Bridge

A bridge is a structure that crosses an obstacle such as a river, lake, railroad, road, or ravine. Its primary function is to transport vehicles, trains, and pedestrians, but bridges may also accommodate pipelines, buildings, wildlife, and canals. Bridge styles include arch, truss, beam, cantilever, suspension, and cable-stayed. Less common types are movable, double-deck, pontoon, and military bridges. They may also be categorized by their materials, which include wood, brick, stone, iron, steel, and concrete.

The history of bridges reflects the evolution of humankind's engineering technologies. The Romans and ancient Chinese built major bridges of stone and timber. During the Renaissance, advances in science and engineering led to wider bridge spans and more elegant designs. Concrete was perfected in the early 1800s, and proved to be superior to stone in many regards. With the Industrial Revolution came mass-produced steel, which revolutionized bridge design and enabled the creation of suspension and cable-stayed bridges that spanned wide obstacles.

The design of a bridge must satisfy many requirements, such as connecting to a transportation network, providing adequate clearances, and safely transporting its users. Additional factors include cost, aesthetics, and longevity. A bridge must be strong enough to support the weight of the bridge itself, as well as the traffic passing over the bridge. It must also tolerate stresses imposed by the environment, such as wind, snow, earthquakes, water currents, flooding, and temperature fluctuations. To meet all these goals, bridge engineers use analytical methods such as limit state design and finite element method.

Many bridges are admired for their beauty, and some serve important roles as iconic landmarks that provide a sense of pride and identity to a community. Bridges are often used as metaphors in art and literature to represent connection or transition.

History

Antiquity

A stone arch bridge passing over a river valley

The Pont du Gard aqueduct in France was built by the Roman Empire Template:Circa, and is still standing.Template:Sfn

The earliest forms of bridges were simple structures for crossing swamps and creeks, consisting of wooden boardwalks or logs.Template:Efn PilingsTemplate:Sndwhich are critical elements of bridge constructionTemplate:Sndwere used in Switzerland around 4,000 BC to support stilt houses built over water.Template:Sfn

Several corbel arch bridges were built Template:Circa 13th century BC by the Mycenaean Greece culture, including the Arkadiko Bridge, which is still in existence.Template:Sfn In the 7th century BC, Assyrian king Sennacherib constructed stone aqueducts to carry water near the city of Ninevah.Template:Sfn One of these aqueducts crossed a small valley at Jerwan with five corbelled arches, and was Template:Convert long and Template:Convert wide.Template:Sfn In Babylonia in 626 BC, a bridge across the Euphrates was built with an estimated length of Template:Convert.Template:Sfn In India, the Arthashastra treatise by Kautilya mentions the construction of bridges and dams.Template:Sfn Ancient China has an extensive history of bridge construction, including cantilever bridges, rope bridges, and bridges built across floating boats.

The ancient Romans built many durable bridges using advanced engineering techniques. Many Roman aqueductsTemplate:Sndsome still standing todayTemplate:Snd used a semicircular arch style. An example is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans used cement as a construction material, which could be mixed with small rocks to form concrete, or mixed with sand to form mortar to join bricks or stones.Template:Sfn Some Roman cements, particularly those containing volcanic ash, could be used in underwater applications. The enormous Trajan's Bridge (105 AD) featured open-spandrel segmental arches made of wood.Template:Sfn

300 to 1400

A graceful stone bridge spanning a river, with trees in the background

The Anji Bridge, which uses a shallow segmental arch, was built in China Template:Circa 600 AD.Template:Sfn

The oldest surviving stone bridge in China is the Anji 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.Template:SfnTemplate:Efn Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America prior to European colonization in the 16th century.

In Medieval Europe, bridge design capabilities declined after the fall of Rome, but revived in the High Middle Ages in France, England, and Italy with the construction of bridges such as the Pont d'Avignon, bridges of the Durance river, the Old London Bridge, and the Ponte Vecchio in Florence.

1400 to 1800

A wooden bridge, covered with a roof, passing over a river

The superstructure of the West Montrose Covered Bridge is made of wood.Template:Sfn

In 15th and 16th century Europe, the Renaissance brought a new emphasis on science and engineering.Template:Sfn Figures such as Galileo Galilei, Fausto Veranzio, and Andrea Palladio (author of I quattro libri dell'architettura) wrote treatises that applied a rigorous, analytic approach to architecture and building.Template:Sfn Their innovations included truss bridges and stone segmental arches, resulting in bridges such as Florence's Ponte Santa Trinita, Rialto Bridge in Venice, and Paris's Pont Neuf. Military and commercial bridges were constructed in India by the Mughal administration.Template:Sfn The Asante Empire in Africa built bridges over streams and rivers using tree trunks and beams.

In the late 1700s, the design of arch bridges was revolutionized in Europe by Jean-Rodolphe Perronet and John Rennie, who designed arches that were flatter than semi-circular Roman arches.Template:Sfn These flatter arches enabled longer spans, fewer piers, and required less material.Template:Sfn These designs were used for bridges such as Pont de la Concorde and New London Bridge.Template:Sfn

With the advent of the Industrial Revolution, cast iron became an important construction material for bridges.Template:Sfn Although cast iron was strong under compression, it was brittle, so it was supplanted by wrought ironTemplate:Sndwhich was more ductile and better under tension. An early iron bridge was built in Shropshire, England crossing the river Severn.Template:Sfn

The abundance of inexpensive lumber in Canada and the United States caused timber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s. Many of these timber bridges were covered bridges. Rail bridges used timber to obtain long spans that utilized strong truss designs, and also tall trestle bridges that spanned deep ravines.

1800 to present

A suspension bridge crossing a deep rocky ravine

The Sidi M'Cid Bridge in Algeria was the highest bridge in the world when it was built in 1912.Template:Sfn

The mass production of steel in the late 1800s provided a new material for bridges, enabling lighter, stronger truss bridges and cantilever bridges, and producing cables strong enough to make suspension bridges and cable-stayed bridges feasible.Template:Efn

ConcreteTemplate:Sndwhich was originally used within the Roman EmpireTemplate:Sndwas improved with the invention of Portland cement in the early 1800s, and replaced stone and masonry as the primary material for bridge foundations. When iron or steel is embedded in the concrete, as in reinforced concrete or prestressed concrete, it is a strong, inexpensive material that can be used for horizontal elements of beam bridges and box girder bridges.

Throughout the 20th century, new bridges by designers such as Othmar Ammann repeatedly broke records for span distances, enabling transportation networks to cross increasingly wider rivers and valleys. Cable-stayed bridgesTemplate:Sndwhich use cable-stays as the exclusive means of supportTemplate:Sndbecame a popular bridge design following World War II.Template:Efn

The late 20th century saw several major innovations in bridge design. Extradosed bridges were introduced and found widespread use, predominantly in Japan.Template:Sfn In China, concrete-filled steel tubes were adopted as a new approach to building arch bridges.Template:Sfn Fiber-reinforced polymersTemplate:Sndwhich do not suffer from the rust problems that plague steelTemplate:Sndwere used in bridges for many applications, such as beams, deck slabs, prestressing cables, wraps on the exterior of concrete elements, and internal reinforcing within concrete.Template:SfnTemplate:Efn In the 21st century a bridge span exceeded Template:Convert for the first time, with the construction of the 1915 Çanakkale Bridge.Template:Sfn

Uses

A bridge carrying canal with water, passing over a valley

The Magdeburg Water Bridge in Germany carries boats across a valley.Template:Sfn

A bridge, topped with soil and vegetation, passing over a highway

This wildlife crossing bridge is in Israel.Template:Sfn

The purpose of any bridge is to traverse an obstacle. A bridge can provide support and transport for railways, cars, pedestrians, pipelines, cables, or any combination of these.Template:Sfn Aqueducts were developed early in human history, and carried water to towns and cities.Template:Sfn Canal systems sometimes include navigable aqueducts (also called canal bridges) to carry boats across a valley or ravine.Template:Sfn

Until the 19th century, the most common use of bridges was to carry pedestrians, horses, and horse drawn carriages.Template:Sfn Following the invention of railways, many rail bridges were built: in England, the number of bridges doubled during the railway-building boom in the mid 1800s.Template:Sfn In the 20th century, the growth of motorway networks required the construction of vast numbers of bridges.Template:Sfn

