A bridge is a structure that spans and provides passage over a physical barrier such as a body of water, valley, or road. Bridges are typically built for the purpose of transporting people or goods from one place to another.
The term “bridge” can also be used metaphorically to refer to other types of connections or linkages between two things.
Bridges come in a variety of shapes and sizes, and they can be made from a variety of materials including wood, stone, steel, and concrete. Some bridges are designed for pedestrians only, while others accommodate both pedestrians and vehicles.
The first known bridges were probably made by nature itself—logs or rocks that had fallen into place across a stream or river. Early human-made bridges were probably just logs or planks laid across a stream or river.
The first major bridge-building project was undertaken by the Roman engineer Vitruvius Pollio, who constructed a series of stone arches across the River Tiber in Rome.
The ancient Chinese also built bridges, including the world-famous Liuguanghe Bridge which has been in use for over 2,000 years. In the Middle Ages, European bridge builders developed new designs and engineering techniques that allowed them to construct larger and more elaborate bridges.
The most famous of these is the Pont du Gard aqueduct in France, which was built by the Roman engineers in the first century AD.
In modern times, bridges are often designed by computer using sophisticated mathematical models to ensure that they are safe and stable. With the advent of new materials and construction methods, bridges have become even more complex and ambitious structures.
Today, there are many different types of bridges including suspension bridges, cable-stayed bridges, cantilever bridges, truss bridges, and arch bridges.
The Golden Gate Bridge in San Francisco is one of the most iconic suspension bridges in the world. It was completed in 1937 and spans almost 2 miles (3.2 kilometers) across the Golden Gate strait.
The Brooklyn Bridge in New York City is another well-known suspension bridge. It was completed in 1883 and was the first bridge to use steel cables to support its deck (the roadway).
Cable-stayed bridges are similar to suspension bridges but instead of suspending the roadway from cables attached to towers, they support the roadway with cables attached directly to the towers themselves. The Millennium Bridge in London is a good example of this type of bridge.
It was completed in 2000 and spans almost 1 mile (1.6 kilometers) across the River Thames.
Cantilever bridges are built using two cantilevers—structures that extend outward from either side of the bridge like arms—to support the roadway between them. The Forth Bridge in Scotland is one of the best-known cantilever bridges.
It was completed in 1890 and spans almost 2 miles (3 kilometers) across the Firth of Forth estuary.
Truss bridges are characterized by their use of triangular shaped trusses (structural frames) to support the roadway above them. The Brooklyn Bridge is an example of a truss bridge; its design includes both suspension and truss elements (the upper part of the bridge is suspended while the lower part is supported by trusses).
Arch bridges are defined by their curved shape; they use an arch to support the weight of the roadway above them. The popular Rainbow Bridge in Tokyo is an example of an arch bridge; it was completed in 1993 and spans almost 1 mile (1 kilometer) across Tokyo Bay.