World Trade Center puzzle lingers

World Trade Center puzzle lingers

Investigations began as the wreckage was cleared

Better construction could have saved hundreds of lives in the World Trade Center (WTC), but investigators are still puzzling over the evidence.

The WTC towers "did very well" to stand as long as they did, the head of the official inquiry into their collapse said in a BBC Horizon programme broadcast in the UK on Thursday.

But other structural engineers say use of more robust materials would have definitely allowed some people trapped on the upper floors of the centre to escape and may have prevented their collapse entirely.

The issue is of vital interest because the WTC's design was revolutionary and its construction methods influenced skyscrapers across the world.

Designed for impact

Leslie Robertson designed the structural elements of the WTC towers to withstand the impact of the largest airliner then in service, the Boeing 707.

Would they ultimately have collapsed? Maybe not

Charles Thornton Structural engineer

"With the 707 however, to the best of my knowledge, the fuel load was not considered in the design," he told Horizon.

But as the programme explained, it was the fuel aboard the hijacked Boeing 767s that led to the towers' collapse.

Each tower had a strong central column and a strong external steel skeleton.

Bound to fall

But neither could ever be strong enough to stand by itself, so the key to stability was the steel floor trusses which linked the two.

Investigators believe that foam fireproofing surrounding these trusses blew away when the planes hit the towers, leaving the steel to buckle in the heat of the fires that followed.

Once the crucial trusses began to fail, the towers were bound to fall.

In the minutes that they remained standing, most of the people below the levels where the planes hit were able to escape.

But those above were trapped as fire blocked the central columns and their stairways.

Only four people from either tower managed to escape from the levels above the impacts.

Squeegee escape

Some investigators believe that better fireproofing of the central columns would have saved hundreds of them, even if the towers had still collapsed in the end.

The drywall fireproofing surrounding the central columns was highly fire-resistant but not very strong.

Researchers believe much of it was dislodged on impact.

And it was weak enough for a man trapped between floors in a lift to hack his way through it with the squeegee he used for cleaning windows.

Gene Corley, head of the official inquiry into the towers' collapse, says that the towers did well not to fall down immediately.

I cannot escape the people who died there

Leslie Robertson WTC structural engineer

"The fact that one of them lasted 55 minutes and the other about an hour and 40 minutes says they did very well," he told the programme.

Mr Corley is due to publish his interim findings in April.

But other engineers appearing in the programme speculate that a better tower design could have survived.

"Had the floor system been a more robust floor system with much stronger connections between the exterior and the inside, I think the buildings... would have lasted longer," said Charles Thornton.

"Would they ultimately have collapsed? Maybe not."

Mr Robertson is plagued by the wisdom of hindsight and deeply distressed by the memory of what happened to those inside his buildings.

"I cannot escape the people who died there... that still to me somehow up there in the air are burning.

"I cannot make that go away," he said.