Railway bridges have unique requirements because of the heavy loads they carryTemplate:Snda single locomotive can weigh 197 tonnes.Template:SfnTemplate:Efn Railway bridges are designed to minimize deflection (bending under load), to maximize robustness (localize the damage caused by accidents), and to tolerate heavy impacts (sudden shocks from, for example, rail wheels striking an imperfection in the track).Template:Sfn These requirements lead railways to avoid curved bridges, suspension bridges, and cable-stayed bridges; instead, straight beam or truss bridges are commonly used.Template:Sfn

Some bridges accommodate uses other than transportation. Pipeline bridges carry oil pipes or water pipes across valleys or rivers.Template:Sfn Many historical bridges supported buildings, such as shrines, factories, shops, restaurants, and houses. Notable examples were the Old London Bridge and Ponte Vecchio.Template:Sfn In the modern era, bridge-restaurants can be found at some highway rest areas; these support a restaurant or shops directly above the highway and are accessible to drivers moving in both directions.Template:Sfn An example is Will Rogers Archway over the Oklahoma Turnpike.Template:Sfn The Nový Most bridge in Bratislava features a restaurant set atop its single tower.Template:Sfn Conservationists use wildlife overpasses to reduce habitat fragmentation and animal-vehicle collisions. The first wildlife crossings were built in the 1950s, and these types of bridges are now used worldwide to protect both large and small wildlife.Template:Sfn

Military bridge

A military vehicle carrying a bridge on its back, extending the bridge over a creek

This portable AM 50 bridge is being laid over a river in Slovakia.Template:Sfn

Military bridges are an important type of equipment in the field of military engineering. They perform a variety of wartime roles, such as quickly traversing obstacles in the midst of battle, or facilitating resupply behind front lines.Template:SfnTemplate:Efn Military bridges can be categorized as wet bridges that rest on pontoon floats, and dry bridges that rest on piers, river banks, or anchorages.Template:Sfn A crude mechanism to cross a small ravine is to place a fascine (a large bundle of pipes or logs) into the ravine to enable vehicles to drive across.Template:Sfn

Some military bridges, referred to as armoured vehicle-launched bridges (AVLB), are carried on purpose-built vehicles.Template:Sfn These vehicles typically have the same cross-country performance as a tank, and can carry a bridge to an obstacle and deploy ("launch") the bridge. The UK Chieftain AVLB could launch a Template:Convert bridgeTemplate:Sndcapable of supporting 60 ton loadsTemplate:Snd in 3 minutes.Template:Sfn

Structures

Basic structures

The basic bridge structures are arch, truss, cantilever, suspension, cable-stayed, and beam. The choice of bridge structure to use in a particular situation is based on many factors, including aesthetics, environment, cost, and purpose.Template:SfnTemplate:Efn

Arch bridge

Template:Multiple image Arch bridges consist of a curved arch, under compression, which supports the deck either above or below the arch.Template:Sfn The shape of the arch can be a semicircle, elliptical, a pointed arch, or a segment of a circle. When the arch is semicircular, as in Roman bridges, the force of the arch is directed vertically downward to the foundation. When the arch is elliptical or a circular segment, the force is directed diagonally, and abutments are often required. Deck arch bridges hold the deck above the arch; tied-arch bridges suspend the deck below the arch; and through-arch bridges position the deck through the middle of the arch.Template:Sfn

Truss bridge

Template:Multiple image A truss bridge is composed of multiple, connected triangular elements.Template:SfnTemplate:Efn The set of triangles form a rigid whole, which rests on the foundation at both ends, applying a vertical force downward.Template:Sfn The deck can be carried on top of the truss ("deck truss") or at the bottom of the truss ("through truss").Template:Sfn Through trusses are useful when more clearance under the bridge is required; deck trusses permit oversized loads and do not interfere with overhead objects, such as electrical lines.Template:Sfn The individual bars can be made of iron or wood, but most modern truss bridges are made of steel.Template:Sfn The horizontal bars along the top are usually in compression, and the horizontal bars along the bottom are usually in tension.Template:Sfn Other bars in the truss may be in tension or compression, depending on the layout of the triangles.Template:Sfn Trusses typically have a span-to-depth ratio (the width of a structure divided by its height) ranging from 10 to 16, compared to beam bridges which typically have a ratio ranging from 20 to 30.Template:Sfn Trusses tend to be relatively stiff, and are commonly used for rail bridges which are required to carry very heavy loads.Template:Sfn

Cantilever bridge

Template:Multiple image Cantilever bridges consist of beams or trusses that are rigidly attached to a support (pier or anchorage) and extend horizontally from the support without additional supports. In ancient Asia, cantilever bridges made of large rocks or timber were used to span small obstacles. In the 1880s, some early cantilever bridges were built from wrought ironTemplate:Sfn but modern cantilever bridges are generally built from steel. A balanced cantilever bridge consists of two connected cantilevers extending outward in opposite directions from a single central support. Other cantilever bridges have two cantilevers, anchored at each end of the span, extending toward the center, and meeting in the center.Template:Sfn Cantilever construction is a method of building a bridge superstructure, which can be utilized for arch and cable-stayed bridges, as well as cantilever bridges. In this technique, construction begins at a support (such as a pier, abutment, or tower) and extends outwards across the obstacle, with no support from below.

Suspension bridge

Template:Multiple image Suspension bridges have large, curved cables attached to the tops of tall towers,Template:Efn and suspend the bridge deck from the cables.Template:SfnTemplate:Efn In the early 1800s, the first modern suspension bridgesTemplate:Sndsuch as the Jacob's Creek BridgeTemplate:Sndwere chain bridges that used iron bars rather than bundled wires for the cables.Template:Sfn After steel wire became widely available, longer cables could be built by stringing hundreds of wires between the towers and bundling them,Template:Sfn enabling suspension bridges to achieve spans Template:Convert long.Template:Efn When the bridge crosses a river, stringing the wires across the large span is a complex process.Template:Sfn The cable of a suspension bridge assumes the shape of a catenary when initially suspended between the bridge towers; however, once the uniform load of the bridge deck is applied, the cable adopts a parabolic shape.Template:Sfn Shorter towers require a smaller sag in the cable, which increases the tension in the cable, and thus requires stronger towers and anchorages.Template:Sfn

Cable-stayed bridge

Template:Multiple image Cable-stayed bridges are similar to suspension bridges, but the cables that support the deck connect directly to the towers.Template:SfnTemplate:Efn The cables may be arranged in a fan pattern or a harp pattern.Template:Efn Modern cable-stayed bridges became popular after WWII, when the design was used for many new bridges in Germany.Template:Sfn When traversing a wide obstacle, designers have a choice of suspension or cable-stayed structures. Suspension bridges provide a longer span (for comparable materials), and require shorter towers (for a given span size). Cable-stayed bridges use less cable for a given span size; do not require anchorages; and the deck can be readily built by cantilevering outward from the towers.

