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The structural route

The US National Institute for Science & Technology's (NIST) overdue fi nal report into the World Trade Center (WTC) disaster has been anticipated for its potential impact on building codes around the world.

Whether that impact will be entirely positive remains to be seen.

Although the UK is now seen as the world leader in structural fire engineering, thanks largely to full scale testing carried out by steel supplier Corus and research organisation BRE at Cardington, Bedfordshire, its potential contribution to the WTC analysis has been largely ignored, Arup Fire believes.

'It was surprising that NIST did not take advantage of UK expertise, ' says Arup associate director Dr Barbara Lane. 'Several UK parties, including BRE, UMIST, University of Edinburgh, Buro Happold and the University of Sheffi eld, as well as ourselves, have been researching real structural response to fire for over 10 years.

'As part of Arup Fire's research work with the University of Edinburgh, we've been doing our own analysis of high-rise building response to real fires and we are finding very interesting structural responses. These so far show very little relationship between time to collapse and quantity of fire proofing, but restrained thermal expansion of structural elements has a major effect.' Arup is concerned by NIST's theory that the Twin Towers' collapse was linked to fire protection being lost due to the impact of the planes. Lane goes on: 'We're afraid that there might be a knee-jerk response to simply increase the thickness and robustness of fire protection to the steelwork.

'This is worrying because we have known in the UK for 10 years or more that in reality fireproofi ng doesn't work the way furnace tests on small specimens indicate. There are very specific structural responses to heat and it is possible to design out weaknesses in the structural frame to enhance its response to fire. This is a more robust approach to improving structural fire response.' In a blaze on the scale of the fires that ravaged the Twin Towers, where many floors were ignited at virtually the same time, Arup believes the fire protection would only slow down the rate at which the protected steel gained temperature. The whole frame in the affected areas would be subject to thermo-mechanical forces as expansion of the structural elements occurred. Arup Fire's finite element analysis (FEA) of building response to fire actually calculates these forces: Arup believes the NIST FEA model does not, based on their latest draft report, and as a result is concerned about NIST's reported collapse mechanisms and the critical variables creating these mechanisms.

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