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 http://wtc.nist.gov.
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.