A viaduct is a type of bridge made up of several small spans supported by piers or arches. Unlike a single-span structure, a viaduct stretches across a wide obstacle such as a valley, a wetland or existing infrastructure to create a continuous elevated route. Viaducts are frequently used to carry roads and railways over low ground, or to form long elevated approaches such as an overpass or a flyover.

Structure and components

At its simplest a viaduct consists of repeating spans and supporting piers. The key elements are the spans (the horizontal members that support traffic), the piers or towers (vertical supports), and the deck or carriageway on which vehicles or trains run. Spans may be constructed as beams, arches or girders and are arranged in series to bridge long distances without a single large opening.

Materials and types

Viaducts have been built with many materials and methods. Common types include:

  • Stone or brick arch viaducts, typical of 18th–19th century railway construction.
  • Steel-girder viaducts and trusses, common in the late 19th and early 20th centuries.
  • Reinforced and prestressed concrete viaducts used in modern highway and high-speed rail projects.
  • Composite and modular designs that combine materials for economy and speed of construction.

History and name

The term derives from Latin roots meaning "road" and "to lead," but it is a relatively modern word. Ancient builders such as the Romans constructed long elevated works and aqueducts; however, the specific English term "viaduct" was coined in the 19th century by analogy with the word aqueduct, an analogy that emphasized a structure for carrying a route across a depression rather than carrying water.

Uses, importance and distinctions

Viaducts remain important for carrying transport routes where ground-level alignment would be impractical. They conserve gradients for railways, cross flood-prone land, and avoid the need for long detours. Compared with ordinary bridges, the term "viaduct" usually implies multiple repeated spans and a considerable overall length rather than a single crossing span. Modern engineering balances cost, environmental impact and aesthetics when choosing viaduct forms for roads and rail networks.