Beam bridge

Template:Multiple image Beam bridgesTemplate:Snd including girder bridgesTemplate:Snd are simple structures consisting of one or more parallel, horizontal beams that span an obstacle. They are the most common type of bridges for both railways and roadways. Beam bridges are ideal for shorter spans (less than about Template:Convert); for longer spans other structures, such as trusses, are generally more efficient.Template:Sfn In many applications, beam bridges can be built rapidly and economically, because the individual beams can be produced offsite and transported to the bridge site. Modern beam bridges are generally made of steel or reinforced concrete, although wood may be suitable for small beam bridges intended for light use. Several different cross-sections may be utilized for beams, including I-beam (common for steel) or flat slabs (sometimes used with concrete).Template:Efn Beams can traverse longer spans when they are designed as hollow box girders; bridges made of box girders are termed box girder bridges. The vertical thickness of beam bridges is generally shallower than comparable deck truss bridges, permitting shorter and lower approach roads to cross an obstacle of a given height. Several beam bridges can be chained together, with supports at each juncture, to form elevated highways or causeways.Template:Efn

Other types

Movable bridge

A tall drawbridge, open, over a river

Tower Bridge in London is a movable bridge of the bascule type.Template:Sfn

Movable bridges are designed so that all or part of the bridge deck can be moved, usually to permit tall trafficTemplate:Sndthat would normally be obstructed by the bridgeTemplate:Sndto pass by. Early movable bridges include drawbridges that pivoted at one end, and required a large amount of work to raise. Adding counterweights on the pivot side of the drawbridge creates a bascule bridge, and makes moving the bridge easier and safer. Swing bridges pivot horizontally around an anchor point on the bank of a canal, or sometimes from a pier in the middle of the water. Lift bridges are raised vertically between two towers by cables passing over pulleys at the top of the towers. Notable movable bridges include El Ferdan Railway Bridge in Egypt, Erasmusbrug bascule in Rotterdam, and Limehouse Basin footbridge in London.Template:Sfn In the modern era, designers sometimes create unusual movable bridges with the intention of establishing signature bridges for a town or locality.Template:Sfn Examples include Puente de la Mujer swing bridge in Buenos Aires, Gateshead MillenniumTemplate:Snda rare example of a tilt bridgeTemplate:Sndover the River Tyne, and Hörn Bridge in Germany.Template:Sfn

Long, multi-span bridge

A large bridge, consisting of multiple tall sections, passing over a wide valley

The Millau Viaduct crosses the Tarn river valley in France.Template:Sfn

There are a variety of terms that describe long, multi-span bridgesTemplate:Sndincluding raised bridge, viaduct, trestle, and causeway. The usage of the terms can overlap, but each has a specific focus.Template:Sfn Viaducts (carrying vehicles) and aqueducts (carrying water) are bridges crossing a valley, supported by multiple arches or piers. Romans built many aqueducts, some of which are still standing today.Template:Sfn Notable viaducts include Penponds Viaduct in England,Template:Sfn Garabit Viaduct in France,Template:Sfn Tunkhannock Viaduct in Pennsylvania,Template:Sfn and Millau Viaduct in France.Template:Sfn

A trestle bridgeTemplate:Sndcommonly used in the 19th century for railway bridgesTemplate:Snd consists of multiple short spans supported by closely spaced structural elements.Template:Sfn A trestle is similar to a viaduct, but viaducts typically have taller piers and longer spans.Template:Sfn A continuous truss bridge is a long, single truss that rests upon multiple supports. A continuous truss bridge may use less material than a series of simple trusses because a continuous truss distributes live loads across all the spans (in contrast to a series of distinct trusses, where each truss must be capable of supporting the entire live load). Visually, a continuous truss looks similar to a cantilever bridge, but a continuous truss experiences hogging stresses at the supports and sagging stresses between the supports. A causeway is a low, raised road, usually crossing a lake or other body of water. The Template:Convert Lake Pontchartrain Causeway in Louisiana is a bridge, but other causeways are built on earthen embankments.

Pontoon bridge

A concrete bridge over a large body of water

Floating concrete pontoons support the weight of the Nordhordland Bridge as it crosses a deep fjord in Norway.Template:Sfn

A pontoon bridge, also known as a floating bridge, uses floats or shallow-draft boats to support a continuous deck for pedestrian or vehicle travel over water. Pontoon bridges are typically used where waters are too deep to build piers, or as a mechanism to implement a movable swing bridge in a canal.Template:Sfn Pontoon bridges were used in ancient China. During the Second Persian invasion of Greece, Persian ruler Xerxes built a large pontoon bridge across the Hellespont, consisting of two parallel rows of 360 boats.

Several pontoon bridges are in use in the modern world. Washington state in the US has several, including Hood Canal Bridge.Template:Sfn In Norway, Nordhordland Bridge crosses a deep fjord by resting on floating concrete pontoons.Template:Sfn Many armies have pontoon bridges that can be rapidly deployed, including the PMP Floating Bridge, designed by the USSR.Template:Sfn

Extradosed

A concrete bridge over a river

The Shin Meisei bridge (foreground) in Japan is an example of an extradosed bridge.Template:Sfn

An extradosed bridge combines features of a box girder bridge and a cable-stayed bridge. Visually, extradosed bridges can be distinguished from cable-stayed bridges because the tower height (above the deck) is relatively low: between 7% and 13% of the span width.Template:SfnTemplate:Efn Extradosed bridges are appropriate for spans ranging from Template:Convert to Template:Convert.Template:Sfn Unlike suspension bridges or cable-stayed bridges, the towers of a extradosed bridge rest on the deck, rather than on a footing; and in some implementations, are solidly connected to the deck.Template:Sfn Because of the relatively flat angle of the cables, the cables of an extradosed bridge compress the deck horizontally, performing a function comparable to prestressing wires that are used within concrete girders.Template:Sfn Extradosed bridges may be appropriate in applications where the deck must have a shallow depth to maximize clearance under the bridge; or where towers must be relatively short to abide by aviation safety constraints.Template:Sfn

Design

Design process

A freeway with several cars driving on it, with two concrete bridges passing overhead

Many overpass bridges in the United States Interstate Highway System are concrete box girder bridges, such as these bridges over Interstate 280 in California.

The process for designing a new bridge typically goes through several iterations, progressively refining the design.Template:Sfn An early step in the design processTemplate:Sndsometimes called conceptual designTemplate:Sndis to consider the multiple requirements that a bridge must satisfy.Template:Sfn

The requirements may be categorized as engineering requirements and non-engineering requirements. Engineering requirements include safety, strength, lifespan, climate, traffic, the size and nature of the obstacle to be traversed, and clearance required for passage underneath. Non-engineering requirements include construction cost, maintenance cost, aesthetics, time available for construction, owner preference, and experience of the builders. Other factors that may be weighed include impact to environment and wildlife; and the bridge's economic, social, and historic relationship to the local community.Template:Sfn

After the requirements of a bridge are established, a bridge designer uses structural analysis methods to identify candidate designs.Template:Sfn Several designs may meet the requirements. The value engineering methodology can be used to select a final design from multiple alternatives.Template:Sfn This methodology evaluates candidate designs based on weighted scores assigned to several different criteria, such as: cost, service life, durability, availability of resources, ease of construction, construction time, and maintenance cost.Template:Sfn After considering all factors, a bridge designerTemplate:Sndin consultation with the ownerTemplate:Snd will select a particular design.

Specifications and standards

One of the requirements a new bridge must satisfy is compliance with the local bridge design specifications and codes. In some cases, these are legally binding requirements.Template:Sfn In many countries, the specifications are developed and published by standards organizations that define acceptable bridge-building practices and designs. In Europe, the organization is the European Committee for Standardization, and the standards it publishes are the Eurocodes.Template:Sfn In the United States, the American Association of State Highway and Transportation Officials (AASHTO) publishes the AASHTO LRFD Bridge Design Specifications.Template:SfnTemplate:Efn Canada's bridge standard is the Canadian Highway Bridge Design Code, developed by the non-profit CSA Group.Template:Sfn

Service life

One of the important requirements established early in the design process is the service life, which is a specific number of years that the bridge is expected to remain in operation with routine maintenance (and without requiring major repairs).Template:Efn For example, wood bridges typically have a service life of 10 to 50 years.Template:SfnTemplate:Efn Concrete highway bridges typically have service lives of 75 to 150 years.Template:Sfn A bridge design methodology incorporates the service life into the design process.Template:Sfn

Aesthetics

A train moving atop a stone bridge in an attractive valley

The Brusio spiral viaductTemplate:Snda part of the Bernina railway in SwitzerlandTemplate:Sndis designated as a World Heritage Site.Template:Sfn

The aesthetics of a new bridge are one of the factors considered when designing a bridge. Attractive bridges can have a positive impact on a community, and some bridges can even be considered as works of art. Bridge designers that are known for emphasizing the visual appeal of their bridges include Thomas Telford, Gustave Eiffel, John Roebling, Robert Maillart, and Santiago Calatrava. Qualities that influence the perceived attractiveness of a bridge include proportion, order, refinement, environmental integration, texture, and color.Template:Sfn

The art historian Dan Cruickshank notes that bridges are regarded as manifestations of human imagination and ambition, and that many bridges bridge transcend their original utilitarian role and become a work of art.Template:Sfn He writes "[a] great bridge has an emotional impact, it has a sublime quality and a heroic beauty that moves even those who are not accustomed to having their senses inflamed by the visual arts."Template:Sfn

Material

An ornate bridge made of iron, passing over a small, lush valley

The Iron Bridge in Shropshire, England, completed in 1781, is the first major bridge made entirely of cast iron.Template:Sfn

A construction site with a halfway built concrete structure

This concrete bridge support is being prepared for a concrete pour. After the concrete cures, the green reinforcing bars will be permanently embedded inside.

