Damage Undermines Towers





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After two hijacked jumbo jets left gaping holes in the sides of both World Trade Center towers, air rushed in, feeding the flames that ultimately led to the towers' collapse an hour after the crashes, UC Berkeley engineers said Tuesday.

Steel columns several feet in diameter buckled under the enormous impact by the airliners. Those columns left standing were weakened by the intense fire, which burned as the airliners exploded upon impact.

"The high temperature could change the characteristics of these materials," said Armen Der Kiureghian, a UC Berkeley civil engineering professor.

"Fire went into places that were critical. I guess one can also imagine that gas lines inside exploded. It's not surprising that steel structures under high stress could lose their strength and fail," Der Kiureghian said.

Under such stress, engineers say, steel loses a great deal of its durability.

"The impact of the plane, primarily on the columns of the building that hold up the vertical load, could bend the columns out of shape," said Jack Moehle, a UC Berkeley civil engineering professor.

"Once they are sufficiently damaged, they lose their ability to support vertical load. Depending on the magnitude, that could be enough to weaken it, because the (columns) are bent out of shape."

Moehle also noted that since the airliners that crashed were all bound for destinations across the country, they were equipped with full tanks of fuel, providing extra volatility for the destruction.

"The fuel is ejected from the plane and is ignited. That leads to a very hot fire. The heat from the fire may have been well enough to soften the steel," Moehle said.

The "Twin Towers," the third-largest buildings in the world, were designed by Leslie Robertson, an engineer who graduated from UC Berkeley.

The structure is mainly supported by the outer ring of steel columns. When that ring is broken, the building is in danger of collapse, said UC Berkeley mechanical engineering professor Filip Filippou.

"It is pretty much like a sugar cane that has its strength on the shell, but you can still not bend it very easily," Filippou said. "If you take one of these faces away because you have an explosion, you do serious damage."

Engineers also noted that the only buildings structurally designed to withstand such impacts are nuclear power plants, which are made from several feet of reinforced concrete.

"They are designed to withstand the impact of a 747, but a normal building design would be simply too expensive, and the risk has been considered too low to require that," Moehle said.

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