How are bridges built?

The design and technology behind these fundamental structures

All over the globe, bridges come in many shapes and sizes. The humble structure was born by simply balancing a horizontal beam across two pillars. However, as demand grew, wider gaps needed to be crossed and more weight had to be carried. This created the need for arches. Utilised by the Ancient Romans, the arch shape could hold massive amounts of weight and was a revelation in creating larger and stronger bridges. However, arches could only reach a certain length. To cross larger expanses, even longer bridges have to be constructed. These are called suspension bridges. These work using a combination of compression and tension forces that run through a cable system. The road or ‘deck’ is laid out across these cables, which are strung together with the correct balance of the two forces so it won’t buckle or snap under pressure. On most suspension bridges, a tower is placed at either end to take the strain and weight. Iconic examples of these bridges include the Golden Gate Bridge in San Francisco and England’s Humber Bridge. Away from the road, bridges are also used for rail and water transport. These bridges are often even longer and stronger as they have to ferry water or huge freight and passenger trains or ferries over vast distances. Seven of the ten longest bridges in the world are located in China, with most of these being rail bridges. As technology and engineering improves, even-more expansive bridges could be in the pipeline. There has been talk of a bridge between England and France and a crossing through the Strait of Gibraltar connecting Europe and Africa. Although it may seem far-fetched, the 16-kilometre (ten-mile) Øresund Bridge that links Denmark and Sweden has demonstrated that cross-country bridges can be constructed successfully.

The key features of a suspension bridge

How the world’s most impressive bridges work

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Types of bridge

Arch

One of the oldest methods of bridge building, the arch bridge is made by compressing stone, steel and concrete with the finished arch working as a wind bracer. They can range from small brick designs to tall and extensive metal constructions.

Suspension

This type of bridge is made of steel plates and cables. Suspension bridges use a combination of tension and compression, which is carried by the cables to towers at either end. The cables transfer the weight to the towers. Their light weight allows them to span long distances.

Beam

Made out of wood or iron, beam bridges are the simplest type of bridge construction. The weight is put on two support girders on either side of the crossing. The earliest examples of beam bridges came in the form of humble logs or tree trunks across streams and rivers.

Cable-stayed

The cable-stayed bridge is one of the most common in contemporary bridge building. They have one or more towers, each of which uses vertical compression to move the forces from the cables through to the foundations, reducing the strain and stress on each part of the bridge.


This article was originally published in How It Works issue 64, written by Jack Griffiths


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