Twin Crossings

Parker, John Stephen; ,

Twin Crossings

by John Stephen Parker, Tech. Dir.; WSP Group, London, England,

Serial Information: Civil Engineering—ASCE, 2002, Vol. 72, Issue 2, Pg. 50-55

Document Type: Feature article

Abstract:

The Hungerford Bridge actually comprises twin pedestrian bridges that are being constructed on either side of an operational 19th century railway bridge over the River Thames in London. The bridge's location� overlooking Big Ben and the Houses of Parliament on the upstream side and St. Paul's Cathedral downstream�demanded a bridge of high aesthetic quality. But the constraints imposed by the railway bridge, by river traffic, and by the nearby tunnels, or tubes, of the London Underground challenged the bridge engineers. The final design comprises 25-m-high steel pylons from which high-strength ductile steel rods support concrete decks, one on each side of the railway bridge. The pylons lean outward, providing a gap for visual interest and taking the new decks about 5 m farther away from the railway tracks than, thereby lessening the noise from trains. The pylons are held at the correct inclination by backstay rods. The greatest challenge faced in the design of the substructure was dealing with ship impact forces. The substructure was designed for the force that would be expected once in 1,000 years: 30 MN at 15 degrees to the line of the river, or 8 MN laterally. In such extreme events the new substructure was allowed to deform and transfer loads to the railway bridge supports. In more common, lower-impact collisions (up to about 2 MN), the new bridge supports have been designed to carry the entire load. By December 2001 the deck on the upstream bridge had been completed, and a large floating crane had lifted the pylons and backstays over the deck so that they could be attached to the substructure. Next, the support rods will be attached, the bridge lined and leveled, temporary works removed, and deck finishes applied. At this stage the upstream bridge will be complete, and pedestrians will be able to use the first of the two beautiful new crossings that will benefit commuters and tourists alike.

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