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Thursday, 4 October, 2001, 17:37 GMT 18:37 UK
WTC collapse forces skyscraper rethink
BBC Graphic
By BBC News Online science editor Dr David Whitehouse

The rapid collapse of the twin towers of the World Trade Center (WTC) will force major building design changes to be adopted in the US, and may reduce the number of new high-rise buildings being constructed, according to experts.

There is going to be a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did

Bob Halvorson, architect
The shocking events of 11 September have prompted a major reassessment of the accepted practices employed in erecting skyscrapers.

British and European architects have criticised the current US regulations for ignoring the valuable information that came out of the 1968 Ronan Point tower block disaster in London.

A detailed analysis of WTC collapse suggests that the failure of fire protection systems used in the towers' central columns contributed to their rapid demise.

'Piledriver' effect

In the days following the New York atrocity, engineers were quoted as saying that the towers behaved as they would have expected given the extraordinary circumstances.

The buildings stood for about an hour after being struck
The towers' central steel spines were weakened by the intense heat from the burning aviation fuel, and eventually gave way when they could no longer support the weight of the floors above the crash zones - that is the accepted analysis. When those upper floors began to fall, they forced everything below them to collapse in a "piledriver" effect.

But should the twin towers have collapsed so quickly? Architects and engineers have been looking at the events in more detail, asking what actually happened and what lessons could be learned.

According to some experts, US engineers have never really taken seriously the idea that buildings can collapse in this way. Khalid Dinno, of the Canadian consulting firm Walters Forensic, said that the US waited for 25 years before introducing regulations that had been adopted in Europe.

After the 1968 Ronan Point tower block partial collapse in London, in which four people died, new British Standard Structural Design Codes for concrete were introduced to prevent it happening again. According to Dinno, these codes were ignored in America.

Lack of oxygen

Professor Wilem Frischmann, of the Pell Frischmann Group and the City University, London, was part of the inquiry into the Ronan Point disaster. He said the twin towers should not have collapsed so quickly. "Prior to 11 September, I scarcely believed that this icon was vulnerable," he said.

He added that the impact of the Boeings, puncturing the outer steel shell of the towers would not in itself have caused the towers to fall. "My current analysis of the collapse sequence [suggests that] damage caused to the outside would not have triggered collapse."

Many high-rise buildings in the USA and Britain will need to be re-evaluated because this flimsy dry-lining type of protection has become commonplace

Professor Wilem Frischmann, City University
Although the explosion caused by the fuel-laden aircraft would have been intense, the lack of available oxygen inside the towers would, according to Professor Frischmann, have limited the fireball's temperature to less than 1,000 Celsius. This was within the design limits that the towers were supposed to withstand.

In an analysis of the events that led to their collapse called Obituary For The Two Tall Towers, a report replete with unanswered questions, Professor Frischmann said that sprayed-water fire protection should have maintained the buildings internal strength for several hours, allowing a more complete evacuation.

But the steel supports in the central cores supporting the towers were protected from fire by plaster that had been sprayed on to them. This plaster could have been cracked by the impact, exposing the structural steel to the fire at an early stage.

"Many high-rise buildings in the USA and Britain will need to be re-evaluated because this flimsy dry-lining type of protection has become commonplace," said Professor Frischmann.

Point of impact

According to some experts, the suicide pilots hit the buildings in just the right place to bring about their collapse.

The north tower was struck at the 80th floor; the south tower at the 60th. Had the aircraft crashed lower down, around the 30th floor, it is possible that the stronger steel shell at those levels may have prevented collapse.

The lessons from the collapse will affect future construction
The regulations concerning the construction of future high-rise buildings are likely to be reviewed. According to Tod Rittenhouse, of Weidlinger Associates, who has written a report called Designing Terrorist Resistant Buildings, the design against progressive collapse must be "top of the agenda".

But Bob Halvorson, of architects Halvorson and Kaye, thinks it may prove too expensive to modify existing buildings.

He said: "There is going to be a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did."

The post-mortem on the twin towers will not be swift and will rely on the plans of the buildings, records of its construction, the testimonies of survivors, video of the collapse and forensic examination of the wreckage.

"We are operating well beyond realistic experience," said Halvorson.

See also:

13 Sep 01 | Americas
How the World Trade Center fell
18 Sep 01 | Sci/Tech
The destructive forces unleashed
24 Sep 01 | Sci/Tech
Laser maps tower rubble
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