Traditionally, three kinds of movable bridges have been constructed. In one, the swing bridge, the deck is rotated around a central span, a large, heavy pier sunk into the river bottom. The swing bridge has one serious disadvantage. The central pier, on which the bridge rotates, is usually located in the deepest part of the waterway. Ships with significant drafts may, therefore, have difficulty passing through such bridges. The swing bridge also has one important advantage. Since it never moves upward in a vertical direction, it will not interfere with air traffic that might be present in the area.
In the second type of movable bridge, the bascule bridge, the deck is raised, either at one end or at two ends. The bascule bridge acts, therefore, something like a cantilever in which the free end is raised to permit passage of seagoing vessels.
In the third type of movable bridge, the vertical-lift bridge, the whole central portion of the bridge is raised straight up by means of steel ropes. One disadvantage of the vertical-lift bridge, of course, is that it can not open entirely above the waterway, but can only be raised to a given maximum height.
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David E. Newton