A bridge is a structure that crosses a river, lake, railroad, road, ravine, or other obstacle. Bridges perform a wide variety of functions, and can carry cars, trains, pedestrians, pipelines, utility lines, buildings, wildlife, and canals. Styles of bridges include arch, truss, beam, cantilever, suspension, and cable-stayed. Less common types of bridges are moveable bridges, double deck bridges, pontoon bridges, and military bridges. Bridges can also be categorized by their materials, which include wood, brick, stone, iron, steel, and concrete.
Bridges have been created by humans throughout history. The greatest bridge builders of antiquity were the ancient Romans, who built many long-standing semicircular-arch 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 the Industrial Revolution, iron became an important construction material for bridges. The abundance of inexpensive lumber in Canada and the United States caused timber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s. Concrete–which was originally used within the Roman Empire–was improved with the invention of portland cement in the early 1800s, and replaced stone and masonry as the primary material for bridge foundations and abutments. 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 be 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 brdige, engineers use processes such as Limit State Design and finite element method.
After a bridge is built, maintenance must be performed to ensure that the bridge remains open to traffic, avoids safety incidents, and achieves its intended lifespan. An important part of maintenance is inspecting for damage or degradation, and taking steps to mitigate any issues. 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.
The earliest types of bridge were probably stepping stones and log boardwalks.[1][2][a] Ancient peoples probably built log bridges consisting of logs that fell naturally or were intentionally felled or placed across streams.[3][1] Some of the first human-made bridges with significant span were probably intentionally felled trees.[1] Crude coffer dams and pilings–which are critical elements of bridge construction–were used in Switzerland in 4,000 BC to support houses built over water.[4]
Several corbel arch bridges were built c. 13th century BC by the Mycenaean Greece culture, including the Arkadiko Bridge, which is one of the oldest bridges still in use.[5] Several intact, arched stone bridges from the Hellenistic era can be found in the Peloponnese.[6] In the 7th century BC, the Neo-Assyrian Empire constructed stone aqueducts To carry the waters of one canal to the city.[7][4] One example was 280 metres (920ft) long and 20 metres (66ft) wide, across a small river-valley at Jerwan which utilized five corbelled arches.[7][4] In Babylonia in 626 BC, a bridge across the Euphrates was built with an estimated length of 120 to 200 metres (390 to 660ft).[7][4]
In India, the Arthashastra treatise by Kautilya mentions the construction of dams and bridges.[8] A Mauryan bridge near Girnar was surveyed by James Princep.[9][9] The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century.[10] Ancient China has a long history of bridge construction, including cantilever bridges[11] and large timber bridges, which were built during the Warring States period.[12]
The greatest bridge builders of antiquity were the ancient Romans.[13] The Romans built semicircular arch bridges and aqueducts–some of which still stand today–that could stand in conditions that would damage or destroy earlier designs.[13]. An example is the Alcántara Bridge, built over the river Tagus, in Spain.[14][15] The Romans also used cement as a construction material, which could be poured into forms, and thus avoided the problem of variable strength found in natural stone. One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock.[16][17] The Roman's enormous Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.[18]
300 to 1500 AD
A number of bridges, both for military and commercial purposes, were constructed in India by the Mughal administration in India.[19]
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.[20]
Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 16th century.[21][22]
The Ashanti built bridges over streams and rivers.[23][24] They were constructed by pounding four large forked tree trunks into the stream bed, placing beams along these forked pillars, then positioning cross-beams that were finally covered with four to six inches of dirt.[24]
In the 1700s, the design of arch bridges was revolutionized in Europe by Jean-Rodolphe Perronet and John Rennie who designed arches that were flatter than semi-circular Roman arches.[33] These flatter arches enabled longer spans, fewer piers, and required less material.[33] These designs were used for bridges such as Pont de la Concorde and New London Bridge.[33]
With the advent of the Industrial Revolution,[34]cast iron became an important construction material for bridges. Although cast iron was strong under compression, it was brittle, so it was supplanted by wrought iron–which was more ductile and better under tension.[35][36]. An early iron bridge was built in Shropshire, England crossing the river Severn.[37]
The abundance of inexpensive lumber in Canada and the United States caused timber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s.[38][39] Many of these timber bridges–especially where snow and ice were common–were covered bridges.[38][39] Rail bridges used timber to obtain long spans that utilized strong truss designs, and also tall trestle bridges that spanned deep ravines.[38][39]
The mass production of steel, starting in the late 1800s, enabled the construction of large suspension bridges, such as Golden Gate Bridge in California.
