The skyscraper of the future

John Hill, the president of the Institution of Structural Engineers, assembled the group after the World Trade Centre collapsed. Its 16 members are made up of experts from the UK, the USA, Australia, New Zealand and Hong Kong. Among them is Gene Corley from the American Society of Civil Engineers, the structural engineer charged with investigating the collapse of the twin towers.

In the chair was John Roberts, director of engineering at the Babtie Group and an expert on the assessment of risk – he specialises in the design of very large roller-coasters. His appointment is an indication of what the group has set out to decide: the acceptable levels of risk that occupants of tall buildings should be exposed to. Once that has been done, experts in specific fields will work out what design changes need to be put in place to meet the targets. These will be published as a series of design change recommendations. Then it will be the turn of governments, who will decide which to enforce through amendments to building regulations.

Apart from improving the safety of those working in tall buildings, it is hoped that the public's confidence in them will be restored.

And with scores of tall buildings either planned or under construction around the world, developers are desperate to incorporate the latest thinking into their proposals – as long as the new regulations do not eat away so much of the floorplate that the building becomes economically unviable.

At that first meeting, a brainstorming session identified three main issues: how to prevent the failure of one part of a building causing the entire structure to collapse, how to slow the spread of a fire, and how to evacuate people within a given time.

"We're not intending to investigate the collapse of the World Trade Centre towers, but to look at the questions their collapse raises for designers," says Roberts.

Progressive collapse
Of the three issues, guidance on the limitation of progressive collapse is perhaps the most urgent. This is because Part A of the Building Regulations, the section that deals with structure, was under review before 11 September, which means that this part of the group's recommendations could come into force quickly. Paul Everall, head of the Building Regulations at the DTLR, was present as an observer at the inaugural meeting.

Progressive collapse is a type of structural failure that occurs, for example, when the supports holding up a building's floor give way. Their collapse causes that floor to crash onto the one below, which in turn collapses, leading to a catastrophic failure of the entire structure.

UK regulations make no difference between a building that is five storeys and one that is 50 storeys

John Roberts, director of engineering, Babtie

One aspect in particular of the regulations that will be examined is whether the rules intended to prevent progressive collapse should be increased depending on the height of a building. "In the UK, there is no differentiation in the regulations for a building that is five storeys and one that is 50 storeys," says Roberts.

A solution proposed by some experts is for tall buildings to have "collapse floors". These would be installed every 10 storeys or so and would be constructed so as to be capable of carrying the weight of the debris from the floors above. Other ideas have include strengthening connections between structural members, or fitting load sensors to warn fire-fighters of an impending collapse. To help reach a conclusion, the group will compare best practice from around the world.

Means of escape
For the fire specialists, the discussion was on whether the fire resistance of a particular structure should be linked to the escape time from it. "At the moment, in the UK, there is no link between the fire resistance of a structure and the time taken for occupants to escape," says Roberts. "To license a football ground for 80,000 spectators, people have to be able to reach a place of safety in eight minutes. There is no such specific requirement in the UK for escape times from tall buildings."

Sophisticated computer modelling programs are being developed to help designers predict escape times. One suggestion is to increase the width of staircases, or even double the number of escape stairs. An even more unusual option being considered is the installation of spiral escape chutes to allow people to exit helter-skelter.

A major factor relating to the time taken for people to escape from a tall building is whether or not lifts should be used. "Current arrangements presuppose people should not use lifts for evacuation," says Roberts. "We're looking at the possibility of changing that arrangement." This is not a new concept: consultant Arup was already investigating the possibility of installing lifts in fireproof shafts to speed evacuation.

Fire resistance
The group will also make recommendations as to whether fire resistance should be related to the height of a building. At the moment, the regulations make no distinction between buildings 30 m high – the maximum height that a fireman's ladder can reach – and those that are hundreds of metres high. "There is no further fire requirement between a 30 m high building, which is quite low, and one that is over 200 m high, in the UK," explains Roberts.

Fundamental to any discussion on fire is the type of blaze a building should be designed to resist. Currently, buildings are designed to withstand fires, the intensity of which is based on the assumption that the fire-load is related to the building's contents. However, the fire at the World Trade Centre was hydrocarbon-based, caused by the combustion of thousands of litres of kerosine. The consequence was a much more intense fire than the building had been designed to contain. "We'll be looking at the advisability of that assumption," says Roberts.

The onus is now on individual specialists within the group to investigate further the issues raised at the meeting. Time is of the essence.