Abridge is astructure that crosses an obstacle such as a river, lake, railroad, road,ravine, or other barrier. The most important function of bridges is totransport cars, trains and pedestrians, but bridges can also accommodate pipelines, utility lines, buildings, wildlife, and canals.
Styles of bridges include arch, truss, beam, cantilever, suspension, and cable-stayed. Less common types of bridges aremoveable bridges, double deck bridges,pontoon bridges, andmilitary bridges. Bridges can also be categorized by their materials, which include wood, brick, stone, iron, steel, and concrete.
The history of bridge building reflects the evolution of humankind's construction technologies. TheRomans andSui dynasty Chinese were renowned for their bridges. The Renaissance in 1500s Europe brought a new emphasis on science and engineering leading to stronger bridges with longer spans. With the advent of theIndustrial Revolution, iron became an important construction material for bridges. The abundance of inexpensivelumber in Canada and the United States causedtimber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s. Concrete – which was originally used within the Roman Empire – was improved with the invention ofportland cement in the early 1800s, and replaced stone and masonry as the primary material for bridgefoundations andabutments. Steel became a common building material for bridges in the late 1800s, leading to suspension bridges and cable-stayed bridges that spanned long distances.
The design of a new bridge must meet many requirements, including strength, traversing the obstacle, connecting to the transportation network, and providing safe transport for its users. Additional factors include cost, aesthetics, longevity, fire resistance, time available for construction, customer preference, and experience of the builders. A bridge design must be strong enough to support many loads and tolerate many stresses, including the weight of the bridge itself, the traffic passing over the bridge, and all forces applied by the bridge's surroundings, including wind, rain, snow, earthquakes, mudslides, water currents, flooding,soil subsidence,frost heaving,temperature fluctuations, and collisions (such as a ship striking the support of a bridge over water). When designing a bridge, engineers use processes such asLimit State Design andfinite element method.
The earliest forms of bridges were simple structures used to cross swamps and creeks, consisting of woodenboardwalks orlogs.[2][a]Pilings – which are critical elements of bridge construction – were used in Switzerland around 4,000 BC to support houses built over water.[4]
Severalcorbel arch bridges were builtc. 13th century BC by theMycenaean Greece culture, including theArkadiko Bridge, which is still in existence.[5] In the 7th century BC,Sennacherib constructed stone aqueducts to carry water near the city ofNinevah.[6] One of these aqueducts crossed a small valley atJerwan with five corbelled arches, and was 280 meters (920 ft) long and 20 meters (66 ft) wide.[6] InBabylonia in 626 BC, a bridge across theEuphrates was built with an estimated length of 120 to 200 meters (390 to 660 ft).[7] In India, theArthashastra treatise byKautilya mentions the construction of dams and bridges.[8] Ancient China has an extensive history of bridge construction, includingcantilever bridges, rope bridges, and bridges built across floating boats.[9]
Theancient Romans were prodigious bridge builders, renowned for their advanced engineering techniques and durable construction methods.[10] Many Romanaqueducts – some still standing today – used a semicircular arch style.[10] An example is theAlcántara Bridge, built over the riverTagus, in Spain.[11] The Romans usedcement as a construction material, which could be mixed with small rocks to formconcrete, or mixed with sand to formmortar to join bricks or stones.[12] Some Roman cements, particularly those containingvolcanic ash, could be used in underwater applications.[13] The enormousTrajan's Bridge (105 AD) featured open-spandrelsegmental arches in wooden construction.[14]
The oldest surviving stone bridge in China is theAnji Bridge, built from 595 to 605 AD during theSui dynasty. This bridge is also historically significant as it is the world's oldestopen-spandrel stone segmental arch bridge.[16][b]Rope bridges, a simple type ofsuspension bridge, were used by theInca civilization in theAndes mountains of South America prior to European colonization in the 16th century.[17]
In the late 1700s, the design of arch bridges was revolutionized in Europe byJean-Rodolphe Perronet andJohn Rennie, who designed arches that were flatter than semi-circular Roman arches.[25] These flatter arches enabled longer spans, fewer piers, and required less material.[25] These designs were used for bridges such asPont de la Concorde andNew London Bridge.[25]
The abundance of inexpensivelumber in Canada and the United States causedtimber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s.[29] Many of these timber bridges werecovered bridges.[29] Rail bridges used timber to obtain long spans that utilized strong truss designs, and also talltrestle bridges that spanned deep ravines.[29]
The mass production ofsteel in the late 1800's provided a new material for bridges, enabling lighter, strongertruss bridges and cantilever bridges, and producing cables strong enough to make suspension bridges and cable-stayed bridges feasible.[31][c] Suspension bridges could span distances far longer than other bridge types – up to 2 kilometres (1.2 mi) – permitting transportation networks to cross deep waters that did not permit multiple piers or supports to be installed.[33][d]
Concrete – which was originally used within the Roman Empire – was improved with the invention ofportland cement in the early 1800s, and replaced stone and masonry as the primary material for bridgefoundations. When iron or steel is embedded in the concrete, as inreinforced concrete orprestressed concrete, it is a strong, inexpensive material that can be used for horizontal elements ofbeam bridges andbox girder bridges.[34]
Straight, diagonal cables – known as "stays" – can be used to directly connect thebridge deck to bridge towers.[35] Stays were used as supplemental supports in some suspension bridges in the 19th century – including theBrooklyn Bridge.[36]Cable-stayed bridges – which used cable-stays as the exclusive means of support – became a popular bridge design followingWorld War II.[37][e]
