Introduction

         The main types of bridges are beam bridges,arches, cable-stayed bridges, cantilever bridges and suspension bridges,and combinations.The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span. 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 towers hold up the cables.  They have to be rigid enough to act as struts between the downward forces from the cables and the upward forces from the foundations.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. 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 diagram shows the length of a bridge and two definitions of span.

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 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.
       Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.

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.

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