bridge

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bridge structure built over water or any obstacle or depression to allow the passage of pedestrians or vehicles. See also viaduct .

Early Bridges

In ancient times and among primitive peoples a log was thrown across a stream, or two vines or woven fibrous ropes (the upper for a handhold and the lower for a footwalk) were thrown across, to serve as a bridge. Later, arched structures of stone or brick were used; traces of these, built from 4000 to 2000 BC, have been found in the E Mediterranean region. The Romans built long, arched spans, many of which are still standing. Bridges built during the Middle Ages usually rested on crude stone arches with heavy piers (intermediate supports) that were a great obstruction to river traffic, and their roadways were often lined with small shops.

The best known early American design is the New England covered bridge, since wood was abundant and cheap, and did not demand trained masons. Colonial American bridge builders were willing to run the risk of rot or fire in exchange for such savings in time and manpower. Beginning with Abraham Darby's bridge at Coalbrookdale in 1779, most bridges began to be built of cast and wrought iron. Robert Stephenson, an English engineer, designed and built a bridge of this type across Menai Strait in North Wales (1850). Another is Victoria Bridge across the St. Lawrence at Montreal. The disadvantage of cast iron for bridges is its low tensile strength.

Modern Bridge Designs

There are six basic modern bridge forms: the beam, the truss, the arch, the cantilever, the cable-stay, and the suspension. A beam bridge is made of long timber, metal, or concrete beams anchored at each end. If the beams are arranged in a lattice, such as a triangle, so that each shares only a portion of the weight on any part of the structure, the result is a truss bridge. An arch bridge has a bowed shape causing the vertical force of the weight it carries to produce a horizontal outward force at its ends. It may be constructed of steel, concrete, or masonry. A cantilever bridge is formed by self-supporting arms anchored at and projecting toward one another from the ends; they meet in the middle of the span where they are connected together or support a third member. In a cable-stayed bridge, the roadway is supported by cables attached directly to the supporting tower or towers. This differs from a suspension bridge, where the roadway is suspended from vertical cables that are in turn attached to two or more main cables. These main cables hang from two towers and have their ends anchored in bedrock or concrete.

The modern era of bridge building began with the development of the Bessemer process for converting cast iron into steel. It became possible to design framed structures with greater ease and flexibility. Single-piece, rolled steel beams can support spans of 50 to 100 ft (15–30 m), depending on the load. Larger, built-up beams are made for longer spans; a steel box-beam bridge with an 850-ft (260-m) span crosses the Rhine at Cologne.

Truss, Arch, and Cantilever Bridges

The truss can span even greater distances and carry heavy loads; it is therefore commonly used for railroad bridges. A large truss span like that over the Columbia River at Astoria, Oreg., can extend to 1,232 ft (376 m). If the truss is shaped into an arch, even longer bridges are possible; the Chaotianmen Bridge in Chongqing, China, the Lupu Bridge, Shanghai, China, the New River Bridge in West Virginia, the Bayonne Bridge between New York and New Jersey, and the Sydney Harbor Bridge in Australia have the longest steel arch spans, at 1,811 ft (552 m), 1,804 ft (550 m), 1,700 ft (518 m), 1,675 ft (510 m), and 1,670 ft (509 m), respectively. Concrete arch bridges tend to be somewhat smaller, the largest being the Wanxian Bridge in China and the Krk Bridge in Croatia at 1,378 ft (420 m) and 1,280 ft (390 m), respectively. The longest concrete arch bridge in the United States is the Natchez Trace Parkway Bridge in Franklin, Tenn., at 582 ft (177 m), although the concrete and steel O'Callaghan-Tillman Memorial Bridge, Nev.-Ariz., near Hoover Dam, has the largest concrete arch, at 1,079 ft (329 m). The cantilever, however, is more common for spans of such lengths. The cantilevered Forth Bridge (1890) in Scotland was the first major structure built entirely of steel, the material that made possible its two record-setting spans of 1,710 ft (521 m) each. They remained the longest in existence until 1917, when the St. Lawrence River at Quebec Bridge was built; it has an 1,800-ft (549-m) span. The longest cantilever bridge in the United States is the Commodore John Barry Bridge in Chester, Penn., which has an 1,644 ft (501 m) span.

