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Tower Bridge is a combined bascule and suspension bridge in London, England, 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.

The bridge consists of two towers which are tied together at the upper level by means of two horizontal walkways which are designed to withstand the horizontal forces exerted by the suspended sections of the bridge on the landward sides of the towers. The vertical component of the forces in the suspended sections and the vertical reactions of the two walkways are carried by the two robust towers. The bascule pivots and operating machinery are housed in the base of each tower. Its present colour dates from 1977 when it was painted red, white and blue for the Queen's Silver Jubilee. Originally it was painted a chocolate brown colour.

Tower Bridge is sometimes mistakenly referred to as London Bridge, which is actually the next bridge upstream. A popular urban legend is that in 1968, Robert McCulloch, the purchaser of the old London Bridge that was later shipped to Lake Havasu City, Arizona, believed that he was in fact buying Tower Bridge. This was denied by McCulloch himself and has been debunked by Ivan Luckin, the seller of the bridge.

The nearest London Underground station is Tower Hill on the Circle and District Lines, and the nearest Docklands Light Railway station is Tower Gateway.

Overview

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.

A Special Bridge or Subway Committee was formed in 1876, chaired by Sir Albert Joseph Altman, to find a solution to the river crossing problem. It opened the design of the crossing to public competition. Over 50 designs were submitted, including one from civil engineer Sir Joseph Bazalgette. The evaluation of the designs was surrounded by controversy, and it was not until 1884 that a design submitted by Horace Jones, the City Architect (who was also one of the judges), was approved.

Jones' engineer, Sir John Wolfe Barry, devised the idea of a bascule bridge with two towers built on piers. The central span was split into two equal bascules or leaves, which could be raised to allow river traffic to pass. The two side-spans were suspension bridges, with the suspension rods anchored both at the abutments and through rods contained within the bridge's upper walkways.

Building The Bridge

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 Arrol & Co. – and employed 432 construction workers. E W Crutwell was the resident engineer for the construction.

Two massive piers, containing over 70,000 tons of concrete, were sunk into the riverbed to support the construction. 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.

Jones died in 1887 and George D. Stevenson took over the project. Stevenson replaced Jones' original brick facade with the more ornate Victorian Gothic style, which makes the bridge a distinctive landmark, and was intended to harmonise the bridge with the nearby Tower of London. The total cost of construction was £1,184,000 (£100 million as of 2010).

Hydraulic system
One of the original steam engines: a 360 hp horizontal twin-tandem compound engine, fitted with Meyer expansion slide valves

The original raising mechanism was powered by pressurised water stored in several hydraulic accumulators.Hydraulics[›][clarification needed]

The system was designed and installed by Sir W. G. Armstrong Mitchell & Company of Newcastle upon Tyne. Water, at a pressure of 750 psi, was pumped into the accumulators by two 360 hp stationary steam engines, each driving a force pump from its piston tail rod. The accumulators each comprise a 20-inch ram on which sits a very heavy weight to maintain the desired pressure.

In 1974, the original operating mechanism was largely replaced by a new electro-hydraulic drive system, designed by BHA Cromwell House. The only components of the original system still in use are the final pinions, which engage with the racks fitted to the bascules. These are driven by modern hydraulic motors and gearing, using oil rather than water as the hydraulic fluid.[10]

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 artefacts.

The third Steam Engine

During World War II, as a precaution against the existing engines being damaged by enemy action, a third engine was installed in 1942:[11] a 150 hp horizontal cross-compound engine, built by Vickers Armstrong Ltd. at their Elswick works in Newcastle upon Tyne. It was fitted with a flywheel having a 9-foot diameter and weighing 9 tons, and was governed to a speed of 30 rpm.

The engine became redundant when the rest of the system was modernised in 1974, and was donated to the Forncett Industrial Steam Museum by the Corporation of the City of London.

Navigation control

To control the passage of river traffic through the bridge, a number of different rules and signals were employed. Daytime control was provided by red semaphore signals, mounted on small control cabins on either end of both Bridge Piers. At night, coloured lights were used, in either direction, on both piers: two red lights to show that the bridge was closed, and two green to show that it was open. In foggy weather, a gong was sounded as well.

Vessels passing through the bridge had to display signals too: by day, a black ball at least 2 feet (0.61 m) in diameter was to be mounted high up where it could be seen; by night, two red lights in the same position. Foggy weather required repeated blasts from the ship's steam whistle.

If a black ball was suspended from the middle of each walkway (or a red light at night) this indicated that the bridge could not be opened. These signals were repeated about 1,000 yards (910 m) downstream, at Cherry Garden Pier, where boats needing to pass through the bridge had to hoist their signals/lights and sound their horn, as appropriate, to alert the Bridge Master.

Some of the control mechanism for the signalling equipment has been preserved and may be seen working in the bridge's museum.

River traffic

The bascules are raised around 1000 times a year. River traffic is now much reduced, but it still takes priority over road traffic. Today, 24 hours' notice is required before opening the bridge. In 2008, a local web developer created a Twitter feed to post live updates of the bridge's opening and closing activities.

A computer system was installed in 2000 to control the raising and lowering of the bascules remotely. Unfortunately it proved less reliable than desired, resulting in the bridge being stuck in the open or closed positions on several occasions during 2005, until its sensors were replaced.

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This entry was posted on Saturday, July 11th, 2009 at 7:29 pm and is filed under Trains. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.