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 is a drawbridge with a counterweight that continuously balances the span, or "leaf," throughout the entire upward swing in providing clearance for boat traffic. Bascule is a French term for seesaw and balance, and bascule bridges operate along the same principle. They are the most common type of movable bridge in existence because they open quickly and require relatively little energy to operate.
Bascule bridges may be single or double leaf. Double leaf bridges usually have any truss structure and counterweights below the deck, while single leaf bridges typically are comprised of a truss bridge with an elevated counterweight.
History
Although the bascule bridge has been in use since ancient times, it was not until the 1850s that engineers developed the ability to move very long, heavy spans quickly enough for practical application. Nikolaevsky Bridge across the Neva in Saint Petersburg was the first large bascule bridge, opened in 1850. Since then, all bridges across the Neva and other major rivers in the city (21 in total) were bascule to facilitate navigation, which prevented the city's inhabitants from traveling across the river at night (this remained so until 2003 when the first cable-stayed bridge across the Neva was opened).
Design
Counterweights may be located above the bridge or below the deck of the bridge. There are two common designs of bascule bridge. One is the fixed-trunnion bascule design, which is where the bridge rotates around a large axle called a trunnion to raise. This bridge type is sometimes called the 'Chicago bascule' as this type was developed and perfected there and is used for many of that city's river crossings. Joseph Strauss was a key person who worked on improving the trunnion bascule bridge. Another form of bascule bridge is the Scherzer rolling lift. The city of Joliet, Illinois has a number of this structure type. The Scherzer rolling lift bridge essentially rolls or rocks like a simple rocking chair on a track to raise.
Types of bascule bridges
- Scherzer rolling lift bascule bridge
- Rall bascule bridge, patented by Theodore Rall in 1901.
- Strauss bascule bridge, patented by Joseph Strauss.
- Chicago or fixed-trunnion bascule bridge.
Tower bridge
Probably the most famous bascule bridge in the world is Tower Bridge across the Thames in London, UK. Originally, Tower Bridge was a hydraulically operated bridge, using steam power from coal-burning boilers to pump river water into six hydraulic accumulators so that power was readily available when required. The water for the boilers was provided by a well. The hydraulic accumulators powered the bascule engines, which raised and lowered the bascules. Today, the bascule mechanism is driven by oil and electricity rather than by water and steam.
See also
- Bridge
- Drawbridge
- List of bascule bridges
- Moveable bridge for a list of other movable bridge types.
- Pegasus Bridge for an example of a rolling bascule bridge, employing a rack and pinion rather than hinge pins.
External links
- Learn about the movable bridges of Chicago, Illinois. Most of Chicago's bridges are bascule bridges and are of the trunnion type.
- Learn about the movable bridges of Joliet, Illinois. Most of Joliet's bridges are bascule bridges and are of the Scherzer rolling lift type.
- Learn about the six Bridges of Alameda County, over the Oakland-Alameda Estuary. Four bridges are for cars, one is for trains and another is the only drawbridge in the United States solely for bicycles and pedestrians.