Beam and slab bridges
A beam and slab bridge is one where a reinforced concrete deck slab sits on top of several steel I-beams, side-by-side, and acts compositely with them in bending. It is presently the most common type of medium span highway bridge being built in the UK. This form of construction is suitable for spans ranging from 13m up to 100m or more, and is economically competitive with reinforced or prestressed concrete construction. The road traffic runs on top of the slab, and the girders and slab effectively form a series of composite T-beams side-byside. The continuity of the slab across all these beams helps to spread the traffic loading between the beams.
For shorter spans (up to 25m for simple spans, up to 30m for continuous spans) the girders can be rolled I-sections (Universal Beams). Larger spans require the use of purpose-made plate girders, each fabricated from two flange plates and a web plate.
Very little fabrication is necessary with rolled section girders, usually only the fitting of stiffeners for support bearings and the attachment of bracing. Although the need to fabricate larger girders from plate necessitates more fabrication, it does give scope to vary the girder sections to suit the loads carried at different positions along the bridge and thus achieve maximum economy. A wide variety of different shapes and arrangements of plate girder bridge have developed.
For example, the designer can choose to vary the depth of the girder along its length. It is quite common to increase the girder depth over intermediate supports or to reduce it in midspan. For spans below about 50m, the choice (constant or varying depth) is often governed by aesthetics. Above 50m, varied depth also offers economy because of the weight savings possible in midspan regions. The variation in depth can be achieved either by straight haunching (tapered girders) or by curving the bottom flange upwards. The shaped web, either for a variable depth girder or for a constant depth girder with a vertical camber, is easily achieved by profile cutting during fabrication.
At the lower end of the span range for plate girder beam and slab bridges, girders are typically spaced between about 3.0 and 3.5m apart transversely and thus, for an ordinary two-lane highway bridge, four girders are provided. This suits an economic thickness of the deck slab that distributes the direct loads from the wheels by bending transversely.
When the spans exceed about 30m, an alternative arrangement with only two main girders is frequently used. Then the slab is supported on crossbeams at about 3.5m spacing; the slab spans longitudinally between crossbeams and the crossbeams span transversely between the two main girders. This form of construction is often called a ‘ladder deck’
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