The shape of reinforced concrete bridges is usually decided by experience aided by typical span-to-depth ratios. The design calculations are only really used to design the reinforcement. A typical simply supported slab has a span-to-depth ratio of around 10-15 but continuous or integral bridges can be shallower. Because the concentrated live load (i.e. the wheel load) the deck has to carry does not reduce with span, the span-to-depth ratio of short span slabs tends to be towards the lower end of the range. However, deck slabs of bigger bridges often have greater span-to-depth ratios than slab bridges. This is economic because the dead weight of the slab, although an insignificant part of the load on the slab, is significant to the global design of the bridge.
There was a fashion for very shallow bridges in the 1960s and 1970s as they were considered to look more elegant. However, unless increasing the construction depth has major cost implications elsewhere (such as the need to raise embankments) it is likely to be more economic to use more than the minimum depth. The appearance disadvantage on short span bridges can be resolved by good detailing of the edges. Bridge decks with short transverse cantilevers at the edges tend to look shallower than vertical sided bridges even if they are actually deeper.
Having decided the dimensions of the bridge, the design calculations then serve primarily to design the reinforcement and the key checks will now be considered. They will be illustrated mainly by considering slab structures but most of the principles apply to all reinforced concrete.
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