Cable-Stayed, Suspension, and Combination Bridges

The cable-stayed bridge is the most modern type, coming into prominence during the 1950s. The longest is the Sutong Bridge, Suzhou–Nantong, Jiangsu, China, which has a main span of 3,570 ft (1,088 m); the Stonecutters Bridge, Hong Kong, spans 3,340 ft (1,018 m). The longest cable-stayed bridge in the United States is the Arthur Ravenel, Jr., Bridge in Charleston, S.C., which has a span of 1,546 ft (471 m).

The suspension bridge is used for the longest spans. The earliest suspension bridges built in America were those constructed by the American builder James Finley. The design of suspension bridges advanced when J. A. Roebling , a German-born engineer who emigrated to the United States, developed the use of wire cables and stiffening trusses. His first completed suspension bridge spanned the Niagara River in 1854. He also designed the Brooklyn Bridge across the East River (completed 1883), which was the world's longest suspension bridge at the time of its construction, having a main span of 1,595.5 ft (487 m).

Today the longest spans in the world are suspended. The longest main spans are the Akashi Kaikyo Bridge, Hyogo, Japan, 6,529 ft (1,990 m); the Xihoumen Bridge, Zhoushan Archipelago, China, 5,414 ft (1,650 m); the Store Bælt Bridge, Denmark, 5,328 ft (1,624 m); the south span of the Runyang Bridge, Jiangsu, China, 4,888 ft (1,490 m); and the Humber River Bridge, Hull, England, 4,626 ft (1410 m). The longest suspension bridges in the United States are the Verrazano-Narrows Bridge , New York City, 4,260 ft (1,298 m); the Golden Gate Bridge , San Francisco, 4,200 ft (1,280 m); the Mackinac Straits Bridge, Mich., 3,800 ft (1,158 m); George Washington Bridge , New York City, 3,500 ft (1,067 m); and the two Tacoma Narrows Bridges, Tacoma, Wash., 2,800 ft (853 m) each.

Combination spans are often used to bridge even longer stretches of water. The San Francisco–Oakland Bay Bridge , noted for its three long spans, of which two are suspension spans and the third a cantilever, has a total length of 8.25 mi (13.2 km). The Chesapeake Bay Bridge-Tunnel has two 1-mi (1.6-km) tunnels along its 17.6-mi (28.2-km) length, and the 8-mi (12.9-km) Confederation Bridge , linking Prince Edward Island to the Canadian mainland, consists of three bridges. The longest cross-sea bridges are the Jiaozhou Bay Bridge, 26.4-mi (42.5-km) long, which connects Qingdao with Huangdao, Shangdong prov., China, and the Hangzhou Bay Bridge, 22.4 mi (36 km) long, which crosses the bay between Zhapu and Cixi, Zhejiang prov., China; both bridges combine causeway with two cable-stayed spans. The longest combination spans in the United States are the twin Lake Ponchartrain Causeways near New Orleans, Louisiana, whose parallel roadways stretch nearly 24 mi (38 km).

Movable Bridges

Movable bridges are generally constructed over waterways where it is impossible or prohibitive to build a fixed bridge high enough for water traffic to pass under it. The most common types of movable bridge are the lifting, bascule, and swing bridges. The lifting bridge, or lift bridge, consists of a rigid frame carrying the road and resting abutments, over each of which rises a steel-frame tower. The center span, which in existing bridges is as long as 585 ft (178 m), is hoisted vertically. The bascule bridge follows the principle of the ancient drawbridge. It may be in one span or in two halves meeting at the center. It consists of a rigid structure mounted at the abutment on a horizontal shaft, about which it swings in a vertical arc. The lower center span of the famous Tower Bridge in London is of the double-leaf bascule type. Because of the need for large counterweights and the stress on hoisting machinery, bascule bridge spans are limited to about 250 ft (75 m). The swing bridge is usually mounted on a pier in midstream and swung parallel to the stream to allow water passage.

Military Bridges

In wartime, where the means of crossing a stream or river is lacking or a bridge has been destroyed by the enemy, the military bridge plays a vital role. Standard types of military bridges include the trestle, built on the spot by the engineering corps from any available material, and the floating bridge made with portable pontoons .

