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structure built over water or any obstacle or depression to allow the passage of pedestrians or vehicles. See also viaduct. Early Bridges
In ancient times and among primitive peoples a log was thrown across a stream, or two vines or woven fibrous ropes (the
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a bridge with a movable span to permit the passage of ships. Drawbridges are usually built across rivers traveled by large ships when conditions make it technically and economically inadvisable to construct a bridge on high piers and with long approaches. The movable span of a drawbridge can be of the vertical-lift, swing, bascule, balance-beam, or rolling-lift type; the choice of type depends on local conditions. Construction of a drawbridge span requires either massive piers or towers to house the mechanisms and engines required for moving the span. Electric and hydraulic drives are the most common, and drawbridges are often equipped with backup drives from internal-combustion engines. The movable span generally has light-duty structural members, with trusses or beams of steel or lightweight alloys.