Bridges perform a wide variety of functions.Aqueducts were developed early in human history, and carried water to towns and cities.[42] Until the 19th century, the most common use of bridges was to carry pedestrians, horses, and horse drawn carriages.[43] Following the invention of railways, manyrail bridges were built: in England, the number of bridges doubled during the railway-building boom in the mid 1800s.[43] In the 20th century, the growth of motorway networks required the construction of vast numbers of bridges.[44]
Canal bridges are used in a canal system to carry boats across a valley or ravine.[45] Conservationists usewildlife overpasses to reducehabitat fragmentation and animal-vehicle collisions.[46] 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.[47]
Some bridges accommodate uses other than transportation. Many historical bridges supported buildings, such as shrines, factories, shops, restaurants, and houses. Notable examples were theOld London Bridge andPonte Vecchio.[48] In the modern era,bridge-restaurants can be found at some highwayrest areas; these support a restaurant or shops directly above the highway and are accessible to drivers moving in both directions.[49] An example isWill Rogers Archway over theOklahoma Turnpike.[50] TheNový Most bridge inBratislava features a restaurant set atop its single tower.[51]
The basic bridge structures are arch, truss, cantilever, suspension, cable-stayed, and beam.[53] The choice of bridge structure to use in a particular situation is based on many factors, including aesthetics, environment, cost, and purpose.[54]
Many bridges are composed of multiple structures, for example, some Roman aqueducts contain dozens of adjacent arches. Longcauseways over large lakes may be composed of hundreds of individual beam structures. Some bridges combine two kinds of structures, for example, it is common for large suspension bridges, such as theVerrazzano-Narrows Bridge, to have beam or truss elements in the approaches; and some large cantilever bridges, such as theForth Bridge, use a truss segment in the middle to connect cantilevers on either side.[55]
Arch bridges consist of a curved arch, under compression, which supports the deck either above or below the arch.[56] The shape of the arch can be asemicircle,elliptical, apointed arch, or asegment of a circle.[57] When the arch is semicircular, as used in Roman bridges, the force of the arch is directed vertically downward to the foundation.[58] When the arch is elliptical or a circular segment, the force is directed diagonally, andabutments are often required.[59] Deck arch bridges hold the deck above the arch;tied-arch bridges suspend the deck below the arch; andthrough-arch bridges position the deck through the middle of the arch.[60]
Atruss bridge is composed of multiple, connected triangular elements.[61][f] The set of triangles form a rigid whole, which rests on the foundation at both ends, applying a vertical force downward.[61] The deck can be carried on top of the truss ("deck truss") or at the bottom of the truss ("through truss").[62] 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.[62] The individual bars can be made of iron or wood, but most modern truss bridges are made of steel.[63] The horizontal bars along the top are usually incompression, and the horizontal bars along the bottom are usually intension.[61] Other bars in the truss may be in tension or compression, depending on the particular layout of the triangles.[64] 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.[65] Trusses tend to be relatively stiff, and are commonly used for rail bridges which are required to carry very heavy loads.[65]
Cantilever bridges consist of one or more trusses, each supported at only one end.[66] Modern cantilever bridges are generally built from steel.[67] In Asia, cantilever bridges made of large rocks or timber were used to span small obstacles.[68] In the 1880s, some early cantilever bridges were built from wrought iron.[66] A basic cantilever bridge has two cantilevers, anchored at each end of the span, extending toward the center, and meeting in the center.[66] Some cantilever bridges have a suspended span (beam or truss) in the center, connecting the two cantilevers where they meet.[69] A balanced cantilever bridge consists of two connected cantilevers extending outward in opposite directions from a single central support.[70] The term "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.[71]
Modernsuspension bridges usually consist of two or more large cables passing over one or more towers.[g] The deck is suspended from the cables by large wire ropes called hangers.[72][h] The earliest suspension bridges were made of ropes or vines.[73] In the early 1800s, the first modern suspension bridges – such as theJacob's Creek Bridge – werechain bridges that used iron bars rather than bundled wires for the main cable.[74] When steel wire became widely available, hundreds of wires were strung between the towers and bundled to form large cables.[75] Each cable can weigh hundreds of tons.[75] When the bridge crosses a river, stringing hundreds of wires across the large span is a complex process.[75] Steel wire cables enabled suspension bridges to achievespans 2 km long.[76] The cable of the bridge is in tension, and the towers are in compression.[72] The cable of a suspension bridge assumes the shape of acatenary when initially suspended between the bridge towers; however, once the uniform load of the bridge deck is applied, the cable adopts aparabolic shape.[77] Shorter towers require a smaller sag in the cable, which increases the tension in the cable, and thus requires stronger towers and anchorages.[72]
Cable-stayed bridges are similar to suspension bridges, but the cables that support the deck connect directly to the towers.[78][g] Cable-stayed bridges offer some advantages over suspension bridges: anchorages are not needed, and constructing the deck can be readily accomplished by cantilevering outward from the towers.[78] The cables may be arranged in a fan pattern or a harp pattern.[81][i] Modern cable-stayed bridges became popular after WW II, when the design was used for many new bridges in Germany.[82] When evaluating the choice of suspension vs. cable-stayed structure for a given application, the suspension option generally uses more cable for a given span size, but provides a longer span (for comparable materials), and requires shorter towers (for a given span size).[83]
