A bridge is a structure designed to span an obstacle, such as a river or railway, allowing vehicles, pedestrians, and other loads to pass across. Most bridges consist of a flat deck, supported by beams, arches, or cables. These structures rest on a foundation that is carefully designed to transfer the weight of the bridge to the subsoil without settling. Bridges can be constructed in a wide variety of forms, determined by the location, intended purpose, and available construction technologies. Simple bridge structures include beam bridges made from logs, and suspension bridges made of ropes or vines. The Romans and ancient Chinese built major arch bridges of timber, stone, and brick. During the Renaissance, advances in science and engineering led to wider bridge spans and more elegant designs. Concrete was perfected in the early 19th century, and arch bridges are now built primarily of concrete or steel. With the Industrial Revolution came mass-produced steel, which enabled the creation of more complex forms – including truss and cantilever bridges – that permitted bridges to cross wide rivers or deep valleys. The longest spans use suspension or cable-stayed designs, both of which rely on high-strength steel cables to support the deck. Over time, the maximum achievable span of bridges has steadily increased, reaching 2 kilometers (1.2 miles) in 2022. Other bridge forms include multi-span viaducts, which can cross wide valleys; trestles, a common design for carrying heavy trains; and movable bridges including drawbridges and swing bridges. The design of a bridge must satisfy many requirements, namely connecting to a transportation network, providing adequate clearances, and safely transporting its users. A bridge must be strong enough to support its own weight as well as the weight of the traffic passing over it. It must also tolerate violent, hard-to-predict stresses imposed by the environment, including winds, floods, and earthquakes. To meet all these goals, bridge engineers typically use limit state design processes and the finite element method. Many bridges are admired for their beauty, and some spectacular bridges serve as iconic landmarks that provide a sense of pride and identity for the local community. In art and literature, bridges are frequently used as metaphors to represent connection or transition. Bridges can create beneficial impacts on a community, including shorter transport times and increased gross domestic product; and also negative effects such as increased pollution and contributions to global warming.
History
Antiquity
The earliest forms of bridges were simple structures for crossing wetlands and creeks, consisting of wooden boardwalks or logs. Pilings – which are critical elements of bridge construction – were used in Switzerland around 4,000 BC to support stilt houses built over water. Several corbel arch bridges were built c. 13th century BC by the Mycenaean Greece culture, including the Arkadiko Bridge, which is still in existence. In the 7th century BC, Assyrian king Sennacherib constructed stone aqueducts to carry water near the city of Nineveh; one of the aqueducts crossed a small valley at Jerwan with five corbelled arches, and was 280 meters (920 ft) long and 20 meters (66 ft) wide. In Babylonia in 626 BC, a bridge across the Euphrates was built with an estimated length of 120 to 200 meters (390 to 660 ft). In India, the Arthashastra treatise by Kautilya mentions the construction of bridges and dams. 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 aqueducts – some still standing today – used a semicircular arch style. Examples include the Alcántara Bridge in Spain and the Pont du Gard in France. 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. Some Roman cements, particularly those containing volcanic ash, were waterproof. The enormous timber and stone Trajan's Bridge (c. 105 AD) crossed the Danube river and was over 900 meters (3,000 ft) long.
300 to 1400
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. 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 like the Pont d'Avignon, bridges of the Durance river, and the Old London Bridge. Surviving examples include the Ponte Vecchio in Florence, the Old Exe Bridge, and the Monnow Bridge in Wales.
1400 to 1750
In 15th- and 16th-century Europe, the Renaissance brought a new emphasis on science and engineering. 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. Their innovations included truss bridges and stone segmental arches, resulting in Florence's Ponte Santa Trinita, the Rialto Bridge in Venice, and Paris's Pont Neuf. Military and commercial bridges were constructed in India by the Mughal administration. The Asante Empire in Africa built bridges over streams and rivers using tree trunks and beams.
1750 to 1900 In the late 18th century, the design of arch bridges was revolutionized in Europe by Jean-Rodolphe Perronet and John Rennie. They designed arches that were flatter than semicircular Roman arches, which yielded faster construction times, better water flow under the bridge, and slimmer piers. These designs were used for the Pont de la Concorde and New London Bridge.
