera
Ray Rivera
Karen Scott
Chet Wilson

W.R. Grace
Robert J. Bettacchi
Michael B. Cohan
Mark A. Shmorhun

Zetlin & DeChiara
David Abramovitz

OTHER CONTRIBUTORS

Noah Bast
Douglas Campbell
Michael Chan
Nicolas Cianca
William Cirone
Jay Comella
Michael Davis
Frank Didik
Alexandra Fisher
David Fitzpatrick
Kevin Flynn
Erik Freeland
Archie Galarza
James Glanz
Jack Glass
Michael Heller
Valerie Hodgson
Shawn Hutchinson
Keith V. Johnson
Lance Karp
Steve Mayer
Steve McCurry
Susan Meiselas
Jeroell Morrien
Jennifer Olsen
Richard Peskin
Roberto Rabanne
Courtenay Redis
Tracey Reilly
Mark Roddenberry
Marcel Saba
Terry Schmidt
Jim Scott
Greg Semendinger
Shepard Sherbell
Bruce Shiller
Scott Sleeper
Steve Spak
Amanda Steinberger
Virginia Stewart
Robert Stolarik
Allan Tannenbaum
Walter Taylor
Chris Thaler
Courtney Thaler
Tim Tobiasen
Mark Walsh
Cynthia Weil
Abe Weinberg
Charles Wisniewski
Aman Zafar
Paul Zucker
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 4:00 am

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. Seven 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 response of the WTC towers, WTC 7, 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 day. 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.
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 4:00 am

ABSTRACT

This is the final report on the National Institute of Standards and Technology (NIST) investigation of the collapse of World Trade Center Building 7 (WTC 7), conducted under the National Construction Safety Team Act. This report describes how the fires that followed the impact of debris from the collapse of WTC 1 (the north tower) led to the collapse of WTC 7; 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 building; and areas in current building and fire codes, standards, and practices that warrant revision. Extensive details are found in the companion reports, NIST NCSTAR 1-9 and NIST NCSTAR 1-9A.

Also in this report is a summary of how NIST reached its conclusions. NIST complemented in-house expertise with private sector technical experts; accumulated copious documents, photographs, and videos of the disaster; conducted first-person interviews of building occupants and emergency responders: analyzed the evacuation and emergency response operations in and around WTC 7; performed computer simulations of the behavior of WTC 7 on September 1, 2001; and combined the knowledge gained into a probable collapse sequence.

The report concludes with a list of 13 recommendations for action in the areas of increased structural integrity, enhanced fire endurance of structures, new methods for fire resistant design of structures, enhanced active fire protection, improved emergency response, improved procedures and practices, and education and training. One of these is new; the other 12 are reiterated from the investigation into the collapse of the WTC towers. Each of the 13 is relevant to WTC 7.
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 4:01 am

List of Acronyms and Abbreviations:

ASCE: American Society of Civil Engineers
ASTM: ASTM International
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
ICC: International Code Council
NFPA: National Fire Protection Association
NIST: National Institute of Standards and Technology
NYC: New York City
NYCBC: New York City Building Code
NYPD: New York City Police Department
NYS: New York State
OEM: New York Mayor's Office of Emergency Management
PANYNJ: The Port Authority of New York and New Jersey
PAPD: Port Authority Police Department
SEC: Securities and Exchange Commission
SFRM: Sprayed fire-resistive material
UL: Underwriters Laboratories
USC: United States Code
USFA: United States Fire Administration
WTC: World Trade Center
WTC 1: World Trade Center 1 (North Tower)
WTC2: World Trade Center 2 (South Tower) WTC7: World Trade Center 7

Abbreviations and Conversion Factors

dB: decibel
°C: degree Celsius / T (°C) = 5/9 [T (°F) - 32]
°F: degree Fahrenheit
ft: feet
gal: gallon / 1 gal = 3.78 x 10[-3]m[3]
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
um: micrometer
min: minute
MJ: megajoule
MW: megawatt
psi: pounds per square inch
s: second
T: temperature
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 4:04 am

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 its 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 its report in May 2002, fulfilling its 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:
o Improvements in the way buildings are designed, constructed, maintained, and used;
o Improved tools and guidance for industry and safety officials;
o Recommended revisions to current codes, standards, and practices; and
o 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 is a nonregulatory agency of the U.S. Department of Commerce. The purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United States, and the focus is on fact finding. NIST investigative teams are authorized 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 nor 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 then NIST Director, Dr. Arden L. Bement, Jr., 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-l , and the key technical components are illustrated in Fig. P- l.

Table P-l . 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. Bukowki / 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 H. 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; Dr. William Grosshandler / 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.


Image

Figure P-1. Technical components of 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, with their terms in parentheses:

• Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety Team Advisory Committee Chair (2003 -2009)
• John Barsom, President, Barsom Consulting, Ltd. (2003 -2011)
• John Bryan, Professor Emeritus, University of Maryland (2003 -2004)
• David Collins, President, The Preview Group, Inc. (2003 -2010)
• Glenn Corbett, Professor, John Jay College of Criminal Justice (2003-2006)
• Philip DiNenno, President, Hughes Associates, Inc. (2003-2006)
• Robert Hanson, Professor Emeritus, University of Michigan (2003-2009)
• Charles Thornton, Co-Chairman and Managing Principal, The Thornton-Tomasetti Group, Inc. (2003-2011)
• Kathleen Tierney, Director, Natural Hazards Research and Applications Information Center, University of Colorado at Boulder (2003-2007)
• Forman Williams, Director, Center for Energy Research, University of California at San Diego (2003-2011)

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. NIST has benefited from the work of many people in the preparation of these reports, including the National Construction Safety Team Advisory Committee. The content of the reports and recommendations, however, are solely the responsibility of NIST.

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. Prior to publishing the final reports, NIST released a draft of the reports for public comment. NIST received many comments from individuals, designers, professional organizations within the building and fire communities, and building owners. The comments were reviewed by the authors and addressed to the extent appropriate. The reports were modified with clarifications and supplemental text where needed, and the changes resulted in an improved final product. Comments on topics outside the scope of the investigation were not addressed.

Information Quality Standards

NIST conducted its World Trade Center (WTC) Investigation in accordance with Office of Management and Budget (OMB) directives and the Department of Commerce's and NIST's Information Quality Standards. This ensured that NIST's findings were objective, had utility to the industry, to emergency response professionals and to the general public, and insured the integrity of the information collected and presented in NIST's reports.

As defined in NIST's Information Quality Standards, "Objectivity consists of two distinct elements: presentation and substance. The presentation element includes whether disseminated information is presented in an accurate, clear, complete, and unbiased manner and in a proper context. The substance element involves a focus on ensuring accurate, reliable, and unbiased information." NIST obtained original source materials whenever possible, including design and renovation drawings, reports, correspondence, photographs and videos, and interviews of eye witnesses. Source materials were obtained from multiple sources, so as to minimize the influence of individual roles or accounts in understanding the design, construction, and operations of the WTC buildings and the events that occurred that day.

NIST conducted detailed analyses to simulate the impact damage caused by aircraft and/or debris and the fires, the building response to fires, and the subsequent collapse. These were extensive, state-of-the-art reconstructions of the events, and were validated using observations obtained from photographs and videos as well as observations made by emergency responders and building occupants.

All relevant data and analyses were presented in detailed reports (NIST NCSTAR 1-9 and 1-9A for WTC 7). The National Construction Safety Team (NCST) Advisory Committee reviewed NIST's technical approach in open session. The individual Advisory Committee members also conducted detailed reviews of NIST's draft reports, findings, and recommendations in their capacities as individual subject matter experts and provided substantive comments that were addressed in preparing the final reports. In addition, the draft NIST reports on WTC 7 were peer reviewed by five individual subjects matter experts, contracted by NIST, whose comments were also addressed in preparing the final reports. Thus, NIST made every effort to ensure that the investigation was conducted objectively, that the findings were reasonable and consistent with observations, and that the recommendations had a rational, scientific basis.

