NIST, FEMA, & ASCE reports & critiques, WTC collapse analyses,
Articles by and about the tower engineers and investigators,
Progressive collapse info, Engineering resources

Main 9/11 Links Page

"I am disgusted, disgusted with the structural engineers who know the truth about this and are keeping their mouth shut. There’s a special place in Hell reserved for them. And they are going to deserve it." –"Scholar for 9/11 Truth" James Fetzer


WTC Building 7


World Trade Center



NCSTAR 1-1 Design, Construction, and Maintenance of Structural and Life Safety Systems
NCSTAR 1-2 Baseline Structural Performance and Aircraft Impact Damage Analysis of the WTC Towers
NCSTAR 1-3 Mechanical and Metallurgical Analysis of Structural Steel
NCSTAR 1-4 Active Fire Protection Systems
NCSTAR 1-5 Reconstruction of the Fires in the World Trade Center Towers
NCSTAR 1-6 Structural Fire Response and Probable Collapse Sequence
NCSTAR 1-7 Occupant Behavior, Egress, and Emergency Communication
NCSTAR 1-8 The Emergency Response Operations


Alternate Collapse Hypotheses
Investigations leading to alternate tower collapse hypotheses, and critiques of NIST's WTC investigation by knowledgeable people who are not conspiracy theorists: FEMA/ASCE, Arup Fire, University of Edinburgh, Usmani, Torero, Lane, Cherepanov, Quintiere, Corbett, Mackey, Greening


Journal and Conference Papers


WTC collapse papers with Zdenek Bazant as lead author

What Did and Did not Cause Collapse of WTC Twin Towers in New York
Authors Bazant, Le, Greening & Benson. Journal of Engineering Mechanics ASCE 134 (2008). Refutation of controlled demolition theory. Discusses matching of video record with progressive collapse equations, "free-fall" claims, concrete crushing (and how much TNT equivalent would be needed to do that crushing), air pressure & ejection of air, spread of dust cloud.

Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions Co-author Verdure. PDF. Journal of Engineering Mechanics ASCE 133 (2007): pp. 308–319
Excerpt (applies to link above and below): The kinetic energy of the top part of the tower impacting the floor below was found to be about 8.4x larger than the plastic energy absorption capability of the underlying story, and considerably higher than that if fracturing were taken into account (Bažant and Zhou 2002a). This fact, along with the fact that during the progressive collapse of underlying stories the loss of gravitational potential per story is much greater than the energy dissipated per story, was sufficient for Bažant and Zhou (2002a) to conclude, purely on energy grounds, that the tower was doomed once the top part of the tower dropped through the height of one story (or even 0.5 m).
Discussion and replies to June 2006 Bazant & Verdure paper: James Gourley, G. Szuladinski. Closing comments from Bazant to Gourley (after detailed rebuttal to his claims):
Although everyone is certainly entitled to express his or her opinion on any issue of concern, interested critics should realize that, to help discern the truth about an engineering problem such as the WTC collapse, it is necessary to become acquainted with the relevant material from an appropriate textbook on structural mechanics. Otherwise critics run the risk of misleading and wrongly influencing the public with incorrect information.
Bazant & Zhou, 2001-2002: Why Did the World Trade Center Collapse?—Simple Analysis J. Engineering Mechanics ASCE, Sept. 28, 2001, addendum March, 2002.


Other explanations of the tower collapses

November, 2007: Structural Engineer Keith Seffen's mathematical model of WTC tower progressive collapse (PDF. Due to appear in the Journal of Engineering Mechanics, Vol. 134, No. 2, February 2008)
BBC News article about Seffen's paper

Dr. Frank Greening's papers on the Collapses, Energy Transfer, "Tipping," Concrete Pulverization (PDFs)

Physicist Manuel Garcia's Counterpunch articles on the physics of the WTC collapses Pt.1 Pt.2 Pt.3

Why didn't the towers, or the upper portions of them, topple over?

