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.

Substructure

The substructure of a bridge is responsible for supporting the superstructure and carrying the loads to the ground through foundations.

To support the superstructure, single span bridges require two ‘abutments’, one at each end of the bridge. Where the bearing strength of the soil is good, these abutments can be quite small, for example a strip foundation on an embankment. Foundations on poor soils must either be broad spread footings or be piled. The abutments may also act as retaining walls, for example to hold back the end of an approach embankment.

Multiple span bridges require intermediate supports, often called ‘piers’, to provide additional support to the superstructure. The locations of these supports are usually constrained by the topography of the ground, though where the superstructure is long the designer may be able to choose the number and spacing of piers for overall economy or appearance. Intermediate supports are generally constructed of reinforced concrete.

Integral construction

Traditionally, movement (expansion) joints have been provided at the ends of the superstructure, to accommodate expansion/contraction. Experience in recent years has been that such joints require on-going maintenance, yet they inevitably leak and result in deterioration of the substructure below the joint. For bridges of modest overall length, it is now common to use integral construction, with no movement joint. In its simplest form, the ends of the superstructure are cast into the tops of the abutments.