NIST's Information Quality Standards define Utility as "the usefulness of information to its intended users, including the public." NIST identified four specific objectives for the WTC Investigation, which are stated at the beginning of the Preface. These goals and objectives were addressed and are reflected in the Principal Findings and Recommendations listed in the NIST NCSTAR I report for the WTC towers and the NIST NCSTAR IA report for WTC 7. Individuals and organizations have responded to NIST's recommendations by developing proposals for changes to codes and standards. At the time of this publication, this effort has resulted in twenty-three changes being adopted into the International Building Code and fifteen introduced into the NFPA 5000 Building Code, NFPA 1 Fire Code, and NFPA 101 Life Safety Code. Some local jurisdictions have incorporated NIST's recommendations in their local codes and some building designers have incorporated the recommendations into their practice. All of these steps will lead to improved safety and security of buildings, building occupants, and emergency responders. The findings and recommendations of the NIST investigation of the World Trade Center disaster have clearly proven to be useful to those charged with ensuring the safety of buildings, building occupants, and emergency responders.

As defined in NIST's Information Quality Standards, "Integrity refers to security - the protection of information from unauthorized access or revision, to ensure that the information is not compromised through corruption or falsification." NIST has been vigilant in protecting all of the information gathered for use in the investigation from outside sources or produced as a part of the investigation (e.g., computer models) from unauthorized access or revision. Access to information has been limited only to those individuals working directly on the investigation. The WTC reports are available electronically as Adobe .pdf files at http: //wtc.nist.gov and have been ''locked'' to prevent revision or other alterations. These measures have ensured the integrity of materials collected or produced during the course of the WTC 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.

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 / 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 May 2003 Progress Report.

August 26-27, 2003 / Gaithersburg, MD / NCST Advisory Committee meeting on status of the WTC investigation with a public comment session.

September 17, 2003 / New York City, NY / Media 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 release of the Public Update with a public comment session.

February 12, 2004 / New York City, NY / Public meeting 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/public briefing on release of 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 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 and public briefing on release of all draft reports for the WTC towers and draft recommendations for public comment.

September 12-13, 2005 / Gaithersburg, MD / NCST Advisory Committee meeting on disposition of public comments and update to draft reports for the WTC towers.

September 13-15, 2005 / Gaithersburg, MD / WTC Technical Conference for stakeholders and technical community for dissemination of findings and recommendations and opportunity for the public to make technical comments.

December 14, 2006 / Teleconference / NCST Advisory Committee meeting on status of WTC 7 investigation and draft annual report to Congress, with a public comment session.

December 16, 2007 / Teleconference / NCST Advisory Committee meeting on status of WTC 7 investigation and draft annual report to Congress, with a public comment session.

August 21, 2008* / Gaithersburg, MD; Teleconference / Media and public briefing on release of all draft reports for WTC 7 and draft recommendations for public comment.

August 26, 2008* / Teleconference / Technical briefing on the probable collapse sequence for WTC 7, draft reports for WTC7, and draft recommendations for public comment.

* Appended to table January 2009


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.

• 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 regulatpru authorities to respond to future disasters.

The desired outcomes are to make buildings, occupants, and first responders safer in future disaster events.

National Construction Safety Team Reports on the WTC Investigation

This report covers WTC 7, with supporting documentation of the techniques and technologies used in the reconstruction located in NIST NCSTAR 1-9 and NIST NCSTAR 1-9A. These two reports provide more detailed documentation of the Investigation findings and the means by which these technical results were achieved. As such, they are part of the archival record of this Investigation. Additional information regarding WTC 7 can be found in the previously published reports: NIST NCSTAR 1-1D, 1-1E, 1-1G, 1- 3D, l-3E, 1-4B, 1-4C, 1-4D, and l-6A. The titles of the full set of Investigation publications are listed in Appendix B.
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 4:05 am

EXECUTIVE SUMMARY

ES.1. WORLD TRADE CENTER BUILDING 7 (WTC 7)


WTC 7 was a 47 story office building located immediately to the north of the main WTC Complex. It had been built on top of an existing Consolidated Edison of New York electric power substation, which was located on land owned by The Port Authority of New York and New Jersey. On September 11, 2001, WTC 7 endured fires for almost seven hours, from the time of the collapse of the north WTC tower (WTC 1) at 10:28:22 a.m. until 5:20:52 p.m., when it collapsed. This was the first known instance of the total collapse of a tall building [1] primarily due to fires.

WTC 7 was unlike the WTC towers in many respects. It was a more typical tall building in the design of its structural system. It was not struck by an airplane. The fires in WTC 7 were quite different from those in the towers. Since WTC 7 was not doused with thousands of gallons of jet firel, large areas of any floor were not ignited simultaneously. Instead, the fires in WTC 7 were similar to those that have occurred in several tall buildings where the automatic sprinklers did not function or were not present. These other buildings did not collapse, while WTC 7 succumbed to its fires.

ES.2. THIS REPORT

This is the final report of the National Institute of Standards and Technology (NIST) investigation into the collapse of WTC 7, conducted under the National Construction Safety Team Act. The report is the result of an extensive, state-of-the-art reconstruction of the events that affected WTC 7 and eventually led to its collapse. Numerous facts and data were obtained, then combined with validated computer modeling to produce an account that captures the key features of what actually occurred. However, the reader should keep in mind that the building and the records kept within it were destroyed, and the remains of all the WTC buildings were disposed of before congressional action and finding was available for this Investigation to begin. As a result, there are some facts that could not be discerned and, thus, there are uncertainties in this accounting. Nonetheless, NIST was able to gather sufficient evidence and documentation to conduct a full investigation upon which to reach firm findings and recommendations.

This report summarizes how NIST reached its conclusions. NIST complemented in-house expertise with private sector technical experts; accumulated copious documents, photographs, and videos of the disaster; conducted first-person interviews of building occupants and emergency responders; analyzed the evacuation and emergency response operations in and around WTC 7: performed computer simulations of the behavior of WTC 7 on September 11, 2001; and combined the knowledge gained into a probable collapse sequence. Extensive details on the reconstruction effort for WTC 7, the uncertainties, the assumptions made, and the testing of these assumptions are documented in NIST NCSTAR 1-9 and NIST NCSTAR 1-9A.

ES.3. PRINCIPAL FINDINGS OF THE INVESTIGATION

The fires in WTC 7 were ignited as a result of the impact of debris from the collapse of WTC 1, which was approximately 110 m (350 ft) to the south. The debris also caused structural damage to the southwest exterior of WTC 7, primarily between Floors 7 to 17. The fires were ignited on at least 10 floors: however, only the fires on Floors 7 through 9 and 11 through 13 grew and lasted until the time of the building collapse. These uncontrolled fires had characteristics similar to those that have occurred previously in tall buildings. Their growth and spread were consistent with ordinary building contents fires. Had a water supply for the automatic sprinkler system been available and had the sprinkler system operated as designed, it is likely that fires in WTC 7 would have been controlled and the collapse prevented. However, the collapse of WTC 7 highlights the importance of designing fire-resistant structures for situations where sprinklers are not present, do not function (e.g., due to disconnected or impaired water supply), or are overwhelmed.

Eventually, the fires reached the northeast region of the building. The probable collapse sequence that caused the global collapse of WTC 7 involved the initiation of the buckling of a critical interior column in that vicinity. This column had become unsupported over nine stories after initial local fire-induced damage led to a cascade of local floor failures. The buckling of this column led to a vertical progression of floor failures up to the roof, and led to the buckling of adjacent interior columns to the south of the critical column. An east-to-west horizontal progression of interior column buckling followed, due to loss of lateral support to adjacent columns, forces exerted by falling debris, and load redistribution from other buckled columns. The exterior columns then buckled as the failed building core moved downward, redistributing its loads to the exterior columns. Global collapse occurred as the entire building above the buckled region moved downward as a single unit. This was a fire-induced progressive collapse, also known as disproportionate collapse, which is defined as the spread of local damage, from an initiating event, from element to element, eventually resulting in the collapse of an entire structure, or a disproportionately large part of it.

