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Responding with strength

Cover Story: After 9.11

The industry is attempting to deliver the more robust buildings being demanded. Dave Parker reports.

It is likely to be years before any structural design codes are significantly amended following last year's World Trade Center disaster.

More research into this type of extreme event will be needed before decisions are made. In the meantime, lacking official guidance, the profession must use its judgement in designing buildings for clients who feel at risk.

Certain trends are beginning to emerge.

'Designers really have only two options, ' says Arup Fire International director Peter Bressington. 'We can design the building to respond to specific events, such as plane impact or a truck bomb, increasing blast resistance and fire protection, or armouring the cores.

'Or we can lay down simple performance parameters: the building must be able to lose one floor without progressive collapse starting, for example, or all occupants must be able to evacuate the building within a specified time; passive fire protection must resist specified impact and flexure loads, no vehicle must be able to get closer than a certain distance (to a building) without passing a security check.'

Meeting such parameters could be easier than agreeing what they should be. There is little debate, however, over the need for more robust fire protection to structural steelwork (see box). And few would argue that sprinklers are capable of coping with major events, despite their success in Milan's Pirelli Building (NCE 25 April).

On one buzzword there is no disagreement - ductility. Increasing the ductility of the building frame, especially its connections, would give it the capacity to absorb more kinetic energy and remain standing even after massive local deformation or loss of major structural elements.

Connections that rotated rather than sheared would carry loads longer and allow alternative load paths to develop. A floor that sags is preferable to one that stays rigid until it suddenly fails, possibly triggering progressive collapse.

But Babtie Group director Dr John Roberts points out that ductility is not an inherent property of any current frame designs, steel or concrete. 'Careful detailing is essential, ' he says. 'This is particularly true where large transfer structures are involved, as on WTC7.'

'We are looking at a medium rise project in London that will sit on a transfer structure supported by just a few large columns, ' says Roberts. 'Analysis of what would happen if one of those columns was lost has so far showed no significant advantage for either steel or concrete.'

According to Steel Construction Institute director Graham Owens, most buildings of up to 50 storeys, in the UK at least, will continue to have concrete cores and steel gravity frames. 'This will still be the most economic option, ' he maintains. 'And UK design codes already produce pretty robust frames.

'But it may be necessary to modify connection design to give more capacity for energy absorption and rotation. I'm confident that we can get a lot of extra ductility just by tweaking current designs.'

Determining what levels of improvement may be needed will be the task of the steel industry's Connections Group (NCE 25 July). Owens admits current codes and guidance documents focus on static performance and can produce 'not very ductile' connections which ultimately fail at the bolts. Final recommendations are likely to take two years to appear - provided someone is willing to supply the essential research funding.

Few designers are likely to adopt a 'Pirelli solution' to the problem of providing secure escape routes after a major event.

Three widely separated cores would be hard to fit economically into a standard medium to high rise project. But, says Roberts, even with a single core, it would make sense to position the escape stairs as far apart as possible.

Fire protection focus

Twelve months on, the UK passive fire protection (pfp) industry has a measured response to most of the doubts raised about the performance of its protective coatings in extreme events. It turns to the petrochemical and offshore oil industries for inspiration and validation.

'We have materials available that would have performed much better in a World Trade Center scenario, : says Association for Specialist Fire Protection (ASFP) chief executive Graham Ellicott.

'These were developed for use in hydrocarbon environments and have to be blast resistant.

'A modern high performance cementitous based system would have more than eight times as much adhesion as the gypsum based material used on the WTC development, while an epoxy intumescent coating would be several orders higher again.'

This extra performance comes at a price, of course. Ellicott says:

'Typically, the steel frame represents about 10% of the construction cost of a high rise building. A standard pfp would come in at about 15% of frame cost, a 'hydrocarbon' pfp would be around double that.

'So specifying a much more robust pfp means overall construction costs would be up 1.5%, everything else being equal.'

However, Jeremy Hodges, managing director of the fire and risk sciences division at building research body BRE, is less convinced that such materials can simply be substituted for standard building pfps. 'The tests the industry quote are less standardised, more ad hoc than the standard furnace tests used for building products, ' he says.

'And the structures these high performance materials are designed for are very different, with much larger elements and thicker steel with different thermal properties. Above all, these structures receive much more frequent inspection and maintenance than any building ever will.'

But Arup Fire International director Peter Bressington is more sanguine. He believes that a 'WTC fire', in which an initial hydrocarbon blaze rapidly evolves into a more familiar office fire, is not properly simulated by any of the standard furnace tests.

'Theoretical modelling using Edinburgh University's ABAQUS programme suggests that a standard intumescent rated at one hour fire resistance could resist a real WTC-type combined fire for at least one hour, ' he says. 'What we need now is another series of full scale fire tests to get hard data on this type of fire.'

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