Why didn't the upper part pivot about it's base? See Bazant & Zhou (2001) Appendix II
Eduardo Kausel (MIT): Why the Towers didn't fall like trees
Frank Greening: An analysis of the tipping of the upper section of WTC 2 (PDF)
A simple graphic explanation of why the top of the south tower didn't fall to the side.
Physicist Dave Rogers on tipping of tower tops.
Structural engineer "Newton's Bit" on "tipping"


Explanations of NIST's Investigation Methods

NIST's Investigation of the Sept. 11 World Trade Center Disaster: FAQ (2002: Goals & Methods)

Ryan Mackey on how the NIST investigation works, and on "Scholars" critics
Ryan Mackey on NIST's degree of certainty, bounding cases, comparisons to other studies
Ryan Mackey on NIST altering model inputs
Ryan Mackey on NIST recovered steel and fire models
JREF Ktesibios on NIST workstation actual tests
JREF RWGuinn on testing models to failure
Ryan Mackey on NIST fuel load estimates
Physicist Dave Rogers on scaling issues with physical models of large structures


NIST's Recommendations, and the Architectural and Engineering Community's Response


The AIA's Response to NIST's Draft Report and Recommendations (PDF)

The World Trade Center Collapse and its Implications for International Standards – W. Gene Corley ISO Focus, January 2004 (PDF)

"Reexamining Premises for High-Rise Design" Security Management magazine (PDF)
In this article from Security Management magazine, Dr. Gene Corley discusses lessons from the World Trade Center investigation and how they might affect future building design.

NIBS Building Code Experts: Translating WTC Recommendations into Model Building Codes

First Comprehensive Set of Model Code Changes Adopted Based on Recommendations from Commerce’s NIST World Trade Center Investigation (June, 2007)
Gene Corley on FEMA/ASCE investigation lessons

"What We Learned: Building Performance Study of the WTC Collapse" (PDF) Outlines preliminary lessons learned in the FEMA/ASCE investigation of the NY World Trade Center collapse.

"Corley, ASCE Take Part in House Hearing" (PDF) On March 6, CTLGroup senior vice president Dr. Gene Corley testified before the Committee on Science of the U.S. House of Representatives. Dr. Corley was one of several members of the ASCE structural investigation team who were asked to discuss the performance of the World Trade Center in the September 11th attacks, and its implications for future building design. An article on the hearing from ASCE News.

"Looking Back on 9/11" (PDF) In this interview, Dr. Gene Corley discusses engineers' roles in the World Trade Center Building Performance Study


Abolhassan Astaneh-Asl's WTC collapse investigations

Tower Wreckage Reveals Clues – Astaneh – Jersey City Scrap Yards (later, some of these conclusions proved wrong)

Abolhassan Astaneh-Asl WTC Collapse, Field Investigations, and Analysis (Dec. 2003)

Now, as the five-year anniversary of the World Trade Center attack draws near, Astaneh-Asl finally expects to have the results of his analysis published in an academic journal. The project, requiring thousands of hours to complete, was self-funded and conducted by Astaneh-Asl, his students, and analysts from the MSC.Software Corporation, which donated the structural analysis software (MD Nastran and Dytran). Did the Building Do it? (Astaneh Study)

Testimony of Abolhassan Astaneh-Asl Before the House Committee on Science, March, 2002



Investigations involving extensive computer modeling of the towers

April, 2007: Purdue University analysis of WTC 1 collapse using LS-Dyna**More on the Purdue WTC study

"Did the Building Do It?" (Article about Abolhassan Astaneh-Asl/Cal FEA study, Sept, 2006)
Now, as the five-year anniversary of the World Trade Center attack draws near, Astaneh-Asl finally expects to have the results of his analysis published in an academic journal. The project, requiring thousands of hours to complete, was self-funded and conducted by Astaneh-Asl, his students, and analysts from the MSC.Software Corporation, which donated the structural analysis software (MD Nastran and Dytran).

NIST's SAP reference models of WTC 1&2, obtained through FOIA request

LU Xinzheng & JIANG Jianjing Simulation for the Collapse of WTC after Aeroplane Impact. Proceedings of the International Conference on the Protection of Structures Against Hazards, Nov. 2002. 57~60

Weidlinger Report Ties WTC Collapses to Column Failures (Matthys Levy). 10/25/02
The engineering team is comprised of: Weidlinger Associates Inc., led by Matthys Levy and Najib Abboud; LZA Technology/Thornton-Tomasetti Group, led by Daniel Cuoco and Gary Panariello; ARUPFire, led by Richard Custer; Hughes Associates Inc., led by Craig Beyler; SafirRosetti, led by Howard Safir; Hillman Environmental Group, led by Christopher Hillmann and John B. Glass Jr.; RWDI, led by Peter Irwin; Dr. W. Gene Corley, who led the ASCE-FEMA study; Professor Sean Ahearn; and Z-Axis Corp., led by Gary Freed and Alan Treibitz.