Factors contributing to the building failure were: thermal expansion occurring at temperatures hundreds of degrees below those typically considered in design practice for establishing structural fire resistance ratings; significant magnification of thermal expansion effects due to the long-span floors, which are common in office buildings in widespread use: connections that were designed to resist gravity loads, but not thermally induced lateral loads; and a structural system that was not designed to prevent fire-induced progressive collapse.

Within the building were emergency electric power generators, whose fuel supply tanks lay in and under the building. However, fuel oil fires did not play a role in the collapse of WTC 7. The worst-case scenarios associated with fires being fed by the ruptured fuel lines (a) could not have been sustained long enough, or could not have generated sufficient heat, to raise the temperature of the critical interior column to the point of significant loss of strength or stiffness, or (b) would have produced large amounts of visible smoke that would have emanated from the exhaust louvers. No such smoke discharge was observed.

Simulations of hypothetical blast events show that no blast event played a role in the collapse of WTC 7. NIST concluded that blast events did not occur, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing a single critical column would have resulted in a sound level of 130 dB to 140 dB at a distance of at least half a mile. There were no witness reports of such a loud noise, nor was such a noise heard on the audio tracks of video recordings of the WTC 7 collapse.

There were no serious injuries or fatalities, because the estimated 4,000 occupants of WTC 7 reacted to the airplane impacts on the two WTC towers and began evacuating before there was significant damage to WTC 7. The occupants were able to use both the elevators and the stairs, which were as yet not damaged, obstructed, or smoke-filled. Evacuation of the building took just over an hour. The potential for injuries to people leaving the building was mitigated by building management personnel holding the occupants in the lobby until they identified an exit path that was safe from the debris falling from WTC 1. The decision not to continue evaluating the building and not to fight the fires was made hours before the building collapsed, so no emergency responders were in or near the building when the collapse occurred.

The design of WTC 7 was generally consistent [2] with the New York City Building Code of 1968 (NYCBC), with which, by PANYNJ policy, it was to comply. The installed thicknesses of the thermal insulation was consistent with the fire rating required by the NYCBC. The stairwells were narrower than those required by the NYCBC, but, combined with the elevators, were adequate for a timely evacuation on September 11, 2001, since the number of building occupants was only about half that expected during normal business hours.

The collapse of WTC 7 could not have been prevented without controlling the fires before most of the combustible building contents were consumed. There were two sources of water (gravity fed overhead tanks and the city water main) for the standpipe and automatic sprinkler systems serving Floor 21 and above, and some of the early fires on those upper floors might have actually been controlled in this manner. However, consistent with the NYCBC, both the primary and back-up source of water for the sprinkler system in the lower 20 floors of WTC 7 was the city water main. Since the collapses of the WTC towers had damaged the water main, there was no secondary supply of water available (such as from the gravity-fed overhead tanks that supplied water to Floor 21 and above) to control those fires that eventually led to the building collapse.

Other than initiating the fires in WTC 7, the damage from the debris from WTC 1 had little effect on initiating the collapse of WTC 7. The building withstood debris impact damage that resulted in seven exterior columns being severed and subsequently withstood fires involving typical office combustibles on several floors for almost seven hours. The debris damaged the spray-applied fire resistive material that was applied to the steel columns, girders, and beams, only in the vicinity of the structural damage from the collapse of WTC 1. This was near the west side of the south face of the building and was far removed from the buckled column that initiated the collapse. Even without the structural damage, WTC 7 would have collapsed from fires having the same characteristics as those experienced on September 11, 2001. The transfer elements such as trusses, girders, and cantilever overhangs that were used to support the office building over the Con Edison substation did not play a significant role in the collapse of WTC 7.

ES.4. RECOMMENDATIONS

Based on the findings of this Investigation, NIST identified one new recommendation (B, below) and reiterated 12 recommendations from the Investigation of the WTC towers. These encompass increased structural integrity, enhanced fire endurance of structures, new methods for fire resistant design of structures, improved active fire protection, improved emergency response, improved procedures and practices, and education and training.

The urgency of these recommendations is substantially reinforced by their pertinence to the collapse of a tall building that was based on a structural system design that is in widespread use.

The partial or total collapse of a building due to fires is an infrequent event. This is particularly true for buildings with a reliably operating active fire protection system such as an automatic fire sprinkler system. A properly designed and operating automatic sprinkler system will contain fires while they are small and, in most instances, prevent them from growing and spreading to threaten structural integrity. The intent of current practice, based on prescriptive standards and codes, is to achieve life safety, not collapse prevention. However, the key premise of NIST's recommendations is that buildings should not collapse in infrequent (worst-case) fires that may occur when active fire protection systems are rendered ineffective, e.g., when sprinklers do not exist, are not functional, or are overwhelmed by the fire, or where the water supply is impaired.

Fire scenarios for structural design based on single compartment or single floor fires are not appropriate representations of infrequent fire events. Such events have occurred in several tall buildings resulting in unexpected substantial losses. Instead, historical data suggests that infrequent fires which should be considered in structural design have characteristics that include: ordinary combustibles and combustible load levels, local fire origin on any given floor, no widespread use of accelerants, consecutive fire spread from combustible to combustible, fire-induced window breakage providing ventilation for continued fire spread and accelerated fire growth, concurrent fires on multiple floors, and active fire protection systems rendered ineffective. The fires in WTC 7 had all of these characteristics.

The subjects of the NIST recommendations are as follows:

A. Development of methods for prevention of progressive collapse and for reliable prediction of the potential for complex failures in structural systems subjected to multiple hazards.

B. (New). Explicit evaluation of the fire resistance of structural systems in buildings under worst-case design fires with any active fire protection systems rendered ineffective. Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: long-span floor systems [3], connections not designed for thermal effects, asymmetric floor framing, and composite floor systems.

C. Evaluation and improvement of the technical basis for determining appropriate construction classification and fire rating requirements (especially for tall buildings), and making of related code changes.

D. Improvement of the technical basis for the standard for fire resistance testing of components, assemblies, and systems.

E. Broadening the scope of the "structural frame" approach to fire resistance ratings by including, as part of the structural frame, floor systems and other bracing members that are essential to the vertical stability of the building under gravity loads.

F. Enhancement of the fire resistance of structures by requiring a performance objective that uncontrolled building fires result in burnout without partial or global (total) collapse.

G. Development of performance-based standards and code provisions to enable the design and retrofit of structures to resist real building fire conditions, and the tools necessary to perform the building evaluations.

H. Enhancement of the performance and redundancy of active fire protection systems to accommodate higher risk buildings.

I. Establishment and implementation of codes and protocols for ensuring effective and uninterrupted operation of the command and control system for large-scale building emergencies.

J. Requirement that building owners to retain building documents over the entire life of the building.

K. Inclusion of all appropriate technical professionals in the building design team.

L. Development and implementation of continuing education curricula for training building professionals in each others' skills and practices.

M. Development and delivery of training materials in the use of computational fire dynamics and thermostructural analysis tools.

Building owners, operators, and designers should immediately act upon the new recommendation (B). Industry should also partner with the research community to fill critical gaps in knowledge about how structures perform in real fires.

_______________

Notes:

1. The term " tall building" is used by architects and structural engineers to indicate buildings that are taller than surrounding buildings, slender in their proportions, and/or require technologies such as wind bracing to carry loads, and are nominally taller than 15 to 20 stories. For fire protection engineers, the term "high-rise building" is used to indicate buildings that are nominally taller than 25 m (75 It), and from which external rescue from fires is not possible. Both terms apply to WTC 7.

