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Tall buildings - Beating the heat

Fire engineering - While the final official World Trade Center report remains unpublished, Dave Parker reports on a team developing its own fire engineering solutions.

It's a mistake to assume concrete never needs fire protection, ' Arup Fire engineer Dr Susan Lamont commented only days before last weekend's blaze in Madrid's 32 storey concrete framed Torre Windsor (News last week). Her colleague, associate director Dr Barbara Lane, now says the building 'behaved well and predictably.' She adds: 'It maintained its overall stability through 17 hours of intense fire. What is of real concern is the apparent lack of fire stopping.' Most analysis is still directed towards steel frames, of course.

Steel's perceived handicap as compared to concrete has always been the need for additional fireproofi ng of some sort, which costs money and can delay construction.

Fire engineering is often seen as a way of cutting down or eliminating fire protection.

Lamont says this is a misconception.

'It's not about leaving off fire protection; it's all about understanding how the whole building works. Fire engineers deal with temperature in the same way structural engineers deal with snow and wind.' A recent example of this approach is Plantation Place South, a typical 21st century hybrid eight-storey London office building featuring a concrete core, composite floors and structural steel frame. There was a time when all exposed steelwork would have been protected in some way as a matter of course - but fire engineering came up with a better solution.

A detailed finite element analysis (FEA) of the most complex asymmetrical sections of the structure showed that after a standard Eurocode parametric fire the structural deflections of protected and unprotected beams were not dramatically different, at maximums of 380mm and 450mm.

Arup then developed the concept of 'protected squares' - lines of beams with fire protection surrounding areas where exposed beams were left virtually naked.

'We ran the simulation with two extreme fires - one with all the windows broken, one with almost all intact, Lane reports.

'Temperatures peaked at 1200ºC.

'Deflections were not significantly greater than the fully protected option, which kept the insurers happy because repair costs after any fire would be no higher.' Modifications to core reinforcement also minimised potential fire damage. After an independent third party review of Arup's proposals the City of London gave its approval.

'The City is very committed to innovation, ' Lamont comments.

'Other authorities are also increasingly willing to accept FEA modelling of fire performance.'

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