Grades 3–5 · Machines
How does a bascule bridge lift its road?
A road can cross a river and still open a path for a tall vessel. In a bascule bridge, a section of road turns around a strong pivot. Bascule comes from a French word for a seesaw.
In the diagramA counterweight balances the road leafIdeal leaf: 100 t × 6 m = 300 t × 2 mCounterweightPivotWaterwayRoad arm300 tat 2 m100 tat 6 mRaised 30°; counterweight moves down
The longer part of the moving structure reaches over the water. A heavy counterweight sits behind the pivot on a shorter arm. As the road rises, the counterweight moves downward into a space below. Its weight helps balance the turning effect of the long road span.
An engine or electric motor supplies the remaining work through a drive system. Balance reduces the effort needed, but it does not remove friction, acceleration or the need for control. The road is closed to traffic before lifting, and the span must be supported and secured for road use.
The illustration shows one idealized leaf. Some bridges have two moving leaves that meet in the middle. The matching weight-and-distance products explain balance here; they are not measurements of a particular bridge.
About this explanation
Starting knowledge: Recognize a pivot and the turning effect of a weight at a distance.
Sources
- Tower Bridge — How does Tower Bridge work?
- Tower Bridge — What type of bridge is Tower Bridge?