Tower Bridge, London

The bridge consists of two bridge towers tied together at the upper level by two horizontal walkways, designed to withstand the horizontal tension forces exerted by the suspended sections of the bridge on the landward sides of the towers.

The vertical components of the forces in the suspended sections and the vertical reactions of the two walkways are carried by the two towers.

Tower Bridge (built 1886–1894) is a combined bascule and suspension bridge in London. The bridge crosses the River Thames close to the Tower of London and has become an iconic symbol of London.

Tower Bridge is one of five London bridges now owned and maintained by the Bridge House Estates, a charitable trust overseen by the City of London Corporation

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.

The total cost of construction was £1,184,000 (equivalent to £120 million in 2015)

CONSTRUCTRION

Construction started in 1887 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

The two side-spans are suspension bridges, each 270 feet (82 m) long, with the suspension rods anchored both at the abutments.

BRIDGE CONSTRUCTION

The bridge is 800 feet (244 m) in length with two towers each 213 feet (65 m) high, built on piers. The central span of 200 feet (61 m) between the towers is split into two equal bascules or leaves, which can be raised to an angle of 86 degrees to allow river traffic to pass. The bascules, weighing over 1,000 tons each, are counterbalanced to minimise the force required and allow raising in five minutes.

Water, at a pressure of 750 psi (5.2 MPa), was pumped into the accumulators by two 360 hp (270 kW) horizontal twin-tandem compound stationary steam engines, fitted with Meyer expansion slide valves. Each engine drove a force pump from its piston tail rod.

The accumulators each comprise a 20 inches (51 cm) ram on which sits a very heavy weight to maintain the desired pressure.

Hydraulic System

The original raising mechanism was powered by pressurised water stored in several hydraulic accumulators. The system was designed and installed by Hamilton Owen Rendel while working for Sir W. G. Armstrong Mitchell & Company of Newcastle upon Tyne.

Modernisation

In 1974, the original operating mechanism was largely replaced by a new electro-hydraulic drive system, designed by BHA Cromwell House.

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

2008–2012 facelift• In April 2008 it was announced that the bridge would undergo a 'facelift' costing £4

million, and taking four years to complete. It is intended that the completed work will stand for 25 years.

• The renovation of the walkway interior was completed in mid-2009. • The renovation of the four suspension chains was completed in March 2010 using a state-

of-the-art coating system requiring up to six different layers of 'paint'.• In mid-2012, The renovation was completed and was used for the opening summer

Olympics and Paralympics

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