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NIST Releases Final WTC 7 Investigation Report

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For Immediate Release

November 20, 2008

WTC7 column 79 Collapse

Graphic showing the buckling of WTC 7 Column 79 (circled area), the local failure
identified as the initiating event in the building’s progressive collapse.
Credit: NIST Building and Fire Research Laboratory
hi-resolution image

The National Institute of Standards and Technology (NIST) today released its
final report on the Sept. 11, 2001, collapse of the 47-story World Trade Center
building 7 (WTC 7) in New York City. The final report is strengthened by clarifications
and supplemental text suggested by organizations and individuals worldwide in
response to the draft WTC 7 report, released for public comment on Aug. 21,
but the revisions did not alter the investigation team’s major findings
and recommendations, which include identification of fire as the primary cause
for the building’s failure.

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, a video describing the WTC 7 investigation findings, a of all comments received on the draft WTC 7 report, a chart tracking the progress toward implementing all of the NIST WTC recommendations, and other materials may be accessed at

Media Contact: Michael E. Newman,, (301) 975-3025.

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