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a bridgelike structure, usually of great length and built on high supporting piers; used when a road crosses ravines, gorges, swampy river valleys, and so on.
Viaducts are sometimes built instead of high embankments if the construction of the latter is economically unfeasible or technically inadvisable—for example, as a result of weakness in the foundation soil. A gradual increase in the height of the piers (and, in certain types of viaducts, also in the size of the spans at the highest point) distinguishes the viaduct from the trestle bridge, which usually has a negligible change in the height of its piers and spans of identical size. Viaducts are built of stone, metal, concrete, or reinforced concrete; for the most part they are of multispan arch design, less frequently of beam construction.
Viaducts were well known even in ancient Rome, where they were built in a system of arches made up of large stone blocks; this gave the viaducts an appearance of austere power, characteristic of the utilitarian Roman structures. Since the end of the 19th century, stone viaducts have been replaced by those built of metal and reinforced concrete. The construction possibilities of these materials and the development of a scientific theory of bridge building have made possible a considerable decrease in the volume and weight of the structure’s main components and have had an effect on the construction of modern viaducts, in which the architectural image is based on the emotional expressiveness of plastically conceived exposed structures that are subordinate to engineering principles (for example, a reinforced-concrete viaduct in Nogent-sur-Marne, France; and the Polcévera Viaduct in Genoa, 1961, engineer R. Morandi).