Template:Multiple image A bridge designer can select from a wide variety of materials, including wood, brick, rope, stone, iron, steel, and concrete.Template:Efn A bridge made from two or more distinct materials (such as steel and concrete) is known as a composite bridge.Template:Sfn For example, some of the largest arch bridges utilize concrete-filled steel tubes.Template:Sfn

Wood is an inexpensive material that is rarely used for modern roadway bridges.Template:Sfn Wood is primarily used in beam or truss bridges, and is also used to build large trestle bridges for railways. When wood is used, it is often in the form of glued laminated timber.Template:Sfn

Masonry includes stone and brick, and is suitable only for elements of a bridge that are under compression, since masonry will crack if under tension. Therefore, masonry is limited to structures such as arches or foundations. In the twentieth century, large masonry bridges Template:Sndalthough superseded by concrete in the WestTemplate:Sndcontinued to be built in China.Template:Sfn

IronTemplate:Sndincluding cast iron and wrought ironTemplate:Sndwas used extensively from the late 1700s to late 1800s, primarily for arch and truss structures. Iron is relatively brittle, and has been replaced by steel for all but ornamental uses.

Steel is one of the most common materials used in modern bridges because it is strong in both compression and tension.Template:Sfn Steel was made in small quantities in antiquity, but became widely available in the late 1800s following invention of new smelting processes by Henry Bessemer and William Siemens. Truss bridges and beam bridges are often made of steel, and steel wires are an essential component of virtually all suspension bridges and cable-stayed bridges. Concrete bridges make extensive use of steel, because all concrete used in bridges contains steel reinforcing bars or steel prestressed cables. Steel bridges are more expensive than comparable concrete bridges, but they are much lighter (for the same strength), faster to build, and offer more flexibility during construction and repair.

Concrete is a strong and inexpensive material, but is brittle and can crack when in tension.Template:Sfn Concrete is useful for bridge elements that are in compression, such as foundations and arches.Template:Sfn Many roadway bridges are built entirely of concrete using a beam structure, often of the box girder variety.Template:Sfn Virtually all concrete used in bridges contains steel reinforcing bars, which greatly increase the strength. Reinforcing bars are set inside the concrete form, and the concrete is poured into the form, and cures with the bars inside. If concrete is used in elements that experience tension, prestressed cables must be embedded within the concrete and tightened. The prestressed cables can be pre-tensioned (stretched beforeTemplate:Sndand whileTemplate:Sndthe concrete cures); or post-tensioned (placed within tubes in the concrete, and tightened after the concrete cures). The prestressed cables compress the concrete. When the beam is placed into the bridge and carries a load, the undesirable tension (produced by the tendency of the beam to sag) is counteracted by the compression from the prestressed cables. Concrete beams can be precast offsite and transported to the bridge site, or cast in place.Template:Sfn High-performance concrete is becoming more commonly used in bridges (compared to conventional concrete) because it suffers less damage from heavy traffic and lasts longer.Template:SfnTemplate:Efn

Double-deck bridge

A long, straight, flat bridge over a large body of water

The Padma Bridge in Bangladesh carries rail traffic on the lower deck and vehicular traffic on the upper deck.Template:Sfn

Designers may choose to use a double-deck design (also known as double-decked or double-decker), which carries two decks on top of each other. This technique may be used to increase the amount of traffic a bridge can carry, or to build in a location where space is limited.Template:Sfn Double-deck bridges permit two different kinds of traffic to be safely carried. For example, motor vehicles can be separated from pedestrians or railways.Template:Sfn

An early double-deck bridge was Niagara Falls Suspension Bridge, which carried rail on the upper deck, and carriages and pedestrians on the lower deck.Template:Sfn George Washington Bridge in New York carries 14 motor vehicle lanes (eight above, six below), and is the world's busiest bridge, carrying over 100 million vehicles annually.Template:Sfn Because of their ability to carry large amounts of motor vehicles, double-deck bridges are often found in large cities, such as Tsing Ma Bridge in Hong Kong,Template:Sfn San Francisco–Oakland Bay Bridge in California,Template:Sfn and Shimotsui-Seto Bridge in Japan.Template:Sfn

Load analysis

A very large suspension bridge passing over a large body of water

The San Francisco–Oakland Bay Bridge is designed to withstand severe earthquakes. The eastern span, shown above, is a self-anchored suspension bridge which can survive a once-in-1,500-year earthquake.Template:Sfn

A bridge design must accommodate all loads and forces that the bridge might reasonably experience. The totality of the forces that the bridge must tolerate is the structural load, which is often divided into three components: dead load, live load, and environmental load. The dead load is the weight of the bridge itself.Template:Efn The live load is all forces and vibrations caused by traffic passing over the bridge, including braking and acceleration. The environmental load encompasses all forces applied by the bridge's surroundings, including weather, earthquakes, mudslides, water currents, flooding, soil subsidence, frost heaving, temperature fluctuations, and collisions (such as a ship striking the pier of a bridge).Template:Efn

Return period

Many load sources vary over time, such as vehicle traffic, wind, and earthquakes. A bridge designer must anticipate the maximum values that those loads are likely to reach during the bridge's lifespan.Template:Sfn For sporadic events like floods, earthquakes, collisions, and hurricanes, bridge designers select a maximum severity that the design must accommodate.Template:Sfn The severity is based on the return period, which is average time between events of a given magnitude. Return periods range from 10 to 2,500 years, depending on type of event and the country in which the bridge is located. Longer return periods are used for bridges that are a critical part of the transportation infrastructure. For example, if the bridge is a key lifeline in case of emergencies, the designer may utilize relatively long return period, such as 2,000 years; in this example, the design must endure the strongest storm that is expected to happen once every 2,000 years.Template:Sfn

Stress and strain

A computer screen running an app that is displaying engineering information

Software applications are used in the bridge design process, such as this app that evaluates stress and strain.Template:Sfn

A two-dimensional graph showing a curved line

Bridge engineers use stress–strain curves to assist with the design process.

The load forces acting on a bridge cause the components of the bridge to become stressed. Stress is a measure of the internal force experienced within a material. Strain is a measure of how much a bridge component bends, stretches, or twists in response to stress. Some strain (bending or twisting) may be acceptable in a bridge component if the material is elastic. For example, steel can tolerate some stretching or bending without failing. Other materials, such as concrete, are inelastic, and their change in shape when stressed is negligible (until the stress becomes excessive and the concrete fails).

A bridge designer must calculate the maximum stress that each bridge component will experience, then select an appropriate design and size for the components to ensure they will safely tolerate the loads on the bridge. Stresses are categorized based on the nature of the force that causes the stress, namely: compression, tension, shear, and torsion. Compression forces compact a component by pushing inward (for example, as felt by a bridge foundation when a heavy tower is resting on it). Tension is a stretching force experienced by a component when pulled (for example by the cables of a suspension bridge). Shear is a sliding force experienced by a component when two offset external forces are applied in opposite directions (for example, during an earthquake when the upper part of a structure is pulled north, and the lower part is pulled south). Torsion is a twisting force.

Traffic

An important component of the live load carried by a bridge is the vehicle and rail traffic the bridge carries. In addition to the weight of the vehicle, other forces must be considered, including braking, acceleration, centrifugal forces, and resonant vibrations. For roadways, the loads imposed by truck traffic far exceed the loads imposed by passenger cars, and so the bridge design process focuses on trucks.Template:Sfn

The loads created by trains and vehicles can be determined by modelling, or by relying on data and algorithms contained in engineering specifications published by organizations such as Eurocode or AASHTO.Template:Sfn Alternatively, weigh-in-motion technology can measure loads on existing bridges with comparable traffic patterns, providing real-world data which can be used to evaluate a proposed bridge design.Template:Sfn

Vibration

An animated video showing wind blowing left to right, creating circular vortexes as it passes by a fixed object

Bridge designers must account for forces caused by wind, such as the vortex shedding shown here.