The mass-production of steel in the late 1800's provided a new material for bridges, enabling lighter, stronger truss bridges and cantilever bridges, and producing cables strong enough to make suspension bridges and cable-stayed bridges feasible.[40][41][b] Suspension bridges could span distances far longer than other bridge types–up to 2 kilometres (1.2mi)–permitting transportation networks to cross deep waters that required ferries. The dynamic nature of suspension bridges requires special design considerations to safely carry rail traffic.[43][44]
Concrete–which was originally used within the Roman Empire–was improved with the invention of portland cement in the early 1800s, and replaced stone and masonry as the primary material for bridge foundations and abutments. When iron or steel are 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.[45][46]
Straight, diagonal cables–known as "stays"–can be used to directly connect the bridge deck to bridge towers.[47] Stays were used as supplemental supports in some suspension bridges in the 19th century–including the Brooklyn Bridge.[48]Cable-stayed bridges–which used cable-stays as the exclusive means of support–became a popular bridge design following World War II.[49][50][c]
An aqueduct is a bridge that carries water. A road-rail bridge carries both road and rail traffic. Overway is a term for a bridge that separates incompatible intersecting traffic, especially road and rail.[54]
Some bridges accommodate other purposes, such as the tower of Nový Most Bridge in Bratislava, which features a restaurant, or a bridge-restaurant which is a bridge built to serve as a restaurant. Other suspension bridge towers carry transmission antennas.[55]
Conservationists use wildlife overpasses to reduce habitat fragmentation and animal-vehicle collisions.[56] 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.[57][58][59]
Canal bridges can be used in a canal system to carry a boats across a valley or ravine.[60]
Bridges are subject to unplanned uses as well. The areas underneath some bridges have become makeshift shelters and homes to homeless people, and the undertimbers of bridges all around the world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and become known as suicide bridges.[61][62]
Basic types
Many bridges use two or more kinds of structures. The Forth Bridge (orange bridge in the foreground) utilizes both trusses (left) and cantilevers (right).
Bridges can be categorized by their structure (arch, truss, beam, suspension, etc) and their materials (stone, wood, iron, steel, concrete, etc). For example, a bridge could be identified as a timber truss bridge, or a reinforced concrete beam bridge, or a stone arch bridge.[63][64]
The basic bridge structures are beam, arch, truss, cantilever, suspension, and cable-stayed.[63][64] The choice of bridge structure to use in a particular situation is based on many factors, including aesthetics, environment, cost, and use.[65]
Many bridges are composed of multiple structures, for example, some Roman aqueducts contain dozens of adjacent arches. Long causeways 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 the Verrazzano-Narrows Bridge, to have beam or truss elements in the approaches; and some large cantilever bridges, such as the Forth Bridge, use a truss segment in the middle to connect cantilevers on either side.[63]
Arch bridges consist of a curved arch, under compression, which supports the deck either above or below the arch.[66] The shape of the arch can be a semicircle, elliptical, a pointed arch, or a segment of a circle.[67][33][68] When the arch is semicircular, as used in Roman bridges, the force of the arch is directed vertically downward to piers or foundations.[67][69] When the arch is elliptical or a circular segment, the force is directed diagonally, and abutments are often required.[67] Deck arch bridges hold the deck above the arch; tied-arch bridges suspend the deck below the arch; and through-deck arches position the deck through the middle of the arch.[70]
A truss bridge is composed of multiple, connected triangular elements.[71][d] The set of triangles form a rigid whole, which rests on foundations or piers at both ends, applying a vertical force downward.[71] The individual bars can be made of iron or wood, but most modern truss bridges are made of steel.[72] The horizontal bars along the top are usually in compression, and the horizontal bars along the bottom are usually in tension.[71] Other bars in the truss may be in tension or compression, depending on the particular layout of the triangles.[73] Trusses typically have a span-to-depth ratio of about 10 to 16, compared to beam brides which typically have a ratio of about 20 to 30.[74][e] Trusses tend to be relatively stiff, and are commonly used for rail bridges which are required to carry very heavy loads.[74]
