Bridges create a span between two points, sometimes with additional supporting points between them. They are commonly used to create a crossing point between two banks of a river or a ravine, but they are also used within and between buildings and other man made structures. They function by using a number of different technologies.
At is simplest, a bridge could be made from a single solid item, such as a plank, which rests on either side of the span, rather like the way a gangplank is used to create a walkway between a boat and a jetty. In such cases, the bridge works thanks to the structural integrity of the material being used. However, with larger spans, such an approach will not always work due to the fact that the bridge won’t have enough strength, particularly when loaded at the middle.
Bridges are able to carry greater loads and cover longer spans when they balance the forces exerted on them from their own mass and from that of anything that is placed on them. These forces tend to be compression, when the bridge is squeezed, and tension, which stretches the bridge. Beam bridges, which have a central horizontal span over two or more supporting abutments, transfer loading forces sideways and downwards. Arched bridges, much loved by Roman architects, transfer their forces rotationally and downwards. Where additional support is required to cover a certain span, additional supports might be added under the middle of the bridge. The correct term for this element is a pier.
More modern designs of bridges include suspension, cable stay and truss bridges. The first two transfer their forces sideways along the deck and up through the towers which make up their structure. At this point loading forces are sent downwards to the ground. Truss bridges, often used for railways, transfer the forces they are exposed to sideways, diagonally along each beam.
Modern bridges are not just designed to have great spans and be capable of dealing with heavy loads. They must also be able to face up to environmental considerations. Bridges over seaways must be able to withstand water pressure and flowing water caused by tidal movements, for example. In addition, wind tunnel modelling is often conducted to ensure that bridges are capable of withstanding gales without collapsing under the strain. Generally speaking, this is done by making them aerodynamic in cross-section and by using materials which prevent a widespread back-and-forth motion in the face of wind which can create destructive resonances.