Video shows a large suspension bridge moving and twisting wildly, pushed by the wind, eventually collapsing.

The Tacoma Narrows Bridge collapsed shortly after opening in 1940 due to failure of the design to properly account for wind forces.

A collapsed concrete bridge, with a broken support pier.

This bridge collapsed in the 1994 Northridge earthquake after the reinforced concrete pier was damaged.

Many loads imposed on a bridgeTemplate:Sndsuch as wind, earthquakes, and vehicular trafficTemplate:Sndcan cause a bridge to experience irregular or periodic forces, which may cause bridge components to vibrate or oscillate. Some bridge components have inherent resonant frequencies to which they are particularly susceptible, and vibrations near those frequencies can cause very large stresses.

Winds can produce a variety of vibrational forces on a bridge, including flutter, galloping, and vortex shedding. Considering wind forces during the design process is especially important for long, slender bridges (typically suspension or cable-stayed bridges).

The Eurocode guideline for bridge design specifies that vibration stress due to moving vehicles should be accounted for by including an additional 10% to 70% of the vehicles' static load; the exact value depends on the span length, the number of traffic lanes, and the type of stress (bending moment or shear force).Template:Sfn

If resonance issues are identified in the design process, they must be mitigated. Common techniques to address vibration include increasing the rigidity of the bridge deck by adding trusses and adding dampers to cables and towers. One mechanism used to combat oscillations is a tuned mass damper, which was first used in the Pont de Normandie in 1995. The Akashi Kaikyo Bridge has twenty tuned mass dampers, weighing nine tonnes each, inside its steel towers.Template:Sfn

Neglecting to account for vibrations and oscillations can lead to bridge failure. The Angers Bridge collapsed in 1850, killing over 200 people, partly due to soldiers marching on the bridge in a manner that increased resonant oscillations.Template:SfnTemplate:Efn The Tacoma Narrows Bridge collapsed in the 1940 in winds of Template:Convert, even though the bridge was designed to withstand winds up to Template:Convert. Investigations revealed that the designer failed to account for wind effects such as flutter and resonant vibrations. The Golden Gate Bridge was damaged in 1951 due to wind forces, and as a result was reinforced with additional stiffening elements.Template:Sfn

Earthquakes can subject bridges to ground motions that cause severe damage. Following seismic events, earthquake engineers study the seismic data to classify and quantify the motions experienced by bridges.Template:Sfn These studies are used by governments to create and revise design standards that specify the types of seismic movements that new bridges must withstand.Template:Sfn Earthquakes can cause long-period velocity pulses, shear cracks, large ground motions, vertical accelerations, and soil liquefaction.Template:SfnTemplate:Efn

Methodologies and tools

A computer app displaying a bridge with engineering data

Engineers use finite element method software tools to evaluate a bridge design.

The process used to design bridges employs structural analysis methods and techniques.Template:Sfn These methods divide the bridge into smaller components, and analyze the components individually, subject to certain constraints.Template:Sfn A proposed bridge design is then modeled with formulas or computer applications. The models incorporate the loads and stresses the bridge will experience, as well as the bridge's structure and material. The models calculate the stresses in the bridge and provide data to the designer indicating whether the design meets the required design goals.

Bridge design models include both mathematical models and numerical models.Template:Sfn The mathematical models that assess bridge loads and stresses are complex formulas that typically include differential equations. Solving these formulas directly is virtually impossible, so numerical models are used to provide approximate, but accurate, results.Template:Sfn The finite element method is a numerical model commonly used to perform detailed analysis of stresses and loads of a bridge design.Template:Efn The finite element method models a proposed bridge by dividing it into numerous small, interconnected pieces, and applying a computer algorithm to the pieces. The algorithm simulates the stresses on the bridge that are caused by the loads, and can iterate over time to simulate dynamic movements.Template:Sfn

A bridge designer evaluates the output of the models to determine if the design meets the design goals. Many criteria are evaluated when determining if a bridge design is sufficient, including deflection, cracking, fatigue, flexure, shear, torsion, buckling, settlement, bearing, and sliding.Template:Sfn The criteria, and their allowable values, are termed limit states. The set of limit states selected for a design are based on the bridge's structure and purpose.

To ensure that a proposed bridge design is sufficiently strong to endure foreseeable stresses, many bridge designers use methodologies such as limit state design (used in Europe and China) or Load and Resistance Factor Design (LRFD) (used in US). These methodologies add a margin of safety to the bridge design by incorporating safety factors into the design process.Template:Sfn The safety factors are applied two ways: (a) increasing the assumed loads and stresses the bridge will experience; and (b) decreasing the assumed strength of the bridge's structure.Template:SfnTemplate:Efn The magnitude of the safety factors are based on several considerations, including the bridge's own dead weight, vehicle traffic, earthquakes, water or ice flows (from rivers or ocean currents) impacting the bridge foundations, rain or snow on the bridge, wind, settling into the soil, and collisions.Template:SfnTemplate:Efn

Construction

A schematic diagram identifying the various parts of a Hypothetical bridge

Some elements of a fictional bridge. 1 Approach, 2 Arch, 3 Truss, 4 Abutment, 5 Bearing, 6 Deck and beams, 7 Pier Cap, 8 Pier, 9 Piling, 10 Footing, 11 Caisson, 12 Subsoil.

The structural elements of a bridge are generally divided into the substructure and the superstructure.Template:Sfn The substructure consists of the lower portions of the bridge, including the footings,Template:Efn abutments, piers, pilings, anchorages, and bearings.Template:Sfn The superstructure rests upon the substructure, and consists of the deck, trusses, arches, towers, cables, beams, and girders.

Construction process

Construction of a bridge is typically managed by construction engineers, who are responsible for planning and supervising the construction process. Important aspects of this role include budgeting, scheduling, periodically conducting formal design reviews, and communicating with the bridge designers to interpret and update the design plans.Template:Efn

The forces experienced by a bridge during construction can be larger or have a different nature than the forces it will experience after completion. The bridge design process typically focuses on the strength of the fully completed bridge, but it should also consider the unusual stresses that individual elements will experience during construction. Special techniques may be required during construction to avoid excessive stresses, such as temporary supports under the bridge, temporary reinforcement, or bracing of specific elements.

When an existing bridge is being replaced or refurbished, the impact on traffic flow can have a detrimental effect on residents and services. Accelerated bridge construction processesTemplate:Sndthat focus on using pre-fabricated components and a rapid timetableTemplate:Sndmay be used to mitigate the impacts.Template:Sfn

Substructure

Two schematic diagrams showing how force is transmitted in a flat bridge compared to an arched bridge

Abutments are an important element of a substructure. Beam bridges (left) direct force vertically into the abutments; some arch bridges (right) direct forces diagonally. 1 Deck, 2 Abutments, 3 Subsoil, 4 Load on bridge, 5 Force from abutment into subsoil.

Construction of all bridge types begins by creating the substructure. The first elements built are typically the footings and abutments, which are typically large blocks of reinforced concrete, entirely or partially buried underground. The footings and abutments support the entire weight of the bridge, and transfer the weight to the subsoil. Based on their height-to-width ratio, footings are categorized as: shallow (height is less than width) or deep (height is greater than width). If the subsoil cannot support the load placed on the footings, pilings must first be driven below the footings: pilings are long structuresTemplate:Sndmade of wood, steel, or concreteTemplate:Sndplaced vertically below footings. Some pilings reach down and rest on bedrock; others rely on friction to prevent the footing from sinking lower.

