Bascule bridge

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.

Bascule bridge

This animation shows the movement of a double-leaf bascule.
Ancestor	Drawbridge, Plate girder bridge, cantilever bridge
Related	Lift bridge, swing bridge
Descendant	None
Carries	Pedestrian, bicycle, automobile, truck, light rail, heavy rail
Span range	Short
Material	Steel
Movable	Yes
Design effort	Medium
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.

History

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.

Types

"Strauss bascule" redirects here. For the company started by its inventor, see Joseph Strauss (engineer) § Early career and the bascule bridge.

A road sign indicating a bascule bridge ahead

There are three types of bascule bridge^[1] and the counterweights to the span may be located 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.^[2]

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.^[3]

The rarer Rall type combines rolling lift with longitudinal motion on trunnions when opening.^[4] It was patented (1901) by Theodor Rall.^[2][4][5] One of the few surviving examples is the Broadway Bridge (1913), in Portland, Oregon.^[4][6]

• 
  The counterweight is often hidden below the road surface in the bridge abutment.^[7]
• 
  Animation of a double-leaf Strauss fixed-trunnion bridge (based on engineering drawings from the Henry Ford Bridge)
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  Animation of a rolling lift bridge (such as the Pegasus Bridge)
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  Bascule bridge in the down position

See also

• Double-beam drawbridge
• Drawbridge
• Johnson Street Bridge
• List of bascule bridges
• Moveable bridges for a list of other movable bridge types
• Straussbrug (in Dutch)