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Fire, not impact, caused collapse of towers: experts
Rob Lever
1:34pm, Thu: WASHINGTON (AFP) - The collapse of the World Trade Center towers resulted
from the intense fire that melted structural support, not the impact of the hijacked
jetliners that crashed into each of the skyscrapers, experts said yesterday.
Engineers and architects said the huge amounts of jet fuel caused an inferno with
temperatures as high as 1,000 degrees Celsius (2,000 Fahrenheit), melting the
supporting steel and eventually causing the 110 story towers to implode.
"The fire brought it down ultimately," Henry Petroski, a civil engineering professor
at Duke University, told AFP. "Buildings like that are built so they can withstand
fire, but not indefinitely. The idea is that the fire will either burn itself out or
cool off before there is any structural damage. In this case, with all the jet fuel,
my guess is that the fire began to soften the steel."
The towers, weighing 290,000 tonnes each, were built for 1.5 billion dollars in the
1970s, using a controversial innovation for the time -- a tubular construction with
most of the structural support in the exterior "skin." New York architect John Young
said the towers were a "vainglorious project" with a number of safety shortcomings
that were overlooked because the structures were exempt from local building codes."
The building was built far larger than local codes would have allowed," he told AFP.
"Many city agencies protested it would be difficult to protect people in an emergency.
From day one, it was talked about as a safety hazard."
Because of the controversy about the structures, terrorists may have been able to
obtain architectural plans to study the weaknesses of the towers. Young noted that the
towers utilized the exterior wall framing for lateral bracing, making more room for
office space without pillars.
"You can see the effect of that when the buildings collapsed, with the lattice
framework crumbling and the interior imploding," he said. "The lattice works so long
as it remains intact as a system: if a part of it goes, then the whole system goes."
But other experts said this system -- also used for the Sears Tower in Chicago and
other skyscrapers -- should not be blamed in the collapse.
"This type of structure gave those precious minutes for people to escape," said Ben
Heimsath, a Texas architect who specialized in building safety." If you take this
amount of momentum and velocity and mass and follow it up with a colossal inferno,
that is a one-two punch that is an impossible situation."
Petroski noted that the Empire State Building survived an impact from a much smaller
B-25 bomber in 1945 with no structural damage, but noted that today's buildings are
generally lighter and use new methods to allow more open floor space. "In order to
build tall buildings economically, you want to make them as light as possible and open
up the building," he said.
He added that the massive quantities of cascading fuel probably created an inferno
that made the collapse inevitable. But he did not think the terrorists had any special
knowledge of the building structure. Experts said it is unlikely that design changes
will be made to new buildings, but that more attention will be paid to evacuation
plans, flammability of materials and fire suppression or sprinkler systems.
But Petroski said the attacks will likely dampen enthusiasm for any massive new
skyscraper projects. "I think it's going to be very difficult to make a proposal that
financiers, the people that supply the money to invest in these buildings, are going
to embrace," said Petroski. Tall buildings are built principally as rental space; and
you don't want something that makes customers anxious. Right now, going into a tall
building is going to have that effect on people."
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