Abutments are usually located at the ends of a bridge deck, where it reaches the subsoil.Template:Sfn They direct the weight into the subsoil, either vertically or diagonally. Abutments may also act as retaining walls, keeping the subsoil under the approach road from eroding.Template:Sfn After footings for the piers have been created, the piers and pier caps are built to complete the substructure.Template:Efn Suspension bridges usually require anchorages, which are large reinforced concrete blocks solidly anchored into the earthTemplate:Sndthey must be exceptionally heavy and tied into the subsoil because they must withstand the lateral pull of the large cables that hold the entire deck and live load.Template:Efn

Building supports in water

When bridge supports (such as piers or towers) are built in a river, lake, or ocean, special technologies must be utilized.Template:Sfn Caissons can be used to provide a workspace while constructing the submerged portion of the supports. A caisson is a large, watertight, hollow structure, open on the bottom. It is usually sunk to the bottom of the water and workers can work inside, preparing the ground for the footings. When excavation is complete, a caisson is typically filled with concrete to create all or part of the footing. Air pressure inside a sealed caisson must be kept high to prevent water from seeping in.Template:Sfn Workers, if they do not properly decompress when exiting the caisson, can get decompression sickness. Early bridge builders did not understand decompression, and deaths were common: thirteen workers died from decompression sickness when building the Eads Bridge (completed in 1874).

An alternative to a caisson is a cofferdam, which is a temporary dam surrounding the support location, open on top, where workers may work while constructing the footings. Another approach for constructing foundations in water was employed for the Akashi Kaikyo suspension bridge: the two foundations for its towers are Template:Convert tall and Template:Convert in diameter. The foundations were partially built on land, then towed by tugboats to the bridge site. They were sunk to the bottom in water Template:Convert deep, and each was filled with 355,000 cubic meters of concrete. The foundations rest directly on the ocean bottom, without pilings or footings.

Bearings

Two cylinders of steel, supporting a large steel bridge, and resting on a concrete support

This bridge superstructure rests on bearings.Template:Sfn

Bearings are often placed between the superstructure and the substructure at the points of contact. Bearings are mechanical devices that enable small movementsTemplate:Sndwhich may result from thermal expansion and contraction, material creep, or minor seismic events. Without bearings, the bridge structure may be damaged when such movements occur. Bearings can be selected to permit small rotational or slipping movements in a specific direction, without permitting movements in other directions. Types of bearings used on bridges include hinge bearings, roller bearings, rocker bearings, sliding bearings, spring bearings, and elastomeric bearings.

Superstructure

A bridge being constructed, with two large cranes on top

Gantries are one technique used to gradually assemble a bridge deck.Template:Sfn

A huge wooden arch structure, over which an arch bridge is being built

This temporary falsework will be removed after an arch is built over it.

After the substructure is complete, the superstructure is built, resting on the substructure. Beam bridge superstructures may be fabricated off-site (common for steel beams) or cast-in-place (for many concrete beams). The beams may be laid across the piers by a crane or gantry. If the span crosses a deep ravine, a technique known as launching may be used: the beams and deck are assembled on the approach road, then pushed horizontally across the obstacle.Template:Efn

Arch bridge superstructure construction methods depend on the material. Concrete or stone arches use a temporary wood structure known as falsework or centering to support the arch while it is built. Some steel arch bridges are constructed without falsework: both sides are built in a cantilever fashion from the abutments, and when they reach the middle, they are jacked slightly apart for the final section to be inserted.

Cantilever bridge superstructures are usually built incrementally by proceeding outward from anchorages or piers. Most cantilever superstructures can be built without temporary support piers, as the bridge can support itself as it extends outward. A similar process is used for steel or concrete cantilevers: prefabricated sections may be positioned at ground (or water) level and hoisted into place with a gantry, or may be transported horizontally along the previously completed portion of the cantilever. Concrete cantilevers require steel prestressing cables to be passed through tubes within each section and tightened, which will put the concrete into compression. Truss bridges are built using a variety of methods, including piece-by-piece, cantilevering, or falsework.Template:Sfn

Cable-stayed bridge superstructures begin with the construction of one or more towers, which rest directly on footings that are part of the substructure. The deck is constructed in pieces beginning at the towersTemplate:Efn and moving outward. As each piece of the deck is added, it is connected to towers with steel cables, and the cables are tightened to take the load of the deck. The deck proceeds outwards in both directions at the same rate, to ensure the forces applied to the tower are balanced. If the deck is made of concrete, steel prestressing cables are inserted through tubes inside each deck section, and tightened to put the concrete into compression.

Suspension bridge superstructure construction usually begins with the towers.Template:SfnTemplate:Efn The towers may be steel or concrete, and rest directly on footings. The large cables are created by hauling a large pulley back and forth across the span, stringing multiple wires between the anchorages in each pass, in a process termed spinning. After the wires are spun, they are bundled together to form the cables.Template:Efn The cables are securely fastened to the anchorages at both ends.Template:Efn Vertical wires called hangers are suspended from the cables, then small sections of the deck are attached to the hangers, and the sections are attached to each other.

Towers

A diagram showing a curved line passing over a curved object on top of a tower; and another diagram showing two lines that each of which end inside a tower.

A cable transfers its load to a tower by either (a) passing over a curved saddle (left image); or (b) the end of the cable is anchored into the tower (right image). Key: 1 Cable, 2 Saddle, 3 Anchor, 4 Tower.Template:SfnTemplate:Efn

Towers are an important component of the superstructure of cable-stayed bridges and suspension bridges.Template:Efn Towers are made of either concrete or steel. Steel towers are much lighter than concrete towers (of the same height). Concrete is generally suitable only for towers up to about Template:Convert tall, whereas steel towers can be much taller.Template:SfnTemplate:Efn

Towers support the bridge cables, whichTemplate:Sndin turnTemplate:Snd hold the weight of the bridge deck and the vehicular traffic. Most of the load imposed on a tower is applied vertically downward on the tower, rather than sideways.Template:Sfn Towers experience a compression stress, in contrast to cables, which experience a tension stress.Template:Sfn There are two mechanisms used to attach a cable to a tower: saddles or anchors. Saddles are curved structures which allow a cable to pass through (or over the top of) a tower. An anchor holds the end of a cable. Saddles are often used in suspension bridges, and anchors are often used in cable-stayed bridges.Template:Sfn

Cables

Template:Multiple image Steel cables are an element of both cable-stayed bridges and suspension bridges. Cables are made of one or more strands, and each strand consists of multiple wires. A wire is a thin, flexible piece of solid steel, of higher tensile strength than normal steel, and with a diameter of 3mm to 7mm.Template:SfnTemplate:Efn Cables are typically constructed at the bridge site by unspooling wires or strands from large reels.Template:SfnTemplate:Efn Large suspension bridges may use cables that are over Template:Convert in diameter and weigh over 20,000 tonnes.

Before building the cables of a suspension bridge, temporary catwalks must be constructed to support the wires while they are drawn across the span and over the tops of the towers.Template:Sfn There are two approaches to pulling the wires across the span: the air spinning method where the individual wires are carried across by pulleys; and the prefabricated parallel-wire strand (PPWS) method where entire strands are individually pulled across.Template:Efn

The air spinning method was used for all suspension bridges until the PPWS method was invented in the 1960s.Template:Sfn The air spinning method is slower because it requires the spinning pulley to cross the span thousands of times, pulling a pair of wires each time. After 300 to 500 wires are pulled, aluminum bands are used to bundle them into strands.Template:Sfn. The PPWS method permits strands to be built away from the bridge site, but the process of pulling the heavy strands across the full span of the bridge is more difficult.Template:Efn

The wires within a strand may be parallel, or they may wrap around each other in a twisted (spiral) pattern.Template:Sfn Air spinning always produces strands that contain parallel wires. The PPWS method can utilize strands with parallel or twisted wires.Template:Sfn

After all the wires have been drawn across the full span and are connected to the towers, they are compacted into a tight bundle by an hydraulic device that moves along the cable and compresses the wires together. Then a wire is usually wrapped around the cable in a helical manner, to provide protection against water intrusion.Template:Sfn The deck is suspended from the cable with vertical strands called hangers. Each hanger is attached to the main cable by a bracket called a cable band.Template:Sfn

Deck

A large concrete arch bridge being constructed

The deck of this arch bridge is being horizontally pushed onto the substructure with jacks.

