Other bridges

Box girder, lattice, cantilever, movable and cable stay bridges are really only variations on the beam bridge, arch bridge or suspension bridge. This can make classification of bridges a difficult task.

This is a type of beam bridge, in which the deck is either built on, or even inside a hollow beam. George Stephenson used this design for the railways in North Wales. Trains still travel through the box girder bridge at Conwy. A larger box girder bridge over the Menai Straits was badly damaged by fire, and reconstructed with two decks: one carrying the railway, and a higher one carrying road traffic. The decks of many suspension bridges are box girders, with vehicles travelling on top of the box.

Again, this is basically a beam bridge, and very similar to a box girder bridge. The deck is the only solid surface of the beam. A lattice of steel girders is used for the sides and sometimes across the top. The side view may be a trapezium, as shown in the diagram, or it may be arch shaped. A common type for railway bridges, but also used for roads in the past. This type of bridge is not usually built for modern roads because of the high cost of steel girders, though there are examples of concrete lattices.

In a simple beam bridge, most of the bending happens in the centre. A cantilever bridge, has hinges at either side of a suspended span. The Forth Railway Bridge is one of the most famous bridges of this type, but it is also a lattice bridge, because it is constructed from a framework of steel girders.

Many more recent bridges of this type are built with reinforced concrete. Look closely at some of the motorway bridges (especially footbridges) which you pass under, and you will see the hinged joints. This type of bridge is very economic in use of materials.

This is the most modern type of design, and is part suspension bridge, and part beam bridge. There is a tall pier near one side of the river, from which cables are attached directly to the deck. There may be a second pier, with additional cables, on the opposite bank. The deck is likely to be a box girder, constructed of reinforced concrete.

The most spectacular bridges of this design in Britain are the Queen Elizabeth Bridge over the River Thames at Dartford and the new River Severn crossing in Avon. Both of these have cables supended from towers at both banks of the river.

In these bridges the deck moves in some way. For more details and photographs click here.

Bridge index.

Forces in bridges.