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However safe your design, it will never be safe enough.

How safe is safe enough? And who decides? Last week a London seminar on 'Safety criteria for buildings and bridges' considered these crucial questions and others, and warned there were no easy answers. Speaker after speaker pointed out the inherent paradox at the heart of all deliberations on structural safety - the public perception of risk. High levels of voluntary risk, such as those attached to contact sports, car driving or cigarette smoking, are widely accepted. Involuntary risk is a different matter.

However low the risk of a bridge failure or a building collapse, it seems it is still too high. But to reduce the risk of collapse to what the public might accept as tolerable would cost a lot of money - resources which might be better spent on kidney dialysis machines or inner city estates.

Highway bridges are a case in point. When the Highways Agency began assessing bridges for their ability to carry the new generation of 40t HGVs, it received a very rude shock. The simple assessment procedures used at first were apparently showing that not only were many highway structures incapable of carrying the new loads, they were nowhere near strong enough to carry current loads.

The potential cost of upgrading all these 'sub-standard' structures was unacceptable. In response, the Agency adopted a two-pronged strategy.

Assessment procedures were improved and refined and for the first time the Agency adopted a strongly risk based approach. It identified those structures where failure, or even the prospect of failure, would cause massive consequential costs in loss of life, disruption, increased accident rates among diverted traffic etc. It concentrated its resources on these.

Even where the risk of failure was almost impossible to calculate with any degree of accuracy, action was still taken if the societal consequences of failure were thought to be high enough. One of these consequences, if a major highway structure collapsed catastrophically, might be an irresistible public demand for all similar structures to be strengthened or demolished.

Something similar happened following the (non-fatal) collapse of the Piper's Row multistorey carpark in (NCE 27 March/3 April 1997). In this particular case the calls were for mandatory regular structural inspections - something that may still happen. The Health & Safety Executive is now widening its use of the societal consequences concept from the nuclear energy sector into all aspects of workplace safety - including structures. Its discussion document Reducing risk, protecting people introduces the concept of societal risk, stating in effect that even if the calculated risk to the individual is low the public is unlikely to countenance the siting of a nuclear power station or an explosives factory in the middle of a densely populated area.

So who should decide on safety - the public, who will react 'illogically', the designer, who will have little control over how the structure is treated after construction, or the politicians, who are paid to take such responsibility? According to one very senior structural engineer at the seminar: 'Politicians expect engineers to ensure mistakes don't happen - and if they do, it will always be the engineers' fault.'

So it is back to the engineers. Those who feel strongly on the subject should get a copy of the HSE's discussion document - R2P2 as it is known. This is available free from HSE Books on (01787) 881165.

davep@construct.emap.co.uk

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