.
Tower Bridge is a
Grade I listed combined
bascule and
suspension bridge in
London,
built between 1886 and 1894, designed by
Horace Jones and engineered by
John Wolfe Barry. The bridge crosses the
River Thames close to the
Tower of London and is one of five London bridges owned and maintained by the
Bridge House Estates, a charitable trust founded in
1282. The bridge was constructed to give better access to the
East End of London, which had expanded its commercial potential in the 19th century.
Prince of Wales and
Princess of Wales opened the bridge in
1894.
The bridge is 800 feet (240 m) in length and consists of two 213-foot (65 m)
bridge towers connected at the upper level by two horizontal walkways, and a central pair of
bascules that can open to allow shipping.
Originally hydraulically powered, the operating mechanism was
converted to an electro-hydraulic system in 1972. The bridge is part of the
London Inner Ring Road and thus the boundary of the
London congestion charge zone, and remains an important traffic route with 40,000 crossings every day. The bridge deck is freely accessible to both vehicles and pedestrians, whereas the bridge's twin towers, high-level walkways and
Victorian engine rooms form part of the Tower Bridge Exhibition.
Tower Bridge has become a recognisable London landmark. It is sometimes confused with
London Bridge, about 0.5 miles (0.80 km) upstream, which has led to a popular
urban legend about an American purchasing the wrong bridge. Several stunt pilots have flown underneath the bridge, including the pioneering
Francis McClean.
A
bascule bridge (also referred to as a
drawbridge or a
lifting bridge) is a
moveable bridge 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 to operate, while providing the possibility for unlimited vertical clearance for marine traffic.
Bascule bridges have been in use since ancient times, but until the adoption of
steam power in the 1850s, very long, heavy spans could not be moved quickly enough for practical application.
There are three types of bascule bridge, and the
counterweights to the span may be located above or below the bridge deck.
The
fixed-Trunnion (sometimes a "Chicago" bascule) rotates around a large
axle 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
rolling lift 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.
