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Last Updated: Friday, 3 August 2007, 13:55 GMT 14:55 UK
How safe are our bridges?
The Magazine answers...

Forth road bridge
Wires inside the Forth and Severn's cables are snapping
The Highways Agency has ordered a review of its road bridges in England, following the collapse of a bridge in Minnesota. Are any in the UK in danger of falling down?

It is highly unusual for a bridge to collapse in the manner of Minnesota, where at least four people were killed in scenes resembling a disaster movie.

But the Highways Agency in England is taking no chances, checking whether the 17,000 road bridges on its motorways and A-roads have any similar characteristics.

Its computers so far indicate no identical steel girder structures in Britain. But unlike in the US, here there are bridges that are hundreds of years old. Are these structures safe?

"The inspection regime is rigorous and undertaken by very professional people," says Tom Foulkes, director-general of the Institution of Civil Engineers.

"It's well-structured, it was updated in the 1970s and the Highways Agency and National Rail and the local authorities are all very conscious of their responsibilities."

The UK's inspection regime is very rigorous and the last major accident was in 1970

He has every faith in the UK's safety regimes but says he had heard about fears over US procedures within the industry there.

In the UK a visual inspection is carried out by engineers, who then run their findings on computer models.

"In any metal structure, cracks are a fact of life but most cracks don't matter," says Mr Foulkes. "What matters is knowing where the cracks are and then monitoring them."

There is a lot of calculation involved - analysis of the cracks and loads, and an assessment of whether the inspection regime properly reflects the age, condition, volume of traffic and modification of the bridge in question.

Question mark
A regular feature in the BBC News Magazine - aiming to answer some of the questions behind the headlines
The critical message to take from events in Minnesota is that we in the UK must look after our old infrastructure properly, says Scott Steedman, a vice-president of the Royal Academy of Engineering.

"I would agree that we should be particularly concerned about our more elderly structures. One of the reasons is the design methods were different and the methodology of the iron isn't nearly as reliable as it would be today."

Cables 'pinging'

Today engineers aim to design "redundancy" into all their structures, to prevent sudden collapse in the event of any single part of it being overloaded, he says. This was a lesson learnt from the Blitz, when bomb damage to the corner of a building could bring down the whole structure.

Clifton Suspension Bridge
Clifton: Triumph of Victorian age
Two famous suspension bridges are undergoing expensive repairs after faults were detected during inspections.

The Forth Road Bridge is having an 8m dehumidification to dry out the cables, after engineers recorded the sound of wires snapping inside seven layers of waterproofing. And the Severn Bridge is getting a similar 20m "blow-dry".

The last major bridge disaster in the UK was the collapse of Pembrokeshire's Cleddau Bridge during construction in 1970, killing four people.

Today the key threats to road bridges come from lorries and salt, says Antony Oliver, editor of New Civil Engineer Plus magazine.

"One of the most destructive things for bridges is road de-icing salts. When you put grit on roads it contains salt and salt mixed with steel corrodes it."

One of the most destructive things for bridges is road de-icing salts
Antony Oliver
New Civil Engineer Plus
The salt seeps through into the structure where there are gaps in the road to allow the bridge to move.

He says the EU introduction of heavier trucks prompted a major inspection about 10 years ago, and concrete was put around motorway piers in the late 90s to prevent the whole structure coming down in the event of a crash.

Overall the UK is very strict about safety, he says, but there is a backlog of work to do on smaller road bridges because councils are finding it hard to come up with the cash.

Mr Oliver believes Victorian bridges are not necessarily the ones most at risk because some were "over-designed". The Clifton Suspension Bridge, for example, has six parallel chains which could each be taken away without causing the bridge to collapse.

It was years later that engineers built lighter and lighter bridges, before safety tightened up again in the 60s.

Rescuers and survivors of Minnesota collapse

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