Cantilever bridges consist of one or more trusses, each supported at only one end.[75] Modern cantilever bridges are generally built from steel.[75][76] In Asia, cantilever bridges made of large rocks or timber were used to span small obstacles.[11][77][78] In the 1880s, some early cantilever bridges were built from wrought iron.[75] A basic cantilever bridge has two cantilevers, anchored at each end of the span, extending toward the center, and meeting in the center.[75] Some cantilever bridges have a small truss in the center, connecting two cantilevers extending from the sides.[79] A balanced cantilever bridge consists of two cantilevers extending in opposite directions from a single central support.[80] The term "cantilever construction" is a method of building a bridge deck which can be used for cable-stay bridges and cantilever bridges. In this technique, deck construction begins at a foundation or tower, and extends outwards across the obstacle, with no support from below.[81][80]
Suspension bridges consist of two large cables, passing over one or more towers. The deck is suspended from the cables by large wires called hangers.[82] The earliest suspension bridges were made of ropes or vines.[83] In the early 1800s, 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 main cable.[84] When steel wire became widely available, cable spinning was used to pull hundreds of small wires between the towers, which were then bundled to form large cables. Steel wire cables enabled suspension bridges to achieve spans 2 km long.[85][86] The cable of the bridge is in tension, and the towers are in compression.[82] 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.[87] Shorter towers require a smaller sag in the cable, which increases the tension in the cable, and thus requires stronger towers and anchorages.[82]
Cable-stayed bridges are similar to suspension bridges, but the cables that support the deck connect directly to the towers.[88] 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.[88] A cable-stayed bridge generally uses less cable than a comparable suspension bridge, but its towers are proportionately higher.[89] The cables may be arranged in a fan pattern or a harp pattern.[90][f] Modern cable stay bridges became popular after WW II, when the design was used for many new bridges in Germany.[89][g]
Beam bridges, also called girder bridges, are simple structures consisting of one or more parallel, horizontal beams that span an obstacle. Beam bridges are the most common type of bridges for both railways and roadways.[91] Beam bridges are ideal for shorter spans (less than about 50 meters), but for longer spans, other structures, such as trusses, may be more efficient.[92] In many applications, beam bridges can be built rapidly and economically, because the individual beams can be mass produced offsite and transported to the bridge site.[91] Modern beam bridges are generally made of steel or reinforced concrete, although wood may be suitable for small beam bridges intended for light use.[91] Several different cross-sections may be utilized for beams, including I-beam (common for steel) or flat slabs (sometimes used with concrete).[91] Beams can traverse longer spans when they are designed as hollow box girders; bridges made of box girders are termed box girder bridges.[91] The vertical thickness of beam bridges is generally shallower than comparable truss bridges, permitting shorter and lower approach roads to cross an obstacle of a given height.[91] Several beam bridges can be chained together, with supports at each juncture, to form elevated highways or causeways.[91][h]
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.[94][95] Early moveable bridges include drawbridges that were pivoted at one end, and required a large amount of work to raise; adding counterweights on the land side of the drawbridge–a bascule bridge–made raising and lowering easier and safer.[94][96]Swing bridges pivoted horizontally around an anchor point on the bank of a canal, or sometimes from a pier in the middle of the water.[94][97]Lift bridges are raised vertically between two towers by cables passing over pulleys at the top of the towers.[94][98] In the modern era, designers sometimes create unusual moveable bridges with the intention of establishing signature bridges for a town or locality.[99] Examples include Puente de la Mujer swing bridge in Buenos Aires, Gateshead Millenniumtilt bridge over the River Tyne, Hörn Bridgefolding bridge in Germany, Erasmusbrug bascule in Rotterdam, and Limehouse Basin footbridge in London.[99]
Padma Bridge in Bangladesh carries rail traffic on the lower deck and vehicular traffic on the upper deck.
Double-deck bridges (also called 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.[101] Double-deck bridges permit two different kinds of traffic to be safely carried, by separating, for example, motor vehicles from pedestrians or railways.[101]
A trestle is a bridge composed of a number of short spans supported by closely spaced supports, typically carrying a railway. A trestle is similar to a viaduct, but viaducts typically have taller piers and longer spans.[111]
A continuous truss bridge is a truss bridge that extends without hinges or joints across three or more piers. 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).[112]
The word "causeway" normally means a road built on top of a dirt or rock embankment, passing over a lake or other body of water.[citation needed] However, the word is sometimes used to designate long, low bridges, as in Lake Pontchartrain Causeway in Louisiana.
Floating concrete pontoons support the weight of the Nordhordland Bridge as it crosses a deep fjord in Norway.