Weidlinger Study – Column failures

Weidlinger study refutes FEMA. Collapse inevitable due to structural damage and fires, not to WTC design defects. (NYT Oct 22, 2002)

First Tower to Fall Was hit at Higher Speed, Study Finds (NYT February 23, 2002)

Silverstein Sending Tower Data to U.S. Agency (NIST/Weidlinger/Insurance Agencies NYT Oct 1, 2002)
Even though the exterior columns all looked identical, both the grade and thickness of their steel varied from place to place, said Dr. Fahim Sadek, a researcher at the institute's building and fire research lab, who is producing a detailed structural model of the towers on a computer using the original blueprints. So there were actually more than 130 different column types, he said, each having to be accounted for in his model. From there, it gets only more intricate. One of Dr. Sadek's detailed models for a single floor -- the 96th floor of the north tower, considered typical -- contains 40,000 separate elements. A coarser representation of the entire tower contains 90,000 elements.

Swiss Re/Exponent Failure Analysis Associates WTC engineering study, Oct. 2002 (Complete copies of the FaAA study are available at Swiss Re by calling 212-317-5663.)

Ming Wang, Peter Chang, James Quintiere, and Andre Marshall "Scale Modeling of the 96th Floor of World Trade Center Tower 1" Journal of Performance of Constructed Facilities Volume 21, Issue 6, pp. 414-421 http://forums.randi.org/showthread.php?p=3161860#post3161860

Applied Reseach Associates NIST World Trade Center Investigations

Aircraft Impact Analysis of the World Trade Center Towers (Univ. of Tsukuba)

Jay Windley's (Jay Utah of BAUT forum) excellent explanation of Finite Element Analysis

Structure Magazine: Computer Modeling of the Collapse of the World Trade Center Towers (March, 2007)

CAD Digest article with CAD graphics (September, 2001) G. Charles Clifton

NIST's SAP reference models of WTC 1&2, obtained through FOIA request



More on WTC engineering, articles by and about engineers
who worked on the buildings and on the investigations

STRUCTURAL ENGINEERS AT GROUND ZERO – NCSEA (August Domel, Nov. 2001)

The Days After, the Days Ahead: Civil engineers respond to 9/11 Attacks (Civil Engineering, Nov. 2001)

Structural engineers first in the aftermath

Civil Engineering Magazine: Dissecting the Collapses

Articles in the National Academy of Engineering's "The Bridge" Spring, 2002 (PDF):
  • Editorial: Engineering and Homeland Defense by George Bugliarello
  • World Trade Center “Bathtub”: From Genesis to Armageddon by George J. Tamaro The engineer who oversaw the construction of the World Trade Center “bathtub” describes the recovery efforts.
  • A 911 Call to the Engineering Profession by Robert Prieto The events of September 11 challenged the future of our heavily engineered environment and the future of the engineering profession.
Early Structural Investigation Teams (through 5/02)

ASCE WTC Disaster Response Team

Engineer Tim Schenck on early GZ work

Multidisciplinary Center for Earthquake Engineering Research (MCEER) The World Trade Center Terrorist Attack: Damage to Critical Facilities and Crisis Response

Georgia Tech Engineering Team Will Conduct Post-Disaster Assessment at World Trade Center Attack Site

Architecture Week: Engineers Explain Collapses (5/2002)

Towers Lost and Beyond: 8 Articles by MIT researchers 2001-2002.

Civil Engineer: WTC & Pentagon Links Portal

Blog: Civil & structural engineers on WTC collapse

Forum on Technical Implications of WTC Collapses (Columbia U., November, 2001)

MIT Masters Thesis: WTC Disaster Analysis & Recommendations (June, 2005)

Designing tall buildings to Resist Earthquakes ($ NYT 2002)

Institute of Structural Engineers: Safety in tall buildings and other buildings of large occupancy ($ 2002)





Others write about sensibly about the collapses

More about the NIST & FEMA investigations

In Collapsing Towers, a Cascade of Failures ($ NYT Nov. 11, 2001)

Experts Urging Broader Inquiry In Towers' Fall (NYT Glanz, steel removal, Dec 25, 2001)

NIST to take over collapse investigation ($ NYT January 17, 2002)

Critique of FEMA WTC report by Ed Schulte in Plumbing Engineer, July, 2005 (PDF)

Learning from 9/11 (March, 2002) Hearing before the House Committee on Science: ASCE/FEMA BPAT study

Mismanagement Muddled Collapse Inquiry, House Panel Says (FEMA/NYC gov't/Fed gov't. $ NYT March 7, 2002)
At one point, Representative Anthony D. Weiner, a Democrat from New York City, asked for the official in charge to raise his hand, and two men, and then three appeared to do so. ''We have very serious problems here,'' added Representative John B. Larson, a Connecticut Democrat.

