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AMERICA ATTACKED | WORLD TRADE CENTER

2 Planes Hit Twin Towers at Exactly the Worst Spot

Collapse: Structural engineers say the terrorists apparently knew they had to strike the World Trade Center as low as possible to cause the most damage.

September 12, 2001|USHA LEE McFARLING | TIMES SCIENCE WRITER

The terrorists who piloted two planes into the World Trade Center apparently managed--either by careful calculation or evil luck--to have hit the buildings at their weakest spot to cause their disastrous collapse, structural engineers said Tuesday.

"It's like hitting someone at the back of the knee," said Nabih Youssef, a structural engineer who heads the Tall Building Council in Los Angeles and is an expert on the design and strength of skyscrapers. "With enough weight above you, you take the entire building down."

Government officials believe the terrorists wrested control of the passenger jets, then skillfully steered the planes toward the doomed towers.

"Whoever took over the plane knew what they were doing," said Greg Fenves, a professor of civil engineering at UC Berkeley.

Had the buildings been hit higher up the towers, they would have sustained damage but probably would not have collapsed because the weight on the damaged portion of the building would not have been enough to overwhelm a tower's structural supports, engineers said.

Planes Had to Clear Nearby Buildings

The planes might have done more damage if they had hit the buildings lower, but they had to fly at a height of about 60 stories to clear nearby buildings. The first tower was hit at about the 80th story. The second tower was hit at about the 60th story.

"They showed some knowledge of physics in the attempt to make the hits as low as possible," said Ron Hamburger, chief structural engineer for ABS Consulting in Oakland and a past president of the Structural Engineers Assn. of California.

To many who saw the buildings fall on television, the collapse resembled a planned demolition, especially in the way that the twin towers imploded--tumbling in on themselves. But engineering experts discounted the notion that additional explosives had been planted around the base of the buildings to ensure that they came down.

Demolition of a building the size of the ones in the World Trade Center would require "literally hundreds of charges around the building," Hamburger said. "It's inconceivable to me anyone would be able to place that many charges--even with years of planning."

Instead, the impact of the planes themselves, and the tremendous heat generated by tons of burning jet fuel--upward of 1,000 degrees Fahrenheit--would suffice to destroy the buildings, said Scott Gustafson, owner of Demtech Inc. of Blue Springs, Mo., one of the world's leading demolition experts.

A Boeing 767 has a fuel capacity of 20,000 gallons. A Boeing 757 has an 11,000-gallon fuel capacity. Because the planes were scheduled for transcontinental flights, they would have been fully loaded with fuel.

Plane Was 'a Highly Explosive Bomb'

"The plane probably made its way halfway to the core of the structure," Gustafson said. "The fuel went through a couple of floors, loaded them with fuel, and the impact opened a corridor to the outside for air. Some fuel probably got into the elevators and spread the fire. One thing led to another, and it just kept snowballing."

"It was very well thought out," said Hank Koffman, who directs the construction engineering department at USC. "These guys were evil geniuses.

"The plane was really a highly explosive bomb," he said. Terrorism experts were calling the attack "low-tech and high-concept."

Even though structural steel used in buildings is coated with a fireproof material, extreme amounts of heat cause the steel to soften and lose its strength. The weight of the floors above then causes them to crash.

"The technical term is progressive collapse--the slang term is pancaking," said Ron Klemencic, president of Skilling Ward Magnusson Barkshire, the Seattle firm that engineered the World Trade Center. "What basically happens is that one floor falls on top of the floor below it, and with one floor falling on top of another there's no way to stop it."

The steel is protected to certain temperatures and for certain periods of time, but "an explosion of this magnitude would have exceeded all those limits," said James C. Anderson, a professor of civil engineering at USC. "Buildings are not designed for this. Not in their wildest dreams."

"Buildings are designed thinking of internally generated heat," added Jon Magnusson, the chairman and chief executive of the Skilling firm. "Nobody anticipates putting jet fuel in a building. If you had to build buildings to withstand this sort of event, you wouldn't be able to build any buildings."

In 1945, a B-25 bomber smashed into the 79th floor of the Empire State Building. It caused an explosion and fire and killed 14 people but did not destroy the building. That plane was not loaded with the huge amount of fuel that Tuesday's jetliners carried.

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