A pontoon bridge, also called a floating bridge, uses floats or shallow-draftboats to support a continuous deck for pedestrian or vehicle travel over water. The buoyancy of the supports limits the maximum load that they can carry.[citation needed] Pontoon bridges are typically used where waters are too deep to build piers, or as a mechanism to implement a moveable swing bridge in a canal were boat traffic needs to pass by.[114]
This portable AM 50 bridge is being laid over a river in Slovakia.
Portable military bridges are an important type of equipment in the field of military engineering, and perform a variety of wartime roles, including smaller bridges built in the midst of battle, and larger bridges built to facilitate supply lines.[120][i]
Military bridges can be categorized as "wet" bridges that rest on pontoon floats, and "dry" bridges that rest on piers, river banks, or anchorages.[120] 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.[122]
Some military bridges, called armoured vehicle-launched bridges (AVLB), are carried on purpose-built vehicles.[122] 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.[123][124][j] The UK Chieftain AVLB could launch a 23 meter bridge–capable of supporting 60 ton loads– in 3 minutes.[122]
The most visible features of a bridge design are its structure (such as arch, beam, or suspension) and its material (such as steel, wood, or concrete). When designing a bridge to traverse a specific obstacle, the designer must identify designs that meet critical requirements, including safety, strength, lifespan, climate, traffic, the size and nature of the obstacle to be traversed, and clearance required for passage underneath.[125][126] There may be several designs that meet those requirements, so additional factors will also be considered, including cost, aesthetics, time available for construction, customer preference, and experience of the builders.[127] After considering all factors and consulting with the customer, the bridge designer will select a particular design.[125][128]
Material
Iron Bridge in Shropshire, England, completed in 1781, is the first major bridge made of cast iron.
Bridges are built from a wide variety of materials, including wood, brick, stone, iron, steel, and concrete. Many bridges use two or more of these materials.[citation needed]
Wood is an inexpensive material that is often used for small bridges that carry moderate loads. Wood is used in bridges primarily in a beam structure or truss structure, and is also used to build huge trestle bridges for railways.[129]
Masonry bridges, built primarily from stone or brick, are 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.[130][131] In the twentieth century, large masonry bridges –although superseded by concrete in the West–continued to be built in China.[132]
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.[133][134][135]
Steel is one of the most common material used for modern bridges.[36] Steel was made in small quantities in antiquity, but became widely available in the late 1800s following invention of new smelting processes by Henry Bessemer and William Siemens. Steel is especially useful for bridges, because it is strong in both compression and tension.[136] 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.[137][138][41] Concrete bridges make extensive use of steel, because all concrete used in bridges contains steel reinforcing bars or steel prestressed cables.[139][140][141] 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.[142][143]
This concrete bridge support is being prepared for a concrete pour. The green reinforcing bars will be embedded inside the concrete after the concrete cures.
Concrete is a strong and inexpensive material, but is brittle and can crack when in tension.[141] Concrete is useful for bridge elements that are in compression, such as foundations and arches.[144] Many roadway bridges are built entirely of concrete using a beam structure, often of the box girder variety.[144] Virtually all concrete used in bridges contains steel reinforcing bars, which greatly increase the strength.[141][139][145][146] Reinforcing bars are set inside the concrete form, and the concrete is poured into the form, and cures with the bars inside. If concrete is used in elements that experience tension–such as the lower region of a horizontal beam or slab–prestressed cables must be embedded within the concrete and tightened.[141][139][145] 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).[139][145] The prestressed cables compress the concrete. When the beam is placed into the bridge and carries a load, the undesirable tension normally produced by the tendency of the beam to sag is counteracted by the compression from the prestressed cables.[139][145]
A bridge design must accommodate all loads and forces that a bridge might experience. The totality of the forces that the bridge must tolerate is represented by the term "structural load". The structural load is usually divided into three components: The dead load, which is the weight of the bridge itself;[k] the live load, which are the forces and vibrations caused by traffic passing over the bridge, including braking and acceleration, and the environmental 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, and collisions (such as a ship striking the support of a bridge over water).[149]
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.[148] For sporadic events like floods, earthquakes, collisions, and hurricanes, bridge designers must select a maximum severity that the design must accommodate.[150] The designer first selects a return period, which typically ranges from 100 to 2,500 years.[150] 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.[151]
This images illustrates the frequence response of a bridge design when subject to certain vibrations.
