|Wrought-iron was widely used once the limitations of cast iron
became apparent. However, this made bridge construction more complicated and
costly. Wrought-iron components have to be rolled and worked into shape. This
made it impractical to produce single large pieces. As a result, bridges
comprise a large number of components which are riveted together. The joints
and rivets add to the overall weight of the structure.
When steel came into use, bridges were designed in the same way. It was only when improved welding techniques were developed during the twentieth century that metal bridges could again be assembled from a few large components.
Plate girder bridges are a particularly common design, comprising wrought-iron or steel plates riveted together to form single girders. These may be sufficiently deep to double as a parapet, but sometimes a railing or non-structural plate extension is required to give adequate parapet height.
Southdown Road Bridge, Shoreham-by-Sea, is a typical plate girder structure carrying a double track line. There are three main beams, two either side and one between the tracks. Cross beams support the bridge deck and tracks. This type of structure, where the deck is supported by two plate girders, is known as a half-though bridge.
Fetcham Road bridge, Leatherhead is a deck-type bridge, where the deck is supported on top of the plate girders. This design is much less common for UK railways than the half-through type. The main beams are directly below the tracks, so a walkway for staff is cantilevered off the top of the outer beams.
Brighton Road bridge, Smitham, is a half-through type, with the original cross beams having been replaced by welded girders. The deck has also been renewed. The contrast between the plate girder main beams, with many rivets and joining pieces, and the cross beams, which are plain apart from the weld lines, is apparent.
Bridge 821A, Margate, one of two over Fulsam Place, is a fine example of a steel, plate girder, half-through bridge, largely unchanged since construction. It is wider than the bridges at Shoreham and Smitham, so the girders are more substantial, and there are members spanning between the cross beams. There is a steel plate parapet mounted on top of the main girders.
Collingwood Road bridge, Sutton, was constructed by the Southern Railway as part of the Wimbledon to Sutton line. The steel beams support a concrete deck.
More Wrought-iron and steel plate
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This page was created 8 January 2010