Beam bridges – includinggirder bridges – are simple structures consisting of one or more parallel, horizontalbeams that span an obstacle. Beam bridges are the most common type of bridges for both railways and roadways.[84] Beam bridges are ideal for shorter spans (less than about 50 meters (160 ft)), but for longer spans, other structures, such as trusses, may be more efficient.[85] 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.[84] Modern beam bridges are generally made of steel or reinforced concrete, although wood may be suitable for small beam bridges intended for light use.[84] Several different cross-sections may be utilized for beams, includingI-beam (common for steel) orflat slabs (sometimes used with concrete).[84][j] Beams can traverse longer spans when they are designed as hollowbox girders; bridges made of box girders are termedbox girder bridges.[84] 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.[84] Several beam bridges can be chained together, with supports at each juncture, to formelevated highways orcauseways.[84][k]
Moveable bridges are designed so that all or part of the bridge deck can be moved, usually to permit tall traffic – that would normally be obstructed by the bridge – to pass by.[89] Early moveable 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 – abascule bridge – made raising and lowering easier and safer.[90]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.[91]Lift bridges are raised vertically between two towers by cables passing over pulleys at the top of the towers.[92] Notable moveable bridges includeEl Ferdan Railway Bridge in Egypt,Erasmusbrug bascule in Rotterdam, andLimehouse Basin footbridge in London.[93] In the modern era, designers sometimes create unusual moveable bridges with the intention of establishing signature bridges for a town or locality.[93] Examples includePuente de la Mujer swing bridge in Buenos Aires,Gateshead Millennium – a rare example of atilt bridge – over theRiver Tyne, andHörn Bridge in Germany.[93]
ThePadma Bridge in Bangladesh carries rail traffic on the lower deck and vehicular traffic on the upper deck.[94]
Double-deck bridges (also known as double-decked, or double-decker) carry 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.[95] Double-deck bridges permit two different kinds of traffic to be safely carried, by separating, for example, motor vehicles from pedestrians or railways.[95]
An early double-deck bridge wasNiagara Falls Suspension Bridge, which carried rail on the upper deck, and carriages and pedestrians on the lower deck.[96]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.[97] Because of their ability to carry large amounts of motor vehicles, double-deck bridges are often found in large cities, such asTsing Ma Bridge in Hong Kong,[98]San Francisco–Oakland Bay Bridge in California,[99] andShimotsui-Seto Bridge in Japan.[100]
There are a variety of terms that describe long, multi-span bridges – including raised bridge, viaduct, trestle, and causeway. The usage of the terms can overlap, but each has a specific focus.[102]Viaducts (carrying vehicles) andaqueducts (carrying water) are bridges crossing a valley, supported by multiple arches or piers.[103] Romans built many aqueducts, some of which are still standing today.[104] Notable viaducts includePenponds Viaduct in England,[105]Garabit Viaduct in France,[106]Tunkhannock Viaduct in Pennsylvania,[107] andMillau Viaduct in France.[101]
Atrestle bridge – commonly used in the 19th century for railway bridges – consists of multiple short spans supported by closely spaced structural elements.[108] A trestle is similar to a viaduct, but viaducts typically have taller piers and longer spans.[109] Acontinuous truss bridge is a long, single truss that rests upon multiplesupports. 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 experienceshogging stresses at the supports andsagging stresses between the supports.[110] Acauseway is a raised road, usually built over a lake or other body of water.[111] Some causeways are bridges, such as the 38.4 kmLake Pontchartrain Causeway in Louisiana;[87] but many – such as theKing Fahd Causeway in Saudi Arabia – are partially or entirely built on solid dirt or rockembankments.[112]
Apontoon bridge, also known as a floating bridge, usesfloats or shallow-draft boats to support a continuous deck for pedestrian or vehicle travel over water.[114] Pontoon bridges are typically used where waters are too deep to build piers, or as a mechanism to implement a moveableswing bridge in a canal.[115] Pontoon bridges were used in ancient China.[116] Duing theSecond Persian invasion of Greece, Persian rulerXerxes built alarge pontoon bridge across theHellespont, consisting of two parallel rows of 360 boats.[117]
Several pontoon bridges are in use in the modern world. Washington state in the U.S. has several, includingHood Canal Bridge.[118] In Norway,Nordhordland Bridge crosses a deepfjord by resting on floating concrete pontoons.[113] Many armies have pontoon bridges that can be rapidly deployed, including thePMP Floating Bridge, designed by theUSSR.[119]
This portableAM 50 bridge is being laid over a river in Slovakia.[120]
Portable military bridges are an important type of equipment in the field ofmilitary engineering. They perform a variety of wartime roles, such as quickly traversing obstacles in the midst of battle, or facilitating resupply behind front lines.[121][l]
Military bridges can be categorized as "wet" bridges that rest on pontoon floats, and "dry" bridges that rest on piers, river banks, or anchorages.[121] A crude mechanism to cross a small ravine is to place afascine (a large bundle of pipes or logs) into the ravine to enable vehicles to drive across.[123]
Some military bridges, referred to asarmoured vehicle-launched bridges (AVLB), are carried on purpose-built vehicles.[123] 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.[124] The UKChieftain AVLB could launch a 23-meter (75 ft) bridge – capable of supporting 60 ton loads – in 3 minutes.[123]
Anextradosed bridge combines features of a box girder bridge and a cable-stayed bridge.[126] 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.[127][m] Extradosed bridges are appropriate for spans ranging from 100 meters (330 ft) to 250 meters (820 ft).[127] Unlike suspension bridges or cable-stayed bridges, the towers of a extradosed bridge rest on the deck, rather than on a pier; and in some implementations, are solidly connected to the deck.[129] 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.[130] 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.[131]
Many overpass bridges in the United StatesInterstate Highway System are concrete box girder bridges, such as these bridges overInterstate 280 in California.
