A bascule bridge (also referred to as a drawbridge or a lifting bridge) is a
moveable bridge
A moveable bridge, or movable bridge, is a bridge that moves to allow passage for boats or barges. In American English, the term is synonymous with , and the latter is the common term, but drawbridge can be limited to the narrower, historical d ...
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
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
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
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be tra ...
in the 1850s, very long, heavy spans could not be moved quickly enough for practical application.
Types
There are three types of bascule bridge and the
counterweight
A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less we ...
s to the span may be located above or below the bridge deck.
The fixed-
trunnion
A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.
Alternatively, a trunnion is a shaft that positions a ...
(sometimes a "Chicago" bascule) rotates around a large
axle
An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...
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.
It was patented (1901) by Theodor Rall.
One of the few surviving examples is the
Broadway Bridge (1913), in Portland, Oregon.
abutment
An abutment is the substructure at the ends of a bridge span or dam supporting its superstructure. Single-span bridges have abutments at each end which provide vertical and lateral support for the span, as well as acting as retaining wal ...
.
See also
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Double-beam drawbridge
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Drawbridge
A drawbridge or draw-bridge is a type of moveable bridge typically at the entrance to a castle or tower surrounded by a moat. In some forms of English, including American English, the word ''drawbridge'' commonly refers to all types of movea ...
*
Johnson Street Bridge
Four known bridges have spanned the narrows between the Inner Harbour and Upper Harbour of Victoria, British Columbia, connecting Johnson Street on the east shore with Esquimalt Road on the west shore. The current bridge is Canada's largest sin ...
*
List of bascule bridges
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Moveable bridges
A moveable bridge, or movable bridge, is a bridge that moves to allow passage for boats or barges. In American English, the term is synonymous with , and the latter is the common term, but drawbridge can be limited to the narrower, historical d ...
for a list of other movable bridge types
*
Straussbrug
References
External links
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{{DEFAULTSORT:Bascule Bridge
Bridges by structural type