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No one designs a skyscraper
to withstand the direct hit of a fully fueled 767, and construction engineers
agree that such an attack would have doomed almost any high-rise.
Each World Trade Center
tower absorbed the impact of a jet with a shudder, as each was designed to
do, and stood.
Inside, though, 2,000-degree
infernos started burning, fed by thousands of gallons of jet fuel.
It then became a question
of time. Would the fuel burn up first or would the steel columns weaken and
buckle under the heat?
For the people on floors
above the crash site, there was another critical factor: an ordinary fire would
take two or three hours to burn through the gypsum wallboard around the stairwells
— but projectiles of plane wreckage almost certainly pierced through, letting
in the fire and smoke. That trapped people on the upper floors.
The south tower collapsed
56 minutes after impact. The north tower lasted an hour and 40 minutes.
Someone probably could
build a fortress skyscraper. "Given enough money, we can design anything,"
said Dr. Charles H. Thornton, chairman of the Thornton-Tomasetti Group Inc.
of New York City, the structural engineering firm that worked on the 1,483-foot-tall
Petronas Towers in Kuala Lumpur, Malaysia.
But no one would pay
to build one, and no one would want to work there. Such a building would probably
have the aesthetic appeal of a containment vessel of a nuclear power plant,
which is designed to survive the crash of a falling 747.
In the decades since
the World Trade Center was built, however, new materials and building techniques
— some used on the more recent super skyscrapers like the Petronas Towers —
may have given people more time to escape.
The key would have been
slowing the fires. The sprinkler systems offered little help.
Even if the pipes survived
the impact, the sprinklers of a typical skyscraper put out a few hundred gallons
of water a minute for half an hour, Dr. Thornton said, and water would have
been useless against a fuel fire in any case. (Water and oil don't mix; droplets
of water sink into the fuel, turn into hot steam and explode, and the fuel
continues burning.)
By contrast, an anti-fire
system at an aircraft hangar can unleash a deluge of 120,000 gallons a minute
of water and foam — which sticks to burning fuel — for two hours straight,
Dr. Thornton said.
Since extinguishing the
fire is impossible, "You have to build a more rugged building," said Dr. R.
Brady Williamson, an emeritus professor of civil engineering at the University
of California at Berkeley.
The main ingredients
of any skyscraper are steel and concrete. Both are strong, but in different
ways. Concrete bears more weight; steel can bend without breaking. The World
Trade Center's supporting columns were made of steel, and the intense heat
would have caused the girders to expand, distorting their shape and sapping
their strength, leading to the collapse.
"It's better to build
in reinforced concrete," said Dr. Mir M. Ali, a professor of architecture at
the University of Illinois. "If there is an impact, crash or explosion, it
can absorb the energy better. That makes the building less vulnerable."
But reinforced concrete
— concrete with steel bars inside — is heavier. When the World Trade Center
was built in the early 1970's, concrete was not a viable option because it
would have required huge, unwieldy pillars to support the towers' weight. But
high-strength concrete developed in recent years has made it more practical.
"The trend is toward
more concrete," Dr. Mir said. "The technology has substantially improved. An
all-concrete structure would have lasted longer."
Each of the two Petronas
Towers has an outer ring of 16 seven-foot- wide columns made of concrete and
at the center of each tower is a 75- foot by 75-foot concrete core — almost
a building within a building — that houses the stairwells and elevator shafts.
Concrete — a mix of cement,
sand and gravel — is not impervious to heat. The cement expands at a different
rate than the sand and gravel, causing cracks. Under intense heat, some types
of concrete can flake apart at about three-quarters of an inch an hour, eventually
exposing the steel inside.
"It ultimately would
lose its strength," Dr. Thornton said.
The concrete core of
the Petronas Towers may have remained intact under a similar crash and provided
a better escape route than the gypsum- walled stairwells of the World Trade
Center.
"In our buildings, most
of the stairways are in the core, which is a very safe haven," Dr. Thornton
said. The cores of the Petronas Towers are also pressurized to keep smoke and
fire out of the stairwells.
In addition to building
more fire- resistant structures, another protection against crashing airplanes
would be to keep the jet fuel from entering the interior of the building.
At the University of
California at Berkeley, Dr. Abolhassan Astaneh- Asl, a professor of structural
engineering, has been developing a new construction technique — bolting half-inch
steel plates to six-inch concrete walls — to create buildings that can better
survive earthquakes. "The concrete wall prevents the steel from buckling,"
Dr. Astaneh- Asl said. "The steel prevents the concrete from cracking and shattering.
When you marry them, they become very good."
In tests, a half-scale,
three-story building proved capable of surviving four magnitude-9 earthquakes.
While the wreckage of the 767's flew into the interior of the World Trade Center,
the extra mass of concrete and steel walls would have absorbed much of the
planes' momentum.
"Most of the fracturing
of the plane will take place outside of the building, not inside," Dr. Astaneh-Asl
said. That is the same fundamental physics that make the S.U.V. the lesser
damaged in a collision with a motorcycle.
Much of the fuel would
have then splashed against the outside of the building instead of igniting
inside, Dr. Astaneh-Asl said.
The World Trade Center
attack may lead developers to regard a terrorist attack as a risk to be planned
for instead of an unthinkable one-time tragedy.
"The perception of the
terrorist threat is where earthquake hazards were in the mid- to late 1960's,"
said Dr. Jeremy Isenberg, president and chief at Weidlinger Associates, a consulting
firm that once helped design resilient military bases and missile silos, and
now offers its expertise for federal and commercial buildings. "It took a series
of three or four damaging earthquakes to drive home to owners of buildings
that they had financial assets at risk."
Developers may now request
that more resilience be built into new buildings and into old ones being remodeled,
Dr. Isenberg said.
While perhaps not much
can protect against kamikaze jetliners, other simple steps may help protect
against lesser attacks. Large, heavy cement flower pots, like those placed
around the World Trade Center after the 1993 bombing, keep a car bomb a safe
distance from the structural columns. Concrete walls around loading docks and
mail rooms can be thickened to protect against bomb blasts. Jackets of graphite
fibers wrapped around columns make them less likely to collapse. Protective
glaze can be added to windows to make them less likely to shatter.
In planning for new buildings,
structural designers are now more likely to add more redundancy where the collapse
of one column does not lead to the collapse of the entire building, as occurred
in the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City.
Dr. Thornton of Thornton-Tomasetti
said that in one sense, taller buildings are safer than midrise buildings;
taller buildings are less likely to topple, because their builders generally
provided more redundancy into the structures. The design of skyscrapers 50
stories or more, has been "generally very robust," said Dr. Thornton. "For
40 or less, it's not."
But the taller buildings
make a more tempting target for terrorists. |
