THE NEW THINKING IS A TAILOR-MADE APPROACH BASED ON RISK
The tragedy of 9/11 has moved structural fire engineering to the top of the agenda for tall-building design. Traditional 'blanket'codes of conduct are being reconsidered and gradually replaced by new approaches that are often specific to a particular building, its location and its context.
Attitudes towards tall buildings were irrevocably changed by 9/11.
Five years on, one would expect the design of tall buildings to have undergone a sea change to incorporate better fire egress. There have been changes, some quite significant, but they have more to do with refinement and common sense than a fundamental rethinking. The changes are global in impact, and some of the new jargon is reminiscent of disaster movie Towering Inferno. Arup has established an Extreme Events Mitigation Task Force to deal with 'imminent catastrophic events'. Fire engineering is a booming discipline, and there are not enough graduates to fill demand.
The main lesson from the World Trade Center attacks relating to structural response is that thermal expansion is the most important parameter affecting stability in fire. Long-span structures are particularly critical as total thermal expansion is a function of span. This means the longer the elements the more they may expand and therefore push the columns outwards, before the columns are pulled back in again if the floor system fails due to heat. This can then lead to progressive collapse. Barbara Lane, design and technical leader of Arup Fire in the UK (which employs 50 people - up from eight people 14 years ago; globally 130 staff in 22 locations) explains that careful analysis of the World Trade Center structural response and photographic evidence from the day provide clear information about what led to the collapse and conclusions can therefore be drawn about how to design towers to minimise the risk of collapse in the future.
'It's nice and simple once you know what the problem is, ' says Lane. The answer is structural fire engineering. For Arup Fire, post-9/11 thinking falls broadly into three areas: not just structural fire engineering, but also evacuation strategies and risk assessment to determine the most appropriate response.
For structural fire engineering, fire is taken as a design parameter just like wind or earthquakes, so that a building's structure is designed to withstand severe fire. This requires a detailed understanding of fire and its temperatures, how heat is transferred from the fire to a structure, and how a structure responds to heat. The fire-proofing of old can be eliminated, resulting in a cost saving. Surprisingly, in most cases, the size of structural elements does not have to be increased. It's all about the geometry of the frame and the detailed design of the connections: strengthening the frame to avoid progressive collapse.
This can also mean a move away from the traditional approach of compartmentalising fire to limit heating of the structure, which can impact strongly on architectural f lexibility. Instead, the structure is designed to withstand heat from realistic fire sizes.
Lane notes that this new performance-based approach gives architects greater freedom. Instead of designing buildings that fit the codes, design teams are 'starting from first principles and can give clients certainty by quantifying what buildings can do'. A new Professorship of Structural Fire Engineering backed by the Ove Arup Foundation and Bovis Lend Lease is being created at the University of Edinburgh to encourage cross-fertilisation of ideas between fire-safety engineers, structural engineers and architects and to help translate this approach into mainstream practice.
In existing buildings, Barbara Lane explains that 'it's all about fisoft changesfl': the way people are managed and how information is disseminated. This can be as simple as posting information in stairs and lifts. Although this sounds easy, it depends on an accurate assessment of where the danger lies to be able to respond quickly with the appropriate evacuation strategy. Prior to 9/11, phased evacuation through stair cores was considered acceptable. Today, total evacuation with a combination of lifts and stairs is considered more effective because lifts are much quicker.
A major focus of the new thinking is a tailor-made approach based on risk which is building- and location-specific.
Lane comments that 'a blanket approach to all buildings is not appropriate, ' explaining as an example that the recently completed World Trade Center 7 would not have the same risk profile as a 15-storey office block in Edinburgh. This building-specific approach does not necessitate an industry-wide adjustment, such as changing the Building Regulations, though Lane acknowledges that this is a major talking point in the industry, driven by the recent 19-point report released by the National Institute of Standards and Technology (NIST) (the American equivalent of the BRE), which was appointed to investigate 9/11. Arup's Tony Jones notes that the UK has addressed disproportionate collapse since the report on the Ronan Point collapse in 1968, and recently with updates to the Building Regulations in 2004. UK practice will not be dramatically affected by the NIST recommendations. Their impact will be more widely felt in America where all buildings will have to conform to current best practice.
Looking to the future, the BRE, along with Arup, Vision Systems, Ansys, Abaqus, the University of Edinburgh and the London Fire and Emergency Planning Authority, has initiated Fire Grid, a three-year initiative to develop a prototype emergency response system using computer software that in the event of a fire or other emergency will be able to synthesise information from various building systems and provide firefighters with accurate information about the extent of damage in a building. The appropriate evacuation strategy can then be implemented and firefighters can prioritise their response based on up-to-theminute information about the exact nature and extent of the fire.
Although the application of Fire Grid may not happen in the immediate future, its ultimate objective is to reduce uncertainty in the face of a crisis so that tall-building managers, the occupants and those charged with their safety can rely on accurate information rather than intuition - as they were forced to do on 9/11. Recent developments in structural fire engineering, evacuation strategies and risk assessment are aiming to achieve that certainty now.