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Fires, not impact, likely caused collapse of World Trade Center, KU professor says

LAWRENCE--The collapse of the World Trade Center's twin towers was more likely caused by the fires from the exploding jet fuel, rather than the actual impact of the planes on the buildings, said a University of Kansas professor who specializes in structural engineering.

Kim Roddis, professor of civil and environmental engineering at KU, said she was able to make that assessment because the towers remained standing after the impact of the crashes and explosions. From viewing television footage of the now-infamous attack, she said it appeared the high-temperature jet-fuel fire was what brought the towers down.

"When you watch those planes and the explosions, it's amazing that those structures stood for an hour and allowed people to evacuate," said Roddis. "That is the first objective of a structural engineer: to hold the public safety paramount."

Roddis, whose research focuses include structural design, infrastructure maintenance, and bridge fatigue, said skyscrapers - including the World Trade Center twin towers - are designed to sustain high winds and so are strong enough to take accidental collisions with airplanes.

However, in the case of the recent attacks, the large amounts of jet fuel stored on the planes for the cross-country flights, caused fires in excess of 1,500 degrees Fahrenheit, which in turn caused the metal support structure to buckle.

To explain her impression of the collapse, Roddis described the towers' design - referred to as "Framed Tube" - as a stack of alternating horizontal bricks and upright empty soda cans. The tops and bottoms of the cans are super-glued to the brick surfaces, so that it is all one unit, and the bottom part of it is buried firmly into the ground to anchor the system.

When an arrow pierces one of the cans, it causes great damage, Roddis said, but the entire structure survives. However, if that same arrow is on fire - like the fire resulting from the jets - it causes the soda can to melt and crumple, resulting in brick falling on brick and ultimately the collapse of the entire structure, Roddis said. In the case of the twin towers attacks, she said, the collapse might have been averted if the strikes were much higher or lower.

If the strike had been toward the top of the building, those floors would have been devastated, but the rest of the building might have remained standing, allowing more people to evacuate.

Had the strike been lower - close to the building's base - the stronger lower framing might have kept the jet from punching into the heart of the tower, limiting the subsequent fire.

Taking all of these factors into consideration, as an engineer Roddis can't help but conclude that whoever orchestrated these horrific attacks intended for the fires to cause the complete collapse of the buildings.

"Obviously, somebody thought very carefully about how big a plane they needed, how much fuel they wanted to have on the plane and where they wanted it to run into the tower," Roddis said.