Relate

There are many types of bridge, and they often look very different. But they are not always as unrelated as they may seem.

Here are some diagrams which show how some types of bridge can be related to each other.

While extending the Great Western Railway to the south west of England, Brunel needed to build cheap bridges quickly. He designed some viaducts using wooden spans. The design reduces the effective span.

Inverting this type of structure gives the same pattern as some types of cable-stayed bridges, with tensions and compressions interchanged. Later the wood could be replaced by more durable materials when income from the railway was forthcoming.

The diagram shows how two halves of two arches could be held together by a cable, forming a pair of cantilevers, with exactly the same stresses as in the arch condition.

But of course this is not an efficient way of making a cantilever, and it does not allow building without a lot of falsework, which is one advantage of a proper cantilever. The white line represents a cable which holds the half-arches together.

The cables in some pre-stressed bridges can be considered as enclosed suspension cables, relating these bridges to the usual type of suspension bridge. The beam is like a self-anchored suspension bridge surrounding its own cables.

This diagram is over-simplified - a real beam would contain a carefully calculated set of cables, designed to ensure that no combination of likely loads could put any part of the concrete in tension.

The diagonal stressing wires in the concrete can be related to diagonal tension members in a truss.

At the bottom of the picture the diagram represents a simple plate girder. In the next diagram some attempt has been made to shape it to suit the bending moments. In the third diagram this is taken further, and in the fourth picture the structure is greatly lightened by changing it into a truss. Finally, at the top, we see a tied arch or bowstring arch.

The point is to get the material as far from the neutral axis as possible in order to oppose the bending moment. Material near the neutral axis isn't doing anything useful in this context. For a tension member, of course, you might as well use a wire as a tube, unless the member is very long and in danger of vibrating.

Each main span includes a tubular arch and a suspension chain, linked by cross-members. It could be regarded as a self-anchoring suspension bridge, a tied arch, or a truss. This novel construction enabled the 455 foot spans to be built on shore, and towed out to the bridge for lifting into position. In this way, falsework was avoided, minimising obstruction to navigation. his idea was an ingenious solution to the requirement for carrying the Great Western Railway across a wide and deep channel, allowing for the passage of large ships.

Building the central pier presented difficult problems. It was built within a huge protective cylinder, which was afterwards removed. The whole procedure is well described in the book "Track Topics" by W G Chapman. The book includes a drawing by W Heath Robinson, illustrating the methods of construction according to his usual style. The new suspension bridge nearby carries the A38 road.

How far from an arch shape can you go and still call it an arch?

What about an almost rectangular portal shape with two halves propped against each other at a hinge?

As we have seen, the distinction between structural types is not always clear cut. With the advent of many new materials and the analytical power of computer programs, there comes the ability and willingness to try new ideas. There is certainly more scope than ever for creating structures that are easy on the eye for those who have to live with them.