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A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle. Designs may be built higher than otherwise needed in order to allow other traffic (particularly ship traffic) beneath.
The purpose of a bridge is to allow people or cargo easy passage over an obstacle by providing a route that would otherwise be uneven or impossible.
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History

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The first bridges were spans made of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement.
The arch was first used by the Roman Empire for bridges and aqueducts, some of which still stand today. The Romans also used cement, which reduced the variation of strength found in natural stone. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost then later rediscovered.
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 engineering book on building bridges 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.
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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 or bro in Danish and Swedish).
The word for the Pope, pontiff, comes from the Latin word pontifex meaning "bridge builder".
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Types of bridges
There are four main types of bridges: beam bridges, cantilever bridges, arch bridges and suspension bridges.
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By use
A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. In some cases there may be restrictions in use. For example, it may be a bridge carrying a highway and forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles.
An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height.
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Decorative and ceremonial bridges
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.
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Index to types of bridges
Image:NagasakiMeganebashi.jpg|Arch bridge
Image:UniversityBridge-1Clip.jpg|Bascule bridge
Image:Small footbridge.jpg|Beam bridge
Image:Saltashrab.jpg|Brunnel Truss bridge
Image:Concrete box girder bridge.JPG|Box girder bridge
Image:Baumgardener's Covered Bridge Inside Center 3008px.jpg|Burr Arch Truss
Image:ThreeTwrBrCenter.jpg|Cable-stayed bridge
Image:ForthBridgeEdinburgh.jpg|Cantilever bridge
Image:Puente del Alamillo.jpg|Cantilever spar cable-stayed bridge
Image:Tarr steps clapper bridge.jpg|Clapper bridge
Image:Australia sydney-404.jpg|Compression arch suspended-deck bridge
Image:CurlingBridgeClip.jpg|Rolling, or curling bridge
Image:Fort ticonderoga drawbridge to demilune.jpg|Drawbridge
Image:NoImageYetRectFramed.png|Extradosed bridge
Image:EastbankEsplanade.jpg|Floating bridge
Image:Hoernbruecke.jpg|Folding bridge
Image:GirderBridge2.jpg|Girder bridge
Image:Guilford vermont bridge covered bridge interior 20040820.jpg|Lattice bridge
Image:BNSFBridgeClip.jpg|Lift bridge
Image:Vallorcine footpath bridge 2003-12-13.jpg|Log bridge
Image:PlateGirderUnderTracks.jpg|Plate girder bridge
Image:ArmyPontoonBr.jpg|Pontoon bridge
Image:Uppsala Ultunabron02 2005-06-16.jpg|Retractable bridge (Thrust bridge)
Image:SegmentalBridgeFtLauderdale.jpg|Segmental bridge
Image:ProposedSFOBBEasternSpan.jpg|Self-anchored suspension bridge
Image:WinnepegBridge.jpg|Side-spar cable-stayed bridge
Image:CapilanoBridge.jpg|Simple suspension bridge
Image:StepStoneBridge.jpg|Step-stone bridge
Image:Holzbrücke bei Essing 1.jpg|Stressed ribbon bridge
Image:BridgeSubmerging4.jpg|Submersible bridge
Image:suspension.bridge.bristol.arp.750pix.jpg|Suspension bridge
Image:Railway swing bridge.jpg|Swing bridge
Image:Tournai Pont levant Notre Dame 20040520-014.jpg|Table bridge
Image:FortPittBridge.jpg|Tied arch bridge (Bowstring bridge)
Image:Millenium_bridge_close.jpg|Tilt bridge
Image:Newport.transporter.750pix.jpg|Transporter bridge
Image:AlhambraTrestle.jpg|Trestle
Image:Eastbound_over_SCB.jpg|Truss arch bridge
Image:LittleManateeRiver.jpg|Truss bridge
Image:Conwy Castle 2.jpg|Tubular bridge
Image:Grammene-vierendeelbridge 20030618.jpg|Vierendeel bridge
Image:BoxerwoodDotComZigZag.jpg|Zig-zag bridge
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Index to bridge related topics
Image:Titanbridger.jpg|Armoured vehicle-launched bridge
Image:Pont_du_gard.jpg|Aqueduct
Image:PontBailey800px.jpg|Bailey bridge
Image:BalsaBridge_Break.jpg|Balsa wood bridge breaking under load
Image:WWI bridge of boats Scheldt.jpg|Bridge of boats
Image:Claude Monet-Waterlilies.jpg|Bridges in art
Image:CaissonSchematic.jpg|Caisson
Image:Guilford vermont covered bridge 20040820.jpg|Covered bridge
Image:IRBSideViewClip.jpg|Inca rope bridge
Image:JetwayAtVancouverBC.jpg|Jetway
Image:16 Bay With Link 4.jpg|Medium Girder Bridge
Image:SFTGMoonBridge.jpg|Moon bridge
Image:Paying Toll on passing a Bridge From a Painted Window in the Cathedral of Tournay Fifteenth Century.png|Toll bridge
Image:Trogbrücke_Magdeburg.JPG|Water bridge
Image:WeighBridge5500.JPG|Weigh bridge
Image:Toronto-bloorviaduct.jpg|Viaduct
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Bridge structural and evolutionary taxonomy

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Bridges may be classified by how the four forces of tension, compression, bending and shear are distributed through their structure. Most bridges will employ all of the principle 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.
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Efficiency

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A bridge's structural efficiency may be considered to be the ratio of load carried to bridge weight, 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 reuse. 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.
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Notable bridges
Chicago Skyway - USA, a 7.8-mile toll bridge bypassing much of the southwest of Chicago
Sunshine Skyway Bridge - USA, longest cable-stayed concrete bridge in the world. Replaced a cantilever truss structure which partially collapsed in 1980 as a result of collision by a freighter.
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Special Installations
Some bridges carry special installations such as the tower of Nový Most bridge in Bratislava which carries a restaurant. On other suspension bridge towers transmission antennas are installed.
A bridge can carry overhead powerlines as the Storstrøm Bridge.
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Catastrophic collapses
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See also
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