See also

Hippodamus of Miletus

Further reading

Manton, E. Lennox. 1988. Roman North Africa. London: Seaby.

Picard, Gilbert, and Yvan Butler. 1965. Living Architecture: Roman. New York: Grosset and Dunlap.

Ward-Perkins, John B. 1988. Roman Architecture. New York: Electa/Rizzoli.

Tower Bridge

London, England

Tower Bridge (1886–1894) is immediately and universally recognizable as an icon of London. Even during its construction, it was nicknamed “Wonder Bridge”—the largest, most advanced bascule bridge ever seen, employing hydraulic power on a scale never before attempted. Spanning the Thames between the Tower of London and Bermondsey, it was built to ease the traffic congestion at London Bridge, which was then (apart from ferries) the only means of crossing at the east end of the growing city. By the early 1870s the problem had become untenable, and travelers faced delays of several hours. Numerous petitions urged either the widening of London Bridge or construction of another crossing, and in 1876 the Corporation of the City of London formed a “Special Bridge or Subway Committee.” It mounted a public design competition to find a solution.

The brief specified that river traffic must not be disrupted during or after construction, that there must be 140 feet (43 meters) clearance for ships, and (perhaps prompted by the problems of Brunel’s Thames Tunnel of 1843), the road approaches must be negotiable for vehicles. There were over fifty entries, presenting a spectrum of ideas: steam vehicle-ferries, tunnels, low-level swing bridges, and high-level bridges with long road approaches or elevators for the street traffic. One entry suggested a railroad on the river bottom with very tall cars that could cross above the high-tide mark. In 1878 the city architect Sir Horace Jones (1819–1887) conceived the idea of a low-level bascule bridge—a double drawbridge with a road deck made up of two hinged arms that could be raised and lowered to accommodate both vehicular and river traffic. The bascule, or draw-span, bridge, although on a much smaller scale, had been common in Europe since the Middle Ages. It was not until October 1884 that Jones offered a developed design for a hydraulically operated openable bascule bridge, spanning between masonry-faced towers and connected to the respective banks by single-span suspension bridges. The following year an act of Parliament authorized the Corporation of the City of London to build the bridge. It took eight years to complete and involved five major contractors, employing nearly 450 construction workers.

Work started in April 1886. Horace Jones was appointed architect but died later in the year, and in 1887 his assistant George Daniel Stevenson took over the design. Sir John Wolfe Barry (1836–1918), son of the famous architect Sir Charles Barry, was appointed engineer. Because of provisions in the Tower Bridge Act about the size of the piers and the width of clearway in the river, the foundation builders faced a difficult and protracted task. Under both massive piers they sunk twelve permanent wrought-iron caissons, combined to follow the profile of the pier, into the watertight London clay 20 feet (6 meters) beneath the riverbed. The caissons and the spaces between were filled with concrete; working in temporary caissons, the masons built the brickwork and stone piers up to a level 4 feet (1.2 meters) above high water. Despite its Gothic Revival appearance, Tower Bridge is a steel structure. Behind the Cornish granite and Portland stone masonry of each of its 293-foot (90-meter) towers, the real work is done by four octagonal 5.5-foot-diameter (1.7-meter) steel columns, resting on granite slabs and secured to the piers with huge holding-down bolts. The towers are connected by two high-level cantilever steel footways, each with a suspended span, 143 feet (44 meters) above high water.

In terms of its operation, the bridge combines the suspension and bascule types. Including the approaches it is 0.5 mile (0.8 kilometer) long; it supports a 35-foot-wide (11-meter) carriageway, flanked by two 12.5-foot-wide (4-meter) sidewalks. Between the abutments the Thames is 880 feet (270 meters) wide, and the central opening span is reached from either direction by a 270-foot (82.5-meter) suspension span, carried on parallel latticed steel trusses