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 B.C., 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 Bayonne Bridge between New York and New Jersey, the Sydney Harbor Bridge in Australia, and the New River Bridge in West Virginia are the longest steel arch bridges, at 1,675 ft (510 m), 1,670 ft (509 m), and 1,700 ft (518 m), respectively. Concrete arch bridges tend to be somewhat smaller, the largest being the Krk Bridge in Croatia and the Gladesville Bridge across the Parramatta River at Sidney, Australia, at 1,280 ft (390 m) and 1,000 ft (305 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). 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 Tatara Bridge in Ehime, Japan, which has a 2,920 ft (890 m) span. The Ponte de Normandie in Le Havre, France, spans 2,808 ft (856 m); the Second Yangtze Bridge in Nanjing, China, spans 2,060 ft (628 m); and the Third Yangtze Bridge in Wuhan, China, spans 2,028 ft (618 m). The longest cable-stayed bridge in the United States is the Dame Point Bridge in Jacksonville, Fla., which has a span of 1,300 ft (396 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); Izmit Bay Bridge, Marmara Sea, Turkey, 5,472 ft (1,668 m); Store Bælt Bridge, Denmark, 5,328 ft (1,624 m); Humber River Bridge, Hull, England, 4,626 ft (1410 m); Tsing Ma Bridge, Hong Kong, 4,518 ft (1,377 m); Verrazano-Narrows Bridge, New York City, 4,260 ft (1,298 m); Golden Gate Bridge, San Francisco, 4,200 ft (1,280 m); Hoga Kusten (or High Coast) Bridge, Västernörrland, Sweden, 3,969 ft (1,210 m); Mackinac Straits Bridge, Mich., 3,800 ft (1,158 m); Minami Bisan-Seto Bridge, Japan, 3,668 ft (1,118 m); Second Bosporus Bridge, İstanbul, Turkey, 3,576 ft (1,090 m); First Bosporus Bridge, İstanbul, Turkey, 3,524 ft (1,074 m); and George Washington Bridge, New York City, 3,500 ft (1,067 m).

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 combination spans are the twin Lake Ponchartrain Causeways near New Orleans, Louisiana, whose parallel roadways stretch nearly 24 mi (38 km). The longest cross-sea bridge is the Hangzhou Bay Bridge, 22.4 mi (36 km) long, which crosses the bay between Zhapu and Cixi, Zhejiang prov., China; the bridge combines causeway with two cable-stayed spans.

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.

bridge, card game derived from whist, played with 52 cards by four players in two partnerships.

Basic Rules

The cards in contract bridge rank from ace down to two; in bidding, suits rank spades, hearts, diamonds, and clubs. After all cards are dealt, so that each player holds 13 cards, the dealer begins the auction, which proceeds in rotation to the left. Each player must bid, pass, double (increase the value of the previously stated contract), or redouble (only after a double, further increasing the point value of the contract). A bid is an offer to win a stated number (over six) of tricks with a named suit as trump or with no trump. The lowest bid is one, the highest seven. Each bid, i.e., "one diamond," "one no-trump," "four hearts," must be higher than the preceding bid, with no-trump ranking above spades. Artificial bids are those that convey certain information to a partner and are not meant to be taken literally. The highest bid of the auction becomes the contract after three consecutive passes end the bidding. The player who first named the suit (or no-trump) specified in the winning bid becomes the declarer. The player to the left of the declarer leads any card face up, and the next hand, that of the declarer's partner, is placed face up on the table, grouped in suits. This is known as the dummy, and the declarer selects the cards to be played from this hand. The object of the game for both partnerships is to win as many tricks as possible, a trick being the three cards played in rotation after the lead. Suits must be followed, but a player who has no cards in the suit led may play any card. Highest trump or, if no trump card is played, highest card of the suit led wins. Points are awarded for the number of tricks won. Numerous conventions—generally accepted forms of bidding—are used in bridge, but the four standard ones are Blackwood, Gerber, Stayman, and grand-slam force.

Competitive Bridge

Duplicate bridge, in which the same prearranged hands are played by individuals, pairs, or teams of four, is the main form of competitive bridge. The laws of contract bridge are promulgated in the Western Hemisphere by the American Contract Bridge League, which holds various bridge tournaments. In international contract bridge matches the Bermuda bowl, the trophy for victory, is the emblem of the world championship. In Olympic years an olympiad championship is held by the World Bridge Federation and replaces the team tournament for the Bermuda bowl.