2. NIST did not conduct an exhaustive review of the plans and specifications for WTC 7 to determine compliance with the NYCBC, for the reasons stated in Disclaimer No. 4 (see page ii).

3. Typical floor span lengths in tall office buildings are in the range of 12 m to 15 m (40 ft to 50 ft); this range is considered to represent long-span floor systems. Thermal effects (e.g., thermal expansion) that may be significant in long-span buildings may also be present in buildings with shorter span lengths, depending on the design of the structural system.
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sat Jul 01, 2017 11:26 pm

Chapter 1: THE NEW YORK CITY WORLD TRADE CENTER BUILDING 7

1.1 THE WORLD TRADE CENTER COMPLEX


The New York City World Trade Center (WTC) complex was located in Lower Manhattan, just north of Wall Street, in the heart of the financial district. It was built by The Port of New York Authority, later to be renamed as The Port Authority of New York and New Jersey (The Port Authority, PANYNJ). Created in 1921, under a clause in the United States Constitution, to run the multi-jurisdictional 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 has the authority to obtain land by eminent domain, raise funds for its projects, and to construct under its own building code. Nonetheless, The Port Authority policy was to comply with the local building code in place at the time of building design, which, for the WTC complex, was the 1968 version of the New York City Building Code (NYCBC).

The original WTC complex consisted of six buildings (Figure 1- 1). The two towers, WTC 1 (North Tower) and WTC 2 (South Tower), which provided the iconic appearance of the complex, were each 110 stories high, dwarfing the other skyscrapers in lower Manhattan. WTC 3, a Marriott Hotel, was 22 stories tall, WTC 4 (South Plaza Building) and WTC 5 (North Plaza Building) were each nine story office buildings, and WTC 6 (U.S. Customs House) was an eight story office building. These six buildings were built around a five acre Plaza. Construction began in 1968, with the first occupancy in 1970.

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.

1.2 WTC 7

1.2.1 The Edifice


In 1967, a Consolidated Edison of New York (Con Edison) substation had been built on Port Authority land on the north side of Vesey Street, between Washington Street on the west, West Broadway on the east, and Barclay Street on the north. This substation would distribute electrical power to Lower Manhattan. In designing the substation, provision was made for a future office tower by including structural capacity to carry the weight of both the substation and the future high-rise building. Twenty years later, the high-rise building, designated WTC 7, was completed. The architectural design was performed by Emory Roth & Sons, P.C. The structural engineer of record was the Office of Irwin G. Cantor, and the mechanical engineer was Syska & Hennessy, P.C. Tishman Construction Corporation was the general contractor. The building was owned by Seven World Trade Company and Silverstein Development Corporation, General Partners.

This 47 story office building was located immediately to the north of the main WTC Complex, approximately 105m (350 ft) from the north side of WTC 1 (Figure 1-1). It was connected to the WTC complex with a 37 m (120 ft) wide elevated plaza, known as the Promenade, at the 3rd floor level, and a 6.7m (22 ft) wide pedestrian bridge, also at the 3m floor level. Figure 1-2 is a photograph of the WTC site, showing the relationship of WTC 7 to the surrounding buildings.

Image
Figure 1-1. The World Trade Center in Lower Manhattan.

Image
Note: Enhancement by NIST
Figure 1-2. Photograph of the World Trade Center Complex, showing WTC 7.


1.2.2 The Con Edison Substation

The Con Edison substation was a steel-framed structure with cast-in-place concrete floors and walls. It was placed on the northern portion of the site and extended approximately 13 m (42 ft) north of the north facade of WTC 7, as shown in Figure 1-3. Its southerly boundary was irregular, but extended approximately two-thirds of the width of WTC 7. The Con Edison Substation was two stories in height, coinciding with the first two floors of WTC 7 (Figure 1-4). Details of the construction and the function of the substation can be found in NIST NCSTAR 1-9, Chapter 2, and Appendix A, respectively.

WTC 7 and the electrical substation were supported on caisson foundations, which were seated in the bedrock, approximately 20 m (60 it) below the surface. Above the caissons were heavy grillages composed of built-up steel girders. The 2.5 m to 9 m (8 it to 30 ft) distance between the caissons was braced by reinforced concrete walls with thicknesses varying from 0.3 m to 0.8 m (1 ft to 2.5 it). Many of the WTC 7 steel columns were embedded in these walls. The areas between the concrete walls were filled with compacted gravel fill and then covered with a concrete slab to form closed cells and bring the structure up to the required elevation. In some cases, the area was left untilled and used to house fuel tanks.

Image
Source: McAllister 2002
Figure 1-3. Footprints of the Con Edison substation and WTC 7.


Image
Source: Con Edison, used with permission. (Photo taken prior to September 11, 2001.)
Note: Enhancement by NIST
Figure 1-4. Aerial view from the north side of WTC 7, showing the Con Edison substation.


Within the substation were nine transformer vaults that housed the units that converted the 138 kV incoming power to 13.8 kV for transmission throughout Lower Manhattan. Access within the substation was provided by stairwells on the west side, in the center, and on the east side of the building. None of these stairs extended into the 47-story building above. In fact, concrete walls fully separated the substation from WTC 7.

1.2.3 The Structure

WTC 7 was an irregular trapezoid, approximately 100 m (329 ft) long on the north face and 75m (247 ft) long on the south face, 44m (144 ft) wide, and 186m (610 ft) tall. The 47 story building contained approximately 200,000 m[2] (2 million ft[2]) of floor area. A typical floor was similar in size to a football field. The gross floor area was about 75 percent of that contained in the Empire State Building. As shown in Figure 1-3, about half of WTC 7 rose outside the footprint of the Con Edison substation. Structurally, WTC 7 consisted of four "tiers."

• The lowest four floors housed two two-story lobbies, one each on the center of the south side of the 1st and 3rd floors. The north side of the 1st and 2nd stories was the Con Edison substation. The remainder of the north, east, and west sides of these four stories was conference space, offices, a cafeteria, etc.

• Floors 5 and 6 were mechanical spaces. Within the volume bounded by the 5th floor slab and the 7th floor slab were three transfer trusses and a series of eight cantilever transfer girders. As their names indicate, these steel assemblies distributed the load of the upper floors of WTC 7 onto the structural frame of the Con Edison substation and the structure of the lowest four floors of WTC 7.

• Floors 7 through 45 were tenant floors, all structurally similar to each other. The exception was a reinforcing belt truss around Floors 22 and 23.

• The 46th and 47th floors, while mainly tenant floors, were structurally reinforced to support special loads, such as the cooling towers and the water tanks for fire suppression.

The structural frame was designed to distribute the weight of the building (gravity loads) and resist (lateral) wind loads. The frame included columns, floor assemblies, spandrel beams, girders, and transfer elements.

From the 7th floor to the 47th floor, WTC 7 was supported by 24 interior columns and 58 perimeter columns (numbered 1 through 57, plus 14A, which was located near the south end of the west face) (Figure 1-5). Twenty-one of the interior columns (numbered 58 through 78) formed a rectangular building core, which was offset toward the west of the building. The remaining three interior columns (79, 80, and 81) were particularly large, as they provided support for the long floor spans on the east side of the building.

In the final design of WTC 7, the layout of the columns did not align with the building foundation and the Con Edison columns. Therefore, a set of column transfers were constructed within the volume bounded by the 5th and 7th floor slabs. These are depicted in Figure 1-6, along with the numbers of the columns to which they connected.

Image
Based on structural drawings (Cantor 1985)
Figure 1-5. Typical WTC7 floor showing locations of the columns, girders, and beams.


Image
Source: McAllister 2002
Figure 1-6. 3D schematic view of transfer trusses and girders between Floors 5 and 7.


