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Definition: bridge from Philip's Encyclopedia

Structure providing a continuous passage over a body of water, roadway or valley. Bridges are built for people, vehicles, pipelines, or power transmission lines. Bridges are prehistoric in origin, the first probably being merely logs over rivers or chasms. Modern bridges take a great variety of forms including beams, arches, cantilevers, suspension bridges and cable-stayed bridges. They can also be movable or floating pontoons. They can be made from a variety of materials, including brick or stone (for arches), steel or concrete.


Summary Article: bridge from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Structure that provides a continuous path or road over water, valleys, ravines, or above other roads. The basic designs and combinations of these are based on the way they bear the weight of the structure and its load. Beam, or girder, bridges are supported at each end by the ground with the weight thrusting downwards. Cantilever bridges are a complex form of girder in which only one end is supported. Arch bridges thrust outwards and downwards at their ends. Suspension bridges use cables under tension to pull inwards against anchorages on either side of the span, so that the span hangs from the main cables by a network of vertical cables. The cable-stayed bridge relies on diagonal cables connected directly between the bridge deck and supporting towers at each end. Some bridges are too low to allow traffic to pass beneath easily, so they are designed with movable parts; swing and draw bridges are examples.

The construction of a bridge is also affected by aesthetics (the preferred or fashionable style at the time of its building) and practicalities such as the materials at hand. The appearance and construction of a bridge reflects the technology and tastes of the era in which it was built; for example, the simple, stocky Pont Neuf constructed over the River Seine in Paris in the 16th century contrasts sharply with the ornate neo-Gothic style of Tower Bridge, built in London in 1894.

History In prehistory, people used logs or wove vines into ropes that were thrown across the obstacle. Clapper bridges, made from flat stones simply laid across or supported by piles of stones, were some of the earliest bridges. By 4000 BC arched structures of stone and/or brick were used in the Middle East, and the Romans built long, arched spans, many of which are still standing. Cast iron bridges were introduced in 1779. The Bessemer process produced steel that made it possible to build long-lived framed structures that support great weight over long spans.

Examples of the main types of bridgesBeam or girder The Rio-Niteroi bridge, Guanabara Bay, Brazil, is the longest continuous box and plate girder bridge, with a centre span of 300 m/984 ft and a length of 13.9 km/8.6 mi

Cantilever Forth Rail Bridge, Scotland, 1,658 m/5,440 ft long with two main spans, two cantilevers each, one from each tower

Arch Sydney Harbour Bridge, Australia, a steel arch with a span of 503 m/1,650 ft

Suspension Humber Bridge, England, with a centre span of 1,410 m/4,628 ft; bridge over the Great Belt waterway in Denmark, 18 km/11 mi

Cable-stayed Dartford Bridge, 30 km/19 mi downstream of central London, main span 450 m/1,476 ft. The longest cable-stayed bridge is the 2,200 m/7,216 ft Pont de Normandie bridge over the Seine estuary in France, completed in 1995. However, the Tatara Bridge over the Seto Inland Sea in Japan, 2,143 metres/7,031 feet long, has a central span of 890 m/2,920 ft, exceeding the 856 m/2,808 ft of the Pont de Normandie's longest span.

Steel is pre-eminent in the construction of long-span bridges because of its high strength-to-weight ratio, but in other circumstances reinforced concrete has the advantage of lower maintenance costs.

The Newport Transporter Bridge, Wales (1906) is a high-level suspension bridge that carries a platform suspended a few feet above the water. The high gantry, from which the platform is suspended, is designed to allow ships to pass underneath.

Construction Girders may be plate, with solid plates between the upper and lower flanges, or lattice (trusses) of constant or varying height. The box girder, forming a closed tube, was first used in the tubular Conwy bridge in 1848, and then in the Britannia tubular bridge over the Menai Straits (1850), which established the superiority of wrought iron over cast iron. Arches built of stone, brick, or reinforced concrete are commonly used for short spans. The longest concrete arch bridge is at Wanxian in Wanzhou, China (1997), and spans 420 m/1,378 ft. The first completed iron bridge was the bridge over the River Severn at Ironbridge, England (1779). It was not until 1864 that a wrought-iron arch bridge of importance was constructed, when a bridge of three spans was built crossing the Rhine at Koblenz, Germany.

