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Thinner model

Research; Funding

Developed originally with giant offshore oil rigs and submersed tube tunnels in mind, British Steel's Bi-Steel concept could offer a solution to the pressing problem of hundreds of aging short span railway bridges - if development funds were forthcoming. Bi-Steel, essentially a steel- concrete-steel sandwich, differs from earlier, similar concepts in the use of friction-welded transverse bars to join the sheets of steel together. Once filled with concrete to create a composite, Bi-Steel is claimed to have many structural advantages, making it a very attractive option for rail infrastructure.

Bi-Steel technical manager Hugh Bowerman explains: 'A Bi-Steel shortspan arch overbridge should be significantly cheaper than the precast concrete alternative, but the real benefit is the thinner deck.'

'Most existing bridges were not designed to modern loadings, and replacing them with the conventional alternatives could mean a significant reduction in headroom - which would mean expensive track lowering.'

A possible 100mm reduction in structural depth on a typical 'half through' overbridge is even more attractive. Bridge strikes are an ever-present problem, track lifting or road lowering is very expensive. A thinner deck will not eliminate all bridge strikes, but Bi-Steel is inherently more resistant to impact than conventional steel or concrete structures, Bowerman says.

He adds that he would use the £IM to build a fully instrumented real bridge on a real rail line and carry out long term monitoring of performance. The steel panels could be assembled and filled with concrete alongside the track and the new bridge slid into position as one unit, or the steel component could be positioned and filled with concrete while trains run below.

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