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Engineer: Impact showed World Trade Center's strength

The structure held up for an hour before collapsing, despite an occurrence that it wasn't designed to withstand.


© St. Petersburg Times, published September 14, 2001

The structure held up for an hour before collapsing, despite an occurrence that it wasn't designed to withstand.

From what he has seen in television accounts, engineer Eugene Corley believes the World Trade Center more than did its job on Tuesday.

Corley, who led the Federal Emergency Management Agency investigation into the bombing of the federal building in Oklahoma City, now has been named to the "forensic team" investigating the World Trade Center's destruction.

"In my opinion, the building did pretty well," said Corley, a structural engineer with Construction Technology Laboratories in Skokie, Ill., near Chicago. "It remained standing for about an hour, allowing people to get out. You have to remember, this (building) was not designed with terrorism in mind."

The World Trade Center was designed to withstand quite a bit of stress -- perhaps even a direct hit from a smaller jet. That design strategy was based on an incident 56 years ago at the Empire State Building when an Army Air Force B-25 crashed between the 79th and 80th floors. Fourteen people died. Damage to the building was $1-million, but the structural integrity of the building was not affected.

The World Trade Center was designed to withstand a hit from a 707 -- a big jet, but much smaller than the plane that hit the building Tuesday.

"At the time this was designed, the design was a stretch; a 707 crashing into it was highly unlikely," Corley said. "Having a 767 crash into a building was completely unimaginable, until someone did it intentionally."

The World Trade Center building's strength came from a central steel core and from steel columns around the perimeter of each building. The impact, the gasoline explosion and heat from the resulting fire softened the steel girders so they could no longer support the weight above. So one floor collapsed on another, and the structure caved in almost straight downward.

Corley has been asked by the American Society of Civil Engineers to accompany a team to investigate "how the building itself performed in terms of the design."

It's unclear how soon their work will begin.

So what did Corley think as he watched the terrorist attack unfold Tuesday?

"My first reaction was, "At least it didn't collapse immediately,"' Corley said. "What happened was bad enough, but it stayed up for a while and people got out."

As an engineer who has examined countless bridges and buildings for structural flaws, Corley knew once the jets hit the building that the World Trade Center would collapse as it did.

"I just didn't know when it was going to happen," he said. "Visually, it was still shocking."

- Researcher Caryn Baird contributed to this report.

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