Which  Bridge  Type  is  Right ?

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Which type of bridge is right for a given location and purpose?  Here are some possible considerations.

Types of loads

Concentrated and heavy, rail

Road traffic

Pedestrians only

Ships and boats on canal, aqueduct

Types of crossings

Over a navigable waterway

Over a non-navigable waterway

Over deep water

Over shallow water

Over slow or stagnant water

Over fast flowing deep water

Over railway tracks

Over a major road

Over a minor road

Over an alluvial plain

Length of crossing

Short

Medium

Long - single span possible

Very long - single span impossible

Intermediate supports possible?

Yes

No

Depth to good ground for supports

Shallow

Deep

Very deep

Quality of ground at abutments

Strong

Weak

Strength of expected winds

Low

Medium

High

Interruption of traffic or navigation allowed during construction?

None

Slight

Medium

Any

Height of abutments above crossing

Low

Medium

High

Appearance and disturbance

Surroundings residential

Surroundings old city

Surroundings AONB

Surroundings tourist area

Surroundings SSSI

Surroundings nature reserve

Surroundings industrial

The choice of bridge construction depends on these and other factors, such as cost.

We can, for example, divide bridges into those which impose horizontal forces on the ground, such as arches and suspension bridges, and those which do not, such as beams, cable-stayed bridges, cantilevers and trusses.  

If the ground is weak, the thrust of an arch or the pull of cables may rule out some types of construction. The alternative is to provide massive and expensive abutments or anchorages. Poor ground may require deep boring in order to find good foundation, as in the case of the south tower of the Humber bridge.

The requirements of clearance for traffic or navigation, together with the height of the ground on either side, may have a great bearing on the choice of bridge type.

Any requirement for clearance, coupled with low ground, will require long and expensive approaches, except in the case of a footbridge.

Any restrictions on interference with traffic or navigation during construction may restrict the design to one which allows large parts to be built elsewhere and moved into position. Tied arches, trusses and beams may be considered. Recently a single span tied arch was constructed near the M42 motorway and slid into place as a complete unit. The spans of the Saltash and Menai Straits railway bridges were also constructed as units, and floated out to the bridges, and then slowly raised into place.

Let's look once more at bridge types.  

Beams, flat trusses and box girders impose only vertical forces on the ground. The total force on the ground is the total weight of the bridge. With any span, every metre of extra span is a metre of extra weight and extra bending moment. The penalty is that these designs are limited to spans far shorter than the maximum possible bridge span.

Why is this? The reason is that the bending moment produces strong forces in opposite directions, which require material to withstand them.

Cantilevers, deep trusses and cable-stayed bridges can produce large spans, such as those of the Forth bridge and the Quebec bridge. This is partly achieved by using a suspended span between two cantilevers. The bending moment referred to above is countered by increasing the depth of the structure, reducing the weight of material required.

By allowing the ground, which costs nothing, to take horizontal forces, an arch can be made quite light and slender, while reaching spans that are comparable with big cantilevers. An arch need be no thicker than is needed to avoid buckling, because it need not handle the bending moment that the other bridges experience, apart from that needed for live loads. 

How can the cable-stayed design do better than the arch and the cantilever? Because it is supported at frequent intervals by cables, against the pull of the earth, the cable-stayed span can be a truss that has only to resist buckling against the dead weight and local live loads. An arch and a cantilever have to provide inherent stiffness without any cables.

But the longest spans of all are those of the largest suspension bridges, again allowing the ground to absorb large horizontal forces.

Why does the suspension bridge do even better than the arch? The reason is that the main stress bearing member is in tension and not in compression. A tie is stable against perturbation, whereas a strut is unstable. In the suspension bridge the ground is used as the compression member. It is large, and it costs nothing, though there is a necessity for ensuring that the interface between bridge and ground spreads the load enough for the ground to cope.

We should always remember the following ideas.

The further a force is from the point at which we want to apply it, the greater the stress we will find somewhere in the system.

The further the direction of a force diverges from the direction we want, the greater will be the stress.

 

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