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Buildings
Report Ties WTC Collapses to Column Failures
enr.construction.com
- 10/25/02
By Nadine
Post
The collapses of the 110-story twin
towers of the World Trade Center after terrorists slammed
hijacked planes into them were separately initiated in the
impact zone of each tower due to failure of the columns, says
a recent engineering report, not the floor trusses. Two WTC,
though hit by the second hijacked plane after One WTC, fell
first "primarily" because the plane struck it at
an off-center angle and caused damage that compromised the
corner of the core of the building, concludes the report's
authors, a team of engineers from several firms working for
Silverstein Properties Inc., the New York City-based leaseholder
of the World Trade Center.
The findings are based on analysis
of original structural drawings, thousands of photos and dozens
of videos, says Silverstein. The team used computer modeling,
some programs developed for the Dept. of Defense, and fire
evaluation techniques to simulate the condition of each tower
at critical points from impact to collapse.
Click here to view diagram
1 Click here to
view diagram 2
The team determined that the initial
hits destroyed 33 of 59 perimeter columns in the north face
of One WTC and 29 of 59 perimeter columns in the south face
of Two WTC. Computer analysis showed that the impact of the
planes also destroyed or disabled some 20 of 47 columns in
the center of the core of One WTC and some five of 47 columns
in the southeast corner of the core of Two WTC. The crashes
stripped fireproofing from columns in the path of debris created
by the planes penetrating the buildings, it continues.
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| One
frame from the computer model of the initiation of the
collapse of Tower 2 performed by Weidlinger Associates |
The team says the towers' columns of
the redundant exterior tube and core columns, connected by
a steel "hat truss" at the top of the buildings,
redistributed loads away from the damaged areas to remaining
columns. This allowed the towers to stand as long as they
did, says the report. In a release, Matthys Levy of Weidlinger
Associates Inc., one of the engineers on the study team, states,
"The fact that tower one stood for 103 minutes.... and
tower two for 56 minutes" after the loss of so many columns,
"is a testament to the strength of the buildings and
the skill of Leslie Robertson and the other engineers who
designed them. I believe that few, if any, other buildings
could suffer that amount of damage and not collapse immediately."
The computer models, says the report,
identify the failure of columns that either lost fireproofing
or were destroyed on impact as the specific cause of the collapse
of each tower. "No fireproofing is designed to withstand
such devastating impacts," says the report.
It adds that the fireproofing on the
structural elements of the towers had been inspected regularly
and that the inspection program "represented a greater
standard of care than is generally followed for high-rise
office buildings in New York City."
The report exonerates the floor trusses
for the collapses. "Failure of the floors...was shown
not to have had any significant role in the initiation of
the collapses," it says. Studies by Hughes Associates
and ARUPFire led the team to conclude that tower floors survived
the initial impact of the planes, suffering only localized
damage. On the basis of a review of smoke plumes and fire
spread, for each tower, the engineers concluded that the fires
did not lead to the collapses of the floors affected before
the towers fell. Additionally, the engineers claim that computer
modeling shows that the failure of columns alone, independent
of the floors explains the collapses.
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| Diagram illustrating the way Tower 1 redistributed loads to
other columns on its north face and started off immediate
collapse, as modeled by Weidlinger Associates |
The findings are intended to build on
the study initiated by the American Society of Civil Engineers
and sponsored by the Federal Emergency Management Agency,
called World Trade Center Building Performance Study. Released
last May, it suggested subjecting the floor truss system to
"more detailed evaluation." But the study also stated
the truss systems "should not be regarded" as design
deficiencies, says the Silverstein report.
The Silverstein report also concludes
that fire temperatures were lower than typical "fully
developed" office fires. The fires were fueled by office
furniture and floor contents initially ignited by the jet
fuel, which burned out quickly. Dust and debris distributed
by the crashes inhibited the fires, which at the impact floors
were between 750°F and 1,300°F.
The engineering team is comprised of:
Weidlinger Associates Inc., led by Matthys Levy and Najib
Abboud; LZA Technology/Thornton-Tomasetti Group, led by Daniel
Cuoco and Gary Panariello; ARUPFire, led by Richard Custer;
Hughes Associates Inc., led by Craig Beyler; SafirRosetti,
led by Howard Safir; Hillman Environmental Group, led by Christopher
Hillmann and John B. Glass Jr.; RWDI, led by Peter Irwin;
Dr. W. Gene Corley, who led the ASCE-FEMA study; Professor
Sean Ahearn; and Z-Axis Corp., led by Gary Freed and Alan
Treibitz.
Silverstein commissioned the reports
for its insurance claim on the World Trade Center. The organization
has already given the reports to the National Institute of
Standards and Technology, which is studying the collapses
of the towers and Seven World Trade as part of a two-year
study.
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