When designing a bridge to traverse a specific obstacle, the designer must identify a design that meets several requirements. 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.[132]
Non-engineering requirements include construction cost, maintenance cost, aesthetics, time available for construction, owner preference, and experience of the builders.[133] 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.[134]
When designing a bridge, the designer will base the design on aservice life, which is a specific number of years that the bridge is expected to remain in operation with routine maintenance, and without major repairs.[135][n] The bridge design methodology incorporates the service life into the design process.[137] Wood bridges may have a service life of ten to 50 years.[138] In the United States, highway bridges typically have service lives of 75 to 150 years.[136]
Many bridges are utilitarian in appearance, but in some cases, the appearance of the bridge can have great importance.[140] Bridges are typically more aesthetically pleasing if they are simple in shape, the deck is thinner in proportion to its span, the lines of the structure are continuous, and the shapes of the structural elements reflect the forces acting on them.[141]
The art historianDan Cruickshank notes that bridges are regarded as manifestations of human imagination and ambition, and that many bridges bridge transcend their original utilitarian role to and become a work of art.[142] 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."[142]
The Iron Bridge inShropshire, England, completed in 1781, is the first major bridge made of entirely ofcast iron.[28]This concrete bridge support is being prepared for a concrete pour. The greenreinforcing bars will be embedded inside the concrete after the concretecures.[143]The small circular holes in thisbox girder will holdprestressing cables, which run the length of the girder and keep the concrete in compression.[144]
Bridges are built from a wide variety of materials, including wood, brick, rope, stone, iron, steel, and concrete.[145] A bridge made from two or more distinct materials (such as steel and concrete) is known as a composite bridge.[146]
Wood is an inexpensive material that is rarely used for modern motor vehicle roads.[147] Wood is used in bridges primarily in a beam structure or truss structure, and is also used to build hugetrestle bridges for railways.[148] When wood is used, it is often in the form ofglued laminated timber.[147]
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.[149] In the twentieth century, large masonry bridges – although superseded by concrete in the West – continued to be built in China.[150]
Iron, including cast iron and wrought iron, was used extensively from the late 1700s to late 1800s, primarily for arch and truss structures. Iron is relatively brittle, and has been superseded by the much stronger steel for all but ornamental uses.[151]
Steel is one of the most common materials used in modern bridges.[152] Steel was made in small quantities in antiquity, but became widely available in the late 1800s following invention of newsmelting processes byHenry Bessemer andWilliam Siemens. Steel is especially useful for bridges, because it is strong in both compression and tension.[153] Steel is widely used for truss bridges and beam bridges, and steel wires are an essential component of virtually all suspension bridges and cable-stayed bridges.[154] Concrete bridges make extensive use of steel, because all concrete used in bridges contains steelreinforcing bars or steelprestressed cables.[155] 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.[156]
Concrete is a strong and inexpensive material, but is brittle and can crack when in tension.[157] Concrete is useful for bridge elements that are in compression, such as foundations and arches.[158] Manyroadway bridges are built entirely of concrete using a beam structure, often of thebox girder variety.[158] Virtually all concrete used in bridges contains steel reinforcing bars, which greatly increase the strength.[143] Reinforcing bars are set inside the concrete form, and the concrete is poured into the form, andcures with the bars inside. If concrete is used in elements that experience tension – such as the lower region of a horizontal beam or slab – prestressed cables must be embedded within the concrete and tightened.[144] 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).[159] 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.[159] Concrete beams can beprecast offsite and transported to the bridge site, orcast in place.[160]
After the requirements of a bridge are established, the bridge designer usesstructural analysis methods and techniques to identify candidate designs.[165] Several designs may meet the requirements. After considering all factors, the bridge designer – in consultation with the owner – will select a particular design.[166] Thevalue engineering methodology can be used to select a final design from multiple alternatives.[167] 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.[168]
A bridge design must accommodate all loads and forces that the bridge might experience. The totality of the forces that the bridge must tolerate is represented by the termstructural load. The structural load is usually divided into three components: Thedead load, which is the weight of the bridge itself;[p] thelive load, which are the forces and vibrations caused by traffic passing over the bridge, including braking and acceleration; and theenvironmental load, which encompasses all forces applied by the bridge's surroundings, including wind, rain, snow, earthquakes, mudslides, water currents, flooding,soil subsidence,frost heaving,temperature fluctuations, andcollisions (such as aship striking the pier of a bridge).[171][q]
Many of the load sources vary over time, such as vehicle traffic, wind, and earthquakes. The bridge designer must anticipate the maximum values that those loads may reach during the course of the bridge's lifespan.[170] For sporadic events like floods, earthquakes, collisions, and hurricanes, bridge designers must select a maximum severity that the design must accommodate.[173] The designer first selects areturn period, which may range from 100 to 2,500 years.[174] 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.[175]
The load forces acting on a bridge cause the components of the bridge to becomestressed. 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 iselastic. For example, steel can tolerate some stretching or bending without failing. Other materials, such as concrete, areinelastic, and their change in shape when stressed is negligible (until the stress becomes excessive and the concrete fails).[177]
The 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 includes forces that compacts 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.[178]