The lack of clear authority has had unfortunate consequences, the House members said. The Giuliani administration started to send World Trade Center steel off to recycling yards before investigators could examine it to determine whether it might hold crucial clues as to why the buildings fell.

The full investigative team set up by FEMA was not allowed to enter ground zero to collect other potentially critical evidence in the weeks after the attack, and it did not get a copy of the World Trade Center blueprints until early January, a delay House members found infuriating.

'The delay in the receipt of the plans did somewhat hinder the team's ability to confirm their understanding of the buildings,'' said Dr. W. Gene Corley, a structural engineer leading the investigative committee organized by FEMA.

A Port Authority spokesman defended the agency, saying that building plans had been given to federal officials within a week of the attack and that the agency was cooperating fully with the inquiry.

The federal officials who testified yesterday -- Dr. Arden L. Bement Jr., the director of the National Institute of Standards and Technology, and Robert F. Shea, the acting administrator of the Federal Emergency Management Agency's Insurance and Mitigation Administration -- repeatedly nodded their agreement.

In response to criticism from one committee member, Mr. Shea said, ''Frankly, I agree with you. There are many things in hindsight we would have done differently.''

The problem, they said, was the lack of clear authority in federal law and financing. None of the investigators, for example, had subpoena power, meaning that they could not order the city to stop sending the steel off for recycling or demand a copy of the building blueprints.

Government Orders Inquiry Into Trade Center Collapse ($ NYT March 23, 2002)

The Trade Center; Towers Fell as Intense Fire Beat Defenses, Report Says (FEMA. $ NYT March 29, 2002)

Interviews will examine why trade center collapsed -100,000 workers issued security passes

Wider Inquiry Into Towers Is Proposed ($ NYT May 2, 2002)

"In Data Trove, a Graphic Look at Towers' Fall, ($ NYT October 29, 2002)

Twin Tower Collapse Theory Challenged (Feb. 2003)

NIST Minutes of December 2-3, 2003, Meeting

New Evidence Is Reported That Floors Failed on 9/11 ($ NYT Dec. 3, 2003)
S. Shyam Sunder, who is leading the investigation for the National Institute of Standards and Technology in the Commerce Department, said, ''We are seeing evidence of floors appearing to be sagging -- or that had been damaged -- prior to collapse.'' Still, Dr. Sunder said, ''The relative role of the floors and the columns still remain to be determined in the collapse.''

NIST Not Ruling Anything Out on WTC Probe

Reliving 9/11, With Fire as Teacher (Modeling. $ NYT, Jan. 6, 2004)

NIST Public Briefing 05/05/04

Fire Testing Is Questioned In Findings On Towers (Current testing standards for new buildings inadequate. $ NYT August 26, 2004)


WTC Twin Tower Construction Details Pertinent to Engineering Investigations

Gregory Urich: Detailed Estimate of Mass and Potential Energy of World Trade Center Tower (pdf)
WTC 1 & 2 core column data
Report of WTC fire code compliance--1993 (pdf)


Truther Claim: the term "Progressive Collapse" is a new one and describes a phenomenon rarely or never seen before 9/11.
Jim Hoffman, whom many truthers describe as one of their best researchers:
You've heard that the Twin Towers pancaked, crushing themselves completely. The experts gave us a fancy-sounding term for this: progressive collapse. If you search with the phrase "progressive collapse" you will find numerous articles, most of them written since 9/11/01 about things like assessing and retrofitting existing structures against progressive collapse. It seems that the only examples of progressive collapse of buildings cited are the Twin Towers, Building 7, and the Murrah Federal Building in Oklahoma City.
A few examples of reality:
"As the hon. Gentleman will know, I accepted the tribunal's recommendations in respect of gas which show that once buildings are strengthened against the possibility of progressive collapse there is no reason why gas should not be used..." –Ronan Point Flats (Report), November, 1968

"New safeguards against the progressive collapse of tall buildings..." –"New Safety Law for Buildings," The Times (London), February 6, 1970

"The engineering term 'progressive collapse' – a potential calamity in high-rise structures – carries a sad double meaning throughout the evening." –'Benefactors' by Frayn, The New York Times, December 23, 1985

'It was a progressive collapse,'' Mr. Cohen said. ''What touched it off, we don't know.'' –"Collapse Inquiry Studies Bracing for New Column," The New York Times, May 1, 1987.

