Салмақты көпірлер және олардың түрлері

Moveable bridge with a counterweight which keeps the span(s) balanced during the upswing

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. 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, but until the adoption of steam power in the 1850s, very long, heavy spans could not be moved quickly enough for practical application.

There are three types of bascule bridge 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. The 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. The rarer Rall type combines rolling lift with longitudinal motion on trunnions when opening. One of the few surviving examples is the Broadway Bridge (1913), in Portland, Oregon.