London Bridge

Design

Elevation, with dimensions

Tower Bridge under construction, 1892
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.
The bridge largely replaced Tower Subway, 400 m to the west, the world's first underground tube railway (1870). Until the bridge was opened, the subway was the shortest way to cross the river from Tower Hill to Tooley Street in Southwark.
A Special Bridge or Subway Committee was formed in 1876, chaired by A. J. 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),[1] was approved.
Jones' engineer, Sir John Wolfe Barry devised the idea of a bascule bridge 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 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 Arrol[2] – and employed 432 construction workers. Two massive piers, containing over 70,000 tons of concrete,[3] were sunk into the river bed to support the construction. Over 11,000 tons of steel provided the framework for the towers and walkways.[4] 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.[5] Stevenson replaced Jones' original brick facade with the more ornate Victorian Gothic style that makes the bridge a distinctive landmark.
The bridge was opened on 30 June 1894 by the Prince of Wales, the future King Edward VII, and his wife, Alexandra of Denmark.

[edit] Hydraulic system

One of the original steam engines: a horizontal twin-tandem compound engine, fitted with Meyer expansion slide valves
The original raising mechanism was powered by pressurised water stored in six hydraulic accumulators.
The system was designed and installed by Sir W. G. Armstrong Mitchell & Company of Gateshead. Water, at a pressure of 750psi, was pumped into the accumulators by two 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.[6]
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.