Contractors on the soon to be completed Millennium Footbridge across the Thames are engaged in a complex balancing act. They have just begun positioning deck sections for the unusual crossing, which is something between a suspension bridge and a permanent high wire balancing act.
When complete each end of the bridge's deck will hang from suspension cables. But the mid section will rest on struts, which will stand on the cables as they descend below deck level.
New Year saw the project make its first physical connection between the north and south banks of the Thames. In February a single cable was pulled across the Thames over temporary A-frames mounted on the bridge piers.
This was followed by others, fitted with pulleys to haul first suspension cables and then deck sections into position.
The system was developed for the project by contractors Monberg & Thorsen and Sir Robert McAlpine.
The cable system meant barges transferring deck sections to the site would not need to be fitted with cranes. Crane mounted barges would havehad to be bigger, and lifting would have been hampered by tidal conditions and other river traffic.
The new system has definitely proved its worth during the installation of the eight permanent suspension cables, explains Roger Ridsdill-Smith, project manager for structural engineer Ove Arup & Partners.
'Each cable was attached to the base of the line from the bank. Then the cable was pulled over via a motorised pulley, rather like a curtain rail, and lowered into place,' he says.
Eight cables were laid across the piers in this way - four supported on each forked arm.
The 120mm diameter cables were manufactured by Bridon off site, built up from hundreds of wires with a 'Z' cross-section.
'The wires give a greater stiffness than circular cross-section wires, as they fit snugly together - also reducing the overall cable diameter,' continues Ridsdill-Smith.
Bridon pre-stretched the cables off site to remove any elasticity. This saved time waiting for the material to 'give' following pre-tensioning to 2,000t against the horizontal concrete beams on either bank. The beams transfer the load into vertical shear walls, then down to concrete piles.
The high tension forces will hold the deck flat and keep it rigid enough to withstand wind gusts of 45m/second - a one in 100,000 year event. This should make a Tacoma Narrows-type situation unlikely, explains Ridsdill-Smith.
'There's no additional horizontal bracing on the bridge - we designed it so the tensioned cables provide it all against wind force,' he says.
Pre-tensioning will only go some way towards creating the final shape of the structure. The design relies on the dead weight of the decking to pull the structure into its final shape.
The 16m long, 20t deck sections are made up of an aluminium deck, supported on horizontal, bent-beam 'legs' that attach to the cables with clamps. Leg angles vary along the bridge to follow the curve of the cables, while keeping the deck section level. This means they rest on the cables at their lowest point, in the middle of the river, but hang from them as they rise to the top of the piers.
'It's not really a suspension bridge, but nor is it a ribbon bridge,' says Ridsdill- Smith. 'It's a new concept altogether.'
The first aluminium deck section was placed at the centre of the main span at the beginning of last week, using the barge and highline system.
The deck assembly sequence is designed to balance loads on the bridge piers, with elements built on each end of the mid-section.
Workmen controlled progress from a temporary gantry spanning the piers. From here they pulled the cables to meet the clamps attached to the deck legs.
Deck sections running through the 'y' shaped piers will be the last to be positioned. When asked whether he is sure they will fit, Ridsdill Smith smiles.
'The cables have markings to guide us, but we have designed tolerances in the cables between each deck that can easily be taken up,' he says.
As the deck is built up, its weight and its interaction with the cables will pull the bridge into shape.
When complete the cables will be 2.5m below the top of the piers at the middle of the 144m central span structure. Then the finishing touches can be applied - such as the addition of handrails, lighting, and the grouting of the piers.
All bridge piers were completed in 1999, except the grouting up of the joint directly between the steel pier heads and the concrete base below. This was left to allow some rotational movement during deck placing.
'This means that, once grouted, the pier piles will be in equilibrium, and only carrying live loads,' explains Ridsdill- Smith.
The bridge may not be fully complete for the Queen's visit in May, but it is expected to open by mid-June when volunteers cross it on a sponsored walk in aid of Save The Children.
More information on: www.arup. com/millenniumbridge
Details of the Save the Children walk from Amy Schick on tel: (020) 7252 4141.