An important component of the live load carried by a bridge is the vehicle and rail traffic the bridge is expected to carry.[179] In addition to the weight of the vehicle, other forces must be considered, including braking, acceleration, centrifugal forces, and resonant frequencies.[180] For roadways, the loads imposed by truck traffic far exceeds the loads imposed by passenger cars, and so the bridge design process focuses on trucks.[181]
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 standards such asEurocode orAASHTO bridge specifications.[182] In addition to algorithmic models contained in specifications, designers may determine traffic loads by utilizing data from real-world measurements on existing bridges that experience traffic comparable to that the proposed bridge will experience: technologies such asweigh-in-motion (WIM) can produce accurate data without the guesswork inherent in an algorithmic model.[183]
TheTacoma Narrows Bridge collapsed shortly after opening in 1940 due to failure of the design to properly account for wind forces.[184]
Many loads imposed on a bridge – such as wind and vehicular traffic – can cause a bridge to experience irregular or periodic forces, which may cause bridge components to vibrate oroscillate.[185] Many bridge components may have inherentresonant frequencies to which they are particularly susceptible, and vibrations near those frequencies can cause very large stresses.[186]
Winds can produce a variety of forces on a bridge, includingflutter and vortexes.[187] Considering wind forces during the design process is especially important for long, slender bridges (typically suspension or cable-stayed bridges).[188] Vibration and resonance concerns are especially important in longer bridges, but still must be accounted for in smaller 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).[189]
The bridge design process must identify potential vibrations and oscillations, and address them with techniques to minimize vibration, such as adding components to dampen movement or stiffen the structure.[190] One technique to combat oscillations istuned mass dampers, which have been installed in bridges such asPont de Normandie, which in 1995 was the first bridge to utilize them.[191] TheAkashi Kaikyo Bridge has twenty tuned mass dampers, weighing ten tons each, inside its steel towers.[192]
Neglecting to account for vibrations and oscillations can lead to bridge failure. TheAngers Bridge collapsed in 1850, killing over 200 people, partly due to soldiers marching on the bridge in a manner that increased resonant oscillations.[193] TheTacoma Narrows Bridge collapsed in the 1940 in winds of 42 mph (68 km/h), even though the bridge was designed to withstand winds up to 128 mph (206 km/h). Investigations revealed that the designer failed to account for wind effects such as flutter and resonant vibrations.[184] TheGolden Gate Bridge was damaged in 1951 due to wind forces, and as a result was reinforced with additional stiffening elements.[194]
The process used to design bridges usesstructural analysis methods and techniques.[165] These methods divide the bridge into smaller components, and analyze the components individually, subject to certain constraints.[165] A proposed bridge design is thenmodeled with formulas or computer applications.[165] 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.[165]
Bridge design models include bothmathematical models andnumerical models.[165] 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.[165] Thefinite element method is the most common numerical model used to perform detailed analysis of stresses and loads of a bridge design.[196][r] 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.[198]
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.[199] The criteria, and their allowable values, are termedlimit states. The set of limit states selected for a design are based on the bridge's structure and purpose.[200]
To ensure that a proposed bridge design is sufficiently strong to endure foreseeable stresses, many bridge designers use methodologies such asLimit State Design (used in Europe and China) orLoad and Resistance Factor Design (LRFD) (used in US).[201] These methodologies add a margin of safety to the bridge design by incorporatingsafety factors into the design process.[202] 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.[203][s] 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, snow or ice on the bridge; winds; settling into the soil; and collisions (such as vehicles on the deck striking a bridge tower; or a ship striking a bridge foundation).[205]
Paint can be used to reduce deterioration of steel components. Steel bridges need to be repainted periodically, as seen in this wire hanger from theGolden Gate Bridge, which is paintedinternational orange.[206]
To achieve a longer lifespan, a bridge should be 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 andspalling on concrete.[207]
Deterioration can be slowed – thus prolonging the life of the bridge – by various measures, primarily aimed at excluding water and oxygen from the bridge elements.[208] Techniques to prevent water-based damage include drainage systems, waterproofing membranes (such as polymer films), and eliminatingexpansion joints.[209][t]
Concrete bridge elements can be protected with waterproof seals and coatings.[211][u] Reinforcing steel within concrete can be protected by using high-quality concrete and increasing the thickness of the concrete surrounding the steel.[213] Steel elements of a bridge can be protected by paints or bygalvanizing with zinc.[214] Paint can be avoided entirely for steel members by using certain steel alloys, such asstainless steel orweathering steel (a steel alloy that eliminates the need for paint, by forming a protective outer layer of rust).[215]
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 acofferdam around the footings, or surrounding the footings withrip-rap.[216][v]
Suspension 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.[218]
Some elements of a fictional bridge. 1 Approach, 2 Arch, 3 Truss, 4 Abutment, 5 Bearing, 6 Deck, 7 Pier Cap, 8 Pier, 9 Piling, 10 Footing, 11 Caisson, 12 Subsoil.[222]
The structural elements of a bridge are generally divided into thesuperstructure and thesubstructure.[223] The superstructure consists of most of the visible parts of a bridge, including the horizontal span,deck,wearing surface (e.g. the asphalt on top surface of the deck), trusses, arches, towers, cables, beams, and girders.[224] The substructure consists of the lower portions of the bridge which support the superstructure, including thefootings,[x]abutments,piers,pilings, anchorages, andbearings.[226]
Abutments are an important element in 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.[59]
