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|
A bascule bridge (sometimes 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. 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 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.
Tower Bridge in London, perhaps the most famous and iconic bascule bridge of all time
The Birkenhead Bridge in Port Adelaide, Australia, fully opened
Cherry Street Strauss Trunnion Bascule Bridge at Toronto Harbour Shipping Channel, Toronto
The Strauss design Johnson Street Bridge across Victoria Harbour, British Columbia, Canada; built in 1924
Mystic River Bascule Bridge, Mystic, Connecticut
Palace Bridge in Saint Petersburg
Pamban Bridge over the Palk Strait in Rameswaram, India
Rolling lift Pegasus Bridge over the Caen Canal, Normandy, France
Bascule bridge in Montceau-les-Mines, France
The Ashtabula lift bridge, a 1925 Strauss bascule built in Ohio, US
Wabash Avenue Bridge in Chicago, Illinois, US