bascules

A bascule is a type of movable bridge in which a leaf or leaves pivot upward to open a passage for water or road traffic, typically balanced by counterweights. The name comes from the French basculer, meaning to tilt or tilt over.

Most bascules have one or two leaves that rotate about a horizontal hinge. Counterweights balance the weight of the leaf, allowing the span to rise with relatively little energy. Opening mechanisms vary: older bridges used hydraulic systems or steam power, while modern examples commonly employ electric motors, winches, or rack-and-pinion drives. Locks and safety devices secure the leafs when closed.

There are single-leaf and double-leaf bascules. In a single-leaf design, one moving segment lifts from a pivot near the shore; in a double-leaf design, two leaves meet at the center and rise together. The compact, robust structure makes bascules suited to urban locations where space is limited, and where rapid opening is advantageous for permitting vessel passage.

Notable examples include Tower Bridge in London, a famous combined bascule and suspension bridge completed in 1894. Bascules remain a common solution for river crossings in North America and Europe, with numerous bridges still using counterweighted leaves to balance opening and closing cycles. See also drawbridges and movable bridges.