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BRIDGES; A new Thames foot crossing will depend on the existing bridge.

Construction of identical twin footbridges across the River Thames flanking London's Hungerford railway bridge will get under way next month.

The designers of the £26M project, architect Lifschutz Davidson and engineer WSP, have created a close relationship between the existing bridge and the new cable stayed structures. The footbridges are five spans wide with a prestressed reinforced concrete deck suspended from pylons rising from footings built around the cast iron caissons of the railway bridge. Four Macalloy backstays supporting the pylons will be fixed via cylindrical anchor points to the tops of the caissons.

Main contractor is a joint venture of Costain and Norwest Holst. First task will be dredging around Hungerford's caissons and around Surrey pier. This, one of the relics of Brunel's original suspension bridge, is incorporated into the existing structure.

Bored concrete piles will be installed in the river bed. As Hungerford bridge looms over the site, the contractor aims during this phase to manoeuvre caissons at low level and under water rather than having to lift them over the cofferdam walls. Sheet-piled cofferdams will then be installed and, after dewatering, pile caps constructed to carry the pylons. Either side of Surrey pier a 10m by 15m island will be constructed to carry a pair of pylons.

Once the pile caps are complete the cofferdams will be removed and precast concrete elements craned in under the railway bridge to create a wall on either side of each set of caissons, linking the pile-caps on the upstream and downstream sides.

The void between each pair of walls will be filled with concrete to create monolithic structures providing protection against ship impact. The deck elements will emerge from a casting yard adjacent to the south bank. The deck for the upstream bridge will be cast first, with elements incrementally launched and supported on temporary concrete trestles. The elements will have ribs on their underside corresponding to the position of suspending cables. Each of the two 315m decks will weigh 1,800t.

Once the deck is in place, 25m tall tubular steel pylons will be erected. At mid-point the pylons will have a diameter of 813mm, tapering to 450mm diameter at either end. A pin mounting will articulate the pylons with their concrete foundations.

Separating pedestrians from the deafening rumbling and squealing of trains crossing the railway bridge by 7m, the pylons will lean out over the water, restrained by four Macalloy rod back stays. Two back stays will attach to each of the cylindrical anchor points, mounted either side of the cast iron bridge caisson. Stability will be increased by rigid members stemming from the same anchor points to the deck and the base of the pylon.

The deck will be supported by 28 cables fanning out from each pylon - 14 either side. These will form 'a feathery fan that will sparkle in the moonlight,' says WSP project director John Parker. Cables will be spaced at 1.25m intervals towards the outside of the fan and 6m at centre.

When the upstream bridge is completed in November 2000 the trestles will be transferred to the downstream side to allow launch of the second bridge deck.

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