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.

== Beam bridges ==

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’