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Buildings
Study Absolves Twin Tower Trusses, Fireproofing
enr.construction.com
- 11/04/02
By Nadine
M. Post
The most comprehensive study yet on
the destruction of the World Trade Center concludes that columns
robbed of fireproofing failed first--not floor trusses--when
the twin 110-story towers collapsed after being hit by terrorist
plane attacks on Sept. 11, 2001. The proof is in the smoke
that emanated from the burning towers before the collapses.
"There is no doubt left about the
sequence of failure," says Matthys P. Levy, chairman
of Weidlinger Associates Inc., the New York City-based engineer
that led the study.
"Failure of the floors...was shown
not to have had any significant role in the initiation of
the collapses," says the report. Levy describes the floor
truss system as "not unsubstantial," acting more
like a membrane than a one-way system. "There was nothing
wrong with it," he says. If the floor trusses had collapsed
first, there would have been a mass of smoke as opposed to
differentiated smoke, floor by floor, he adds.
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IMPACT
SEQUENCE
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 |
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| ONE
WTC |
TWO
WTC |
| HITS
Planes caused different damage (Graphics courtesy of Weidlinger
Associates Inc.) |
The report also exonerates the steel's
sprayed-on fireproofing. Computer models that identify the
columns affected by the planes' impacts and flying debris
confirm that columns with intact fireproofing did not succumb
to the jet- fuel-triggered fire. The report also says, of
the fireproofing knocked off the steel, that "no fireproofing
is designed to withstand such devastating impacts."
Levy echoes preliminary reports. "The
buildings were well-designed, rugged and withstood a tremendous
impact," he says. "The fact that they did not collapse
on the planes' impacts saved tens of thousands of lives."
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| SMOKE
PROOF Engineers say smoke patterns are evidence
that columns failed first, not floors. (Photo by Tom Sawyer
for ENR) |
Questions brought into the limelight
by Sept. 11 include whether there is a better way to fight
fires in tall buildings, says the engineer. "It's always
been a problem," says Levy.
Another issue is whether less-frangible
fireproofing should be considered for steel structures considered
vulnerable to blasts and attacks. Experts might also reconsider
location of fire stairs and the strengthening of the core,
says Levy. But he cautions, "You can never anticipate
exactly what the threat is going to be."
Regarding building materials, Levy says:
"Concrete is not foolproof either."
The Weidlinger-led study was commissioned
by Silverstein Properties Inc., the New York City-based leaseholder
of the World Trade Center, to help support a $7-billion insurance
claim. The research team also included LZA Technology/Thornton-Tomasetti
Group; ARUPFire; Hughes Associates Inc.; SafirRosetti; Hillman
Environmental Group; RWDI; W. Gene Corley, who led the ASCE-FEMA
WTC study; Professor Sean Ahearn; and Z-Axis Corp.
Silverstein's insurers claim the collapse
of the south tower, Two WTC, rendered the north tower, One
WTC, unsalvageable even before it collapsed. If they prevail,
Silverstein would receive only $3.5 billion (ENR 10/7 p. 11).
Click here to view
one WTC collapse sequence (SECOND DOWN North tower lasted
longer due to impact site. (Graphics courtesy of Weidlinger
Associates Inc.))
The insurers commissioned their own
engineering study, written by Exponent Failure Analysis Associates
Inc., Los Angeles. Also released, the report disagrees with
the Weidlinger findings, but mostly on points relating to
the insurance battle. Engineers from Wiss, Janney, Elstner
and Associates Inc., Northbrook, Ill., also working for the
insurers, would not comment on their work.
In the Silverstein study, engineers
put forth similar but not exact failure scenarios for both
towers: The planes and flying debris hobbled the buildings
at the zones of impact. Intact columns, their fireproofing
knocked off by flying debris, ultimately lost strength and
failed in the fuel-triggered fire.
Though hit by the second plane later
than One WTC, Two WTC fell first, "primarily" because
the plane struck it off-center and at an angle and caused
damage that compromised the southeast corner of the core.
"This confirms an earlier theory," says Levy. Click
here to view two WTC collapse sequence (FIRST DOWN Plane
took out corner of core, which hastened collapse. (Graphics
courtesy of Weidlinger Associates Inc.)).
At each tower, exterior wall and core
columns, connected by a steel "hat truss" at the
building's top, initially redistributed loads away from the
damaged areas to remaining columns. In Two WTC, the hat truss
eventually could not deal with the situation of the corner
columns gone, says Levy.
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 Silverstein findings are based on
analysis of original structural drawings, thousands of photos
and dozens of videos. The team used computer modeling, including
a program called FLEX developed by Weidlinger for the Dept.
of Defense, and fire evaluation techniques to simulate the
condition of each tower at critical times, creating impact
and collapse sequences.
The National Institute of Standards
and Technology, which recently began a two-year technical
study on the World Trade Center disaster, is using both team's
studies to perform a "very systematic" analysis,
says S. Shyam Sunder, chief of NIST's materials and construction
research division, Gaithersburg, Md. "The real question
is whether there was one dominant failure mechanism or a combination,"
he adds.
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