Engineer explores structural reasons for WTC collapse

Noted structural engineer and visiting University lecturer, Dr. Charles Thornton, spoke yesterday about the engineering analysis of the September 11 terrorist attacks on the World Trade Center. Thornton, chairman of the engineering firm responsible for assessing the damage at the WTC site, explained the reasons for the catastrophic collapse of the twin towers.

Thornton confirmed the widely reported structural explanation for the towers' collapse. "No aircraft alone will knock over an office building" as massive as a tower at the World Trade Center, he said.

The burning jet fuel weakened the building's structural supports, but as to what specifically failed, he admitted, "I don't know if anybody will ever know for certain."

He noted several measures that could have improved the buildings' chances for survival under such an assault. "I might use a concrete core [rather than the steel in the WTC], or I might encase the beams in concrete," Thornton said.

Reinforced concrete can survive intense heat far better than steel, he said. The beams of the twin towers were made of steel with an applied fire retardant coating.

Thornton referred to the Petronas towers in Malaysia, which his company — the Thornton-Tomasetti Group — designed. Those twin towers, currently the tallest in the world, have concrete beams arranged in a circular pattern, which gives added strength against wind and other stress. "There's no way an aircraft goes through a thirty-inch concrete wall," he added.

Brian Tokarczik, a Thornton-Tomasetti engineer who had been at ground zero from day one, also spoke. Elaborating on entering the effected region of Lower Manhattan, he said, "You leave New York City and enter a different world. There are pieces of debris wedged thirty feet down into the roadway" after falling from the towers' upper stories.

After Tokarczik spoke, Thornton took questions from the audience. One, posed by Dean of the Engineering School James Wei, inquired as to whether the terrorists could have known that ramming planes into the buildings would have caused such damage.

"No. They absolutely lucked out. They probably thought they would destroy the tops of the buildings, but it must have been beyond their faintest dreams that they could take out all seven buildings" of the World Trade Center.

Thornton discussed the difficulties facing engineers trying to help the recovery workers clear the site. One involves the commuter train tracks (PATH) that run under the Hudson River from the west, into the Center. If crews are not careful, the damaged train tunnel could collapse and be inundated with water."The PATH system could very well fill up with water all the way back into New Jersey," he said.

As to whether future building codes will require greater safety measures capable of preventing such disasters, Thornton said he was unsure. "The question is, do we penalize the whole office building industry" by requiring such features as redundant building support walls, reinforced concrete cores and compartmentalized design.

In addition to his position at the Thornton-Tomasetti group, Thornton is a visiting lecturer here at the University, teaching CEE 461 Design of Large Scale Structures: Buildings, a required senior level course for structural engineers.