Bridge form

There are three primary load-carrying systems for bridges. These are based on hanging cables or stays (in tension), arches (in compression) and beams or trusses which combine both tensile and compressive action. Sometimes the bridge deck and supporting piers are rigidly joined so that they act together to form a portal frame structure.

Although these primary load-carrying systems are quite distinct, most steel bridges have secondary structure that is of a different type. For example, in suspension bridges, the longitudinal cables are the primary system spanning between the main supports but the bridge deck is suspended from tensile hangers placed at regular intervals and must span between them as a continuous beam. The deck must also act as a horizontal beam, resisting the lateral wind and / or seismic loads, in this case spanning between the main supports.

Bridges can be classified according to structural type including tensile, compressive, beam, hybrid and special.

Typical bridge types can be classified as:

• Tensile: Suspension cable, cable-stayed (fan, harp and modified fan)
• Compressive: Ribbed arch, box arch, arch truss
• Beam: Plate girders, hollow box girders, trusses, through tubes, rigid frames
• Hybrid: "Bowstring" arch
• Special: Bascules, swing bridges, lift bridges, integral bridges, buildings as bridges.

In the Jackfield Bridge, 1 km downstream of the historic Ironbridge, the composite steel and concrete deck is suspended from cable stays in tension. The configuration of stays in this case is of the modified fan type (i.e. radiating from points at regular intervals down the pylon) for the main stays and harp type (i.e. with parallel stays at similar intervals down the pylon) for the backstays.

This trussed arch bridge of approximately "funicular" form carries the Pan-American highway over the Rio San Francisco. The 157m central span rises to 100m above the valley bottom.

In this simple plate girder motorway bridge, in Normandy, the steel girders act compositely with a reinforced concrete slab. Three separate beam sections were welded together on site to produce a continuous structure.

Although generally deeper in section, trusses are more efficient than beams for long spans, as their members are subject primarily to axial tension and compression forces rather than bending. In this example the deep trusses form the sides of the walkway with the deck supported on the bottom chords.

In some cases, the primary structure employs more than one of these basic systems. Such bridges are classified as hybrids.

The Barqueta Bridge was one of the bridges constructed to give access to the site of Expo '92 in Seville, Spain. Its 214m span arch is formed from a single welded box section steel arch, which bifurcates near the supports to allow vehicles to pass below. The slender deck, which acts as a tie between the supports, is suspended from the arch by a line of cables running along the centre of the deck. The bridge was completely assembled off site and floated into position along the river. (Designers: Juan Jose Arenas and Marcos Pantaleon.)

A further example of a hybrid structure is Brunel's Saltash Bridge, near Plymouth, where the deck is suspended from a large, elliptical-section tube arch and suspension cables. The two systems share the task of carrying the vertical loads from the deck, the suspension cables tie the ends of the arch tube together, forming a "bowstring" arch, and the tube acts as a strut resisting the inward pull of the cables. A closed force system results so that with vertical loading only vertical forces are imposed on the supports.

There are other special types of bridge that move, generally to allow the passage of boats through a navigation channel.

Tower Bridge in London is an internationally recognisable landmark. As a bascule bridge its main spans open by rotating about a horizontal axis to allow river traffic to pass. The weight of spans is counterbalanced by large iron and lead weights concealed in the base of the towers. Movement was originally achieved using hydraulic motors but these have now been replaced by much smaller electric motors.

An alternative means of providing headroom for navigation is to lift the bridge deck out of the way completely to allow boats to pass as in the Martrou Road Bridge, La Rochelle, France. Here, the horizontal deck, which is supported on the bottom chord of the main lattice trusses, is raised between two towers. A similar bridge was recently constructed in the UK - the Lowry Footbridge in Salford, over the Manchester Ship Canal (Building, 9 July 1999). Here cheaper tubular steel towers were substituted for the prestressed concrete originally proposed.

A special category of building is required when construction has to take place over a road or railway. These are effectively inhabited bridges.

An interesting example is Exchange House, Broadgate, London, which is a 10-storey office building spanning 78m over Liverpool Street Station (Architect and Structural Engineer: Skidmore Owings and Merrill).The building frame of Exchange House is supported on a primary structure of four parabolic segmented tied arches spanning the full 78m across the railway tracks entering Liverpool Street Station. Two external arches are visible and set 2m from the building façades and two internal arches pass through the offices and are exposed in the atria. Transverse, open-webbed beams span between the arches and vertical "columns" transfer the floor loads to the arches by either compression or tension, depending on their position.