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
"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
"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.
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."
PROOF Engineers say smoke patterns are evidence
that columns failed first, not floors. (Photo by Tom Sawyer
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).
here to view one WTC collapse sequence
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
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,
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,"