History

Bridge probably originated in the Middle East in the 19th cent. Auction bridge, one form of the game, was developed by the British in India and later was popular in England and the United States. It is still played but has largely been supplanted by contract bridge, which achieved popularity after important innovations were made in 1925 by Harold S. Vanderbilt. Its phenomenal popularity owed much to the activities of Ely Culbertson. The craze subsided but was later revived; books, tournaments, and newspaper columns on bridge abound. Culbertson devised the honor count system to evaluate a hand for bidding. The point count (or standard American) system introduced by Charles H. Goren in the 1940s has generally replaced honor count.

or natural arch

Naturally created arch formation resembling a bridge. Most are erosion features that occur in sandstone or limestone. Some are formed by the collapse of part of a cavern roof. Others may be produced by entrenched rivers eroding through meander necks to form cutoffs. Still others are produced by exfoliation (separation of successive thin shells) and may be enlarged by wind erosion.

Learn more about natural bridge with a free trial on Britannica.com.

Card game similar to whist. Bridge is any one of several games, including games such as auction bridge and contract bridge, which retain the essential features of whist: Four players participate, two against two in partnership. They play with a 52-card pack, all cards of which are dealt face downward one at a time, clockwise. When play begins, the object is to win tricks, consisting of one card from each player in rotation. The players must, if able, contribute a card of the suit led, and the trick is won by the highest card. All tricks taken in excess of the first six tricks are known as odd tricks. Before play begins, a suit may be designated the trump suit, in which case any card in it beats any card of the other suits. In all types of bridge a certain number points are needed to win a game, and two games won by the same team allows them to win the rubber.

Learn more about bridge with a free trial on Britannica.com.

Natural area, southern Utah, U.S. Located on the Navajo Indian Reservation near the Utah-Arizona border, the monument was established in 1910 and occupies 160 acres (65 hectares). It centres on a rainbow-shaped bridge of pink sandstone 290 ft (88 m) above a creek that winds toward the Colorado River. The bridge is 275 ft (84 m) long and is one of the world's largest natural bridges. Embedded among canyons, the area is accessible only on foot, by horseback, or by boat on Lake Powell.

Learn more about Rainbow Bridge National Monument with a free trial on Britannica.com.

Suspension bridge spanning the Golden Gate, San Francisco. From its completion in 1937 until the completion of New York's Verrazano-Narrows Bridge in 1964, it had the longest main span in the world, 4,200 ft (1,280 m). It remains incomparable in its magnificence. Its construction, supervised by Joseph B. Strauss (1870–1938), involved many difficulties: rapidly running tides, frequent storms and fogs, and the problem of blasting rock under deep water to plant earthquake-resistant foundations.

Learn more about Golden Gate Bridge with a free trial on Britannica.com.

Suspension bridge built (1869–83) over the East River to link Brooklyn to Manhattan island. It was designed by the cable manufacturer John A. Roebling and his son Washington. A brilliant feat of 19th-century engineering, the bridge was the first to use steel for cable wire and the first in which explosives were used inside a pneumatic caisson during construction. In 1869 John was killed in one of at least 27 fatal construction accidents; his son saw the project to completion. The bridge's main span of 1,595 ft (486 m) was the longest in the world to date. It opened to such fanfare that within 24 hours an estimated quarter-million people crossed over it, using an elevated walkway designed to give pedestrians a dramatic view of the city.

Learn more about Brooklyn Bridge with a free trial on Britannica.com.

A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges will vary depending on the function of the bridge and the nature of the terrain where the bridge is to be constructed.

History

The first bridges were made by nature — as simple as a log fallen across a stream. The first bridges made by humans were probably spans of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Most of these early bridges could not support heavy weights or withstand strong currents. It was these inadequacies which led to the development of better bridges.

Epic literature of India provides mythological accounts of bridges constructed from India to Lanka by the army of Rama. The Arthashastra of Kautilya mentions the construction of dams and bridges. A Mauryan bridge near Girnar was surveyed by James Princep. The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor Chandragupta I. The bridge also fell under the care of the Yavana Tushaspa, and the Satrap Rudra Daman. The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the Mughal administration in India.

