By KENNETH CHANG
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,OOO-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 rninutes.
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
construction began on the World Trade Center in the late 1960'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 7-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. Thomton 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- AsI 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 remodelled, 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 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.
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Copyright 2001 The New York Times Company