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US design standards neglected progressive collapse

News : Cover story

UNITED STATES design standards failed to acknowledge the risk of progressive collapse of the kind which felled both towers of the World Trade Center (WTC) until 1995, a leading Canadian forensic engineer told NCE this week.

But designers of the WTC should have been able to prevent the total collapse of both structures which were each struck by hijacked jets on 11 September.

Intense heat from burning aviation fuel is thought to have weakened the towers, triggering a catastrophic progressive collapse (NCE last week).

'Engineers in the US have never taken progressive collapse very seriously, ' said Khalid Dinno, senior forensic engineer at Walters Forensic - the largest forensic consultant in Canada.

'Now I think there will be a revival of interest in designing to avoid such a collapse.'

The British Standard structural design code CP110 was fully revised in 1972 in the light of investigations into the 1968 progressive collapse at Ronan Point in East London.

But the United States authors waited up to 25 years before upgrading the American Concrete Institute (ACI) design standard in line with CP110:1972.

In a paper on progressive collapse published earlier this year, Dinno stated: 'The first provisions to deal with progressive collapse were those in the [United States] BOCA (Building Officials & Code Administrators) 1981 Code. However, it was not until 1989 that the ACI-318 Code made a first mention of the problem and not until the issuance of the 1995 Code that provisions comparable to those of BS-CP 110: 1972 were adopted.

Dinno told NCE this week: 'While the British, quickly followed by the Europeans, picked up the new theory soon after Ronan Point, the US did not really appreciate that you could have progressive collapse. The US was intransigent to the need for change.'

He added: 'I am sure that there will have to be much greater emphasis on what happens to a structure when damaged. Local collapse, partial collapse, deformations are all acceptable but a total collapse of the type we have seen must be avoided.'

Major changes were made to design codes in the UK after four people were killed and 19 injured in 1968.

A gas explosion on the 19th floor of the 23 storey residential tower in Newham, London, triggered a domino effect collapse of precast concrete wall and floor panels.

The subsequent investigation, led by leading structural engineer Sir Alfred Pugsley, was a seminal point in structural design philosophy and led to a major change in thinking about high rise construction.

Changes in UK structural design codes made it mandatory for buildings of five or more storeys to be designed for the possibility of progressive or disproportionate collapse. The need to boost these standards in the UK has been a constant recommendation of the ICE/ IStructE Standing Committee on Structural Safety in the last decade.

But unlike the UK, Dinno said, the full recommendations of the Ronan Point collapse inquiry have yet to find their way into many states' legislation across the US. The American Institute of Steel Construction standard for steel construction still fails to mention such precautions, he said.

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