Bibliography

See D. Plowden, The Spans of North America (1984); H. Petroski, Engineers of Dreams: Great Bridge Builders and the Spanning of America (1995); J. Dupré and F. O. Geary, Bridges: A History of the World's Most Famous and Important Spans (1996); S. A. Ostrow, Bridges (1997); F. Gottemoeller, Bridgescape: The Art of Designing Bridges (1998); K. Willard, Bridges: Designing the Future (1999). See also bibliographies for articles on individual bridges.

bridge, an elevated platform built above the upper deck of a powered vessel, from which a ship is normally navigated and from where all activities on deck can be seen and controlled by the captain or officer of the watch. Except in the case of some fishing and similar vessels, where it may be in the form of a wheelhouse, it usually runs athwartships. The bridge of a modern ship is normally totally enclosed by glass screens or windows to give protection from the weather, but in earlier vessels the bridge was usually open and protected from the weather only by a canvas dodger and, in very hot weather, from the sun by a canvas awning. The main magnetic compass and a repeater from the gyroscopic compass are normally situated on the bridge together with the steering wheel, a chart table for chart work, GPS displays, and radar scanners. In very large ships, such as ocean liners etc., the bridge structure may contain two or even more bridges extending the full width of the ship.

A flying bridge in motor yachts is a steering platform located a level above the deck. In the USA a bridge deck is the deck between the cockpit and the cabin in a monohull and between the hulls in a catamaran.

In the days of sail, ships were controlled from the quarterdeck, with the steering wheel in the after part of the waist, but as steam propulsion was developed, first in the paddle steamer, it was discovered that the platform between the two paddle boxes, known at the time as a bridge, gave a much better all-round view of operations on deck. When the propeller replaced the paddle wheel the elevated structure amidships was retained as the navigational control position, and as further development took place, particularly in the number of boilers required to produce steam in sufficient quantity, this central bridge was moved forward and raised to keep it clear of the funnel smoke.

The modern tendency in larger ships, particularly tankers and bulk carriers, and some cruise ships, is to construct the bridge well aft in the ship, mainly in order to keep the upper deck as clear as possible in order to give an uninterrupted view of it, and to provide easier and more economical working conditions at sea and in harbour. This is made possible because the propulsion machinery is placed as far aft as possible, with funnels or diesel exhausts, which affect the position of the bridge in the ship, close to the stern.

See also dodger.

bridge¹ / brij/ • n. 1. a structure carrying a road, path, railroad, or canal across a river, ravine, road, railroad, or other obstacle: a bridge across the river a railroad bridge. ∎  something that makes a physical connection between two other things. ∎  something that is intended to reconcile or form a connection between two things: a committee that was formed to create a bridge between rival parties. ∎  a partial denture supported by natural teeth on either side. See also bridgework. ∎  the support formed by the hand for the forward part of a billiard cue. ∎  a long stick with a frame at the end that is used to support a cue for a shot that is otherwise hard to reach. ∎  Mus. an upright piece of wood on a string instrument over which the strings are stretched. ∎  Mus. a bridge passage. 2. the elevated platform on a ship from which the captain and officers direct operations. 3. the upper bony part of a person's nose: he pushed his spectacles further up the bridge of his nose. ∎  the central part of a pair of glasses, fitting over this: these sunglasses have a special nose bridge for comfort. 4. an electric circuit with two branches across which a detector or load is connected. • v. [tr.] be a bridge over (something): a covered walkway that bridged the gardens. ∎  build a bridge over (something): earlier attempts to bridge the channel had failed. ∎  make (a difference between two groups) smaller or less significant: bridging the gap between avant garde art and popular culture. DERIVATIVES: bridge·a·ble adj. bridge² • n. a card game descended from whist, played by two partnerships of two players who at the beginning of each hand bid for the right to name the trump suit, the highest bid also representing a contract to make a specified number of tricks with a specified suit as trumps.

bridge A unit that supports a low-level link of two regions of a single network. In networks using a broadcast protocol, in which all network nodes receive all messages, it is helpful to subdivide the network into a number of regions in which the majority of traffic is between pairs of nodes within that region, with only a small amount of traffic leaving the region. A bridge can be inserted between two such regions: it allows interregion traffic to cross the bridge but will not forward into the second region traffic that is addressed to a destination in the same region as the sender. To achieve this, a bridge must be capable of interpreting the sender and receiver addresses in the data. It must therefore be capable of interpreting the network protocol, and will almost certainly need to store an entire packet before forwarding it. The bridge will be designed so as to function at the lowest possible level within the protocol stack, consistent with achieving correct partitioning of the network.

