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Movable Bridges

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.



Brash, Sarah, Matthew Cope, Charles Foran, Dónal Kevin Gordon, and Peter Pocock. How Things Work: Structures. Alexandria, VA: Time-Life Books, 1991.
"Bridge," McGraw-Hill Encyclopedia of Science & Technology, 7th ed. New York: McGraw-Hill Book Company, 1992, vol. 2.

Corbett, Scott. Bridges. New York: Four Winds Press, 1978.

DeLony, Eric. Landmark American Bridges. Boston: Little, Brown and Company, 1993.

MacGregor, Anne, and Scott MacGregor. Bridges: A Project Book. New York: Lothrop, Lee & Shephard Books, 1980.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

David E. Newton


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—Heavy supporting structures usually attached to bedrock and supporting bridge piers.

Cable-stayed bridge

—A type of bridge that is a mix of cantilever and suspension bridge, in which the deck is supported both by one or more central towers and cables suspended from the tower(s).

Dead load

—The force exerted by a bridge as a result of its own weight.

Dynamic load

—The force exerted on a bridge as a result of unusual environmental factors, such as earthquakes or strong gusts of wind.

Live load

—The force exerted on a bridge as a result of the traffic moving across the bridge.


—Vertical columns, usually made of reinforced concrete or some other strong material, on which bridges rest.


—Ropes or steel wires from which the deck of a bridge is suspended.


—A very light, yet extremely strong structural form consisting of triangular elements, usually made of iron, steel, or wood.

Additional topics

Science EncyclopediaScience & Philosophy: Boolean algebra to Calcium PropionateBridges - Forces Acting On A Bridge, Dynamic Loads, Model Testing, Types Of Bridges, Cantilever Bridges