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Oklahoma bomb lessons were ignored say engineers


AMERICAN STRUCTURAL engineers have failed to incorporate progressive collapse lessons from the 1995 Oklahoma City bombing, leading members of the profession warned this week.

But they are unconvinced that the collapse of the twin World Trade Center (WTC) towers in New York on 11 September will force major design code changes.

'It will be very difficult to review existing buildings and my guess is that some may choose to and some may not, ' said Tom Rittenhouse, a principal at New York based Weidlinger Associates.

Rittenhouse is an expert on the design of terrorist-resistant buildings and advised the United States General Services Administration on structural design after the 1995 Oklahoma bomb.

He said the structural collapses witnessed at the WTC meant the need to design against progressive and disproportionate collapse must still be 'right at the top' of the agenda.

In a 1995 paper written just after the bombing, Rittenhouse concludes: 'Commercial buildings should be designed to sustain a certain amount of attack, meaning they are designed to allow for limited localised damage - not total failure - to permit rescue teams to evacuate the victims.'

The paper adds: 'Acceptable damage is a relative term. In any case, the ultimate goal is to prevent widespread structural failure or progressive collapse.'

All federal building codes have since been changed to 'harden' public buildings and, where appropriate, make them more resistant to collapse.

'But not for commercial buildings, ' Rittenhouse said this week. 'New building design still does not require progressive collapse to be taken into account - it is not specifically in the design codes.'

Conclusions from investigations of the bomb-induced collapse of the Alfred P Murrah Federal Building in Oklahoma found that up to 80% of the 168 deaths were caused when the structure disintegrated.

Rittenhouse emphasised that while it would be almost impossible to have prevented the WTC's twin towers from collapsing after aircraft strike, he was concerned that so many nearby buildings came down.

The need for the US structural engineering profession to reassess its approach to progressive collapse was confirmed by Bob Halvorson, former partner in charge of structures at high rise specialist Skidmore Owings & Merrill.

'I think we will find the issue coming into codes explicitly now, ' said Halvorson, now a partner at Halvorson & Kaye.

'There are a few general requirements in the earthquake codes dealing with continuity of members now, but there isn't anything that specifically says consider progressive collapse.'

Although the issue would 'clearly now be on people's minds', designing against the small risk of a catastrophic event would lead to big problems for clients and designers, he said.

'I think that (disproportionate collapse) will become a more important issue based on what has happened. But there will no doubt be a gradation of requirement depending on the risks.'

Halvorson pointed out that strengthening work to existing buildings would be highly disruptive. He added that he would be 'very surprised' if building codes were changed to force owners to strengthen existing buildings.

'There is going to be a debate about whether or not the (WTC) towers should have collapsed in the way that they did, ' he added.

'But I am not sure that engineers really have any proven tools to look at this type of failure. We are operating well beyond realistic experience.'

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