Steel 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	Expressways, highways, streets, railways
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 for crossing a valley, dry or wetland, or forming a flyover.

The term is conventional for a rail flyover as opposed to a flying junction or a rail bridge which crosses one feature. In Romance languages, the word viaduct refers to a bridge which spans only land. A bridge spanning water is called ponte.

The term viaduct is derived from the Latin via for road and ducere, to lead. The ancient Romans did not use the term; 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.

The longest viaduct in antiquity may have been the Pont Serme which crossed wide marshes in southern France.

Viaducts are commonly used in many cities that are railroad centers, such as Chicago, Atlanta, Birmingham, London 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 many 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. Such bridges also lend themselves for use by rail traffic, which requires straighter and flatter routes. Some viaducts have more than one deck, such that one deck has vehicular traffic and another deck carries rail traffic. One example of this is the Prince Edward Viaduct in Toronto, Canada, that carries motor traffic on the top deck as Bloor Street, and metro as the Bloor-Danforth subway line on the lower deck, over the steep Don River valley. Others were built to span settled areas, crossing over roads beneath—the reason for many viaducts in London.