Construction of all bridge types begins by creating the substructure. The first elements built are typically the footings and abutments, which are often large blocks of reinforced concrete, entirely or partially buried underground, which support the entire weight of the bridge, and transfer the weight to thesubsoil.[227] Based on their height-to-width ratio, footings are categorized as:shallow (height is less than width) ordeep (height is greater than width).[228] If the subsoil cannot support the load placed on the footings,pilings must first be driven below the footings: pilings are long structures – made of wood, steel, or concrete – placed vertically below footings.[227] Some pilings reach down and rest onbedrock; others rely on friction to prevent the footing from sinking lower.[229]
Abutments are usually located at the ends of a bridge deck, where it reaches the subsoil.[230] They direct the weight into the subsoil, either vertically or diagonally.[59] Abutments may also act as retaining walls, keeping the subsoil under the approach road from eroding.[230] After footings for the piers have been created, the piers and pier caps are built to complete the substructure.[231][y] Suspension bridges usually require anchorages, which are large reinforced concrete blocks solidly anchored into the earth – they 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.[233][z]
Gantries are one technique used to gradually assemble a bridge deck.[235]The deck of this arch bridge is beinghorizontally pushed onto the substructure withjacks.[236]This temporaryfalsework will be removed after an arch is built over it.[237]
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).[238] The beams may be laid across the piers by a crane organtry.[239] If the span crosses a deep ravine, a technique known aslaunching may be used: the full span (beams and deck) are assembled on the approach road, then pushed horizontally across the obstacle.[236][aa]
Arch bridge superstructure construction methods depend on the material. Concrete or stone arches use a temporary wood structure known asfalsework orcentering to support the arch while it is built.[237] 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.[240]
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.[241]
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 towers[ab] 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.[242]
Suspension bridge superstructure construction usually begins with the towers.[243][ab] 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.[ac] The cables are securely fastened to the anchorages at both ends.[ad] 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.[246]
This concrete bridge pier is being built within acofferdam (the rusted, vertical steel walls).[247]
To build a bridge pier in water,caissons may be used to hold workers and machinery during excavation.[248]
When bridge supports (piers or towers) are built in a river, lake, or ocean,caissons are often 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.[249] Air pressure inside a sealed caisson must be kept high to prevent water from seeping in.[250] Workers, if they do not properlydecompress when exiting the caisson, can getdecompression sickness.[251] Early bridge builders did not understand decompression, and deaths were common: thirteen workers died from decompression sickness when building theEads Bridge (completed in 1874).[251]
An alternative to a caisson is acofferdam, which is a temporary dam surrounding the support location, open on top, where workers may work while constructing the footings.[252] Another approach for constructing foundations in water was employed for theAkashi Kaikyo suspension bridge: the two foundations for its towers are 70 meters (230 ft) tall and 80 meters (260 ft) 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 60 meters (200 ft) deep, and each was filled with 355,000 cubic meters of concrete. The foundations rest directly on the ocean bottom, without pilings or footings.[253]
Two types ofbridge bearings are used in this application: a hinge bearing above a pair of roller bearings.
Bearings are often placed between the superstructure and the substructure at the points of contact. Bearings are mechanical devices that enable small movements – which may result fromthermal expansion and contraction,material creep, or minorseismic events. Without bearings, the bridge may be damaged when 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, andelastomeric bearings.[254]
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.[255]
The cables of theVerrazano-Narrows Bridge are passing over the saddles at the top of the tower. The individual strands are wrapped with bands.[citation needed]
This suspension bridge cable, shown in cross-section, consists of 37 strands, and each strand consists of multiple wires.[256]
Towers are an important component of the superstructure of cable-stayed bridges and suspension bridges.[ae] 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 250 meters (820 ft) tall, whereas steel towers can be much taller.[257][af]
Towers hold the bridge cables, which – in turn – 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.[259] There are two mechanisms used to attach a cable to a tower: saddles and 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.[260]
Cables are made of one or more strands, and each strand consists of multiple wires. Each wire is a thin, flexible piece of solid steel, of higher tensile strength than normal steel, and with a diameter of 3mm to 7mm.[261] Cables for large suspension bridges can reach over 1 meter (3 ft 3 in) in diameter and weigh over 20,000 tonnes.[262] Cables are typically constructed at the bridge site by unspooling wires or strands from largereels.[263][ag]
After the towers of a suspension bridge are built, the next step is to build a temporarycatwalk to support the wires while they are drawn across the span and over the tops of the towers.[264] There are two approaches to pulling the wires across the span: the air spinning (AS) method where the individual wires are carried across by pulleys; or the prefabricated parallel-wire strand (PPWS) method where entire strands are individually pulled across.[265][ah]
The AS method was used for all suspension bridges until the PPWS method was invented in the 1960s. The AS method is slower because it requires the spinning pulley to cross the span thousands of times, pulling a pair of wires each time.[267] After 300 to 500 wires are pulled, aluminum bands are used to bundle them into strands.[268]. 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.[267] The PPWS method was used for the Akashi Kaikyo Bridge, where each strand weighed 94 tonnes and was 4 kilometres (2.5 mi) long.[269]
The wires within a strand may be parallel, or they may wrap around each other in a twisted (spiral) pattern.[270] Air spinning always produces strands that contain parallel wires. The PPWS method can produce strands with parallel or twisted wires.[271]
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.[272] Then a wire is usually wrapped around the cable in a helical manner, to provide protection against water intrustion.[273] 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.[274]
This pier of a bridge in Germany is degrading, and the internal steelreinforcing bars are exposed and rusting.
