Bascule bridges

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, automobile, truck, light rail, heavy rail
Span range	Short
Material	Steel
Movable	Yes
Design effort	Medium
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 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.

There are three types of bascule bridge designs, and counterweights required to balance a bascule's span may be located either 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.

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

Mystic River Bascule Bridge, Mystic, Connecticut

Rolling lift Pegasus Bridge over the Caen Canal, Normandy, France