of the twin 110-story towers of the World Trade Center was
stripped of its fireproofing by debris from the aircraft impact
and weakened by the resulting fires, eventually causing the
towers to collapse, according to an interim report by the
National Institute of Standards & Technology. The report
says the region of dislodged fireproofing was determined from
the predicted path of the debris.
Had the fireproofing not
been dislodged, the temperature rise of the structural components
would likely have been insufficient to cause the global collapse
of the towers, says NIST in the Oct. 19 release of another
interim report of its $16-million study of the WTC destruction
on Sept. 11, 2001, by terrorists. Fireproofing dislodged
by debris left the components more sensitive to heat than
any areas where there was missing or thin fireproofing before
the aircraft impacts, says the report.
Many experts familiar with the
twin towers design are not surprised by the findings. But
they are worth noting, say sources, because there are others,
both structural engineers and fire experts, who have questioned
whether the design by Skilling Helle Christiansen Robertson
in some way contributed to the collapse.
According to S. Shyam Sunder, NISTs
lead investigator for the study, an ordinary office fire would
likely have resulted in burn-out, not collapse.
Sunder says the working hypothesis
for a conventional fire, where sprinklers are not working,
is that it IS more likely TO have burn-out without collapse.
"If the sprinkler system were not operational in a multifloor
fire, our current working hypothesis, without having done
the calculations," is that the building would not have
collapsed, he says. On 9/11, the building collapsed because
the fireproofing on the steel was dislodged by direct debris
impact. The steel heated up and softened and lost strength.
In addition, NIST has determined
that the majority of the steel was stronger than minimum requirements.
The safety of the towers was most likely not affected
by the small percentage of steel below the minimum,
says the report. Building designs routinely allow structures
to withstand greater loads than are expected by including
significant factors of safety. Moreover, the structural loads
on Sept. 11, 2001, were well below this design level.
In fire tests in August, NIST also
determined that the floor systems in the towers met the New
York City building code of the time (ENR 9/13 p. 16).
The findings include an explanation
for the time delay between the collapses of the two towers.
(The south tower, Two WTC, survived for 56 minutes; the north
tower, One WTC, for 103 minutes). NIST says the difference
was primarily due to five items: the asymmetrical structural
damage of the aircraft impact to Two WTC compared to the aircraft
damage to One WTC; the time it took for heat to soften, buckle
and shorten core columns that had fireproofing dislodged by
debris impact; the structures ability to redistribute
loads as the core columns shortened; the time it took for
fires to traverse from their initial location to the face
of the towers where perimeter columns were bowing inward (as
seen only minutes before the collapse of each tower); and
the time it took for heat to soften and buckle those columns.
NIST plans to release its final
draft of the twin towers report in December or January.
A four to six-week public comment period will follow. The
final release is expected in May. The draft report on Seven
WTC is set to be released in May. The final report is expected
out in July.