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A telling experience

News analysis: 11 September lessons

An Institution of Structural Engineers report into lessons to be learned from the World Trade Center attacks is brimful of recommendations.

Diarmaid Fleming reports on what could be a blueprint for dealing with disasters in tall buildings.

A SIGNIFICANT new study is needed into progressive structural collapse, fire protection, escape from buildings and emergency management, says a new report by the Institution of Structural Engineers (IStructE).

The report, Safety in tall buildings and other buildings with large occupancy, published yesterday, is the outcome of investigations by an international working group of eminent engineers, architects and other building professionals. The group was convened last October to assess the engineering implications posed by the risk of further attacks after the World Trade Center (WTC) attacks last September.

Rather than retrace the steps of the American Society of Civil Engineers (ASCE) team report into the WTC collapses last May (NCE 9 May), this study, which could serve as an outline best practice guide to designers of tall structures, helps move the debate on.

While criticism was made in some quarters that the ASCE report did not go far enough beyond a forensic analysis of the modes of collapse of the WTC buildings, the IStructE report differs significantly. It is packed with recommendations.

The IStrucE working party included Dr Gene Corley, who headed the ASCE investigation team, while another eminent member was Ysrael Seinuk of New York consultant Cantor Seinuk. The draft of the IStructE report, according to the chairman of the working party, was produced before the ASCE report was published, highlighting its independent view.

'Our aim was to develop some initial guidance as soon as possible, drawing on the lessons of the tragedies in New York, ' says Babtie Group chairman John Roberts, who chaired the working party.

The report focuses on marrying engineering, building management and emergency response. The separation of these elements in New York is believed to have raised the cost in lives at the WTC, primarily in the emergency services. A sharing of engineering knowledge, for example regarding the 'active' fire protection systems in the Twin Towers - which were made largely redundant after the attacks - could have led to a decision not to attempt to reach the upper storeys, probably saving the lives of scores of fire fighters.

The number of New York Fire Department deaths led to an immediate realisation of the importance of communication by both engineers and fire fighting experts. This should be repeated here, says Roberts.

'It became obvious to us at an early stage that the key issues in terms of protecting buildings and occupants from 'extreme events' like the WTC attacks are all interlinked. But building designers in the past have not had enough information about how fire response teams fight fires, while fire fighters have had insufficient training in engineering. That is why now you have firefighters in New York being taught structural engineering - telling them how buildings are put together so they have a greater understanding of what can happen, ' says Roberts.

The report makes some startling observations about the role of emergency rescue teams. 'For some major emergencies, there are some circumstances where no benefit would be gained by allowing emergency services personnel to enter the building - for example if dedicated fire fighting lifts are inoperable.

Such considerations need to be taken into account in emergency response plans, ' it notes.

Among 24 areas identified where extra study is needed, the report highlights vulnerability to progressive collapse, escape management, and fire protection. Specifics include the development of 'analytical tools to support performance based engineering design of buildings for extreme events' and analysing the behaviour of whole buildings in real fires using fire modelling. It calls for the development of detection systems to monitor building performance before and during an emergency, and communication strategies between emergency managers, occupants and the emergency services.

Roberts says that the UK is in a reasonably good position because lessons learned about progressive failure after the collapse of the Ronan Point tower block in London in 1968 have been incorporated into design practice. Other lessons have been learned from terrorist attacks in the UK.

'The UK is in a good position because there has been explicit guidance since the early 1970s into progressive collapse. The requirements are quite simple however - to produce reasonably robust buildings. But they require you just to tie buildings together, not to design ductile buildings, and it is ductility which gives you the ability and energy to withstand damage without total collapse, ' he says.

'Quite a lot of work has been done, but it is not in an easy form for designers to use. More work needs to be done to disseminate this knowledge more easily.'

INFOPLUS The report Safety in tall buildings and other buildings with large occupancy is available from the Institution of Structural Engineers £30.

www. istructe. org. uk

Recommendations

Further incidents of 'extreme damage caused by malicious acts can be expected in the future', says the report, meaning that protection must be provided against them. A range should be considered, varying from bomb attacks to attempts to contaminate air conditioning systems.

A structure's vulnerability to structural collapse must be considered, taking account of redundancy, available alternative load paths, strength, ductility and energy absorption capacity of the building (that is, of the structural elements and their connections), and the retention of structural integrity in fire.

The 'trigger threshold' above which a building could fail must be raised after considering a structure's vulnerability. Specific consideration must be given to elements fundamental to the survival of the structure.

Provide robust, resilient and durable passive fire protection.

Treat active systems such as sprinklers as additional and not as a substitute for passive protection. They should not be considered in extreme events.

Provide gas and smoke-tight compartment protection in buildings.

Design buildings to survive complete 'burn out' of contents.

lndependent regular audits of fire protection systems and improvements should be carried out.

Provide smoke protection to escape routes which should be robust, adequately sized and in diverse locations with additional protection for final exit routes.

Plans should exist for largescale as well as localised evacuation, including use of lifts and stairwells. Vulnerable building services facilities should have back-up power and redundancy.

Strategies should be developed and tested for extreme event emergencies which include communication with building occupants and the emergency services.

Building plans should be available in a remote, accessible location.

Engineering advice should be available quickly.

Communications systems with floors, lifts and stairwells should be functioning. Full emergency management training should be provided and regularly practised.

Cladding design including glazing should take account of potential explosions.

Security measures to prevent unauthorised entry by potential attackers and physical barriers to prevent vehicle bombs near a building should be in place.

Design and construction of all safety features should be subject to third party checks to eliminate errors or building defects that could compromise safety.

Consideration should be given to adopting a licensing system for tall buildings similar to that in place for stadia, incorporating the above recommendations.

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