Some Progressive Collapse Studies & Recommendations
Thanks to JREF forum member cmcaulif for his contributions

Progressive Collapse Basics (R. Shankar Nair. 2004 North American Steel Construction Conference. PDF)

Fire Induced Progressive Collapse. (Arup. PDF) "This paper considers issues related to fire induced progressive collapse of tall buildings in extreme events."

Fire induced progressive collapse analysis of high rise buildings


Abolhassan Astaneh-Asl: Progressive collapse prevention in new and existing buildings (Includes use of catenary action of cables to resist collapse. 2003. pdf).

Practical Means for Energy-Based Analyses of Disproportionate Collapse Potential
Journal of Performance of Constructed Facilities, Volume 20, Issue 4, pp. 336-348 (November 2006)

Progressive Analysis Procedure for Progressive Collapse
Journal of Performance of Constructed Facilities, Volume 18, Issue 2, pp. 79-85 (May 2004)

Progressive Collapse of Structures: Annotated Bibliography and Comparison of Codes and Standards
Journal of Performance of Constructed Facilities, Volume 20, Issue 4, pp. 418-425 (November 2006)

Cheol-Ho Lee, Seonwoong Kim, Kyu-Hong Han, Kyungkoo Lee 2009. "Simplified nonlinear progressive collapse analysis of welded steel moment frames"
Journal of Constructional Steel Research, Volume 65, Issue 5, 1130-1137

Feng Fu, "Progressive collapse analysis of high-rise building with 3-D finite element modeling method" Journal of Constructional Steel Research, In Press, Corrected Proof, Available online 13 March 2009

A.G. Vlassis, B.A. Izzuddin, A.Y. Elghazouli, D.A. Nethercot, "Progressive collapse of multi-storey buildings due to failed floor impact" Engineering Structures, In Press, Corrected Proof, Available online 12 March 2009
Abstract excerpt: "The application of the proposed methodology is demonstrated by means of a case study, which considers the impact response of a floor plate within a typical multi-storey steel-framed composite building. ...The application study illustrates the extremely onerous conditions imposed on the impacted floor resulting in an increased vulnerability to progressive collapse for structures of this type."

B.A. Izzuddin, A.G. Vlassis, A.Y. Elghazouli, D.A. Nethercot, "Progressive collapse of multi-storey buildings due to sudden column loss — Part I: Simplified assessment framework" Engineering Structures, Volume 30, Issue 5, May 2008, Pages 1308-1318

A.G. Vlassis, B.A. Izzuddin, A.Y. Elghazouli, D.A. Nethercot, "Progressive collapse of multi-storey buildings due to sudden column loss—Part II: Application" Engineering Structures, Volume 30, Issue 5, May 2008, Pages 1424-1438

Santiago Pujol, J. Paul Smith-Pardo 2009. "A new perspective on the effects of abrupt column removal" Engineering Structures, Volume 31, Issue 4, 869-874

Hyun-Su Kim, Jinkoo Kim, Da-Woon An, "Development of integrated system for progressive collapse analysis of building structures considering dynamic effects" Advances in Engineering Software, Volume 40, Issue 1, January 2009, Pages 1-8

Jinkoo Kim, Taewan Kim, "Assessment of progressive collapse-resisting capacity of steel moment frames" Journal of Constructional Steel Research, Volume 65, Issue 1, January 2009, Pages 169-179

Jeom Kee Paik, Bong Ju Kim, "Progressive collapse analysis of thin-walled box columns" Thin-Walled Structures, Volume 46, Issue 5, May 2008, Pages 541-550

Some Engineering & physics fundamentals & calculation aids

Efunda: Engineering Fundamentals
Online scientific calculator and unit converter
Shear Force And Bending diagrams 1
Ductile fracture and fragmentation in the reconstruction of the World Trade Center Attack (Wierzbicki)
Elastic Bending Theory
JREF: PixyMIsa on truther misuse of the term "entropy"
JREF Structural Engineer Newtons Bit on DCR/dead/live loads/safety factor
Steel: Ultimate strength vs. yield strength (strength is in the geometry)