Designing London's Tower Bridge.
From Roman times until the mid-18th century, as London grew to become one of the world's most important cities, only one bridge crossed the River Thames: London Bridge. The Roman and Norman timbers had been replaced by stone, but the structure was woefully inadequate to handle the massive transfer of goods, carriages, and pedestrians across the river. By the mid-19th century, nearly 170,000 people and more than 20,000 vehicles crossed the bridge each day. Westminster Bridge opened in 1750, providing a crossing point for travelers to the west of London, but the British Parliament recognized the need for a new bridge east of London Bridge, where most of the commercial activity was. Tower Bridge was the answer.
The design had to accommodate several difficulties. Tall-masted ships brought cargoes from around the world to wharves upstream of the designated site, making it the busiest stretch of river in the world. Yet a bridge high enough to allow tall ships to sail beneath would have a roadway too steep for horse-drawn carriages to climb. With one-third of London's population living east of London Bridge, the city wanted a structure that tens of thousands of pedestrians could cross unimpeded by the frequent openings of a drawbridge.
In 1878, Horace Jones, designer of some of London's most important buildings and the architect for the City, presented his plan for a revolutionary part-opening, part-suspension bridge. He teamed up with John Wolfe Barry, considered the preeminent engineer in Britain, and together they made design changes to meet the stringent requirements of the Parliamentary Bridge Committee. Barry testified before the committee that he had consulted "six or seven engineers of repute, and therefore the bridge [being proposed] is not merely Mr. Jones's and my proposal, but it is a proposal that before it took any tangible shape was endorsed by all those gentlemen."
Like Jones, Barry came from a family whose professional references were impeccable. His father, Sir Charles Barry, designed the British Houses of Parliament, and his brothers Edward and Charles had created several London landmark buildings, including the Royal Opera House. Barry had contributed his engineering expertise on bridge, railway, and dock projects in London, Wales, Scotland, Ireland, and as far afield as the Suez Canal.
The winning design met all the criteria established by Parliament. It had a 200-foot-wide clear span for shipping, piers that neither interfered with the passage of ships nor the flow of water, approach-road inclines shallow enough for horse-drawn vehicles, and a vertical clearance of 135 feet above high water when the bridge was open. Steel, manufacturing methods for which had recently been improved, was chosen for the bridge's frame. To allay critics' claims that such a huge structure would detract from the adjacent Tower of London, the stone fortress dating back to William the Conqueror in the 11th century, the steel towers would be clad in stone.
Jones and Barry called their design a bascule bridge, borrowing the French word for seesaw. The roadways, or bascules, would be fully raised to allow ships to pass, yet pedestrians could cross uninterrupted, using the upper footbridge. The latest technology from the Industrial Revolution, hydraulic power, would be used to lift the bascules and the pedestrian footbridge elevators. The riverside towers bore the weight of the high-level walkways, and shoreside columns the weight of the suspension chains.
In an age when the building of bridges was at its peak, public adulation and national pride were lavished on their engineers and designers. Across the Atlantic, the Brooklyn Bridge, which opened in 1883, was compared to the Roman Colosseum and the hanging gardens of Babylon, and according to Harper's Weekly, it was "more wonderful than the Pyramids."
Barry estimated the bridge would take four years to complete, at a cost of [pounds]585,000 (about $2.8 million at the time). To complement the design team, some of the century's most notable figures were enlisted: Henry Brunel, son of the great engineer Sir Isambard Brunel, and George Stevenson, a master mason,' who was responsible for the design of the bridge's stone and granite cladding. Sir William Armstrong, considered the father of hydraulic engineering, would design and build the systems to open the bridge, and Sir William Arrol, the brilliant engineer and steel expert who had built the magnificent Forth Rail Bridge and other impressive structures from Scotland to Australia, was chosen as the contractor.
TOWER BRIDGE IS GOING UP
Work on Tower Bridge began on April 22, 1886. To build the foundations for the bridge piers, each of which is 189 feet long and 70 feet wide, 12 caissons were erected in place on timber staging and gradually lowered into the fast-flowing river at the site of each pier. Workers inside the caissons dug out the gravel and clay 25 feet deep, sending the mud topside by cranes. As the caissons sank deeper into the river bed, additional sections were added so that they ultimately became iron boxes 57 feet high, which were then filled with concrete. Because of a contract provision calling for a 160-foot clear passageway for ships at all times, the two piers could not be built simultaneously, so work proceeded slowly. Construction of the abutment piers was done inside coffer dams, and it took the contractor almost four years to build the abutment and main tower piers to a height of four feet above high water.
Steel for the framework was manufactured in Scotland, much of it in Arrol's own foundry, and sent by ship to docks within half a mile of the bridge site. Each piece was limited to a 5-ton maximum to make handling easier. Arrol developed a revolutionary hydraulic riveter that significantly enhanced the productivity of the steel assembly workers. Temporary bridges were built out to the piers to transport the massive girders into place. Before adding the masonry cladding, the steel columns were washed in cement and wrapped in oiled canvas to prevent rust, expansion, and contraction. The walkway 140 feet above the river used cantilevers built out from each tower, section by section. Construction was so precise that the two halves met exactly in the center. The chains weighed 1 ton per foot run; constructing and raising them was one of the project's most difficult tasks.
As impressive as these structural achievements were, it was operating integrity that would determine Tower Bridge's place in history. Each of the two bridge piers contains two large rooms that house twin bascule drive engines. Each of the eight engines has three single-acting cylinders and its own integral brake. Water under pressure enters the cylinders, which turn the crankshafts. The crankshafts turn the gear wheels, which in turn move the toothed bascule drive quadrants, which lift the two 1200-ton bascule roadways. The ends of the roadways move at two feet per second. To allow the bridge to be raised with minimal effort, each bascule has a 400-ton counterweight that moves inside huge chambers in the towers. The counterweights permit the bascules to be raised to a nearly vertical 83-degree opening angle in just 60 seconds with relatively little effort applied to the gear wheels. Four locking bolts provide stability at the point where the two bascules meet when the bridge is closed. Resting blocks on the edges of the piers in front of the main pivot shafts and two tail locks that engage the undersides of each counterweight serve as additional safety precautions.
