Tower Bridge

photograph of tower bridge from the south bank of the River Thames

Tower Bridge, London

Tower Bridge, London, is one of the most famous bridges in the world and is instantly recognisable with its twin gothic towers of stone, high level walkway and suspension links spanning the River Thames.  It was built between 1886 and 1894 at a time when the nearest bridge, London Bridge, had become seriously congested. Since 1874 public bodies had been petitioning the Corporation of the City of London to provide a new bridge for the growing East End population, downriver from London Bridge. A survey of London Bridge in August 1882 had shown that 22,242 vehicles and 110,525 pedestrians had passed over the bridge in a 24 hour period, causing delays of hours. It was not until an Act of Parliament was passed in 1885, however, that building work on Tower Bridge was allowed to proceed.

Design

The bridge was designed by Horace Jones, the City Architect, in collaboration with John Wolfe Barry. It is a combined suspension and bascule bridge, designed to allow the passage of tall ships along the River Thames without restricting pedestrian traffic across the river. Despite its stone exterior it is actually a steel bridge and it is the steel superstructure built by Sir William Arrol & Company Ltd. that gives it its strength. Decorative cladding in Cornish granite and Portland stone was added to ‘soften’ the look of the bridge and allow it to blend in more to its historic surroundings  with the Tower of London nearby.

Architectural drawing of Tower Bridge

Tower Bridge Design

The bridge consists of 3 spans, of which the 200 foot central opening span is formed of 2 bascules which can be raised to provide a headway of 140 feet at high tide between the river and the high level walkways above. The two side spans, each 270 feet long are suspended on chains between the small abutment towers on shore and the tall towers built on piers in the river.  The chains are anchored into the ground at the shore ends and connected by tie bars running along the upper walkway between the tall towers where they anchor each other.  The upper walkways themselves are formed of two cantilevers extending 55 feet from each tower with suspended girders completing the remaining distance of 120 feet.

Construction

Arrol’s were just one of 8 contractors on the bridge, the main ones being:
Piers and abutments – Sir John Jackson
Steel Superstructure – Sir William Arrol & Company Ltd.
Hydraulics – Sir William Armstrong, Mitchell & Company Ltd.
Masonry superstructure – Perry & Company.

Piers

Work first began on the bridge in April 1886 with the building of the piers. Progress was slow as the piers had to be built one at a time in order that a clear passage was left for ships. Eight caissons were used to build foundations for each pier. These were sunk into the clay, 19 feet below the riverbed, and filled with concrete.  Brickwork piers were then built on top of the concrete and clad with granite to a height of 4 feet above high water level.  A further complication in building the piers was that a massive chamber had to be created in each one to house the short, counterbalancing arms of the bascules and the hydraulic machinery used to move them. Finally, in April 1890 the piers were complete and work on the steel superstructure could begin.

Black and white photograph showing the metal structure of the towers and arms of the upper walkways extending towards each other.

Construction of Tower Bridge towers and high level walkway.

Steelwork

Over 11,000 tons of steel provided the framework for the towers and walkways and all the components were made at Arrol’s Dalmarnock Works in Glasgow, and shipped to London.  Between 50 and 100 tons were sent by steamer each week to within 0.5 miles of the site where it was loaded onto barges to be transported to the bridge.  Any components needed urgently were delivered by train. Each section had its rivet holes pre-drilled in Glasgow, to within 0.5 inch of their final diameter to allow for final adjustments to be made once the girder was in position.  Arrol subcontracted the manufacture of the ornamental cast iron parapets and decorative panels for the upper walkways to Fullerton, Hodgart & Barclay, Paisley.

Each tower consisted of 4 octagonal columns, connected horizontally at the landing levels with 6 foot wide plate girders and smaller diagonal girders used to cross-brace the structure between.  Each tower had 3 levels, the uppermost providing access to the high level walkways, which were reached by steel stairs or hydraulic lifts built inside the towers. The suspension chains consisted of lower and upper girders, linked together and cross-braced between to provide rigidity.  These were built in their final positions and supported by a wooden framework which could not be removed until all the sections were in position and pinned together.

As with the building of the piers, during the steelwork construction a waterway of 160 feet wide had to be left clear for passage of ships.  This meant that the arms of the bascules had to be built in the vertical position. Each arm consisted of 4 parallel girders 160 feet long, spaced 13.5 feet apart and each bascule of a total weight of 12oo tons.

It was dangerous working at heights up to 140 feet above the high water mark and exposed to the wind, rain and cold. Plating, fitting and riveting were all highly skilled, high risk jobs, being carried out at these great heights on temporary platforms.  Many of the workers at Tower Bridge had previously been engaged on other Arrol bridge constructions or were from the shipyards and experienced in working at heights. With time workmen would become indifferent to the danger but working in a confined space over a busy waterway they had to be careful not to drop rivets, materials or tools which could damage boats and injure people passing below.  Over the summer of 1893, at the peak of construction, there were over 280 men and boys working on the steelwork.  They worked in 2 independent teams, one on the north bank and one on the south bank, working an average of 56.5 hours a week, Monday to Saturday.

The steel was given three layers of brown paint to protect it from the elements and manholes were built into the steel columns so that even their interior could be painted.  The stone cladding also offered some protection although during construction care had to be taken that the masonry was not cemented to the steel.  Any adhesion to the steel could have led to hazardous movement in the stonework and so a canvas layer was placed between the steel columns and the stone walls. Where the stonework prevented access for painting, the steelwork was coated with a layer of Portland cement.

 Hydraulics

The long arms of the bascules were balanced at the short end by ballast boxes loaded with 300 tons of lead and 130 tons of cast iron. The counterbalanced system meant that no energy was required to lift the actual weight of the arms.  However, energy was required to overcome the pressure from wind applying an additional load to the arms and also, to a lesser extent, to overcome the forces of inertia to get the basules moving and friction on the racks and pinions on which the bascules pivoted.

Power to move the bascules and operate lifts inside the towers was provided by hydraulics, designed and installed by Sir William Armstrong, Mitchell & Company Ltd., Newcastle upon Tyne. There were 2 sets of hydraulics equipment which ensured that if one failed, another was available as backup. From the pumping station on the south shore, two 360hp steam engines, powered by coal-fed boilers, pumped water to the six accumulators, two in the accumulator tower and two in each pier.  The accumulators were large water tanks having a 20 inch ram topped by a heavy weight which stored the water under pressure. When the bridge had to be opened the high pressure water was fed to the driving engines which raised the bascules. This system allowed the bridge to be raised and lowered within 5 minutes.

Completion

The bridge was finally completed in 1894, after 8 years construction, at a cost of £1,184,000 (equivalent to £122 million in 2015).  It was officially opened by Edward, Prince of Wales on 30 June 1894 followed by a 10 gun salute and flotilla of ships sailing under the bridge.

In the 2 years following the official opening the bascules were raised an average of 18 times a day.  However, the bridge was so successful in providing a fast turnaround in opening and closing the bascules that people chose to wait and cross the bridge at the lower level rather than using the upper walkways and by 1910 these were closed to pedestrians.

Today the bridge is as popular as it was when built, although the horse carriages have been replaced by cars and the decline in tall ships has meant that the bascules do not need to be raised as often.  Hydraulics are still used to raise the bascules but these are now powered by electricity.  The bridge has become one of the most iconic symbols of Britain, a lasting remnant of ingenious Victorian engineering.