With the advent of the Industrial Revolution, iron became an important construction material for bridges. Both cast iron (which is strong under compression, but brittle) and wrought iron (which was more ductile and better under tension) were used for building bridges. The Iron Bridge in England – made of cast iron and completed in 1781 – was the first major bridge made entirely of metal. Several long suspension bridges were built in the early 19th century using iron eyebars (steel wire, vastly superior, became available later in the century). The age of railways began in the 1820s, and led to major innovations in bridge design. Britain is representative of how railways influenced bridge-building in industrialized nations: led by designers Isambard Kingdom Brunel, Robert Stephenson, and Joseph Locke, British railway bridges steadily grew in size as the decades passed. Notable bridges of that era include the High Level Bridge (1849), Royal Border Bridge (1850), Britannia Bridge (1850), Royal Albert Bridge (1859), and Clifton Suspension Bridge (1864). The number of railway bridges in Britain increased from 30,000 to 60,000 during the Railway Mania era. Railway bridges primarily used masonry and stone arch designs, because those could withstand the tremendous loads imposed by trains, but iron beam designs (on masonry or stone piers) were also used. The abundance of inexpensive lumber in North America led that continent to favor timber as a bridge material: using truss designs (for long spans) and trestle designs (for spanning deep ravines). The mass production of steel in the late 19th century provided a new material for bridges, enabling lighter, stronger truss bridges and cantilever bridges; and steel wires replaced iron bars as the preferred material for suspension bridge cables. Concrete – which was originally used within the Roman Empire – was improved with the invention of Portland cement in the early 19th century, 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.
1900 to present
Throughout the 20th century, new bridges – by designer Othmar Ammann and others – repeatedly broke records for span distances, enabling transportation networks to cross increasingly wider rivers and valleys. Cable-stayed bridges – which use cable-stays as the exclusive means of support – became a popular bridge design following World War II. The late 20th century saw several major innovations in bridge design. Extradosed bridges were introduced and found widespread use, predominantly in Japan. In China, concrete-filled steel tubes were adopted as a new approach to building arch bridges. Fiber-reinforced polymers – which do not suffer from the rust problems that plague steel – were 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. In the 21st century, a bridge span exceeded 2 kilometers (1.2 mi) for the first time with the construction of the 1915 Çanakkale Bridge.
Uses 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. Aqueducts were developed early in human history, and carried water to towns and cities. Canal systems sometimes include navigable aqueducts (also called canal bridges) to carry boats across a valley or ravine.
Transportation
Until the early 19th century, most bridges were designed to carry pedestrians, horses, and horse-drawn carriages. Following the invention of railways, many rail bridges were built; in Britain the number of bridges doubled during the railway-building boom in the mid-19th century. Railway bridges have unique requirements because of the heavy loads they carry – a single locomotive can weigh 197 tonnes (217 short tons). 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). These requirements led railways to avoid curved bridges, suspension bridges, and cable-stayed bridges; instead, straight beam or truss bridges are commonly used. The explosive growth of motorway networks in the 20th century required bridges to span ever longer distances to reach islands and cross valleys, along with the urban introduction of elevated railways and monorails.
Grade separation
An important application of bridges is improving safety and traffic flow at traffic junctions where roads or railways cross at ground level. Such intersections require vehicles to stop, and lead to slower traffic, wasted fuel, and higher incidence of collisions. One technique to mitigate these issues is to build a bridge, enabling one of the roads to pass over the other: this process is known as grade separation. Grade separation can be implemented at railway-road intersections or road-road intersections.
Pedestrians Some bridges, known as footbridges, are devoted to pedestrian traffic. They range from simple boardwalks enabling passage over marshy land to elevated skybridges – including the Minneapolis Skyway System – which shield pedestrians from harsh winter weather. When used to cross roads in busy urban areas, footbridges are generally safer than crosswalks, but have been criticized by urbanists and disability advocates for inconveniencing pedestrians, hindering accessibility, diminishing the quality of city life, and perpetuating car dependency.
Military Military bridges are an important type of equipment in the field of military engineering. They perform a variety of wartime roles, namely quickly traversing obstacles in the midst of battle, or facilitating resupply behind front lines. Military bridges can be categorized as wet bridges that rest on pontoon floats, and dry bridges that rest on piers, river banks, or anchorages. 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.
Armoured vehicle-launched bridges are carried on purpose-built vehicles. 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's Chieftain vehicle could launch a 23-meter (75 ft) bridge – capable of supporting 54-tonne (60-short-ton) loads – in 3 minutes. Military bridges have found use in civilian applications. The Bailey bridge was originally invented in 1940 for use in World War II, but continues to be used in peacetime. Bailey bridges are used as small, permanent bridges, as well as temporary bridges used while a permanent bridge is being replaced or repaired. During wartime, bridges are often damaged by bombing or by combat engineers. Bridges can be valuable targets because they are immobile, relatively easy to spot from the air, and damage to the bridge can disrupt the enemy's transportation network. Bridges have been targeted by artillery since antiquity, and the advent of newer technologies – specifically, aircraft and drones – has made bridges easier to attack. Germany used the Stuka aircraft to perform dive-bomb attacks on bridges during World War II, and Ukraine damaged the Crimean Bridge with a drone attack in 2023.
