A bascule bridge (sometimes referred to as a drawbridge) is a moveable bridge with a counterweight that continuously balances a span, or "leaf", throughout its upward swing to provide clearance for boat traffic. It may be single- or double-leafed.
This animation shows the movement of a double-leaf bascule.
|Ancestor||Drawbridge, Plate girder bridge, cantilever bridge|
|Related||Lift bridge, swing bridge|
|Carries||Pedestrian, bicycle, automobile, truck, light rail, heavy rail|
|Falsework required||Site and prefabrication specific|
The name comes from the French term for balance scale, which employs the same principle. Bascule bridges are the most common type of movable span because they open quickly and require relatively little energy to operate, while providing the possibility for unlimited vertical clearance for marine traffic.
Bascule bridges have been in use since ancient times. However, it was not until the adoption of steam power in the 1850s that very long, heavy spans could be moved quickly enough for practical application.
There are three types of bascule bridge designs, and counterweights required to balance a bascule's span may be located either above or below the bridge deck.
The fixed-trunnion (sometimes a "Chicago" bascule) rotates around a large axle that raises the span(s). The Chicago bascule name derives from the location where it is widely used, and is a refinement by Joseph Strauss of the fixed-trunnion.
The rolling lift trunnion (sometimes a "Scherzer" rolling lift), raises the span by rolling on a track resembling a rocking chair base. The "Scherzer" rolling lift is a refinement patented in 1893 by the American engineer William Donald Scherzer.
The rarer Rall type combines rolling lift with longitudinal motion on trunnions when opening. It was patented (1901) by Theodor Rall. One of the few surviving examples is the Broadway Bridge (1913), in Portland, Oregon.
|Wikimedia Commons has media related to Bascule bridges.|
- Koglin, Terry L. (2003). "4. Bascule Bridges". Movable bridge engineering. John Wiley and Sons. ISBN 978-0-471-41960-0. Retrieved May 25, 2009.
- "Landmark Designation Report: Historic Chicago Bridges" (PDF). Commission on Chicago Landmarks. September 2007 [September 2006]. pp. 12, 15 (pdf pages 14, 17). Retrieved April 21, 2013.
- US grant 511713, Scherzer, William, "Lift-Bridge", issued 26 December 1893
- Wood Wortman, Sharon; Wortman, Ed (2006). The Portland Bridge Book (3rd Edition). Urban Adventure Press. pp. 32, 35. ISBN 0-9787365-1-6.
- "Patent number 669348: T. Rall movable bridge". United States Patent and Trademark Office (referenced online by Google Patents). 1901. Retrieved April 21, 2013.
- Historic American Engineering Record. "Broadway Bridge, Spanning Willamette River at Broadway Street [sic], Portland, Multnomah County, OR". Library of Congress. Retrieved April 21, 2013.