Railway Structures
Truss girder bridges
| Truss girder bridges were used in locations where the span is
too great for a plate girder bridge. The depth of a solid plate beam would give
a very heavy bridge that would have to resist significant wind loading. The
structural principle of a truss bridge is the same as a plate girder one, but
the beam has separate upper and lower members.
The most common type of truss used on UK railway bridges is the Pratt truss, named after its inventor. Verticals tie the upper and lower members, with a single diagonal brace between each pair, giving an 'N' pattern, but with cross-bracing in the middle. There is often horizontal bracing between the upper members, but this is often quite light weight.
The Medway Bridge at Maidstone East is a typical Pratt truss bridge. It has a single bay in the centre of the bridge with cross bracing. The bridge is skewed, so the two sides are offset by one bay.
London Road bridge at Morden South has a 120 feet Pratt truss span. There are two bays with cross bracing. The bridge is skewed, so the sides are offset by two bays. Like most truss bridges it is of riveted steel.
There are several impressive truss bridges on the Quarry Line. The finest is probably Nutfield Lane bridge, Merstham, which has a curved 'hogback' shape to the top of the girders and a single bay with cross bracing. Large brick pilasters hide the ends of the spans. There is a considerable skew, so there are no girders linking the upper edges of the trusses.
One of the oldest types of truss is the Warren truss, also named after its inventor. There are no vertical members and the bracing forms triangles, alternately apex upwards and downwards. The design has returned to favour and has been used extensively on new railways in Germany. This Warren truss bridge carries the new northbound East London line over the main line at New Cross Gate. It comprises a number of large components of welded steel which have been bolted together on site.
The Medway bridge at Rochester has four lattice truss spans. The bracing is much lighter weight than in other types of truss, but there are many more pieces. |
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This page was created 8 January 2010