The floor slabs were reinforced concrete of varying thickness. The 1st floor slab was 0.36 m (14 in.) thick. The concrete on almost all of the other floors was poured on top of 76 mm (3 in.) deep corrugated metal decking. Floors 2, 3, 4, and 6 had a 0.15 m (6 in.) total slab thickness; on Floor 5, the concrete was 0.36m (14 in.) thick; and all Floors 8 through 47, the concrete was 0.14 m (5.5 in.) thick. On Floor 7, the south half of the floor had a poured 0.2 m (8 in. ) slab, and the north half had an 0.2 m (8 in.) total slab thickness on a 76 mm (3 in.) deep metal deck. The floor slabs were supported by the structural floor framing shown in Figure 1-5. The floor beams were connected to the concrete deck by shear studs, which caused the floor beams and concrete slab to act together, or compositely. This type of floor system is thus referred to as a composite floor. The floor beams were framed into (connected to) girders with a variety of types of shear connectors [1], through which the floor beams transferred gravity loads from the floors to the girders. The girders also framed into the columns with a variety of types of shear connectors and transferred the gravity loads to the columns. Interior columns were connected with splice plates, welds and bolts. The exterior frame had moment connections in each face of the building.

1.2.4 Fire Protection

There were both passive and active fire protection systems in WTC 7 (NIST NCSTAR 1-9, Chapter 4). The former was in the form of sprayed fire-resistive material (SFRM) applied to the structural steel and to the underside of the metal floor decking. The latter comprised fire sensors and alarms, notification systems, automatic fire sprinklers, water supplies, and smoke management.

According to the 1968 version of the NYCBC and Local Law 16 (1984), a fully sprinklered high-rise building could follow the fire resistance requirements for Type I C construction. For this construction category, columns were required to have a 2 h rating as established by the Standard Fire Test (ASTM E 119); beams were required to have a 1-1/2 h rating. The instructions to the bidders for the WTC 7 job were to bid on a 3 h rating for the columns and a 2 h rating for the metal deck and floor support steel, which corresponded to the more stringent fire resistance requirements for Type IB (unsplinklered) construction. These ratings were to be achieved by application of Monokote :MK-5, a gypsum-based SFRM that contained a vermiculite aggregate. According to the Underwriters Laboratories (UL) Fire Resistance Directory (1983), these ratings required a thickness of 22 mm (7/8 in.) of Monokote MK-5 to be applied to the heavy columns, 48mm (1-7/8 in.) to be applied to the lighter columns, 13 mm (1/2 in.) to be applied to the beams, and 10 mm (3 /8 in.) to be applied to the bottom of the metal deck. Private inspectors found that the applied SFRM thicknesses were consistent with these values.

Within the building was an array of smoke detectors which, when triggered, would sound alarms on the floor of detection and the floor above, provide an alarm and signal location to the Fire Command Station in the 3rd floor lobby, and transmit a signal to the fire department. There were barriers to smoke spread (in the form of walls and smoke dampers), as well as air movement equipment to exhaust the smoke.

WTC 7 contained a three-zone system of sprinklers and standpipes.

• Water to the low zone (1st floor through the 20th floor) came from the water main. The backup water also came from the water main via a supplemental pump.

• Water to the mid-level zone (21st floor through the 39th floor) was supplied from two large storage tanks located on the 46th floor. Backup water could be pumped from the water main.

• The sources of the primary and backup water supplies to the high zone (40th floor through the 47th floor) were the same as for the mid-level zone.

The NYCBC requirement was for a 30 min water delivery duration at a delivered density of 0.04 L/min_m[2] (0.10 gal/min-ft[2]), and Investigation calculations determined that the installed system met these requirements. This would have been sufficient to control fires of four clusters of six workstations each, either all on one floor or single clusters on four floors.

These fire protection measures addressed the conventional approaches to preserving life safety. However, in the U.S., neither architectural nor structural engineering practice explicitly required (then or currently) an evaluation of the structural system response to heating (fires) as part of the building design.

1.2.5 The Workplace

Many of the roughly 8,000 people who worked in or visited WTC 7 on a given day would have arrived via trains that stopped in the large station under the main WTC complex. They could enter the first floor of the building through street-level doors along Vesey Street, Washington Street, and West Broadway. Alternatively, from the main WTC complex, they could cross Vesey Street via the Promenade or the covered pedestrian bridge and enter the 3rd floor lobby of the building.

Within the building core were 32 elevators, 28 of which would have taken them to their various offices. The floors that the elevators served are indicated in Figure 1-7.

Image
Figure 1-7. Schematic drawing of the elevators in WTC 7.

There were also two stairwells, each 1.42 m (56 in.) to 1.47 m (58 in.) wide. The west stairwell was entered through a side door on Washington Street. The stairs rose to the 5th floor, where there was a short transfer corridor. From there, the stairs were vertically continuous to the 47th floor. The east stairwell was entered from West Broadway and had transfer corridors on the 5th and 23rd floors, before continuing to the 47th floor.

WTC 7 was operated by Silverstein Properties, Inc., from the date of its completion. Table 1-1 indicates the tenants of WTC 7 as of September 11, 2001.

Table 1- 1. Use of floors in WTC 7

Floor(s) / Tenant or Function [a]

46, 47 / Mechanical space, Citigroup
26 through 45 / Citigroup
25/ U.S. Internal Revenue Service, Department of Defense, Central Intelligence Agency
24 / U.S. Internal Revenue Service
23 / New York City Mayor's Office of Emergency Management (OEM)
22 / Federal Home Loan Bank of New York
21 / First State Management Group
19 through 21 / The Hartford Insurance Company 18 / Equal Employment Opportunity Commission, Teleport, Metropolitan Fiber Systems
15 through 17 / Citigroup
14 / Vacant
13 / U.S. Securities and Exchange Commission, Provident Financial Management. American Express
11, 12 / U.S. Securities and Exchange Commission
10, 9 / U.S. Secret Service
7, 8 / American Express
5, 6 / Mechanical floors
4 / Meeting spaces, cafeteria
1 through 3 / Lobbies, conference center

a. Among those interviewed by the Investigation Team, there was limited recollection of the organizations occupying some of the floors, especially those occupying smaller spaces, and no one had copies of all the tenant leases.


1.2.6 The Combustible Contents

The layout of most of the floors featured clusters of workstations, or cubicles, throughout the space surrounding the building core (NIST NCSTAR 1-9, Chapter 3). Often, there were walled offices at the perimeter. The layout in Figure 1-8 is indicative of these floors. While there were almost certainly different types of workstations in the building, they were all fundamentally similar. Each cubicle typically was bounded on four sides by privacy panels, with a single entrance opening. Within the area defined by the panels was a self-contained workspace: desktop (almost always a wood product, generally with a laminated finish), file storage, bookshelves, carpeting, chair, etc. Presumably there were a variety of amounts and locations of paper, both exposed on the work surfaces and contained within the file cabinets and bookshelves.

The combustible fuel load [2] for these open landscaped floors was dominated by the workstations. The architectural drawings showed densities of workstations similar to those on most of the fire floors in the WTC towers. The estimated combustible fuel load for these floors was about 20 kg/m[2] (4 lb/ft[2]). Simulations of the fires with a higher combusted fuel load (NIST NCSTAR 1-9, Chapter 9) resulted in poor agreement with the observed fire spread rates.

Image
Based on a floor plan and additional information from American Express.
Figure 1- 8. Schematic of Floor 8.


On a number of other floors. the space was almost completely subdivided into individual offices. A typical layout is depicted in Figure 1- 9.

Image
Based on a floor plan and additional information from the Securities and Exchange Commission.
Figure 1-9. Schematic of Floor 11.


On the 11th and 12th floors, which will be seen later to have been the sites of significant and sustained fires, the mass of additional paper materials was described as very high. As indicated in NIST NCSTAR 1-9, Chapter 3, the Investigation Team estimated a combustible fuel load of approximately 32 kg/m[3] (6.4 lb/ft[2]). Simulations of the fires with a lower combustible fuel load showed little effect on the rate of fire progression.

