MOVABLE BRIDGE

either a drawbridge, a vertical-lift bridge, a transporter bridge, or a swing (pivot) bridge. The drawbridge, or bascule, is the best known; it may be single- or double-leafed. It originated in medieval Europe, probably Normandy, as a defensive feature of castles and towns. It was operated by a counterweight and winch. The drawbridge that formed one span of Old London Bridge was occasionally raised to permit passage of a ship having masts too tall to pass under at this point. In the late 19th century drawbridges began to be built specifically to aid navigation; the Tower Bridge, London, and the Van Buren Street Bridge, Chicago, were built almost simultaneously. Both were double-leaf bascules, and their success led to wide imitation; more than 20 were built to span the Chicago River alone. At the same time, another movable bridge was pioneered in Chicago: the vertical lift, designed by J.A.L. Waddell. For several years it was unimitated; later, when its great strength for railroad loading was appreciated, it was repeated widely, in increasing span lengths, many exceeding 500 feet (152 m). The vertical lift also relies on counterweights; the entire bridge roadway is elevated by counterweights and machinery in two towers. The transporter bridge consists of a car suspended from a trolley traveling along an overhead bridge superstructure. It carries passengers and vehicles across a waterway. For exceptionally long spans, the pivot, or swing bridge, which turns on a table, is suitable. Several of more than 500 feet have been built in the United States, but the turntable obstructs the river, limiting its use.