|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|
A bascule bridge (also referred to as a drawbridge or a lifting bridge) 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.
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, but until the adoption of steam power in the 1850s, very long, heavy spans could not be moved quickly enough for practical application.
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 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.
Animation of a double-leaf Strauss fixed-trunnion bridge (based on engineering drawings from the Henry Ford Bridge)
Animation of a rolling lift bridge (such as the Pegasus Bridge)
- 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 ed.). 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.
- Van Zantvliet, P. S. (June 2015). "Analysis of the force distribution on operating mechanisms in a bascule bridge" (PDF). Archived (PDF) from the original on 2021-09-21.
- Media related to Bascule bridges at Wikimedia Commons