When it opened in 1961, the 335m span Tamar Bridge was the longest suspension bridge in the UK. Built in two years at a cost of £1.8M, it carried up to 4,000 vehicles a day, the heaviest of which weighed 25t. Alongside the slender curves of Brunel's nearby 'suspenarch' rail bridge the road crossing looks stubby and solid, its deep lattice truss deck suggesting massive reserves of strength.
So it came as a nasty surprise to the Tamar Bridge and Torpoint Ferry Joint Committee when in 1995 the crossing failed an assessment of its ability to carry the new generation of 40t HGVs. The results were so poor it looked as though a 17.5t weight limit would eventually have to be imposed (see box).
'Basically the truss is too stiff and is fighting the cables for a bigger share of the loads,' says Hyder Special Structures head Jolyon Gill. 'The main towers and cables are fine: it's the concrete deck and the truss itself we were worried about.'
With only one lane in each direction separated by a central tidal flow lane and traffic flows now topping 40,000 daily, traffic management was a key factor in any strengthening proposals. Hyder's solution (see box), with its addition of permanent 6m wide cantilevers each side of the existing roadway, promised some extra long term bonuses.
Cornwall County Council deputy project manager Tim Wood explains: 'The cantilevers take two lanes of traffic while the contractor is working on the existing roadway. But it would be crazy to dismantle them afterwards, especially as leaving them in position will allow us to cure the third worst accident blackspot in Cornwall.'
This is close to the Saltash shore where local Plymouth-bound traffic slips onto the A38 soon after the main road traffic emerges from the Saltash tunnel. When the project is complete local traffic will stay on the east- bound cantilever until the toll area on the Plymouth shore, where traffic is slower. Cyclists and pedestrians will use the west-bound cantilever.
Hyder's solution also satisfied another key demand, Wood adds. 'We had no funding from the Highways Agency, no EU grants, just the reserves built up from toll income. The cost of the chosen solution could be funded out of the reserves without having to put up tolls.'
With cost certainty the top priority for the client, an open book partnering agreement was the chosen procurement route. By March last year Kvaerner Cleveland Bridge had been selected from a short list of five to join the Tamar Bridge Project Team, and by September a £23.5M, 31.5 month contract had been agreed.
'We also had a major input on buildability during the final design stages,' says Kvaerner Cleveland Bridge contracts manager Alan Platt.
'For example, when the original fabricated I-beam under the bottom chord was changed to a trough section, we suggested it should be pressed at our works rather than welded up - even though the steel is up to 40mm thick.'
Platt's main concerns at the moment, however, centre around logistics. The small areas of land at each end of the bridge that have been made available to the contractor will be largely occupied by fabrication sheds for the cantilever sections. Restrictions on lane closures and the presence of suspension cables and hangars rule out conventional craneage for cantilever construction. And there are tight weight restrictions on temporary works.
'There'll be about 300t of scaffolding in all, two-thirds of it on the main span,' Platt reports. Temporary works erection was due to be completed during Eclipse week, when other operations would be restricted by a total ban on lane closures across the bridge. Normally one lane can be closed outside peak hours and, with a free bus service now shuttling pedestrians across the bridge, materials can be stored on walkways.
Currently most operations are concentrated on the approaches to the crossing. Concrete walls are being cut away above the anchorages to make space for the cantilever beams. The incongruous plate girders on the anchorage spans - which allow the original cables to pass outside the deck on their way to the anchorages - have to have a new truss constructed around and through them to carry the new cantilevers. The side towers and anchorages are being drilled to take the Macalloy bars which will anchor the new link beams (see box).
Platt says he has only one hydraulic bed transporter to service cantilever construction on both sides of the bridge.
'Construction has to be balanced, with both sides advancing from both ends at the same rate,' he points out. 'The cantilever sections are 3m wide and 16m long when they are delivered.
'We weld them together into 16m by 6m sections weighing 20t, and these are picked up by the transporter and moved to the bridge.'
At each corner of the bridge Platt plans to install a 'rolling' set of seven davits, designed to leapfrog forward as cantilever construction advances. These will pick up the cantilever sections from the transporter and transfer them to runway beams for their final positioning.
After unloading the transporter will then nip smartly across the bridge and pick up a unit from the other fabrication shed.
'We have to install the cables before we can complete the cantilevers, but we can advance as far as the main towers without the cables,' says Platt.
Bottom chord strengthening has already begun. The first cables are due to be in place by the end of November this year, some 24 months before the planned handover.