Other Some bridges accommodate uses other than transportation. Pipeline bridges carry oil pipes or water pipes across valleys or rivers. Many historical bridges supported buildings, including shrines, factories, shops, restaurants, and houses. Notable examples were the Old London Bridge and Ponte Vecchio. Some bridges built in Europe in the Middle Ages incorporated chapels into their design. 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. Examples include the Will Rogers Archway over the Oklahoma Turnpike and the several Illinois Tollway oasis locations. The Nový Most bridge in Bratislava features a restaurant set atop its single tower. Conservationists use wildlife bridges 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.
Structure and form
Bridges are primarily classified by their basic structural design: arch, truss, cantilever, suspension, cable-stayed, or beam. Several other terms can be used to designate various aspects of a bridge's form or design, including viaduct, trestle, and causeway.
Basic structures The choice of bridge structure to use in a particular situation is based on many factors, including aesthetics, environment, cost, and purpose. Some bridge spans combine two types of basic structures; for instance, the Brooklyn Bridge is primarily a suspension structure, but also uses cable-stays. Some multi-span bridges – called hybrid bridges – use different basic structures for different spans.
Arch bridge
Arch bridges consist of a curved arch, under compression, which supports the deck either above or below the arch. The shape of the arch can be a semicircle, ellipse, pointed arch, or segment of a circle. Arches exert a diagonal force at both ends, requiring strong supports or abutments to prevent the arch from spreading or collapsing. 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.
Truss bridge
A truss bridge is composed of multiple, connected triangular elements. The set of triangles form a rigid whole, which rests on the foundation at both ends, applying a vertical force downward. The deck can be carried on top of the truss ("deck truss") or at the bottom of the truss ("through truss"). Through trusses are useful when more clearance under the bridge is required; deck trusses permit oversize loads and do not interfere with overhead objects, such as electrical lines. The individual bars can be made of iron or wood, but most modern truss bridges are made of steel. The horizontal bars along the top are usually in compression, and the horizontal bars along the bottom are usually in tension. Bars connecting the top and bottom may be in tension or compression, depending on the layout of the triangles. 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. Trusses tend to be relatively stiff, and are commonly used for rail bridges which are required to carry very heavy loads.
Cantilever bridge
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 iron, but steel became common starting in the late 19th century. 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. 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 (specifically a pier, abutment, or tower) and extends outwards across the obstacle, with no support from below.
Suspension bridge
Suspension bridges have large, curved cables attached to the tops of tall towers, and suspend the bridge deck from the cables. In the early 19th century, the first modern suspension bridges – such as the Jacob's Creek Bridge – were chain bridges that used iron bars rather than bundled wires for the cables. After steel wire became widely available, longer cables could be built by stringing hundreds of wires between the towers and bundling them, enabling suspension bridges to achieve spans 2 kilometers (1.2 miles) long. When the bridge crosses a river, stringing the wires across the large span is a complex process. 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. Shorter towers require a smaller sag in the cable, which increases the tension in the cable, and thus requires stronger towers and anchorages.
Cable-stayed bridge
Cable-stayed bridges are similar to suspension bridges, but the cables that support the deck connect directly to the towers. The inclined cables may be arranged in a fan pattern or a harp pattern. Modern cable-stayed bridges became popular after World War II, when the design was used for many new bridges in Germany. When traversing a wide obstacle, designers have a choice of suspension or cable-stayed structures. Suspension bridges can achieve a longer span, but 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
Beam bridges are simple structures consisting of one or more parallel, horizontal beams or girders that span an obstacle. A box girder bridge is a variant that is generally shallower than an I-beam equivalent, permitting shorter and lower approach roads to cross an obstacle of a given height. Beam bridges are commonly used for both railways and roadways. Beam bridges are often used for spans shorter than about 50 meters (160 ft); for longer spans, trusses or similar structures are generally more efficient. The majority of beam bridges have a flat, horizontal bottom; but some have a bottom that arches upward, called haunching. Haunching looks more graceful than a flat bottom, and can provide greater clearance below the bridge, but it tends to be more costly because flat bottom beams are easier to build.