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 processes – collectively referred to as bridge management – include technical activities such as maintenance, inspection,monitoring, and testing.[275]
In addition to technical tasks, management encompasses planning, budgeting, and prioritization of maintenance activities.[275] Bridge designers useLife-Cycle Cost Analysis (LCCA) methodologies to estimate the maintenance costs of a bridge throughout its lifetime.[276] Annual maintenance costs increase as the bridge ages and degrades.[277]
An important part of maintenance is inspecting 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, carbonation of concrete, vehicular traffic, corrosion, mechanical abrasion, poor bridge design, and improper repair procedures.[278]
A variety of inspection techniques can be utilized to measure degradation of bridge elements.Destructive testing removes material samples from the bridge – such ascores drilled from concrete, or a small piece of steel wire cut from a cable – to a laboratory.[ai] In the laboratory, the samples are analyzed using microscopes, sonic devices, or X-ray diffraction.[279]
During bridge construction, permanent sensors may be placed within bridge elements at critical locations, and may be measured at any time to obtain data about stresses and chemical degradation in hard-to-reach locations.[283] Many long-span bridges are now routinely monitored with a range of sensors, includingstrain transducers,Sodar,accelerometers,tiltmeters, andGPS.[284]
Scaffolding is erected under theSitterviadukt rail bridge in Switzerland while maintenance on the deck truss is performed.[285]
To evaluate the condition of large steel cables, as used in suspension bridges or cable-stayed bridges, electrical coils are moved along the cable, measuring the induction of the cable, which can reveal corrosion issues.[286]
A variety ofstructural 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.[287] Detailed measurements of the external surface of a bridge can be recorded usingLidar technology. Comparing measurements taken at multiple points in time can reveal long-term changes.[288]
TheNanfang'ao Bridge in Taiwan collapsed because of excessive corrosion that went undetected.[289]
Bridge failures are of special importance tostructural engineers, because theanalyses of the failures providelessons learned that serve to improve design and construction processes.[290] 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.[291] In the United States, 25% of all bridges failed during the 1870s.[292] Over time, bridge failures led to improvements in bridge design, construction, and maintenance practices.[293] In the modern era, the majority of bridge failures in the United States are due to water-related causes, such as damage from floods, or scouring (due to water currents) undermining bridge supports.[294]
Bridges occur extensively in art, legend, and literature, often employed in a metaphorical manner.[296] InNorse mythology, the home of the gods – Asgard – is connected to the earth byBifröst, a rainbow bridge.[297] Many bridges in Europe are named "Devil's Bridge", and sometimes have folkloric stories that explain why the bridge is associated with the devil.[298] There are many stories, mostly apocryphal, relating bridges to Christiansaints.[299] Stories and poems often employ a bridge as a metaphor of the human lifespan, or human experiences.[300] Bridges are often the setting for pageants, celebrations, and processions.[301]
Bridges are often venerated as symbols of humankind's heroism and accomplishments.[302] The inspirational nature of bridges has led them to be featured in the works of poets, painters and writers.[303] Bridges feature prominently in paintings – often in the background – as in theMona Lisa.[304] Authors have used bridges as the centerpiece of novels, such asThe Bridge on the Drina byIvo Andrić, andThornton Wilder'sThe Bridge of San Luis Rey.[295]
Many bridges – known as signature bridges – are strongly identified with a particular community.[305]In some cases they are deliberately built with an especially magnificent design, to serve as a landmark or icon.[306] The art historian Dan Cruickshank writes: "I am ... enthralled by [the] ability [of bridges] to transform a place a community and amazed by the way a bold bridge 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."[142] Large suspension bridges, in particular, are often regarded as iconic landmarks that symbolize the cities in which they are located. Notable examples include theBrooklyn Bridge in New York; theGolden Gate Bridge inSan Francisco; theClifton Suspension Bridge inBristol; and theSzéchenyi Chain Bridge inBudapest.[307]
The 500euro banknote displays a cable-stayed bridge.[308]
Bridges have been featured on coins since antiquity.[309] In the modern era, brides are prominently displayed oneuro banknotes. 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. Bridges were chosen because they symbolize links between states in the union and with the future. The designs were supposed to be devoid of any identifiable characteristics, so as to not show favoritism to specific countries. The initial designs by Kalina were discovered to be of specific bridges, including theRialto and thePont de Neuilly, and were subsequently changed to be more generic. Each banknote depicts a bridge design representative of a certain architectural era: €5 Classical,€10 Romanesque,€20 Gothic, €50 Renaissance, €100 Baroque and Rococo,€200 19th century (iron and glass), and €500 20th century.[308]
^Examples of early bridges include theSweet Track and thePost Track in England, approximately 6,000 years old.[3]
^The Anji bridge is also called the Zhaozhou Bridge.[16]
^Long before the steel era, people made suspension bridges from vines or ropes. Iron was used in a few early suspension bridges in the form of iron rods or chains (rather than steel wires or cables).[32]