Unlike the case for the two WTC towers, there was no widespread spraying of jet fuel to ignite numerous workstations or offices simultaneously. Rather, in the earlier hours of the fires, following the debris impact due to the collapse of WTC 1, the fire would have spread from one individual workstation or office to another. [3] Thus, the fire spread would have been dependent on the office walls, the specific spacing of the cubicles, the ease of ignition of the furnishings, their combustible mass, and the extent of surface occlusion by foreign matter.

There were emergency power generators in WTC 7 (NIST NCSTAR 1-1J). The diesel fuel for these generators was stored within and under WTC 7. Properties of the emergency power systems are summarized in Table 1-2.

Table 1-2. Emergency power systems in WTC 7.

-- / Base Building System / Salomon Brothers System [4] / Mayor's OEM System


Fuel Storage Tank Capacities / Two 12,000 gal tanks / Two 6,000 gal tanks / Single 6,000 gal tank

Tank Locations / Below the loading dock / Below the loading dock / 1st floor

Locations of Generator(s) / Two on 5th floor / Nine on the 5th floor / Three on the 7th floor

Day Tanks and Locations / Single 275 gal tank on the 5th floor / None [a] / Single 275 gal day tank on the 7th floor

Day Tank Pump Locations and Capacities / Two on the 1st floor; 4.4 gal/min / Two circulating pumps on 1st floor, 70 gal/min / Two on the 1st floor, 12 gal/min

Ambassador (U.S. Secret Service) Modification / Generator and 50 gal day tank on 9th floor; two pumps on the 1st floor, 2.4 gal/min / -- / --

American Express Modification / Generator and 275 gal day tank on 8th floor [ b]; two pumps on the 1st floor, 2.8 gal/min

a. The NYCBC had a limit of one day tank per floor. Since there was a day tank on the 5th floor for the base generators, the SSB system used a pressurized fuel distribution system, in which pumps continuously circulated fuel whenever the generators were running. There was enough fuel (35 gal) in the valve rig and piping on the 5th floor to start the diesel engines, which, in turn, would supply power to operate the circulating pumps.

b. The generator and day tank had been removed prior to September 1, 2001.


Figure 1-10 depicts the locations of the electrical generators, the day tanks, and the fuel lines that connected them to the below-ground fuel tanks.

The base building tanks were full on September 11, 2001. Several months following the attacks on the WTC, a contractor recovered an estimated 23,000 gal of fuel from these tanks. NIST estimated that approximately 1000 gal ± 1000 gal was unaccounted. The fate of the fuel in the three day tanks is unknown, so NIST assumed they were full on September 11, 2001.

The fate of the fuel in the two tanks for the Salomon Brothers system was also unknown. Thuss, NIST assumed that all of the fuel would have been available to feed fires either at ground level or on the 5th floor.

No trace of the Mayor's OEM system tank or fuel was found. Since the pumps used to fill the day tank on the 7th floor would only have run when the low fuel switch came on, NIST assumed that all the fuel was available. This tank was enclosed in 4 h fire rated construction and was provided with a total flooding fire suppression system.

Image
Based on Figure 8-1 from NIST NCSTAR 1-1J
Figure 1-10. Section view of diesel fuel distribution components in WTC 7.


1.3 REFERENCES

Cantor 1985. Irwin G. Cantor P.C., Structural Engineers, Structural Drawings, 7 World Trade Center.

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations. FEMA 403. Federal Emergency Management Agency. Washington, DC, May.

_______________

Notes:

1. Shear connections are designed to transfer only vertical gravity loads, whereas moment connections are designed to transfer loads and moments (forces resulting from bending of a beam) induced by both (vertical) gravity and (horizontal) wind loads.

2. In the fire simulations, the entire combustible fuel load can be burned. In actuality, not all of, e.g., a wood desk is consumed. Thus, the combusted fuel loads estimated for these simulations are somewhat lower than the actual fuel loads in prior surveys of office buildings. (See NIST NCSTAR 1-5.)

3. The ingress of dust and debris through broken windows would have slowed this spread, especially near the south face of WTC 7, by depositing on horizontal surfaces and, thus, making ignition more difficult.

4. Renovations were made in 1988 and 1989 to the space leased by Salomon Brothers Inc. in WTC 7. In 1998, Smith Barney Inc. merged with Salomon Brothers Inc. to form Salomon Smith Barney. In 1999, Salomon Smith Barney Inc. merged with Citicorp Inc. to form Citigroup Inc.
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Re: Final Report on the Collapse of World Trade Center Build

Postby admin » Sun Jul 02, 2017 9:26 pm

Chapter 2: THE ACCOUNT OF WTC 7

2.1 INTRODUCTION


Shortly before 9:00 a.m. on Tuesday, September 11, 2001, about 4,000 people were at work in WTC 7. This was about half of the roughly 8,000 people who worked there. It was the first day of school for many local children, and it also was a primary election day in New York. The weather was clear and comfortable, so some had taken time to do early morning errands.

At 8:46:30 a.m. EDT, five hijackers flew American Airlines Flight 11 (AA 11), a Boeing 767-200ER aircraft with 11 crew and 76 passengers on board, over the top of WTC 7 and into the north face of WTC 1. Moving at about 710 km/h (440 mph), the nose hit the exterior of the tower at the 96th floor. The aircraft cut a gash that was over half the width of the building and extended from the 93rd floor to the 99th floor. At 9:02:59 a.m., 16-1/2 minutes after the first impact, five other hijackers flew United Airlines Flight 175, a Boeing 767-200ER with 9 crew and 51 passengers on board, into the south face of WTC 2 at about 870 km/h (540 mph), about 160 km/h (100 mph) faster than AA 11. The center of the nose of the plane struck at the 81st floor slab. Tins entry wound stretched over nine floors, from Floor 77 to Floor 85 (NIST NCSTAR 1).

The account that follows is the result of an extensive, state-of-the-art reconstruction of the events that affected WTC 7 and eventually led to its collapse at 5:20:52 p.m. Numerous facts and data were obtained, then combined with validated computer modeling to produce an account that is believed to be close to what actually occurred. However, the reader should keep in mind that the building and the records kept within it were destroyed, and the remains of all the WTC buildings were disposed of before congressional action and funding was available for this Investigation to begin. As a result, there are some facts that could not be discerned, and thus there are uncertainties in this accounting. Nonetheless, NIST was able to gather sufficient evidence and documentation to conduct a full investigation upon winch to reach firm findings and recommendations. The reconstruction effort for WTC 7, the uncertainties, the assumptions made, and the testing of these assumptions are documented in NIST NCSTAR 1-9.

2.2 ACTIVITY AT THE WTC 7 SITE

2.2.1 8:46 a.m. to 9:59 a.m. EDT


People throughout WTC 7 heard the impact of the aircraft hitting WTC 1, which was only about 110 m (350 ft) to the south. Lights flickered, the building shook, and some windows on the south side of WTC 7 were broken. However, few, if any, of the workers felt their lives were in immediate danger. This perception changed as the occupants became aware of the subsequent attacks on WTC 2 and the Pentagon, and people began using the elevators and stairs to leave the building. The elevators alone could have evacuated the building in about 20 min. The stairwells, although somewhat narrow for the maximum possible 14,000 occupants (estimated using the formula in the NYCBC), were more than adequate to evacuate roughly one-third of that number in the building that morning (NIST NCSTAR 1-9, Chapter 7).

At about 9:45 a.m., a manager from the OEM ordered the evacuation of WTC 7. It was not clear whether this order was broadcast over the public address system, because the building was already nearly empty.

Initially, as the occupants had tried to leave WTC 7, building officials kept them in the lobby for fear that they might be hit by debris falling from WTC 1. The lobby quickly filled with evacuating WTC 7 occupants, occupants of WTC 5 and WTC 6 who had crossed Vesey Street using the Promenade and covered walkway, and injured people from WTC 1 in a medical triage post, established by the OEM. After the second aircraft was flown into WTC 2, the people in WTC 7 were directed down the turned-off escalators to the 1st floor lobby, out the loading dock doors on the west end of the south side of the building, and across Washington Street. There, they moved north under protection of scaffolding on the Verizon building. By the time WTC 2 collapsed at 9:59 a.m., all the building occupants who intended to leave WTC 7 had done so (NIST NCSTAR 1-9, Chapter 7).

