Bridge

A bridge is an engineered structure built to span a gorge, valley, road, railroad track, river or other body of water, or any other physical obstacle.

Table of contents

1 History
2 Etymology
3 Types of bridge

3.1 Movable bridges

History

The first bridges were simple wooden logs or planks spanning a stream or such; the next examples found use stone, but again as a simple support and crossbeam arrangement. The arch was first used by the Roman Empire for bridges, and many Roman bridges and aqueducts still exist today. The Romans also had cement, which reduced the variation of strength found in natural stone. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost.

With the advent of the Industrial Revolution of the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, with its high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel, which were first shown at the Eiffel Tower in Paris France.

Etymology

The Oxford English Dictionary traces the origin of the word bridge to an Old Norse word brygga, meaning "landing stage, gangway, or movable pier".

Types of bridge

A bridge is usually either designed for trains, pedestrian or road traffic. In the latter case there may be restrictions in use; for example, it may be a bridge carrying a highway and forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles. A bridge which has a series of spans, typically arches, is called a viaduct. An aqueduct is a kind of bridge that carries water, resembling a viaduct. Sometimes a bridge carries a pipeline only. When a bridge spans a road or railroad track, it is often called an overpass (US) or flyover (UK).

Movable bridges

To allow ships to pass which can not pass under it, a bridge may be constructed such that it (or part of it) can be turned up (drawbridge; either one part or two) or sideways (swing bridge). A third method is that the bridge deck is lifted while staying horizontal (lift bridge or lifting bridge). (Alternatively, if road traffic is very light, a transporter bridge may be used.)

For small bridges these movements may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridge-man/woman, sometimes remotely using video-cameras and loudspeakers.

There are often traffic lights for the road and water traffic, and moving barriers for the road traffic.

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