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Introduction The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. You will have noticed that this list does not include truss bridges. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.
At various places in this website there are sections which explain that the boundaries between the various types of bridges are not completely impervious, and that in principle at least, bridges can be built that are not obviously in a simple category. The reason that the types of most bridges are obvious is that these types have become popular because they are successful, and success is greatest in the broad central regions of the available variable-space. For example, if you make an extremely flat suspension bridge, you could put the wires in a concrete matrix, and you would have a pre-stressed beam requiring no anchorages. The same is true of arches - an extremely flat arch would generate enormous thrust, and a beam would be a better solution. The same difficulty applies to many other other human activities, and indeed of many natural groups of species: although there are many genera and species which tax the powers of biologists to classify them, the vast majority fall more easily into groups. On the other hand, where there are very many closely related species, there may be sporadic disputes between "lumpers" and "splitters".
This chart shows the relative lengths of the
longest bridges of different types, in 2004. The completion of new bridges
may mean that the diagram needs updating. The spans are measured on the
vertical axis, while the
If we look at the distribution of the longitudinal forces within a span, they can be summarized as follows. Arch - the average line of the forces should be as near the centre line as possible, and certainly within the kern. Cable - the forces will automatically be distributed across the cable. Beam - the forces should be as far from the neutral axis as possible. Cantilever - the forces should be as far from the neutral axis as possible. This requirement leads to the use of constructions such as I-beams, truss girders and trusses. |
This diagram shows the length of a bridge and two
definitions of span.
horizontal axis merely counts the spans in order
of length. The types of materials used are greatly dependent on the span.
The designer of a small footbridge may have greater freedom of choice than
the designer of a large cable-stayed bridge, for example, though economic
principles always play a part.