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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 (sometimes referred to as a drawbridge) is a moveable bridge with a counterweight that continuously balances the span, or "leaf," throughout the entire upward swing in providing clearance for boat traffic.
Bascule is a French term for seesaw and balance, and bascule bridges operate along the same principle. They are the most common type of movable bridge in existence because they open quickly and require relatively little energy to operate.
Bascule bridges may be single or double leaf.
Although small bascule bridges have been in use since ancient times, it was not until the 1850s that engineers developed the ability to move very long, heavy spans quickly enough for practical application. The Blagoveshchensky Bridge across the Neva River in Saint Petersburg was the first large bascule bridge, opened in 1850. Since then, all bridges across the Neva and other major rivers in the city (21 in total) were bascule to facilitate navigation, which prevented the city's inhabitants from travelling across the river at night (this remained so until 2003 when the first cable-stayed bridge across the Neva was opened).
Counterweights may be located above the bridge or below the deck of the bridge. There are two common designs of bascule bridge. One is the fixed-trunnion bascule design, which is where the bridge rotates around a large axle called a trunnion to raise. This bridge type is sometimes called the Chicago bascule, as this type was developed and perfected there and is used for many of that city's river crossings. Joseph Strauss was a key person who worked on improving the trunnion bascule bridge. Another form of bascule bridge is the Scherzer rolling lift bridge, also known as a Rolling Bascule Bridge. The city of Joliet, Illinois has a number of this structure type. The Scherzer rolling lift bridge essentially rolls or rocks like a simple rocking chair on a track to raise.
- Scherzer rolling lift bascule bridge
- Rall bascule bridge, patented by Theodor Rall in 1901.
- Strauss bascule bridge, patented by Joseph Strauss.
- Chicago or fixed-trunnion bascule bridge.
The least common of these types was the Rall, 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.
Tower Bridge 
Tower Bridge across the Thames in London is a famous bascule bridge. Originally, Tower Bridge was a hydraulically operated bridge, using steam power from coal-burning boilers to pump river water into six hydraulic accumulators so that power was readily available when required. The water for the boilers was provided by a well. The hydraulic accumulators powered the bascule engines, which raised and lowered the bascules. Today, the bascule mechanism is driven by oil and electricity rather than by water and steam.
A double-leaf bascule bridge (Burnside Bridge, Portland, Oregon)
Rolling lift Pegasus Bridge
Single-leaf through truss with overhead counterweight, Seattle, Washington
The Ashtabula lift bridge, a Strauss bascule bridge built in 1925
Bascule bridge in Montceau-les-Mines, France
A view from a Bridge command center on the Fremont Bridge in Seattle, Washington, where the Bridge Operator is controlling the bridge.
See also 
|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.
- 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.
- "Tower Bridge Exhibition". Archived from the original on 12 May 2009. Retrieved May 25, 2009.