The first major steel bridge, Captain Ead's great bridge over the Mississippi at St Louis, USA, was completed in 1874. The arches are formed of open triangulated ribs, supporting the roadway by vertical columns at the apexes of the arch bracing. The Sydney Harbour Bridge, which opened in 1932, is one of the great single-arch bridges of the world, with a span of 503 m/1,650 ft. Another great arch bridge in steel was built in 1932, the Bayonne Bridge in New York, which exceeds the span of Sydney Harbour Bridge by 0.6 m/2 ft. Tied arches avoid the need for strong abutments to support the thrust of the arch which is sustained by the carriageway instead. The earliest tied (bowstring) arch was built at Lugao, Hungary, in 1833, where a cast-iron arch was tied at deck level by a chain. The longest tied-arch bridge, at Port Mann, Vancouver, was constructed in 1964, has a central span of 366 m/1,200 ft and rises 76 m/249 ft; however, the Fremont Bridge (1973) in Portland, Oregon, has a central span of 382.5 m/1,255 ft.

Cantilever bridges were, in a crude form, known in very early times by the Chinese, and were constructed of timber. The first memorable example is the Forth railway bridge built in 1890. The longest cantilever bridge for a railway was opened in 1919 at Québec, after the original design collapsed during construction in 1907. Its cantilevers rest upon points, and derive their whole stability from the land anchorages upon the shore. The year 1927 saw the opening of two important cantilever bridges – in May the bridge over the Carquinez Straits, California, and in August the International Peace Bridge between Buffalo, USA, and Fort Erie, Canada.

Suspension bridges, with long, narrow floors hung from, or carried upon, ropes or chains were used by the Incas of Peru for spans of up to 60 m/197 ft in the 16th century. In more modern times, one of the earliest suspension bridges of note was the Menai Bridge, Wales (1819). In 1926 the highway bridge from Philadelphia to Camden over the Delaware River, USA, was completed. The Hudson River Bridge from New York City to New Jersey has steel towers 194 m/636 ft above high water and a clearance of 65 m/213 ft at the centre, allowing the largest ocean-going ships to pass beneath. Until 1964, the Golden Gate Bridge, San Francisco, was the world's longest suspension bridge, but its length was exceeded by that of the 4.8 km/3 mi, twin-decked Verrazano-Narrows Bridge, which spans the mouth of New York's harbour from Brooklyn to Staten Island. It was opened in 1964 and has a central span of 1,006 m/3,300 ft, carrying four traffic lanes and a cycle track. The Severn Suspension Bridge, which joins southern England to South Wales, was designed in conjunction with the Forth Road Bridge, Scotland, and completed in 1966. The Bosporus Bridge, Istanbul, Turkey (1973) joins Europe to Asia; its lightness relative to its span (1,074 m/3,523 ft) demonstrates the progress made in the art of bridging since the Golden Gate Bridge was built. The Verrazano-Narrows Bridge is now rated seventh in the world by length of main span (distance between towers). The world record-holder is the Akashi-Kaikyo Bridge across the Akashi Straits in Japan, with a main span of 1.99 km/1.24 mi.

Suspension bridges for long spans and box-girder bridges for shorter spans have led to a hybrid form, cable-stayed bridges, which use high-strength cables to reduce the bending action in the mid-span section of a box girder. Usually two towers support the cables, which may be arranged in a fan form, or in a parallel form called ‘modified fan’. The 2,143-m/7,031-ft Pont de Normandie, which spans the Seine estuary, France, opened in January 1995, and was at the time the world's longest cable-stayed bridge. The Tatara Bridge in Japan opened in 1999 with the world's longest central span for a cable-stayed bridge of 890 m/2,920 ft. The Rio-Antirio Bridge in Greece, opened in 2004, has an overall length of 2,880 m/9,449 ft. Cable-stayed concrete bridges have been built for shorter spans; Tiel Bridge on the Waal River, the Netherlands (1975), has a span of 267 m/876 ft supported by a fan arrangement of cables.

Precast concrete has been used extensively since World War II for bridge construction. Concrete, while having a high resistance to compressive forces, is relatively weak in tension. In pre-stressed concrete the tension forces are taken up by high-tensile steel wires, and this has resulted in the construction of a number of elegant bridges. The longest cantilever span in pre-stressed concrete is the Urato Bridge, Shikoku, Japan (1972), which has a central span of 230 m/754 ft and a mid-span depth of only 4.5 m/15 ft. The Confederation Bridge, spanning an eight-mile gap between Prince Edward Island and New Brunswick, is the world's largest continual multi-span bridge. Completed in 1997, the bridge's massive concrete spans are 197 m/646 ft long and weigh up to 7,500 tonnes.

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