The ancient Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs. Some of them still stand today. An example is the Alcántara Bridge, built over the river Tagus, in Spain. Most earlier bridges would have been swept away by the strong current. The Romans also used cement, which reduced the variation of strength found in natural stone. One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost then later rediscovered.

Although large Chinese bridges of wooden construction existed at the time of the Warring States, the oldest surviving stone bridge in China is the Zhaozhou Bridge, built from 595 to 605 AD during the Sui Dynasty. This bridge is also historically significant as it is the world's oldest open-spandrel stone segmental arch bridge. European segmental arch bridges date back to at least the Alconétar Bridge (approximately 2nd century AD), while the enormous Roman era Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.

Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 1500s.

During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The first book on bridge engineering was written by Hubert Gautier in 1716.

With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

Etymology

The Oxford English Dictionary traces the origin of the word bridge to an Old English word brycg, of the same meaning, derived from a hypothetical Proto-Germanic root brugjō. There are cognates in other Germanic languages (for instance Brücke in German, brug in Dutch, brúgv in Faroese or bro in Danish, Norwegian and Swedish).

Another theory suggests that "bridge" comes from Turkish "köprü" (lit. bridge). It is highly possible that Turkish lent this word to Eastern European languages and then, in time, it arrived in English. "Köprü" itself is derived from "köprük (köpük)" which literally means "foam".

Types of bridges

There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges.

Beam bridges

Beam bridges are horizontal beams supported at each end by piers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on top of the beam pushes straight down on the piers at either end of the bridge.

Cantilever bridges

Cantilever bridges are built using cantilevers — horizontal beams that are supported on only one end. Most cantilever bridges use two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. The largest cantilever bridge is the Quebec Bridge in Quebec, Canada.

Arch bridges

Arch bridges are arch-shaped and have abutments at each end. The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments at either side. Dubai in the United Arab Emirates is currently building the largest arch bridge in the world, which is scheduled for completion in 2012.

Suspension bridges

Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river. The longest suspension bridge in the world is the Akashi Kaikyo Bridge in Japan.

Cable-stayed bridges

Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The first known cable-stayed bridge was designed in 1784 by C.T. Loescher. The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China.

Truss bridges

Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed girders for the sides. Early truss bridges were made of wood, and later of wood with iron tensile rods, but modern truss bridges are made completely of metals such as wrought iron and steel or sometimes of reinforced concrete. The Quebec Bridge, mentioned above as a cantilever bridge, is also the world's longest truss bridge.

By use

A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic.

Bridges are subject to unplanned uses as well. The areas underneath some bridges have become makeshift shelters and homes to homeless people, and the undersides of bridges all around the world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and become known as suicide bridges.

Decorative or ceremonial

To create a beautiful image, some bridges are built much taller than necessary. This type, often found in east-Asian style gardens, is called a Moon bridge, evoking a rising full moon. Other garden bridges may cross only a dry bed of stream washed pebbles, intended only to convey an impression of a stream. Often in palaces a bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind. A set of five bridges cross a sinuous waterway in an important courtyard of the Forbidden City in Beijing, the People's Republic of China. The central bridge was reserved exclusively for the use of the Emperor, Empress, and their attendants.

The differences & similarities in bridge structure

Bridges may be classified by how the forces of tension, compression, bending, torsion and shear are distributed through their structure. Most bridges will employ all of the principal forces to some degree, but only a few will predominate. The separation of forces may be quite clear. In a suspension or cable-stayed span, the elements in tension are distinct in shape and placement. In other cases the forces may be distributed among a large number of members, as in a truss, or not clearly discernible to a casual observer as in a box beam. Bridges can also be classified by their lineage, which is shown as the vertical axis on the diagram to the right.

Efficiency

A bridge's structural efficiency may be considered to be the ratio of load carried to bridge mass, given a specific set of material types. In one common challenge students are divided into groups and given a quantity of wood sticks, a distance to span, and glue, and then asked to construct a bridge that will be tested to destruction by the progressive addition of load at the center of the span. The bridge taking the greatest load is by this test the most structurally efficient. A more refined measure for this exercise is to weigh the completed bridge rather than measure against a fixed quantity of materials provided and determine the multiple of this weight that the bridge can carry, a test that emphasizes economy of materials and efficient glue joints (see balsa wood bridge).

