Buildings
Columns
Likely Failed First in Terrorist-Triggered WTC Fires
(enr.construction.com - 06/28/04 issue)
By Nadine M. Post
...when One WTC fell. The theory “suggests”
a “classic progressive collapse,” says the report.
An initial local failure was at the lower floors, below 13,
due to fire and/or debris-induced structural damage of a critical
column, which supported a large-span floor bay covering 2,000
sq ft. >>Click
here to view image
NIST estimates 17,400 occupants,
plus or minus 1,200, were present in the WTC towers at the
time of the attacks, which killed nearly 3,000 people. Some
87% of the WTC tower occupants, including more than 99% of
those below the impact floors, were able to evacuate. NIST
estimates “about 20% or more” of those trapped may
have still been alive just prior to the collapse.
For the twin towers, NIST is still
studying “how and why” One WTC stood nearly twice
as long as Two WTC. NIST also is exploring whether any factors
related to “normal” building and fire safety considerations,
not unique to the terrorist attacks, could have delayed or
prevented the collapses. Another question, says NIST, is whether
the undamaged towers would have remained standing in a “normal
major” building fire.
>>Click
here to view image
The study has determined, based
on photographs, that perimeter columns bowed inward on One
WTC’s south face and Two WTC’s east face, “some”
minutes prior to collapse. “Initiation of global collapse
was first observed by the tilting of building sections above
the impact regions” of both towers, says the report.
One WTC tilted to the south and Two WTC tilted to the east
and south and twisted in a counterclockwise motion.
The report also finds “a lack
of technical basis” in the selection of fireproofing
thickness to meet the two-hour fire rating of the composite
floor system.
Sources critical of the investigation
say its goal to improve building safety and building practice
sounds noble enough but question whether it is achievable
(ENR 12/15/03 p. 10). There are concerns that findings may
somehow be used indirectly in pending lawsuits. No part of
any report resulting from an NIST investigation into a structural
failure or from an investigation under NCSTA may be used in
any lawsuit or action for damages arising out of any matter
mentioned in a report.
“This study has its value
in coming up with information that will help us resist unanticipated
attacks for particular buildings,” says Tomasetti. “Anything
learned from this study does not indicate previous designs
or previous codes were in any way deficient.”
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