^A truss can be considered as a deep beam, out of which numerous triangular holes have been cut to reduce the weight.[61]
^abMost suspension bridges and cable-stayed bridges have two or more towers, but some have only one tower. A single-tower cable-stayed bridge is theFlehe Bridge in Germany,[79] and a single-tower suspension bridge is the east span of theSan Francisco-Oakland Bay Bridge.[80]
^ in a harp pattern all the cables are parallel; in a fan pattern the cables all radiate from near the top of the tower. TheSeverins Bridge was the first cable-stayed bridge that arranged its cables in a fan pattern, rather than a harp pattern.[38]
^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 proved greater clearance below the bridge, but it tends to be more costly because flat bottom beams are easier to build.[86]
^Another definition of an extradosed bridge is one where the "stiffness ratio" (load carried by stay cables divided by total vertical load) is less than 30%.[128]
^Routine maintenance may include replacing certain parts or elements that are designed to be replaced, such as the wearable surface of the deck.[136]
^A list of some bridge-related specifications in the US is found in thePlanning and Design of Bridges book.[163]
^The dead load also includes any permanent fixtures on the bridge, such as light poles, traffic signage, and guardrails;[170]
^There are other ways to classify loads in addition to dead/live/environmental. One is permanent loads (bridge structure) and transient loads (traffic and environment).[170]. Another is dead (bridge structure) and live (vehicles and environment).[172]
^An alternative to the finite element method is the simpler, but less powerful,finite strip method.[197]
^The strength of a bridge component is referred to as "resistance" in the context of LRFD.[204]
^Expansion joints relieve stress due to thermal expansion and contraction, but permit water to seep into vulnerable bridge elements, which can lead to corrosion and degradation.Integral bridge concepts are an alternative to expansion joints.[210]
^As an example of measures taken to combat scour: the underwater foundations of theAkashi Kaikyo Bridge are surrounded withrip rap 8 meters (26 ft) thick.[217]
^Originally named Bureau des dessinateurs du Roi, it was given its current name in 1775.
^The term "foundation" is sometimes used to represent footings, but in most contexts "foundation" means all or most of the substructure.[225]
^A pier cap is a block of concrete at the top of a pier, upon which rests the deck.[232]
^Self-anchored suspension bridges do not require anchorages.[234]
^Incremental launching may be employed for several types of bridges: beam bridges, deck arch bridges, and cable-stay bridges with short spans. In all cases, the substructure is completed first, then the deck is pushed horizontally across the top of the substructure.[236]
^abSuspension bridges and cable-stayed bridges may have one or more towers.
^The term "pylon" is sometimes used for a bridge tower.[257]
^Most towers are rigidly attached to the footings below them, but some relatively short towers have bearings at their base which permit pivoting.[258]
^Some cables consist of a single strand, in that situation, if the strand is delivered to the bridge site on a reel, there is no need to construct the cable at the bridge site.
^For large suspension bridges, the length of wire or strand on a reel may not reach across the full span, so when a reel reaches its end, the wires (or strands) must be spliced to the wires (or strands) of a new reel.[266]
^An example of a small piece of wire being cut-out of the large cable of a heavily trafficked suspension bridge is seen inthis video.
Birnstiel, Charles (2000)."Moveable Bridges". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 663–698.ISBN0727727745. Retrieved1 September 2025.
Collings, David (2000)."Composite Construction". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 407–448.ISBN0727727745. Retrieved1 September 2025.
Elnashai, Amr (2000)."Seismic Response and Design". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 519–548.ISBN0727727745. Retrieved1 September 2025.
Farquhar, Daniel (2000)."Cable-Stay Bridges". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 549–594.ISBN0727727745. Retrieved1 September 2025.
Jones, Vardiman; Howells, John (2000)."Suspension Bridges". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 595–662.ISBN0727727745. Retrieved1 September 2025.
Mulheron, Mike (2000)."Protection". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 805–848.ISBN0727727745. Retrieved1 September 2025.
Ryall, Michael (2000)."Loads and Load Distribution". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 43–94.ISBN0727727745. Retrieved1 September 2025.
Shanmugam, N. E. (2000)."Structural Analysis". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 95–224.ISBN0727727745. Retrieved1 September 2025.
Schlaich, Mike (2019). "General". In Schlaich, Mike (ed.).Extradosed Bridges(PDF). International Association for Bridge and Structural Engineering. pp. 3–8.ISBN9783857481680. Retrieved16 October 2025.
Tytler, I.F.B. (1985).Vehicles and Bridging. Battlefield Weapons Systems and Technology. Brasey's Defense Publishers.ISBN0080283225. Retrieved13 September 2025.
Vassie, Perry (2000)."Bridge Management". In Ryall, Michael (ed.).The Manual of Bridge Engineering. Thomas Telford. pp. 849–882.ISBN0727727745. Retrieved1 September 2025.
Omer, Muhammad; et al. (2018)."Performance Evaluation of Bridges Using Virtual Reality".Proceedings of the 6th European Conference on Computational Mechanics (ECCM 6) & 7th European Conference on Computational Fluid Dynamics (ECFD 7), Glasgow, Scotland. International Center for Numerical Methods in Engineering.
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