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A finite benefit

Finite element analysis is still a developing technique. Are we getting too dependent on it, asks Dave Parker.

Next month a leading fire engineer will take the stage at NCE's Fire Engineering conference and publicly challenge the official analysis of the exact collapse mechanisms involved in the World Trade Center disaster (NCE 17 February).

Dr Susan Lamont's criticisms will be aimed specifically at the US National Institute of Standards & Technology's computer generated finite element analysis (FEA) of the Twin Towers, and their response to the impact of the doomed Boeings and the massive fires that followed.

She will question some of the apparent assumptions made by US structural analysts during the creation of the FEA model, and offer what she claims is a more accurate alternative.

Last month several structural engineers also expressed their reservations about the official FEA analysis of the structural stresses in the section of Terminal 2E at Paris Charles de Gaulle Airport before its sudden collapse (NCE 24 February).

They said it made no allowance for the inevitable minor misalignments and distortions built in during the construction process - which in this type of complex structure could lead to much higher stress concentrations than those indicated in the official analysis.

So is FEA all it has been claimed to be - or should engineers use it with more care than is sometimes the case?

'It's still a developing technique, especially in fire analysis, ' says Buro Happold partner John Morrison. 'The problem is that some believe if it's FEA then it must be right.

'And the biggest danger is young engineers working with commercial software packages who don't understand the simplifying assumptions made by the software - like concrete being elastic throughout its load range, or concrete slabs never cracking.' Arup software technology group associate director Chris Kaethner blames the push for higher productivity for some of the problems. 'There's less engineering input at the front end of the design process, ' he says.

'And the computers are getting so good and capable of handling so many elements that it's quite hard to spot when the results are misleading.' Agreement omes om Whitbybird director Mike Hitchens. 'Structural FEA programmes are very sophisticated now and there's not very much you can't mimic in the model - concrete creep and shrinkage, for example. But a lot of people use them for basic analysis only. The real problem is lack of engineering judgement.' But all is not bad. Most experts agree that effective management of the design process can reduce the risk of misuse of FEA and enhance its benefits. Buro Happold insists on backing up its FEA results with hand calculations, Morrison says.

Software is rapidly developing its ability to help engineers understand the results, says Kaethner. 'This is through a much greater emphasis on graphical presentation, ' he adds.

'And there will always be some expectation of what the right answer should be.' Developing the role of fire engineering. 12 April. www.nce-fireengineering. co. uk

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