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
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Contact: Michael E. Newman
michael.newman@nist.gov
301-975-3025
National Institute of Standards and Technology (NIST)
12-Apr-2005


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