Bascule bridge

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title: "Bascule bridge" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["bascule-bridges", "bridges-by-structural-type"] description: "Moveable bridge with a counterweight which keeps the span(s) balanced during the upswing" topic_path: "general/bascule-bridges" source: "https://en.wikipedia.org/wiki/Bascule_bridge" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

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

|type_name= Bascule bridge |image=File:Wells Street Bridge 070512.jpg |image_title=The Wells Street Bridge (Chicago), a double-leaf fixed-trunnion bascule bridge |sibling_names= Lift bridge, swing bridge |descendent_names= None |ancestor_names= Drawbridge, Plate girder bridge, cantilever bridge |carries= Pedestrian, bicycle, automobile, truck, light rail, heavy rail |span_range= Short |material= Steel |movable= Yes |design= Medium |falsework= Site and prefabrication specific |}}

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

::figure[src="https://upload.wikimedia.org/wikipedia/commons/8/81/Vienna_Convention_road_sign_Aa-5-V1.svg" caption="A road sign indicating a bascule bridge (or other moveable bridge) ahead"] ::

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. 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 **** 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 bascule consists of a trunnion integral to the bascule leaf which moves horizontally away from the channel along a track. A pivotally connected strut links the leaf to a fixed point in the structure, constraining the motion of the leaf. As the trunnion travels along the track, this linkage causes the leaf to rotate about the trunnion, converting horizontal motion into angular motion. It was patented in 1901 by Theodor Rall. One of the few surviving examples is the Broadway Bridge (1913), in Portland, Oregon, though it is driven through the pivotal link rather than a motor on the trunnion.

File:Amaliabrug_(Waddinxveen)_-_opening_2.jpg|The counterweight is often hidden below the road surface in the bridge abutment. File:Ford Bridge Schematic open-close.gif|Animation of a double-leaf Strauss fixed-trunnion bridge (based on engineering drawings from the Henry Ford Bridge) File:MovableBridge roll.gif|Animation of a rolling lift bridge (such as the Pegasus Bridge) File:Bascule bridge.jpg|A rolling-lift bascule bridge in the down position

References

References

1. Koglin, Terry L.. (2003). "Movable bridge engineering". John Wiley and Sons.
2. (September 2007). "Landmark Designation Report: Historic Chicago Bridges". [[Commission on Chicago Landmarks]].
3. "Lift-Bridge".
4. (1901). "Patent number 669348: T. Rall movable bridge". [[United States Patent and Trademark Office]] (referenced online by [[Google Patents]]).
5. Wood Wortman, Sharon. (2006). "The Portland Bridge Book". Urban Adventure Press.
6. Historic American Engineering Record. "Broadway Bridge, Spanning Willamette River at Broadway {{Sic". [[Library of Congress]].
7. Van Zantvliet, P. S.. (June 2015). "Analysis of the force distribution on operating mechanisms in a bascule bridge".

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