|The Chertsey Loop crosses the M25 motorway at a very acute
angle, 28 degrees, so a most unusual design for a railway bridge was adopted, a
cable stay structure In this type of bridge the deck is supported by cables
directly from one or more towers. That contrasts with a suspension bridge,
where cables are suspended between two towers and the deck is hung off them on
Suspension bridges are not suitable for railway use, principally because the deck is insufficiently rigid. This was established in the early days of railway construction, not least by Captain Samuel Brown who built a suspension bridge over the River Tees in 1830 for the Stockton & Darlington Railway's Middlesbrough branch. It rapidly proved unsuitable and had to be replaced. The best-known railway suspension bridge was that in use from 1855 to 1897 over the Niagara Gorge, but it was an exceptional design. The deck was an extremely deep truss, with a road below the railway, giving considerable rigidity. It was also stayed by cables from below as well as from the towers.
Cable stay bridges came into widespread use from the 1950s, with notable examples being used for road bridges over the River Rhein at Düsseldorf and Köln. The first large structure in Britain was the Erskine road bridge, over the River Clyde. The Lyne Bridge, as the M25 bridge at Chertsey became known, was the first cable stayed structure on a British railway line. A pair of concrete towers, one either side of the railway, are founded in the centre reservation of the motorway, so are offset from the point of view of the rail user. The concrete edge beams for the bridge are suspended from the towers, each beam being supported by four cables, in upper and lower pairs, from its tower. The edge beams support a reinforced concrete deck slab. The bridge is 120 yards long and the towers 72 feet high. It was designed by Stressed Concrete Design Ltd and built by Redpath Dorman Long. The railway was temporarily diverted while the bridge was being constructed, and realigned over it in February 1979.
While of great technical interest, Lyne Bridge is not a thing of beauty, being of unadorned concrete. The deck itself appears from the motorway as a horizontal slab and the cable housings make the towers thicker at the top than at the bottom. At the time of construction it was the only cable-stayed railway bridge in Europe, but it has been surpassed, both in size and elegance, by the 8 km Oresund bridge, between København and Malmö, opened in 2000. That carries a motorway and railway, and has a cable-stayed centre span of 490 metres supported from towers 204 metres high.
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This page was created 10 January 2010