Selection of Type - Structural Steel in Bridges.

Suspension or cable-stayed bridges are suitable for the longest spans, but are less suitable to support heavy loading across short or medium spans.At the same time medium-span footbridges can appropriately be suspension, cable-stayed or arch types because concentrated loading is absent. Some of the considerations given to long spans such as aerodynamics need to be applied to footbridges. For footbridges the phenomenon of pedestrian-excited vibrations needs to be considered and tends to affect the design of spans exceeding about 25m. The possibility of horizontal  oscillation needs consideration if the structure is laterally flexible or is mounted  on slender supports. Other bridge types such as arches or portals may be suitable in special locations. For example, an arch is a logical solution  for a medium span across a steep-sided ravine.

Through trusses are suitable for medium spans where the available construction depth is limited. The vast majority of short- and medium-span highway bridges are formed with composite construction because the highway profile can be arranged to suit the depth available.

For short and medium spans the most important factor which influences the type is the available construction depth.This particularly affects railway bridges because it is rarely feasible to modify existing track levels.Where depth is limited then types such as arch, truss or half-through plate girder offer an alternative solution. Types of steel bridge are shown in Fig. 4.2 with their normal economic span range and the world’s longest. Each is briefly described below.

1 Suspension bridges   Suspension bridges (Fig. 4.3) are used for the longest spans across river estuaries where intermediate piers are not feasible. The cables form catenaries supporting both sides of the deck and are tied to the ground usually by gravity...

2 Cable-stayed bridges   Cable-stayed bridges (Fig. 4.4) are of a suspension form using straight cables which are directly connected to the deck.The structure is self-anchoring and therefore less dependent upon...

3 Arch bridges   Arch bridges (Fig. 4.5) are suitable in particular site conditions. An example is  a medium single span over a ravine where an arch with spandrel  columns will efficiently carry a deck with...

4 Portal frame bridges   Portal frame bridges (Fig. 4.6) are mainly suitable for short or medium spans. In a three-span form with sloping legs they can provide an economic solution by offering a reduction in...

5 Truss bridges   Through trusses are used for medium spans where a limited construction depth precludes use of a composite deck bridge.They are suitable in flat terrain to reduce the height...

6 Girder type   Girder bridges (Fig. 4.8) predominate over the previously described types for short and medium spans, and generally provide the most economic solution. For highway bridges...

Normal span range of bridge types
Fig. 4.2 Normal span range of bridge types

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