The load forces acting on a bridge cause the components of the bridge to become stressed. The bridge designer must calculate the maximum stress that each bridge component will experience, and ensure that the components are sufficiently strong to tolerate the stresses.[152]
Stresses are categorized as 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, as felt by a metal joint during an earthquake when one element of the joint moves north, and another element moves south). Torsion is a twisting force.[152]
Traffic
An important component of the live load carried by a bridge is the vehicle and rail traffic the bridge is expected to carry.[153] In addition to the weight of the vehicle, other forces must be considered, including braking, acceleration, centrifugal forces, and resonant frequencies.[154] 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.[155]
The loads created by trains and vehicles can be determined by modelling or by relying on data and algorithms contained in engineering specifications published by organizations such as Eurocode or AASHTO.[153] 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 as weigh-in-motion (WIM) can produce accurate data without the guesswork inherent in an algorithmic model.[156]
Vibration and resonance
Tacoma Narrows Bridge collapsed shortly after opening in 1940 due to failure of the design to properly account for wind forces.
Many loads imposed on a bridge, including winds and vehicular traffic, can cause a bridge to experience irregular or periodic forces, which may cause bridge components to vibrate or oscillate.[157][158][159] Many bridge components may have inherent resonant frequencies to which they are particularly susceptible, and vibrations near those frequencies can cause very large stresses.[160][161][159] 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.[162][163][164]
Winds can produce a variety of forces on a bridge, including flutter and vortexes.[165][166][167] Considering wind forces during the design process is especially important for long, slender bridges (typically suspension or cable-stayed bridges).[165][166][159] 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 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).[168]
Neglecting to account for vibrations and oscillations can lead to bridge failure. The Angers Bridge collapsed in 1850, killing over 200 people, partly due to soldiers marching on the bridge in a manner that increased resonant oscillations.[164] The Tacoma Narrows Bridge collapsed in the 1940 due to failure of the designer to properly account for wind forces.[169][161] The Golden Gate Bridge was damaged in 1951 due to wind forces, and as a result was reinforced in the 1950s with additional stiffening elements.[162]
Engineers use finite element method software tools to evaluate bridge designs. The colors in this output image illustrate displacement in a bridge under load.
The process used to design bridges uses structural analysis methods and techniques.[170] These methods divide the bridge into smaller components, and analyze the components individually, subject to certain constraints.[170] A proposed bridge design is then modeled with formulas or computer applications.[170] 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.[170]
Bridge design models include both mathematical models and numerical models.[170] 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.[170] The finite element method is the most common numerical model used to perform detailed analysis of stresses and loads of a bridge design.[171][172][173][l] 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.[173]
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.[175] The criteria, and their allowable values, are called limit states. The set of limit states selected for a design are based on the bridge's structure and purpose.[175]
To ensure that a proposed bridge design is sufficiently strong to endure foreseeable stresses, most bridge designers use the Limit State Design methodology (called Load and Resistance Factor Design in United States).[176] This methodology adds a margin of safety to the bridge design by incorporating safety factors into the design process.[177] 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.[178][m] 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).[179]
Other requirements
In addition to satisfying primary requirements–such as safety, strength, and utility–a bridge design must also support secondary requirements such as cost, schedule, ease of maintenance and repair, longevity, fire resistance, and prevention of undesirable flex, sway, and vibration.[180][citation needed][n]
Protection
Paint can be used to reduce deterioration of steel components. Steel bridges need to be repainted periodically, as seen in this wire hanger from the Golden Gate Bridge, which is painted international orange.
To achieve a longer lifespan, a bridge can 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 and spalling on concrete.[181]
The 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.[182] Techniques to prevent water-based damage include drainage systems, waterproofing membranes (such as polymer films), and eliminating expansion joints.[183][o]
Concrete bridge elements can be protected with waterproof seals and coatings.[185] Reinforcing steel within concrete can be protected by using high-quality concrete and increasing the thickness of the outer concrete.[186] Steel elements of a bridge can be protected by paints or galvanized coatings.[187] Paint can be avoided entirely for steel members by using certain steel alloys, such as stainless steel or weathering steel (a steel alloy that eliminates the need for paint, by forming a protective outer layer of rust).[188]
Bridge scour is a potentially serious problem when bridge foundations are located in water. Currents in the water can cause the sand and rocks around and below the foundation to wash-away over time. This effect can be mitigated by placing a cofferdam around the foundations, or surrounding the foundations with rip-rap.[189]
Most bridges are utilitarian in appearance, but in some cases, the appearance of the bridge can have great importance.[190] 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.[191]
The art Historian Dan Cruickshank writes that bridges are regarded as objects of beauty by many people:[192]
Bridge construction remains... the most absolute expression of the beauty and excitement invoked by man-made constructions.... Bridges that are leaps of faith and imagination.... They are an act of creation that challenge the gods, works that possess the very power of nature itself. They are objects in which beauty is the direct result of functional excellence, conceptual elegance and boldness of design and construction.... A great bridge–one that defies and tames nature — becomes almost in itself a supreme work of nature. Bridges embody the essence of mankind’s structural ingenuity.... 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.[192]
Maintenance, inspection and monitoring
Samples of concrete from a bridge (obtained during construction, or later during bridge operations) can be analyzed to identify possible degradation issues.
