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.

In response to comments from the building community, NIST conducted an additional computer analysis. 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.

Other revisions to the final WTC 7 report included:

• Expanding the discussion of firestopping, the material placed between floors to prevent floor-to-floor fire spread;
• Clarifying the description of thermal expansion as it related to WTC 7’s shear studs and floor beams; and
• Explaining in greater detail the computer modeling approach used to define where and when the fire in WTC 7 started and the extent of window breakage as a result of fire.

With the release of the final WTC 7 report, NIST has completed its federal building and fire safety investigation of the WTC disaster that began in August 2002. A three-year study of the collapses of the WTC towers (WTC 1 and 2) was completed in October 2005. More than 20 changes in the U.S. model building and fire codes have already been adopted based on the findings and recommendations from the investigation.

NIST will now work with various public and private groups toward implementing additional changes to the U.S. model building and fire codes including those based on the 13 recommendations from the WTC 7 report (one new and 12 reiterated from the towers investigation).

The complete text of the final WTC 7 report, and a video describing the WTC 7 investigation findings are available on the NIST website.

Note: If no author is given, the source is cited instead.

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• Collapse of the World Trade Center
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• Levee failures in Greater New Orleans, 2005