The Fire Department of the City of New York (FDNY) had arrived on the scene by 8:50 a.m. and took control of the site, since this had been identified as a fire incident. Eventually, roughly 1,000 fire fighters would arrive. The New York City Police Department (NYPD) helicopters and the first of about 50 ground staff reached the site by 8:52 a.m. and began establishing a security perimeter around the WTC site. Port Authority Police Department (PAPD) and staff from the OEM were already present. There was extensive emergency response activity during this roughly 70 min interval. Aside from the medical triage unit set up in the WTC 7 lobby, most of their efforts were directed at the WTC towers (NIST NCSTAR 1- 9, Chapter 6).

2.2.2 9:59 a.m. to 10:28 a.m. EDT

WTC 2 collapsed at 9:58:59 a.m. from the damage inflicted by the aircraft and the intense, multi-floor fires that followed. A few windows on lower floors of the south face of WTC 7 were broken, and dust and small debris were deposited in the 3rd floor lobby. None of the large pieces of debris from WTC 2 hit WTC 7 because of the large distance between the two buildings, and there was no evidence of structural damage to WTC 7.

When WTC 1 collapsed at 10:28:22 a.m., most of the debris landed in an area not much larger than the original WTC 1 building footprint. However, some fragments were forcibly ejected and traveled distances up to hundreds of meters. Pieces of WTC 1 hit WTC 7, severing six columns on Floors 7 through 17 on the south face and one column on the west face near the southwest corner. The debris also caused structural damage between Floor 44 and the roof. The damage to the building face is depicted in Figure 2-1. Based on photographic evidence, witness accounts, and engineering judgment, it is likely that the structural damage (steel and floor slabs) did not penetrate beyond the perimeter of the building core. At the southwest corner, the structural damage extended only about one-third of the distance from the exterior wall to the building core. The debris also broke a large number of windows on the south face. Compared to the airplane impact damage to the WTC towers, there was relatively little damage to the interior of WTC 7. For instance, damage to the sprayed fire resistive material (SFRM) was limited to the immediate vicinity of the WTC 1 debris impact. There was no superficial or structural damage to the north and east faces (NIST NCSTAR 1-9, Chapter 5, Section 5.5.2).

Image
Figure 2-1. Observed damage to WTC 7 following the collapse of WTC 1. West face (left) and south face (right)

The damage is color coded as follows: green (_) - no visible damage, yellow ( ) - window glass broken, orange (_) - granite and underlying truss damage, red (_) - damage to exterior structural steel, gray (_) - possible structural damage, and blue (_) - not visible due to smoke, dust, and intervening buildings.

The collapses of the two towers further focused the emergency responders' activities south of Vesey Street. FDNY moved its Command Post north on West Street towards Chambers Street, as a succession of FDNY officials took command of the incident.

2.2.3 10:29 a.m. to 5:21 p.m. EDT

The emergency responders quickly recognized that WTC 7 had been damaged by the collapse of WTC 1. A number of fire teams entered WTC 7 to examine the damage, locate fires and possibly extinguish them, and search for occupants.

As early as 11:30 a.m., FDNY found that there was no water supplied by the hydrant system to fight the fires that were visible. With the collapses of the towers fresh in their minds, there was concern that WTC 7 too might collapse, risking the lives of additional firefighters. Within the next two hours, serious discussions were underway regarding the cessation of any efforts to save WTC 7, and the final order to cease was given at about 2:30 p.m. The Con Edison substation was shut down at 4:33 p.m. (NIST NCSTAR 1-9, Chapter 6).

2.3 PROGRESS OF THE FIRES IN WTC 7

General


Even though available images showing fires in WTC 7 did not allow the detailed description of fire spread that was possible for the WTC towers, there was sufficient information to derive general descriptions of fire ignition and spread on various floors of the building (NIST NCSTAR 1-9, Chapter 5). It must be kept in mind that the fire observations were based on images of the exterior faces, which provided little indication about the behavior of fires well removed from the exterior walls. It is likely that much of the building took place beyond the views of the windows. Relatively little smoke was seen emanating from the windows on the north face, even when flames were observed, indicating that the hot combustion products were not exhausting through the nearest openings, but instead were passing through the building interior to other exits. The wind direction was from the north, and since there would have been office furnishings spread across the tenant spaces, some of the air would have penetrated to the building interior and supported combustion of the furnishings located there. Features of this interior building were reconstructed from the computer simulations of the fires, as described in NIST NCSTAR 1-9, Chapter 9.

Most likely, the WTC 7 fires began as a result of burning debris from the collapse of WTC 1 at about 10:28:22 a.m. Soon after that, there were numerous vehicles around WTC 7 that were on fire, presumably ignited by burning debris from the tower. It is likely that nascent fires were also growing within WTC 7 around the same time, although visual evidence of fires in the building was not available until around noon. From the fire spread patterns, it is also likely that the fires began around the western half of the south face.

Fires broke out on at least 10 floors of WTC 7, near the damaged southwest corner of the building (NIST NCSTAR 1-9, Chapter 5). They were typically observed as single floor fires, and observations supported a local fire origin on an given floor. Unlike the WTC towers, there was no dispersion of jet fuel in WTC 7 causing simultaneous fire initiation over extensive areas of a single floor or over multiple adjacent floors. The early fire on each floor was small, probably involving a single cubicle or office. On the floors which were mostly furnished with clusters of cubicles (such as Floors 7 and 8), the initial fire spread would have been by flame contact with an adjacent cubicle within the cluster. Once a cluster was burning, a nearby cubicle, across an aisle, would have been ignited by thermal radiation from the flames. By the time this second cluster was fully involved, the prior cluster would have passed its peak burning rate. The path of the fires would likely have jumped from cluster to cluster, meandering toward the windows, toward the building core, or parallel to the facade. Eventually, the upper air layer over enough of the large open space would have become hot enough for the thermal radiation from the hot air to have heated and ignited multiple cubicles simultaneously, leading to faster fire growth.

On those floors that were mostly subdivided into offices (such as Floors 11 and 12), the fire would have grown within a single office, reaching flashovers within several minutes. After about 5 to 15 min, the ceiling tile system would have failed from the heat, and the hot air would have flowed over the office wall. Soon the hot air would fail the ceiling of an adjacent office, and eventually the thermal radiation would ignite the contents in this office. Fire spread would have been similar for offices separated by a corridor, although this would have taken longer, since the hot air would have to travel further and would be cooling along the way.

Between 12:10 p.m. and 1:00 p.m., there were fires at the southwest corners of the 19th, 22nd, 29th, and 30th floors. These fires grew large enough to break glass from nearby windows, but did not spread far before dying out. These fires might have also burned along the south sides of the floors, where they would not have been seen, due to limited visibility from smoke obscuration. It is possible that the fires on the 22nd, 29th, and 30th floors were limited by automatic sprinklers, whose water came from the storage tanks on the 47th floor. At any rate, after about 1:00 p.m., there was no evidence of fires on these floors on the east, north, or west faces of the building.

Between roughly 2:00 p.m. and the collapse of WTC 7 at 5:20:52 p.m., fires were observed spreading on the 7th floor through the 13th floor, with the exception of the 10th floor. Since the collapses of the towers had resulted in the loss of city water that was the sole supply for the automatic sprinkler system on the lower 20 floors of WTC 7, these fires continued to spread unabated. All of these fires reached the northeast sector of the building between approximately 3:00 p.m. and 4:00 p.m. The intensities of the fires on the 11th, 12th, and 13th floors were higher than those on the 7th, 8th, and 9th floors because of the higher loading of combustibles and a larger burning area. There was also a small fire on the north side of the vacant 14th floor shortly before the collapse of the building. There was no visual evidence of fires on other floors, other than near the debris-damaged southwest corner of the building. The following sections describe the timing and paths of these fires (NIST NCSTAR 1-9, Chapter 5).

