Creative Images - Stock Photo Library / London-011

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 London-011

London-011.jpg

Tower bridge

Tower Bridge is a combined bascule and suspension bridge over the River Thames. It is close to the Tower of London, which gives it its name. It has become an iconic symbol of London and is sometimes mistakenly called London Bridge, though London Bridge is in fact the next bridge upstream. In the second half of the 19th century, increased commercial development in the East End of London led to a requirement for a new river crossing downstream of London Bridge. A traditional fixed bridge could not be built because it would cut off access to the port facilities in the Pool of London, between London Bridge and the Tower of London.

Jones' engineer, Sir John Wolfe Barry devised the idea of a bascule bridge 800ft (244m) in length with two towers each 213ft (65m) high, built on piers. The central span of 200ft (61m) between the towers was split into two equal bascules or leaves, which could be raised to an angle of 83 degrees to allow river traffic to pass. The bascules, weighing over 1,000 tons each, were counterbalanced to minimize the force required and allow raising in five minutes. The two side-spans are suspension bridges, with the suspension rods anchored both at the abutments and through rods contained within the bridge's upper walkway.

Construction started in 1886 and took eight years. With five major contractors – Sir John Jackson (foundations), Baron Armstrong (hydraulics), William Webster, Sir H.H. Bartlett, and Sir William Arro and employed 432 construction workers. Over 11,000 tons of steel provided the framework for the towers and walkways. This was then clad in Cornish granite and Portland stone, both to protect the underlying steelwork and to give the bridge a pleasing appearance.

The Prince of Wales, the future King Edward VII, and his wife, Alexandra of Denmark, opened the bridge on 30 June 1894. The original raising mechanism was powered by pressurised water stored in six hydraulic accumulators. In 1974, the original operating mechanism was largely replaced by a new electro-hydraulic drive system. The only components of the original system still in use are the final pinions, which engage with the racks fitted to the bascules. Modern hydraulic motors and gearing using oil rather than water as hydraulic fluid.

Some of the original hydraulic machinery has been retained, although it is no longer in use. It is open to the public and forms the basis for the bridge's museum, which resides in the old engine rooms on the south side of the bridge. The museum includes the steam engines, two of the accumulators and one of the hydraulic engines that moved the bascules, along with other related artifacts. The high-level walkways between the towers were closed in 1910; they have been reopened as part of the Tower Bridge Experience, an exhibition mostly housed in the bridge's twin towers. River traffic is now a fraction of what it used to be, but it still takes priority over road traffic, 24 hours' notice is required before opening the bridge.



Copyright ©2004 John Reali