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Probable Cause Sequences For WTC Collapses Finalized

At a press briefing in New York City on April 5, the National Institute of Standards and Technology (NIST) presented its analysis of how the World Trade Center (WTC) towers collapsed after two aircraft were flown into the buildings by terrorists on Sept. 11, 2001.

The study is the most detailed examination of a building failure ever conducted.

"Like most building collapses, these events were the result of a combination of factors," said Shyam Sunder, lead investigator for the agency's building and fire safety investigation into the WTC disaster. "While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts. This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled."

The probable collapse sequences, which update and finalize hypotheses released by NIST last October, were presented by Sunder at a press briefing in New York City.

The specific factors in the collapse sequences relevant to both towers (the sequences vary in detail for WTC 1 and WTC 2) are:

  • Each aircraft severed perimeter columns, damaged interior core columns and knocked off fireproofing from steel as the planes penetrated the buildings. The weight carried by the severed columns was distributed to other columns.
  • Subsequently, fires began that were initiated by the aircraft's jet fuel but were fed for the most part by the building contents and the air supply resulting from breached walls and fire-induced window breakage.
  • These fires, in combination with the dislodged fireproofing, were responsible for a chain of events in which the building core weakened and began losing its ability to carry loads.
  • The floors weakened and sagged from the fires, pulling inward on the perimeter columns.
  • Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle--a process that spread across the faces of the buildings.
  • Collapse then ensued.

The sequences are supported by extensive computer modeling and the evidence held by NIST, including photographs and videos, recovered steel, eyewitness accounts and emergency communication records. Additionally, this information was used to document a variety of factors affecting the performance of the buildings, the efforts of emergency responders and the ability of occupants to escape prior to the collapses. In turn, NIST has identified a number of future practices and technologies that potentially could have enhanced building performance and life safety capabilities on 9-11 had they been available for implementation. All are being considered for NIST's upcoming recommendations.

NIST also released drafts of 15 reports from three projects of the investigation: analysis of building and fire codes and practices; occupant behavior, egress and emergency communications; and fire service technologies and guidelines.

Recommendations for improvements to building and fire codes, standards and practices derived from these and the other five projects in the investigation will be released for public comment in June, along with the draft of the final investigation report and drafts of 27 reports from the remaining five projects.

The NIST WTC investigation's goal is to recommend improvements in the way people design, construct, maintain and use buildings, especially high-rises.

Sunder's full press briefing presentation (including the complete probable collapse sequences for both WTC towers), the text of the 15 reports issued, and all previous WTC investigation findings are available at

As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST develops and promotes measurement, standards and technology to enhance productivity, facilitate trade and improve the quality of life.

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