Bridge design

== Basic features of bridges ==

Superstructure

The superstructure of a bridge is the part directly responsible for carrying the road or other service. Its layout is determined largely by the disposition of the service to be carried. In most cases, there is a deck structure that carries the loads from the individual wheels and distributes the loads to the principal structural elements, such as beams spanning between the substructure supports.

Road bridges carry a number of traffic lanes, in one or two directions, and may also carry footways. At the edge of the bridge, parapets are provided for the protection of vehicles and people. The arrangement of traffic lanes and footways is usually decided by the highway engineer. Traffic lane and footpath widths, along with clear height above the carriageway are usually specified by the highway authority. Whilst the bridge designer has little influence over selecting the layout and geometry of the running surface, he does determine the structural form of the superstructure. In doing so, he must balance requirements for the substructure and superstructure, whilst achieving necessary clearances above and across the obstacle below.

Rail bridges typically carry two tracks, laid on ballast, although separate superstructures are often provided for each track. Railway gradients are much more limited than roadway gradients and because of this the construction depth of the superstructure (from rail level to the underside or soffit of the bridge) is often very limited. This limitation frequently results in ‘half through’ construction. Railway loading is greater than highway loading and consequently the superstructures for railway bridges are usually much heavier than for highway bridges.

Footbridges are smaller lighter structures. They are narrow (about 2m wide) and are usually single span structures that rarely span more than 40m.