Despite the complexity of the unit, and the delay it introduces, large networks almost invariably include bridges since their presence greatly reduces the total network traffic. A bridge may be adaptive, determining the addresses in each region by examining the contents of the address fields of the packets. A bridge may operate as a filtering bridge, with a fixed set of node identities that will be allowed to send packets across the bridge, providing a limited form of safeguard against unwanted attempts to connect to a sensitive system. See also router, firewall.

bridge a structure carrying a road over a river or other obstacle, often in figurative use. Bridge-building is the promotion of friendly relations between groups.
Bridge Fraternity from the 12th century, a confraternity of laymen dedicated to the building or maintenance of a bridge, especially one carrying a pilgrimage route over a river. The best-known example of such a fraternity is that of the bridge of Avignon, founded by St-Bénézet in the late 12th century and linking Avignon with Villeneuve on the opposite bank of the Rhone.
bridge of boats a bridge formed by mooring boats side by side across a river; the bridge of boats over the river Rhône at Arles was in use throughout the 2nd and 3rd centuries ad.
Bridge of Sighs in Venice, a bridge connecting the Doge's palace with the state prison originally crossed by prisoners on their way to torture or execution.
cross that bridge when one comes to it deal with a problem when and if it arises; from the proverbial saying don't cross the bridge till you come to it (see cross3).
it is good to make a bridge of gold to a flying enemy it is wiser to give passage to an enemy in flight, who may be desperate; the idea is attributed to Aristides (480 bc), who warned Themistocles against destroying the bridge of boats which the Persian king Xerxes had constructed across the Hellespont for the invasion of Greece. The saying is recorded from the late 16th century.

See also everyone speaks well of the bridge that carries him over at speak, water of Chancery.

bridge An apparent structure of stars or gas linking one galaxy with another. In some cases this may be the result of physical (tidal) interaction between the two galaxies, as in the case of the Antennae. In other cases the feature may be an observational artefact, or a chance, line-of-sight superposition of luminous material in the galaxies that is unrelated to any interaction. A combination of both artefact and line-of-sight effects probably accounts for the apparent bridge between the quasar Markarian 205 and the spiral galaxy NGC 4319, which have very different velocities of recession (21 000 and 1700 km/s respectively). A chain of galaxies linking two clusters or groups of galaxies is also sometimes known as a bridge.

bridge 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.

bridge A hardware device which allows data to flow between two networks, usually LOCAL AREA NETWORKS. It examines the destination address of data that flows through it and forwards it to the network that it bridges if the address matches. If it does not, then the data is sent to other networks which contain bridges so that they can check whether the data is intended for them.

bridge.
1. In str. instr., the piece of wood that supports the str. and communicates their vibrations to the belly.

2. A term, usually ‘bridge passage’, in comp., meaning a short section which links together—perhaps by a key change—2 important sections of a large-scale sym. or similar work.

bridge. Cúchulainn has to cross the Bridge of Leaps to reach the realm of Scáthach. It is high, narrow, and slippery as an eel's tail; anyone failing to cross it will fall to sea-monsters below. Cúchulainn does cross, and on a second crossing kills a hag who opposes him.

Bridge, mechanical device (called an elevator in America) for raising heavy pieces of scenery or a tableau of actors from below up to stage level. The electrically controlled bridges of a large theatre may reach a high degree of engineering complexity.

bridge1 elevated structure (often arched over water) forming a passageway between two points. OE. bryċġ = OS. bruggia, OHG. brucca (G. brücke), ON. bryggja :- Gmc. *bruʒjō.
Hence vb. OE. bryċġian.

bridge n. the elevated, enclosed platform on a ship from which the captain and officers direct operations.

Bridge Kent. Brige 1086 (DB). ‘(Place at) the bridge’. OE brycg.

bridge2 card game based on whist. XIX. of unkn. orig.

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