A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having a bridge (instead of, for example, a ferry, or a longer road route) compared to its cost. The lifetime cost is composed of materials, labor, machinery, engineering, cost of money, insurance, maintenance, refurbishment, and ultimately, demolition and associated disposal, recycling, and replacement, less the value of scrap and reuse of components. Bridges employing only compression are relatively inefficient structurally, but may be highly cost efficient where suitable materials are available near the site and the cost of labor is low. For medium spans, trusses or box beams are usually most economical, while in some cases, the appearance of the bridge may be more important than its cost efficiency. The longest spans usually require suspension bridges.

Double-decker bridge

Double-decker bridges have two levels, such as the San Francisco – Oakland Bay Bridge, with two road levels. Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their lower decks there are two lanes and a pair of tracks for MTR metro trains. Some double-decker bridges only use one level for street traffic; the Washington Avenue Bridge in Minneapolis reserves its lower level for automobile traffic and its upper level for pedestrian and bicycle traffic (predominantly students at the University of Minnesota).

Robert Stephenson's High Level Bridge across the River Tyne in Newcastle upon Tyne, completed in 1849, is an early example of a double-deck bridge. The upper level carries a railway, and the lower level is used for road traffic.

Another example is Craigavon Bridge in Derry, Northern Ireland. The Oresund Bridge between Copenhagen and Malmö consists of a four-lane highway on the upper level and a pair of railway tracks at the lower level.

The George Washington Bridge between New Jersey and New York has two roadway levels. It was built with only the upper roadway as traffic demands did not require more capacity. A truss work between the roadway levels provides stiffness to the roadways and reduced movement of the upper level when installed.

Tower Bridge is different example of a double-decker bridge, with the central section consisting of a low level bascule span and a high level footbridge.

More than just a bridge

• Some bridges carry special installations such as the tower of Nový Most bridge in Bratislava which carries a restaurant. Other suspension bridge towers carry transmission antennas.
• A bridge can carry overhead power lines as does the Storstrøm Bridge.
• Costs and cost overruns in bridge construction have been studied by Flyvbjerg et al. (2003). The average cost overrun in building a bridge was found to be 34%.
• In railway parlance, an overbridge is a bridge crossing over the course of the railway. In contrast, an underbridge allows passage under the line.

Visual index

Index to types

Index to related topics

See also

• Architectural structure
• Cost overrun in bridge construction
• Landscape architecture

References

General references

• Brown, David J. Bridges: Three Thousand Years of Defying Nature. Richmond Hill, Ont: Firefly Books, 2005. ISBN 1-55407-099-6.
• Sandak, Cass R. Bridges. An Easy-read modern wonders book. New York: F. Watts, 1983. ISBN 0-531-04624-9.
• Whitney, Charles S. Bridges of the World: Their Design and Construction. Mineola, NY: Dover Publications, 2003. ISBN 0-486-42995-4.

Unabridged republication of Bridges : a study in their art, science, and evolution. 1929.

• Dikshitar, V. R. R. Dikshitar (1993). The Mauryan Polity. Motilal Banarsidass. ISBN 8120810236.
• Dutt, Romesh Chunder (2000). A History of Civilisation in Ancient India: Vol II. Routledge. ISBN 0415231884.
• Nath, R. (1982). History of Mughal Architecture. Abhinav Publications. ISBN 8170171598.
• Kinney, A. R.; el al. (2003). Worshiping Siva and Buddha: The Temple Art of East Java. University of Hawaii Press. ISBN 0824827791.
• Buck, William; el al. (2000). Ramayana. University of California Press. ISBN 0520227034

External links

• en.Broer.no a site for bridges
• Digital Bridge: Bridges of the Nineteenth Century, a collection of digitized books at Lehigh University
• Structurae - International Database and Gallery of Engineerings Structures with over 10000 Bridges.
• U.S. Federal Highway Administration Bridge Technology
• Descriptions and photographs of five bridge disasters
• Bridge enthusiast site
• Interesting Flexible Arch Bridge design
• Video on how bridges are made (Grade school level educational film by National Association of Manufactures.)