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This animation shows the movement of a double leaf bascule.
|Ancestor||Drawbridge, Plate girder bridge, cantilever bridge|
|Related||Lift bridge, swing bridge|
|Carries||Pedestrian, automobile, truck, light rail, heavy rail|
|Falsework required||Site and prefabrication specific|
A bascule bridge (commonly 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.
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.
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. The Blagoveshchensky Bridge across the Neva River in Saint Petersburg was the first large bascule bridge, opened in 1850.
There are two common designs of bascule bridge: the fixed-trunnion, where the bridge rotates around a large axle (its trunnion) to raise, and the rolling lift, which raises by rolling on a track resembling the base of a rocking chair. Counterweights required to balance a bascule's span may be located above or below the bridge deck.
A third and rare type is the Rall, patented by Theodor Rall in 1901, which combined rolling with longitudinal motion on trunnions when opening. One of the few surviving examples is the 1913-opened Broadway Bridge, in Portland, Oregon.
The fixed trunion is sometimes called the Chicago bascule for its refinement there by Joseph Strauss and popular use throughout that city. A patented version of the rolling lift is known as the "Scherzer" (after the american engineer William Donald Scherzer)
Tower Bridge across the Thames in London is a famous bascule bridge. It was originally hydraulically operated, using steam power from coal-burning boilers to pump river water into six hydraulic accumulators that drove engines which raised and lowered its spans. Water for the boilers was provided by a well; hydraulic accumulators were employed to provide power on demand. Today, oil and electricity are used rather than water and steam.
Mystic River Bascule Bridge, Mystic, Connecticut
The double-leaf Burnside Bridge, Portland, Oregon
Rolling lift Pegasus Bridge over the Caen Canal, Normandy, France
Single-leaf through truss with overhead counterweight, Seattle, Washington
The Ashtabula lift bridge, a Strauss bascule built in Ohio in 1925
The Strauss design Johnson Street Bridge across Victoria Harbour, British Columbia, built in 1924
Bascule bridge in Montceau-les-Mines, France
Command center of the Fremont Bridge in Seattle, Washington
Wabash Avenue Bridge in Chicago, Illinois, honored for its elegance by the American Institute of Steel Construction in 1930
Pamban Bridge in Rameswaram, India, over the Palk Strait
|Wikimedia Commons has media related to Bascule bridges.|
- Wood Wortman, Sharon; Wortman, Ed (2006). The Portland Bridge Book (3rd Edition). Urban Adventure Press. pp. 32, 35. ISBN 0-9787365-1-6.
- "Landmark Designation Report: Historic Chicago Bridges" (PDF). Commission on Chicago Landmarks. September 2006 (revised September 2007). pp. 12, 15 (pdf pages 14, 17). Retrieved April 21, 2013.
- "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.
- Koglin, Terry L. (2003), "4. Bascule Bridges", Movable bridge engineering, John Wiley and Sons, ISBN 978-0-471-41960-0, retrieved May 25, 2009.
- "Tower Bridge Exhibition". Archived from the original on 12 May 2009. Retrieved May 25, 2009.