SECURING THE WAY TO THE WEST

When it opened in 1961, the 335m long Tamar Bridge was the longest suspension bridge in the UK. It carried up to 4000 vehicles a day, the heaviest of which weighted 25t. Unfortunately, in 1995, the bridge failed an assessment of its ability to carry 40t lorries, and it even seemed possible that a 17.5t weight limit might eventually have to be imposed. The bridge's main towers and cables are fine, but there are problems with the concrete bridge deck and the truss. There are only three traffic lanes, of which the central lane is a tidal-flow lane, and daily traffic now exceeds 40,000 vehicles. Therefore, Cornwall County Council and Plymouth City Council commissioned Hyder Special Structures and Kvaerner Cleveland Bridge to strengthen the bridge. Hyder's solution is to add permanent 6m wide cantilevers, each weighing 2000t, on each side of the existing roadway; the only realistic alternative was to construct a completely new bridge. 4000t of bridge weight is being saved by removing the existing 150mm thick concrete deck slab, and replacing it by an orthotropic steel slab weighing only 2200t. After the bottom of the truss has been stiffened, the net gain in structural weight will be only 500t. Construction of the cantilevers must be balanced, with both sides advancing from both ends at the same rate.