The deck of a bridge is the horizontal, continuous surface that extends across the full span of a bridge, and upon which vehicles or pedestrians travel. Decks generally rest on beams or box girders. When a deck is rigidly attached to its supporting beams or girders they function together as a single structure.Template:Efn

The two most common types of decks are concrete decks and orthotropic steel decks.Template:Efn Concrete decks are flat slabs of reinforced concrete. The slabs may precast off-site, or cast-in-place by pouring concrete into forms on the bridge superstructure.Template:Efn Orthotropic steel decks are built of numerous small ribs of steel, running in the direction of the bridge roadway.Template:Efn On top of the ribs is a flat steel plate, coated with a wearing surface. Below the ribs are floor beams, placed crosswise to the ribs.Template:SfnTemplate:Efn Orthotropic steel decks are more expensive than concrete steel decks, but weigh less. They are useful in applications where weight is critical, a thin deck is required, or the environment is subject to earthquakes or extreme cold weather.Template:Sfn

Many decks have a wearing surface on top, which is a layer of material designed to be periodically replaced after it is worn away by vehicular traffic. Wearing surfaces are typically made of aggregate (small rocks) mixed with a binder such as asphalt, polyurethane, epoxy resins, or polyester.Template:Efn Railway bridge decks are categorized as open decks (the ties rest directly on beams or girders, with air gaps between) and ballast decks (the ties rest on ballast rocks, and the ballast rests on a deck slab).Template:Sfn

Constructing the deck (and its supporting beams or girders) can be difficult when the bridge is over water or a deep valley. A variety of techniques are available, and the choice depends on factors such as the topography of the site, the deck material (concrete or steel), traffic or obstacles under the bridge, and whether sections can be built off-site and transported to the bridge. Methods of deck construction include building atop temporary supports, jacking up from the ground, incremental launching (building the entire deck on the approach road and pushing it horizontally), lifting from below with a hoist mounted on the bridge, cantilevering (incrementally extending the deck, starting from towers or abutments), and lifting with a floating crane.

Protection

A thick, old wire cable, with paint that is partially worn off

Paint can be used to reduce deterioration of steel components. Steel bridges need to be repainted periodically, as seen in this wire hanger from the Golden Gate Bridge, which is painted international orange.Template:Sfn

To achieve a longer lifespan, a bridge is protected from deterioration by incorporating certain features into the design. Bridges can deteriorate due to a variety of causes, including rust, corrosion, chemical actions, and mechanical abrasion. Deterioration is sometimes visible as rust on steel components, or cracks and spalling in concrete.Template:Sfn

Deterioration can be slowed with various measures, primarily aimed at excluding water and oxygen from the bridge elements.Template:Sfn Techniques to prevent water-based damage include drainage systems, waterproofing membranes (such as polymer films), and eliminating expansion joints.Template:SfnTemplate:Efn

Concrete bridge elements can be protected with waterproof seals and coatings.Template:SfnTemplate:Efn Reinforcing steel within concrete can be protected by using high-quality concrete and increasing the thickness of the concrete surrounding the steel.Template:Sfn Steel elements of a bridge can be protected by paints or by galvanizing with zinc. Paint can be avoided entirely for steel members by using certain steel alloys, such as stainless steel or weathering steel (a steel alloy that eliminates the need for paint, by forming a protective outer layer of rust).

Bridge scour is a potentially serious problem when bridge footings are located in water. Currents in the water can cause the sand and rocks around and below the footings to wash-away over time. This effect can be mitigated by placing a cofferdam around the footings, or surrounding the footings with rip-rap.Template:Efn

Suspension bridges and cable-stayed bridges have large cables containing hundreds of steel wires. Several techniques are used to minimize corrosion inside the cables, such as wrapping the cables with galvanized wire, injecting the cables with grout or epoxy, using interlocking S-profile wires, and circulating dry air through the interior of the cable.Template:Sfn

Operation

Management

After a bridge is completed and becomes operational, management processes are employed to ensure that it remains open to traffic, avoids safety incidents, and achieves its intended lifespan. These processesTemplate:Sndcollectively referred to as bridge managementTemplate:Snd include technical activities such as maintenance, inspection, monitoring, and testing.Template:Sfn In addition to technical tasks, management encompasses planning, budgeting, and prioritization of maintenance activities.Template:Sfn Bridge managers use methodologies such as bridge management systems and Life-Cycle Cost Analysis to manage a bridge and estimate the maintenance costs of a bridge throughout its lifetime. Annual maintenance costs increase as the bridge ages and degrades.Template:Sfn

Maintenance

Maintenance activities seek to prolong the life of the bridge, reduce lifecycle costs, and ensure the safety of the community.Template:Sfn Maintenance tasks can be categorized as corrective tasks and preventive tasks.Template:Sfn Corrective tasks are implemented in response to unexpected issues that arise, such as repairing structural elements (piers, beams, girders, towers, or cables) and replacing bearings.Template:Sfn

Preventive tasks include washing, painting, lubricating bearings, sealing the deck, filling cracks, removing snow, filling potholes, and repairing minor issues with structures and electrical fixtures.Template:Sfn Some preventive tasks are performed on a periodic schedule. Example intervals for periodic bridge maintenance tasks include: washing entire structure (1–2 years); sealing deck surface (4–6 years); lubricating bearings (4 years); painting steel bridge components (12–15 years); replacing the deck's wearing surface (12 years); sealing sidewalks (5 years); filling cracks (4 years); and cleaning drains (2 years).

Inspection and monitoring

A large block of concrete, partially crumbling, with internal steel bars exposed

This pier of a bridge in Germany is degrading, and the internal steel reinforcing bars are exposed and rusting.

An important part of maintenance is inspecting a bridge for damage or degradation, and taking steps to mitigate any issues detected. Degradation can come from a variety of sources: expansion/contraction from freeze/thaw cycles, rain and snow, oxidation of steel, saltwater spray, carbonatation of concrete, vehicular traffic, corrosion, mechanical abrasion, poor bridge design, and improper repair procedures. Some countries mandate periodic inspection schedules, for example, routine inspections every 24 months, or inspecting underwater foundations for scouring every 60 months.Template:Sfn

Relying solely on visual inspection to assess degradation of a bridge can be unreliable, so inspectors use a variety of nondestructive testing techqniques.Template:Sfn These techniques include hammer strike tests, ultrasonic pulse velocity tests, seismic tomography, and ground penetrating radar. Magnetometers can be used to detect the location of reinforcing steel within concrete.Template:Sfn Various electrical tests, such as permeability and resistance, can give insight into the condition of surface concrete.Template:Sfn X-rays can be passed through concrete to obtain data about concrete density and condition.Template:Sfn Videography using slender probes can be used where access is available.Template:Sfn

A tall bridge covered in temporary scaffolding

Scaffolding is erected under the Sitterviadukt rail bridge in Switzerland while maintenance on the deck truss is performed.Template:Sfn

Measurements of the state of a bridge may be made automatically and periodically using structural health monitoring (SHM) technologies.Template:Sfn SHM places permanent sensors at critical locations in the bridge, which may be sampled at any time to obtain data about stresses and chemical degradation.Template:Sfn The sensors may be placed in the bridge during construction, or while it is in operationTemplate:Sndfor example, to monitor the quality of a repair. Many long-span bridges are routinely monitored with a range of sensors, including strain transducers, sodar, accelerometers, tiltmeters, and GPS.Template:Sfn

To evaluate the condition of large steel cables, electrical coils are moved along the cable, measuring the induction of the cable, which can reveal corrosion issues.Template:Sfn Detailed measurements of the external surface of a bridge can be recorded using lidar technology. Comparing measurements taken at multiple points in time can reveal long-term changes.Template:Sfn

A variety of structural tests may be performed to evaluate a bridge's condition. One test involves placing loads in selected locations on the bridge, and measuring the resulting deflections: sensitive instruments measure how much the bridge elements bend or twist, and the results can reveal if the element is not performing within expected limits. Another test involves jacking the bridge deck off its supports slightly, and measuring the force required. Cables can be evaluated by vibrating them and measuring their dynamic response.Template:Sfn

Some testingTemplate:Sndtermed destructive testingTemplate:Sndrequires removing samples from the bridge and taking them to a laboratory for analysis with microscopes, sonic devices, or X-ray diffraction.Template:Sfn Destructive testing is performed on samples such as cores drilled from concrete, or a small piece of steel wire cut from a cable.Template:SfnTemplate:Efn

Failures

A broken bridge, which has fallen into the water over which it used to pass

The Nanfang'ao Bridge in Taiwan collapsed because of excessive corrosion that went undetected.Template:Sfn

Bridge failures are of special importance to structural engineers, because the analyses of the failures provide lessons learned that serve to improve design and construction processes. Bridge failures are caused by a variety of factors, which can be categorized as natural factors (flood, scour, earthquake, landslide, and wind) and human factors (improper design and construction method, collision, overloading, fire, corrosion, and lack of inspection and maintenance).Template:Sfn Over time, bridge failures have led to significant improvements in bridge design, construction, and maintenance practices.