Tower Bridge was designed with backup systems to ensure operational reliability, including two of everything required to open and close the bascules. The duplicate set of machinery was always maintained with water flowing to it, although the water was not always under pressure. Four coal-burning double-furnace Lancashire boilers, each 7 1/2 feet in diameter and 30 feet long, were placed in the arches of the engine house. The 20 tons of coal they consumed each week was delivered on special trucks that ran on rails between the riverside barge dock and the engine house. The boilers, two of which were fired up at all times, generated steam to drive two 300-hp pumping engines, each one sufficiently powerful to pump the water at 750 pounds per square inch. Barry said that such pressure "will be appreciated when we remember that in the boiler of a locomotive engine, the steam pressure is usually not more than about one-fifth that amount."
The highly pressurized water was sent by hydraulic pumps to six accumulators - two in the special accumulator room next to the engine house and two in each pier. A constant pressure was maintained in the pipes, and the stored surplus power could be instantly released by simply turning a lever when the bascules needed to be raised. Each accumulator contained a central vertical cylinder in which a piston was fitted. The upper end of the piston was connected to an outer sleeve weighing 100 tons. As water was pumped into the cylinder, the piston rose and lifted the outer sleeve, which, because of its weight, maintained the constant 750-psi pressure.
Whenever power was needed, the high-pressure water was carried via two 6-inch pipes to the lifting mechanism in the piers. Pipes carried the water along the fixed span on the south-side machinery chamber, up the steel columns of the south tower, across the top of the high-level walkways some 140 feet above the Thames, and down the steel columns of the north tower to the bascule mechanism on the north pier. Special joints capable of adjusting to the varying conditions were installed at those points in the system where unequal loading of the spans or temperature changes could affect the pipes.
In December 1879, the Tay Railway Bridge in Scotland had collapsed, plunging a passenger train into the icy waters and killing all 75 people on board. An investigation charged that the bridge had been constructed with inadequate provision for stress from high winds. The government subsequently mandated that all bridges be designed to withstand wind pressure of 56 pounds per square foot. Tower Bridge's duplicate machinery provided for power twice the government's requirements, and the engineers could switch to the larger bascule-drive engines if the bridge needed to be opened in high winds.
Tower Bridge finally opened on June 30, 1894, eight years after construction began. The cost, [pounds]1 million ($4.8 million), was underwritten by the extraordinarily wealthy Bridge House Estates trust, so no taxpayers' money was used. The Prince of Wales (who later became King Edward VII) opened the bascules to the sound of brass bands and the cheers of hundreds of thousands of spectators. Word of London's spectacular engineering accomplishment spread throughout the world, and travelers to Britain made a visit to Tower Bridge a required stop on their itinerary. The Times of London called the bridge "one of the structural triumphs of this age of steel" while Engineering Review said Tower Bridge was "undoubtedly the most successful structure to date for reconciling the requirements of shipping with those of pedestrians and vehicles."
Unfortunately, for all the stringent criteria designed to accommodate the tall ships, the bridge was completed just as the age of steam caused the rapid disappearance of sailing vessels. Still, in its first year of operation, Tower Bridge opened 6160 times, and an average of 8000 horse-drawn vehicles and 60,000 pedestrians crossed it each day. The bridge had a crew of 120 men commanded by the bridge master, who lived in an apartment in the bridge, a tradition that continues today. The bridge has never charged a toll for either land or river vehicles, and the Tower Bridge Act of 1892 provides that river transport has precedence over those who cross by land. To this day, the bridge must be opened to accommodate any vessel, day or night, provided they give 24 hours notice, says Bridge Master Lt. Col. Chris Stevens, himself a mechanical engineer. Traffic currently calls for about 10 bridge openings each week.
One might expect a structure designed for the Victorian age of sailing ships to be of little use in today's era of supersonic jet travel, particularly a bridge that has been subjected to such destructive forces as water, pollution, and the devastating nightly blitz of World War II. Today, London has 17 bridges spanning the Thames, but more than 10,000 commercial vehicles thunder across Tower Bridge each day, many of them 30-ton juggernauts, traveling on bascules designed for horses and carts. Yet Tower Bridge has never once failed to operate during its 100-year history, a tribute to its mechanical design.
Two modifications have brought the bridge up-to-date with developing technology. In 1909, the boilers' steam output was condensed, so a channel was built to divert water from the Thames into reservoirs, which then recirculated it to condensers. In 1975, the Corporation disconnected the bridge's high-pressure water system and converted it to electro-oil hydraulic operation, which can go from idle to full pressure within two minutes. The original boiler-driven system is still maintained in immaculate condition and may be used as a backup at any time.
The bridge is opened to its near-vertical angle only for special occasions, such as to welcome Queen Elizabeth aboard the Royal Yacht Britannia after her coronation in 1953. On January 10, 1965 the bascules fully opened and dockside cranes dipped in salute as the body of Sir Winston Churchill was carried to its final resting place.
Today, visitors can descend nearly to river level and examine the museum, including the engine house, two massive accumulators, and the enormous boilers, resplendent in their red, green, black, and gold livery. The high-level walkways, closed in 1910 when pedestrian traffic declined to a trickle, are open again, offering one of the most spectacular panoramic vistas of London.
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|Author:||Forward, David C.|
|Date:||Feb 1, 1995|
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