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Better than new

BRIDGES

'This is first and foremost a strengthening project. The widening is a bonus.' So says Hyder project manager Simon James, adding that with no alternative route for diverted traffic within reach the only realistic alternative to the cantilevers was to construct a completely new bridge.

Total weight of the cantilevers is 2,000t. Against this can be set the 4,000t saved by removing the existing 150mm thick concrete deck slab and replacing it with an orthotropic steel alternative weighing only 2,200t.

But by the time the bottom chord of the truss has been stiffened with a pressed steel 600mm by 400mm trough section attached at 9m centres and the ends of the diagonal stiffeners beefed up, the net gain in structural weight will be just over 500t.

To take this and the additional traffic loads Hyder decided to add extra cables from the towers to the deck. At first glance these appear to be straightforward cable stays, in pairs, running in both directions from the top of the towers in virtually the same plane as the existing suspension cables. This is only partially true, Gill explains.

'The inner pairs are indeed conventional cable stays, with the horizontal forces being taken by the deck in compression.

'But at the Saltash tower there's an expansion joint, which would have closed up solid if the outer pair of cables worked the same way.'

Moving the joint to a more convenient position could have been very expensive, he adds, as the deck truss would need extensive modification.

'So we decided to stick with the existing articulation and find a way of reducing horizontal forces in the deck.'

A closer look therefore reveals that the outer backspan cables, rather than terminating at deck level, in fact carry on through the deck down to new anchorages low on the existing side towers.

New 850m by 800mm insitu concrete link beams take the horizontal forces from the cables from the side towers back to the original suspension cable anchorages.

A further refinement is the use of a horizontal cable to prestress the bottom chord of the truss over centre span. Taken together, Gill says, the effects of this particular cable layout are enough to allow the expansion joint to function as designed.

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