Other forms In addition to the basic bridge structures, there are many other forms of bridges. The following sections describe some of the more common forms, but are not an exhaustive list.
Movable bridge
Movable bridges are designed so that all or part of the bridge deck can be moved, usually to permit tall traffic – such as tall boats or ships – to 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. In the modern era, designers sometimes create unusual movable bridges with the intention of establishing signature bridges for a town or locality. Examples include the Puente de la Mujer swing bridge in Buenos Aires, the Gateshead Millennium – a rare example of a tilt bridge – over the River Tyne, and the Hörn Bridge in Germany.
Long multi-span bridge
There are a variety of terms that describe long, multi-span bridges – including viaduct, trestle, continuous, and causeway. The usage of the terms can overlap, but each has a specific focus. Viaducts (carrying vehicles) and aqueducts (carrying water) are bridges crossing a valley or underpass, supported by multiple arches or piers. Romans built many aqueducts, some of which are still standing today. Notable viaducts include Penponds Viaduct in England, Garabit Viaduct in France, Tunkhannock Viaduct in Pennsylvania, and Millau Viaduct in France. A trestle bridge – commonly used in the 19th century for railway bridges – consists of multiple short spans supported by closely spaced structural elements. A trestle is similar to a viaduct, but viaducts typically have taller pier supports and longer spans. 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 simple 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 road, usually crossing a bog, marsh, lake, or other body of water. Many causeways are earthen embankments, but some are raised bridges, such as the 38.4-kilometer (23.9-mile) Lake Pontchartrain Causeway in Louisiana.
Extradosed
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 seven and thirteen percent of the span width. Extradosed bridges are appropriate for spans ranging from 100 meters (330 ft) to 250 meters (820 ft). Unlike suspension bridges or cable-stayed bridges, the towers of an extradosed bridge often rest on the deck (rather than on a footing) and are solidly connected to the deck. 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. 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.
Pontoon bridge
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. 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. In Norway, Nordhordland Bridge crosses a deep fjord by resting on floating concrete pontoons. Many armies have pontoon bridges that can be rapidly deployed, including the PMP Floating Bridge, designed by the USSR.
Design
Design process
The process for designing a new bridge typically goes through several stages, progressively refining the design. An early step in the design process – sometimes called conceptual design – is to consider the multiple requirements that a bridge must satisfy. Requirements that are directly related to function include lifespan, safety, climate, soil condition, traffic volume, the size and nature of the obstacle to be traversed, and clearance required for passage underneath. Other constraints may include construction cost, maintenance cost, aesthetics, time available for construction, owner preference, and experience of the builders. Some bridge designs consider factors such as impact on the environment and wildlife, and the bridge's economic, social, and historic relationship to the local community. After the requirements of a bridge are established, a bridge designer uses structural analysis methods to identify candidate designs. Several designs may meet the requirements. The value engineering methodology can be used to select a final design from multiple alternatives. This methodology evaluates candidate designs based on weighted scores assigned to several different criteria, including cost, service life, durability, availability of resources, ease of construction, construction time, and maintenance cost. An important requirement considered during 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). For example, wood bridge superstructures typically have a service life of 10 to 50 years. Concrete highway bridges can have service lives of 75 to 150 years. A bridge design methodology incorporates the service life into the design process.
Specifications and standards One of the requirements a new bridge must satisfy is compliance with the local bridge design specifications and codes which – in some countries – may be legally binding requirements. In many countries, these 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. In the United States, the American Association of State Highway and Transportation Officials (AASHTO) publishes the AASHTO LRFD Bridge Design Specifications. Canada's bridge standard is the Canadian Highway Bridge Design Code, developed by the non-profit CSA Group. Agencies that regulate aviation or waterways may also impose standards that dictate some aspects of a bridge design, such as requirements for aviation warning lights at the top of bridge towers, or navigational warning lights on bridge supports located in navigable waterways.
Aesthetics
A bridge's appearance is one of the factors considered during its design. 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, color, texture, order, refinement, environmental integration, and functionality. The art historian Dan Cruickshank notes that bridges are regarded as manifestations of human imagination and ambition, and that many bridges transcend their original utilitarian role and become a work of art. 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."
Material
A bridge designer can select from a wide variety of materials, including wood, brick, rope, stone, iron, steel, and concrete. A bridge made from two or more distinct materials (for example, steel and concrete) is known as a composite bridge. Some of the largest arch bridges are composite, because they are made from concrete and steel. Wood is an inexpensive renewable resource with a high strength-to-weight ratio, but it is rarely used for modern roadway bridges because it is prone to degradation from the environment, and is much weaker than steel or concrete. Wood is primarily used in beam or truss bridges including covered 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. Masonry includes stone and brick, and is suitable only for elements of a bridge that are under compression (as opposed to tension), therefore, masonry is limited to structures such as arches or foundations. In the 20th century, large masonry bridges – although superseded by concrete in the West – continued to be built in China.
