Engineering

How the bascules, towers and walkways work.

A bridge in three parts

Tower Bridge combines two bridge types. The central span is formed by two bascules, which are movable leaves that pivot upward. The two side spans are suspension spans, hung from chains anchored in the towers and at the shore ends.

The bascules

A bascule works like a drawbridge: each leaf pivots at its base and is balanced by a heavy counterweight, so relatively little force is needed to raise it. When a large vessel needs to pass, the road is closed to traffic and both leaves rise to let it through. Each leaf weighs about 1,000 tonnes. They can be raised to an angle of 86 degrees, and opening or closing takes about a minute.

The towers

Each tower stands on a large pier in the river. Underneath the stone cladding is a steel frame, which carries the weight of the bridge. The cladding is Portland stone and Cornish granite, added for appearance and to protect the steelwork.

The high-level walkways

Two walkways link the tops of the towers high above the roadway. They were designed so that pedestrians could cross even when the bascules were raised. Today they form part of the visitor attraction, with views along the Thames.

Power

The bridge was designed to be raised using hydraulic power. Steam engines pumped water into accumulators, which stored energy under pressure, and the stored energy drove the engines that lifted the bascules. Modern electric and oil-based hydraulic drives replaced the steam system in the 1970s. The original engines and boilers are preserved and can be seen on a visit.