Before the advent of bridge engineering procedures based on rigorous, scientific principles, bridges frequently failed. Failures were most common in the mid 1800s, when the rapidly expanding railway networks were building hundreds of new bridges every year around the globe.Template:Sfn In the United States, 40 bridges per year failed in the 1870s, amounting to 25% of all bridges built in that decade.

In the modern era, in spite of advances in bridge engineering methodologies, bridge failures continue to be a global issue. In Australia, the King Street Bridge collapsed in 1962, a year after opening, due to improper welding techniques.Template:Sfn In Palau, the Koror–Babeldaob Bridge collapsed in 1996, three months after a repair operation made major changes to the bridge.Template:Sfn In 1998, the Turag-Bhakurta Bridge in Bangladesh collapsed due to river waters scouring away the soil around the bridge supports.Template:Sfn The Millennium Bridge in London opened in 2000, but closed two days later due to excessive swaying.Template:Efn It did not open until two years laterTemplate:Sndafter dampers were installed. About half of all bridge failures in the early 21st century in the US were due to water-related causes, such as flood damage or scouring (water currents undermining the bridge supports).

Society and culture

Economic and environmental impact

Bridges can have a significant impactsTemplate:Sndboth positive and negativeTemplate:Snd on a community's environment, society, and economy. Positive effects can include shorter transport times, employment opportunities, improvements to social equity, improved productivity, and increases to the gross domestic product. Negative impacts of bridges can include contributions to global warming, increased traffic accidents, workplace injuries, corruption, and increased pollution (during construction, from maintenance work, and from vehicular traffic). During the bridge design process, these effects may be modeled with sustainability methodologies such as life cycle sustainability assessment or building information modeling, and the results can be used to improve the bridge's sustainability.

In underdeveloped regions with mountainous topography, construction of bridges that cross deep valleys can bring major benefits to the communities they connect. Without bridges, such areas often have a core region that is more prosperous, surrounded by less developed peripheral regions. Building bridges over deep valleys can reduce developmental disparities between areas, as well as generate economic development, and improve accessibility to goods and services.Template:Sfn

Metaphor and symbol

Reaching for the world, as our lives do,
As all lives do, reaching that we may give
The best of what we are and hold as true:
Always it is by bridges that we live.

Philip Larkin "Bridge for the Living" (1981)

Bridges occur extensively in art, legend, and literature, often employed in a metaphorical manner. In Norse mythology, the home of the godsTemplate:SndAsgardTemplate:Sndis connected to the earth by Bifröst, a rainbow bridge.Template:Sfn Many bridges in Europe are named Devil's Bridge, and sometimes have folkloric stories that explain why the bridge is associated with the devil.Template:Sfn Christian legend holds that St. Bénézet lifted a huge boulder to begin construction of the Pont Saint-Bénézet bridge, and went on to found the apocryphal Bridge-Building Brotherhood.Template:Sfn Stories and poems often employ a bridge as a metaphor of the human lifespan, or human experiences.Template:Sfn Bridges are often the setting for pageants, celebrations, and processions.Template:Sfn Some nations have chosen to designate bridges that connect them as friendship bridges or peace bridges.Template:SfnTemplate:Efn In 1996, the European Commission held a competition to select art for the euro banknotes. Robert Kalina, an Austrian designer, won with a set of illustrations of bridges, chosen because they symbolize links between states in the union and with the future.

Bridges are often venerated as symbols of humankind's heroism and accomplishments.Template:Sfn The inspirational nature of bridges has led them to be featured in the works of poets, painters and writers.Template:Sfn Bridges feature prominently in paintingsTemplate:Sndoften in the backgroundTemplate:Sndas in the Mona Lisa.Template:Sfn Authors have used bridges as the centerpiece of novels, such as The Bridge on the Drina by Ivo Andrić and Thornton Wilder's The Bridge of San Luis Rey.Template:Sfn British poet Philip Larkin, inspired by the construction of the Humber Bridge near his home, wrote "Bridge for the Living" in 1981.Template:Sfn

Signature bridges

A large bridge crossing a river, in nighttime, with skyscrapers in the background

The Dagu Bridge in China was designed to be a signature bridge.Template:Sfn

Many bridgesTemplate:Sndknown as signature bridgesTemplate:Sndare strongly identified with a particular community.Template:Efn Some visually impressive bridges, such as the Dagu Bridge in China, are designed with the express goal of creating a landmark for the host city. The art historian Dan Cruickshank notes that some bridges have the ability to "transform a place a community and ... can make its mark on the landscape and in men's minds, capture the imagination, engender pride and sense of identity and define a time and place."Template:Sfn Large suspension bridges, in particular, are often regarded as iconic landmarks that symbolize the cities in which they are located. Notable examples include the Brooklyn Bridge in New York; the Golden Gate Bridge in San Francisco; the Clifton Suspension Bridge in Bristol; and the Széchenyi Chain Bridge in Budapest.Template:SfnTemplate:Efn

Profession and regulation

File:The Institution of Civil Engineers porch 2025-10-03.jpg

The Institution of Civil Engineers, located in London, is the world's oldest professional civil engineering association.Template:Sfn

The profession of civil engineeringTemplate:Sndwhich includes the discipline of bridge buildingTemplate:Snd began to be formalized in the 1700s when a school of engineering was created in France within the Corps des Ponts et Chaussées at the École de Paris, under the direction of Jacques Gabriel.Template:Sfn In 1747 the first school dedicated to bridge building was founded, also in France: the École Nationale des Ponts et ChausséesTemplate:Efn led by engineers Daniel-Charles Trudaine and Jean-Rodolphe Perronet.Template:Sfn The first professional organization focused on civil engineering was the Institution of Civil Engineers founded in 1818 in the UK, initially led by Thomas Telford.Template:Sfn

In the modern era, bridge engineering is regulated by national organizations, such as the National Council of Examiners for Engineering and Surveying (US), the Canadian Council of Professional Engineers (Canada), and the Engineering Council (UK).Template:Sfn In many countries, bridge engineers must be licensed or meet minimal educational requirements.Template:Sfn Some countries require engineers to pass qualification examinations, for example, in the US engineers must pass the Fundamentals of Engineering exam followed by the Principles and Practice of Engineering exam.Template:Sfn In Poland, bridge engineers are required to obtain certification by accumulating several years of experience under a senior engineer, and passing an exam administered by the Polish Chamber of Civil Engineers (pl:Polska_Izba_Inżynierów_Budownictwa).Template:Sfn International cooperation in the field of engineering is facilitated by the World Federation of Engineering Organizations.Template:Sfn

Suicide

Suicides are sometimes committed by jumping off bridges. This method can account for 20% to 70% of suicides in urban areas with access to tall bridges.Template:Efn In some regions, suicide by jumping disproportionately affects young adults, who tend to have lower inhibitory control. Specific bridges can gain notoriety and attract persons experiencing a suicidal crisis, which creates a feedback loop. High-risk bridges often have suicide prevention barriers installed,Template:Efn which dramatically decrease the suicide rate on the bridge.Template:Efn Installing barriers on a high-risk bridge generally reduces the jumping suicide rate in a region, although in some instances, other bridges become substitutes.Template:Sfn

References

Sources

Books

• Reprinted in 1993.

Journals and websites

• Template:Cite thesis

• Template:Cite magazine

• PDF version

• Template:Cite magazine

• Template:Cite magazine

Unknown author

• Document states 50,241,454 eastbound vehicles in 2024.