Iron – including cast iron and wrought iron – was used extensively from the late 18th century to late 19th century, 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. Steel was made in small quantities in antiquity, but became widely available in the late 19th century following invention of new smelting processes. 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. Steel is a critical component in concrete bridges, because steel reinforcing bars or steel prestressed cables must be embedded within concrete to make it sufficiently strong. 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 commonly used in modern bridges, and many roadway bridges are built primarily with a reinforced concrete beam structure, often of the box girder variety. The shape of concrete elements is determined by the formwork (mold) into which the concrete is poured (cast): the concrete will adopt the shape of the formwork as it cures. Beams can be precast offsite and transported to the bridge site, or cast in place. Bridges use concrete that contains embedded steel reinforcing bars – placed within the concrete when it is initially poured – which greatly increase the strength. Concrete is a strong and inexpensive material, but is brittle and can crack when in tension. If concrete is used in elements that may experience tension, prestressed cables are usually embedded within the concrete and tightened, which compresses the concrete. When a horizontal 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. The prestressed cables can be pre-tensioned (stretched before – and while – the concrete cures); or post-tensioned (placed within tubes in the concrete, and tightened after the concrete cures).
Double-deck bridge
Designers may choose to use a double-deck design (also known as double-decked or double-decker), that carries two decks on top of each other. This technique can be used to increase the amount of traffic a bridge can carry; or when the location constrains the size of the bridge. Double-deck bridges also permit two different kinds of traffic to be safely carried. For example, motor vehicles can be separated from pedestrians or railways. Some double-deck bridges carry rail on one deck, and vehicles on the other deck. An early example was the Niagara Falls Suspension Bridge, and a modern example is the Dom Luís I Bridge in Portugal. Because of their ability to carry large amounts of motor vehicles, double-deck bridges are often found near large cities carrying cars on both decks, for example, the DuSable Bridge in Chicago, Tsing Ma Bridge in Hong Kong, the Øresund Bridge connecting Copenhagen and Malmö, and the Shimotsui-Seto Bridge near Kurashiki. The 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.
Load analysis
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. The live load is all forces and vibrations caused by traffic passing over the bridge, including weight, 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. For sporadic events like floods, earthquakes, collisions, and hurricanes, bridge designers select a maximum severity that the design must accommodate. 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, for instance, 2,000 years; in this example, the design must endure the strongest storm that is expected to happen once every 2,000 years.
Stress and strain
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. In contrast, concrete is inelastic, and the change in its shape when stressed is negligible (until the stress becomes excessive and the concrete fails). A critical phase of the design process is calculating the maximum stress that each bridge component will experience, and selecting 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. The bridge design process typically employs structural analysis methods that divide the bridge into smaller components, and analyze the components individually, subject to certain constraints. A proposed bridge design is then usually modeled with formulas or computer applications. The models incorporate the loads the bridge will experience, calculate the stresses in the bridge, and provide data to the designer indicating whether the design meets the required design goals. To ensure that a proposed bridge design is sufficiently strong to endure foreseeable stresses, many bridge designers use limit state design methodologies (used in Europe and China) or Load and Resistance Factor Design (LRFD) methodologies (used in US).
Vibration
Many loads imposed on a bridge – wind, earthquakes, and vehicular traffic – can 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). 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. Neglecting to account for vibrations and oscillations can lead to bridge failure. The Tacoma Narrows Bridge collapsed in 1940 in winds of 68 km/h (42 mph), even though the bridge was designed to withstand winds up to 206 km/h (128 mph). Investigations revealed that the designer failed to account for wind-induced flutter and resonant vibrations. Bridges can suffer severe damage when subjected to earthquake ground motions. During a seismic event, several phenomena can occur, such as long-period velocity pulses, shear cracks, large ground motions, vertical accelerations, and soil liquefaction. To mitigate risks, earthquake engineers study seismic data to classify and quantify the motions experienced by bridges. These studies are used by governments to create and revise design standards that specify the types of seismic movements that new bridges must withstand.
Construction
The structural elements of a bridge are generally divided into the substructure and the superstructure. The substructure consists of the lower portions of the bridge, including the footings, abutments, piers, pilings, anchorages, and bearings. 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. When an existing bridge is b