Viaduct

Viaduct
Laigh Milton Viaduct in Ayrshire is the oldest surviving railway bridge in Scotland
Ancestor:	Trestle bridge, Box girder bridge
Related:	None
Descendant:	None
Carries:	highways, streets, railroads
Span range:	Short (multiple)
Material:	reinforced concrete, prestressed concrete, masonry
Movable:	No
Design effort:	medium
Falsework required:	available for use, since viaducts are all composed of low bridges.

A viaduct is a bridge composed of several small spans. The term viaduct is derived from the Latin via for road and ducere to lead something. However, the Ancient Romans did not use that term per se; it is a modern derivation from an analogy with aqueduct. Like the Roman aqueducts, many early viaducts comprised a series of arches of roughly equal length. Viaducts may span land or water or both.

Viaducts are commonly used in many large cities that are railroad centers, such as Chicago, Atlanta, Birmingham, Alabama and Manchester. These viaducts cross the large railroad yards that are needed for freight trains there, and also cross the multi-track railroad lines that are needed for heavy railroad traffic. These viaducts keep highway and city street traffic from having to be continually interrupted by the train traffic. Likewise, some viaducts carry railroads over large valleys, or they carry railroads over cities with lots of cross-streets and avenues.

Many viaducts over land connect points of similar height in a landscape, usually by bridging a river valley or other eroded opening in an otherwise flat area. Often such valleys had roads descending either side (with a small bridge over the river, where necessary) that become inadequate for the traffic load, necessitating a viaduct for "through" traffic.^[1] Such bridges also lend themselves for use by rail traffic, which requires straighter and flatter routes.^[2]

Viaducts over water are often combined with other types of bridges or tunnels to cross navigable waters. The viaduct sections, while less expensive to design and build than tunnels or bridges with larger spans, typically lack sufficient horizontal and vertical clearance for large ships. See the Chesapeake Bay Bridge-Tunnel.

The Millau Viaduct is a cable-stayed road-bridge that spans the valley of the River Tarn near Millau in southern France. Designed by the French bridge engineer Michel Virlogeux, in collaboration with architect Norman Robert Foster, it is the tallest vehicular bridge in the world, with one pier's summit at 343 metres (1,125 ft)—slightly taller than the Eiffel Tower and only 38 m (125 ft) shorter than the Empire State Building. It was formally dedicated on 14 December2004 and opened to traffic two days later.

In Romance languages, the word viaduct refers to a bridge which spans only land. A bridge spanning water is called ponte.

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