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Fire engineering hots up

STEEL - Large-scale fire tests carried out in the mid-1990s have helped push UK designers to the forefront of fire-safety engineering, says Nick Barrett

JUST when you thought building designs incorporating fire engineering were at the leading edge of safety, along comes a new approach to knock you back to the drawing board.

The new approach, structural fire engineering, uses finite element analysis-based software to help develop safer buildings subjected to fire loading.

The input of engineers with an appreciation of how structural members behave and how whole frames interact in fire is crucial though, and the UK is leading the world in developing the necessary skills.

Dr Barbara Lane, an associate at Arup Fire, is one of a small but fastgrowing group of designers with the knowledge to apply whole-frame analysis. She claims the techniques have applications for any structure subject to fire risk, including commercial buildings, tunnels and high-rise construction.

'We do not compromise safety by this approach but we can save overdesign.That means there is more money available for other safety measures, perhaps unrelated to fire risk, ' she says.

The new approach is based on a fundamental rethink of what is required for protecting a structure from fire. It has been used in a City of London scheme designed by Arup Associates, a commercial development called Plantation Place South.

The basic idea is to analyse how the structure behaves in terms of the structural element joints and continuity that it possesses.This analysis is related to how it will perform in a real fire and used to place protection only where it is needed.This contrasts with the traditional approach of protecting everything and not thinking about real structural behaviour.

The investigation into a major fire at Broadgate in 1990 and the results of large-scale fire testing carried out by Corus and research body BRE at its Cardington centre in Bedfordshire between 1994 and 1997 helped put the UK on the fast track for this type of advanced analysis.

Much was learned from the fire at Broadgate, which broke out during construction.According to accepted wisdom at the time, the building should have collapsed like a house of cards. But the structure stood up far better than expected, despite a lack of protection on much of the steelwork, and less that 5 per cent of the restoration cost was structural repair. Large deflections of beams were seen, but no collapse.

The Cardington fire tests were inspired by the enhanced structural capability in fire demonstrated at Broadgate and the results of these tests have validated the new approach.

As a result of the Cardington studies it is known that steel-frame buildings with composite metal deck floors survive longer in a real fire than standard fire tests suggest. Columns are recognised as critical stabilising elements in the new approach and must be given protection where full floor fires are a concern.

But the tests also revealed that not all beams need to be protected; it can generally be omitted from secondary beams.

Composite metal deck structures gain particularly from this approach because the floor continuity considerably boosts the load-carrying capacity in fire.

The work carried out at Plantation Place South on Mincing Lane in the City of London is the most sophisticated example yet of the use of wholeframe analysis and is attracting a considerable level of interest.

'The traditional approach is prescriptive and based on tests on single unrestrained structural elements, ' explains Dr Lane.'But this does not capture the real structural response to fire.Our approach has been to use structural fire engineering on a case-by-case approach, based on analysis of each structure.

'We can use our understanding of the structural behaviour of buildings in fires and can predict, using structural analysis, how they will react.As we are able to do a mechanical analysis of the structure when it has been heated we know a structure's weaknesses and strong points. So we strengthen the weak points to make them better able to withstand a fire.

Mincing Lane is one of the first times that this has been done.'

As a direct result of the innovative analysis, 50 per cent of the beams on every floor, except the ground floor and top plant level, were left unprotected.

It has taken considerable investment from organisations such as Arup Fire to understand and validate an approach to structural fire-resistance using whole-frame behaviour.

Arup Fire organised seminars for local authority building control officers, the fire brigade and insurers, to help them appreciate what was being proposed for Mincing Lane.All were won over once the approach had been adapted to include their feedback.

The company is now in the process of developing marketing programmes to help spread the knowledge gained on this project.

'Plantation Place showed the robustness of the techniques very well, ' says Dr Lane.

'We assumed a really severe fire, with total failure of the sprinkler system, resulting in a whole floor fire and temperatures of up to 1,200 deg C.This gives comfort to people who have concerns that the approach is still new.'

Mincing Lane is not a high-rise building and is not regarded in the fire engineering world as a high-risk structure, but Dr Lane insists that the principles of structural fire engineering developed on the scheme are particularly applicable to high-risk situations, including major developments with high public safety risks.

'All building types are suited to this approach and we are already analysing other structural forms this way, including tunnels and bridges, ' she says.

Dr Lane admits structural fire engineering is still in its infancy compared with the traditional approach, but claims developers and designers alike are waking up to the potential of this more efficiently engineered approach.

'The structural fire-engineering group at Arup will soon be about the same size as the fire-engineering department, as demand is growing so fast.At the moment it is a sophisticated niche area but it will be mainstream structural engineering within the next 10 years, ' she says.