World Trade Center excavation

Last Report Completes NIST's Study of 9/11 Fires, Collapses

The National Institute of Standards and Technology has released its final report on the fires and building collapses caused by the Sept. 11, 2001, attacks on the World Trade Center in New York City. Skeptics continue to argue burning jet fuel could not have felled the buildings, but this report -- "strengthened by clarifications and supplemental text suggested by organizations and individuals worldwide" in response to the draft report on the collapse of World Trade Center building 7, released for public comment on Aug. 21, the agency said -- concludes fire alone caused WTC 7 to fall. "The extensive three-year scientific and technical building and fire safety investigation found that the fires on multiple floors in WTC 7, which were uncontrolled but otherwise similar to fires experienced in other tall buildings, caused an extraordinary event. Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down," NIST concluded.

NIST, a Department of Commerce unit, said it conducted additional computer analysis in response to comments from the building community. "The goal was to see if the loss of WTC 7's Column 79 -- the structural component identified as the one whose failure on 9/11 started the progressive collapse -- would still have led to a complete loss of the building if fire or damage from the falling debris of the nearby WTC 1 tower were not factors. The investigation team concluded that the column's failure under any circumstance would have initiated the destructive sequence of events."

The final report includes an expanded discussion of firestopping, the material placed between floors to prevent floor-to-floor fire spread. This report completes the six-year NIST investigation of the 9/11 disaster; NIST says more than 20 changes in U.S. model building and fire codes have been adopted based on findings and recommendations from the investigation. A chart of the progress on those recommendations is available at http://wtc.nist.gov/recommendations/index.htm.

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