Bascule bridge

Template:Short description Template:BridgeTypePix

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.

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

Template:Redirect

There are three types of bascule bridge<ref>Template:Cite book</ref> 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.<ref name="chicago landmarks">Template:Cite web</ref> There are 44 movable bridges in Chicago, however 12 are not in operation. The Jackknife Bascule Bridge in Fort William, Ontario (now Thunder Bay, Ontario) was the first double-decked bascule bridge in the world, accommodating rail on the bottom and road/foot traffic on top. It was designed by Joseph Strauss for the Canadian Pacific Railway.

The Template:Vanchor 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.<ref>Template:Cite patent</ref>

The rarer Rall type combines rolling lift with longitudinal motion on trunnions when opening.<ref name="wood-wortman"/> It was patented (1901) by Theodor Rall.<ref name="chicago landmarks"/><ref name="wood-wortman">Template:Cite book</ref><ref name="Rall patent">Template:Cite web</ref> One of the few surviving examples is the Broadway Bridge (1913), in Portland, Oregon.<ref name="wood-wortman"/><ref name="haer-broadwaybridge">Template:Cite web</ref>

The counterweight is often hidden below the road surface in the bridge abutment.<ref>Template:Cite web</ref>
Animation of a double-leaf Strauss fixed-trunnion bridge (based on engineering drawings from the Henry Ford Bridge)
Animation of a rolling lift bridge (such as the Pegasus Bridge)
A rolling-lift bascule bridge in the down position