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Steel on the ropes after World Trade Center report

News special

STEEL FRAMED construction could lose out to concrete in the major building market, following the publication last month of the first official report into the World Trade Center disaster, experts have told NCEI.

Produced by the American Society of Civil Engineers (ASCE) for the Federal Emergency Management Agency (FEMA), the report analyses the events of 11 September.

Analysis includes the mysterious collapse of the 47 storey WTC7 building and the collateral damage to several other buildings, located close to the twin towers.

The report's conclusions and recommendations are highly qualified, but include the need for new design tools which can predict whole building response to fire much more accurately.

It also called for enhanced performance standards for fireproofing materials.

All of this will be bad news for the structural steel lobby, according to tall buildings expert Dr John Knapton.

'Any major building designer who puts forward a steel frame solution will have a hard time justifying it until all the research is done and new codes produced, ' he said. Knapton is advising the insurers of the twin towers following the 11 September attacks.

'It will be much simpler to opt for a classic reinforced concrete design - which is fine up to 50 floors at least.'

According to Knapton, the key recommendation of the ASCE/FEMA report was the need for much greater integration of fire engineering and structural design.

'Fire engineers need to learn the same lessons about redundancy that structural engineers learned the hard way, ' he added.

Knapton's opinions on the future preferences of structural engineers were supported by Connell Mott MacDonald divisional director James Bennett.

'Everyone must have been having second thoughts since 11 September, ' he said.

'Currently, we are looking at several air rights projects, where steel may seem the obvious choice as it is usually much more convenient than concrete when building over live railway lines.

'But now we're considering concrete, simply because it doesn't need added fire protection.'

This may seem unfair to steel, given that the report emphasises that most of the steel framed buildings involved in the WTC catastrophe performed relatively well in extraordinary circumstances.

Even the still unexplained collapse of WTC7 occurred after its fires had raged unchecked for almost seven hours.

But with so many questionmarks still hanging over such issues as connection design and measures to resist progressive collapse, engineers could hardly be blamed for choosing a less controversial option, Knapton concluded.

Recent steel industry campaigns in the US and Britain to cut the amount of fire protection for structural steel are now 'dead in the water', said construction research body BRE's fire and risk sciences division managing director Jeremy Hodges.

'The campaigns were based on trade-offs between sprinklers and passive protection, and the WTC disaster showed the flaws in the concept, ' he added.

Also in doubt is the future of special structural systems, developed to make steel more attractive in the commercial office market.

The heart of such systems is a complex beam which derives much of its fire protection from the precast concrete floor units it supports. The lower face of the bottom flange is exposed and unprotected. But spraying this area with the current generation of fire protection coatings may not provide a solution.

'Current test methods are static rather than dynamic, ' said Hodges. 'The WTC report shows just how vulnerable some sprayon coatings are to dynamic impact - but developing a realistic and representative impact test is going to take five years of research, then perhaps another five years to get it into the standards.'


For more on the WTC and for links to the ASCE/FEMA report go to www. nceplus. co. uk/magazine

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