7th Floor

The fire on the 7th floor spread in a clockwise direction. Shortly after 2:00 p.m. , there was a fire on the west side of the 7th floor, spreading north along the west face. The fire turned the northwest corner and by 3:00 p.m. was spreading east across the north face. Around 3:15 p.m., the fire, which had passed the midpoint of the north face, stopped and died down. About an hour later, the fire appeared a little farther to the east, then died down by 4:40 p.m. Although no further images were available, it is likely that the fire continued to burn toward the east.

8th Floor

The 8th floor fire also spread clockwise. At about 3:40 p.m., a broad fire was first seen spreading east from the center of the north face. A few minutes later, there was a fire on the north end of the west face, suggesting that the fire had burned at the interior of the floor, initially bypassing the northwest corner, then burning back to that corner after the fire became established on the north face. The fire on the north face spread rapidly eastward, reaching the east face around 3:55 p.m., and then burned intensely on the east face. Soon after 4:00 p.m., the observable burning near the center of the north face had died down.

9th Floor

There were no indications of fire on the 9th floor until shortly before 4:00 p.m., when a small fire appeared on the west side of the north face. The fire grew rapidly and spread to the east, reaching the midpoint of the north face by around 4:10 p.m. Ten minutes later, the fire was halfway to the northeast corner, but by 4:38 p.m., there were only spot fires visible, located on the east side of the north face.

11th Floor

The fire on the 11th floor generally spread counterclockwise. Fire was first observed at 2:08 p.m. at the south end of the east face. Over the next 20 min, the fire spread slowly northward to the midpoint of the east face. Over the next two hours, images showed no burning. At 5:09 p.m., the fire reappeared near the center of the north face, spreading slowly to the west and not reaching the northwest corner when WTC 7 collapsed at 5:21 p.m. In the meantime, at 4:38 p.m., a fire appeared spreading east from the center of the north face, once again suggesting that the prior burning had progressed along the interior of the building before backtracking to combust furnishings near the perimeter. By 4:52 p.m., the observable flames in the area had died down.

12th Floor

The fire on the 12th floor followed a path similar to that of the fire on the 11th floor, but with different timing. Fire was first seen on the 12th floor at 2:08 p.m., toward the south end of the east face. Further south on this face, the window glass was still intact, indicating that this fire had burned in the building interior as it turned the southeast corner.

By around 2:30 p.m., the visible flames had diminished, but the fire had spread both south into the southeast corner and north, reaching two-thirds of the way to the northeast corner. By 3:00 p.m., the fire had spread internally past the northeast corner and onto the north face. In less than 15 min, the fire simultaneously spread rapidly to the east to engulf the northeast corner of the floor and more slowly westward about one-third of the way across the north face. The fire continued spreading westward in starts and stops, approaching the northwest corner of the floor around 3:45 p.m. At around 5:00 p.m., the fire had reached the northwest corner.

13th Floor

Like the fires on the 11th and 12th floors, fire on the 13th floor also moved counterclockwise. Fire was seen at about 2:30 p.m. on the east side of the floor. Somewhat later, smoke and flames were coming from windows across much of the east face. Around 3:41 p.m., the fire had turned the northeast corner and was one-fourth of the way across the north face. Soon after 4:00 p.m., flames had reached at least to the midpoint of the north face; and at 4:38 p.m., the fires to the east had died down to the point where they could no longer be observed. Around 5:00 p.m., there was intense burning to the west of the center of the north face. A couple of minutes prior to the collapse of the building at 5:20:52 p.m., flames jetted from windows in the same area, indicating that there had been fire toward the interior of the floor.

14th Floor

A fire was seen briefly on the north face of the 14th floor, about halfway between the midpoint and the northeast corner, at 5:03 p.m. No fire was evident in images taken a few minutes before and a few minutes after this time.

2.4 THE PROBABLE COLLAPSE SEQUENCE

The following is the NIST account of how the fires in WTC 7 most likely led to the building's collapse. The collapse of WTC 1 damaged seven exterior columns, between Floors 7 and 17 of the south and west faces of WTC 7. It also ignited fires on at least 10 floors between Floors 7 and 30, and the fires burned out of control on Floors 7 to 9 and 11 to 13. Fires on these six floors grew and spread since they were not extinguished either by the automatic sprinkler system or by FDNY, because water was not available in WTC 7. Fires were generally concentrated on the east and north sides of the northeast region beginning at about 3 p.m. to 4 p.m.

As the fires progressed, some of the structural steel began to heat. According to the generally accepted test standard, ASTM E-119, one of the criteria for establishing the fire resistance rating for a steel column or floor beam is derived from the time at which, during a standard fire exposure, the average column temperature exceeds 538 °C (1000 °F) or the average floor beam temperature exceeds 593°C (1100 °F). These are temperatures at which there is significant loss of steel strength and stiffness. Due to the effectiveness of the SFRM, the highest column temperatures in WTC 7 only reached an estimated 300 °C (570 °F), and only on the east side of the building did the floor beams reach or exceed about 600 °C (1100 °F). The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, primarily at or below 400°C (750 °F), damaging the floor framing on multiple floors.

The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. This buckling arose from a process that occurred at temperatures at or below approximately 400 °C (750 °F), which are well below the temperatures considered in current practice for determining fire resistance ratings associated with significant loss of steel strength. When steel (or any other metal) is heated, it expands. If thermal expansion in steel beams is resisted by columns or other steel members, forces develop in the structural members that can result in buckling of beams or failures of connections. Fire-induced thermal expansion of the floor system surrounding Column 79 led to the collapse of Floor 13, which triggered a cascade of floor failures. In this case, the floor beams on the east side of the building expanded enough that they pushed the girder spanning between Columns 79 and 44 to the west on the 13th floor. (See Figure 1-5 for column numbering and the locations of girders and beams.) This movement was enough for the girder to walk off of its support at Column 79.

The unsupported girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the 5th floor (which, as noted in Section 1.2.3, was much thicker and stronger). Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. This left Column 79 with insufficient lateral support, and as a consequence, the column buckled eastward, becoming the initial local failure for collapse initiation.

Image
Figure 2-2. Eastward buckling of Column 79, viewed from the southeast.

Due to the buckling of Column 79 between Floors 5 and 14, the upper section of Column 79 began to descend. The downward movement of Column 79 1ed to the observed kink in the east penthouse, and its subsequent descent. The cascading failures of the lower floors surrounding Column 79 led to increased unsupported length in, falling debris impact on, and loads being redistributed to adjacent columns; and Column 80 and then column 81 buckled as well. All the floor connections to these three columns, as well as to the exterior columns, failed, and the floors fell on the east side of the building. The exterior facade on the east quarter of the building was just a hollow shell.

The failure of the interior columns then proceeded toward the west. Truss 2 (Figure 1-6) failed, hit by the debris from the falling floors. This caused Column 77 and Column 78 to fail, followed shortly by Column 76. Each north-south line of three core columns then buckled in succession from east to west, due to loss of lateral support from floor system failures, to the forces exerted by falling debris, which tended to push the columns westward, and to the loads redistributed to them from the buckled columns. Within seconds, the entire building core was buckling.

The global collapse of WTC 7 was underway. The shell of exterior columns buckled between the 7th and 14th floors, as loads were redistributed to these columns due to the downward movement of the building core and the floors. The entire building above the buckled-column region then moved downward as a single unit, completing the global collapse sequence.

_______________

Notes:

8. Flashover is the point in an enclosure fire when the fire changes (often abruptly) from being a local fire, perhaps involving one or two combustibles, to becoming a fire involving virtually all the combustibles.


 
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