LONDON - A vulnerability in the structure of the World Trade Centre towers may have contributed to their collapse after the September 11 attacks, a leading structural engineer said yesterday.

A combination of events, including the aircraft colliding into the Manhattan buildings, a weakening of the structure by the physical impact, the thickness of fire-proofing insulation and the fires, have been blamed for the catastrophe in which about 2800 people were killed.

But Dr Asif Usmani, a structural engineer at the University of Edinburgh's School of Engineering and Electronics, told an engineering conference the twin towers seem to have been "unusually vulnerable" to a major fire.

"There was a vulnerability in the design of the structural system. It is not materials. It is not about fire protection. It is about the design of the structure," he said.

In a computer-based analysis of the buildings, Usmani studied the structure of the towers that collapsed after they were hit by two hijacked planes in attacks blamed on Osama bin Laden's al Qaeda network.

"We are saying let's look at the structure itself and see if it has anything unusual that makes it more vulnerable to fire than other structures."

Usmani said the towers' exterior and internal columns, which made up the core of each building, were joined by a very long and thin expanse of floor. Supporting the floors were lightweight trusses which, when exposed to fire, expand.

The way the towers were built meant the trusses did not have anywhere to expand without buckling.

"This was good for constructing a building very quickly and they were strong enough to carry all the loading that was put on them. But when there is a fire underneath the trusses and the steel gets hot ... it expands quite a lot and those expansion forces can be quite high if the restrains to expansion are high."

Even at low temperatures of about 200C-300C the steel can begin to expand and, because the building itself is keeping them in place, pressure builds up to a critical point.

"This caused a buckling of the floors which were providing not only the load-carrying capacity for the furniture and the people, but were also providing lateral support for the columns," Usmani explained.

"That lateral support vanished as soon as they buckled and that is why the building fell in on itself."

Usmani, whose research is to appear in the Fire Safety Journal, said there might be other buildings which had a vulnerability to fire of an extraordinary nature that should be looked at.


Herald Feature: The Sept 11 attacks

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