Relationships  Between  Bridge  Types

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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.

 

Cable-stayed bridge and wooden viaduct

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.

CableBrunel.gif (12004 bytes)

 

Cable-stayed bridge and suspension bridge

The suspension bridge needs a strong anchorage at each end to hold the main cables.

If the ground is weak this can create a big problem.  The cable-stayed bridge is self-anchored.

CableSuspA.gif (9995 bytes)

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Arch bridge and suspension bridge

The diagram below shows a simple suspension structure with a stiff deck, and a simple deck-stiffened segmented arch.

These are similar except that tensions and compressions are interchanged, and the suspension bridge requires towers.  Both types give a graphic demonstration of the concept of the funicular.

ArchSuspR.gif (8486 bytes)

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Arch bridge and cantilever bridge

Loyn.jpg (38359 bytes)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.

Suspension bridge and pre-stressed bridge

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.  In other cases, such as Donnington bridge, the structure is more like a fanned cable-stayed bridge.

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

SuspPreB.gif (6844 bytes)

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Truss and pre-stressed concrete cantilever

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

TrussPreB.gif (5697 bytes)

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Beam relationships

Arches6B.gif (5521 bytes)

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.

 

Brunel's Saltash Bridge over the river Tamar

RABridge1849Sm.jpg (228902 bytes) RATrain35S.jpg (306951 bytes)

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 possibly a lenticular 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.

 

Three-pinned arch

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Estcourt3.jpg (43741 bytes)

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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.

 

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