Bridge maintenance encompasses the activities that ensure an operating bridge remains open to traffic, avoids safety incidents, and achieves its intended lifespan.[193] Maintenance responsibilities include planning, budgeting, and prioritizing all monitoring and inspection tasks.[193] An important part of maintenance is inspecting for damage or degradation, and taking steps to mitigate any issues. 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.[194][181]
A variety of inspection techniques can be utilized to measure degradation of bridge elements. Destructive testing removes material samples from the bridge–such as cores drilled from concrete, or fragments from broken steel wires–to a laboratory. In the laboratory, the samples are analyzed using microscopes, sonic devices, or X-ray diffraction.[195]
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.[199] Many long-span bridges are now routinely monitored with a range of sensors, including strain transducers, accelerometers,[200]tiltmeters, and GPS.
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.[201]
Structural evaluation tests can be conducted which measure deflections in bridge elements when test loads are placed in certain points. 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.[202]
An option for structural-integrity monitoring is "non-contact monitoring", which uses the Doppler effect. A laser beam from a Laser Doppler Vibrometer is directed at the point of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.[203]
Detailed measurements of the external surface of a bridge can be recorded using Lidar technology. Comparing measurements taken at multiple points in time can reveal long-term movements or defelections.[204]
Bridge failures are of special importance to structural engineers, because they provide lessons learned that serve to improve design and construction processes.[205] Before the advent of bridge engineering procedures based on rigorous, scientific foundations, 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.[206] In the United States, 25% of all bridges failed during the 1870s.[206] A notable disaster was the 1876 Ashtabula River railroad disaster, in which a railway bridge failed, resulting in over 80 deaths.[206] Over time, bridge failures gradually led to improvements in bridge design, construction, and maintenance practices.[207]
Bridges occur extensively in art, legend, and literature–often in a metaphorical manner. In Norse mythology, the home of the gods–Asgard–is connected to the earth by Bifröst, a rainbow bridge.[208] Many bridges in Europe are named "Devil's Bridge", and sometimes have folkloric stories that explain why the bridge is associated with the devil.[209] There are many stories, mostly apocryphal, relating bridges to Christian saints.[210] Stories and poems often employ a bridge as a metaphor of the human lifespan, or human experiences.[211] Bridges are often the setting for pageants, celebrations, and processions.[212]
Bridges are often venerated as heroic symbols of humankind's accomplishments.[213] The inspirational nature of bridges has led them to be featured in the works of poets, painters and writers.[214] Bridges feature prominently in paintings–often in the background–as in the Mona Lisa.[215] Authors have used bridges as the centerpiece of novels, such as The Bridge on the Drina by Ivo Andrić, and Thornton Wilder's The Bridge of San Luis Rey.[216]
Many bridges–called "signature bridges"–are strongly identified with a particular city or country. In some cases they are deliberately built with an especially magnificent design, to serve as a landmark or icon. 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."[192]
↑ Examples of early stepping stone bridges include the Sweet Track and the Post Track in England, approximately 6000 years old.[2]
↑ 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).[42]
↑ The dead load also includes any permanent fixtures on the bridge, such as light poles, traffic signage, and guardrails;[148]
↑ An alternative to the finite element method is the simpler, but less powerful, finite strip method.[174]
↑ The strength of a bridge component is referred to as "resistance" in the context of LRFD.[citation needed]
↑ The term "serviceability" is used to describe these requirements that are physical or structural in nature, such as flex, sway, and vibration.[citation needed]
↑ Expansion joints relieve stress due to thermal expansion and contraction, but permit water to seep into vulnerable bridge elements. Integral bridge concepts are an alternative to expansion joints.[184]
↑ Omer; etal. (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.
Birnstiel, Charles (2000). "Moveable Bridges". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.663–698. ISBN0727727745. Retrieved 1 September 2025.
Collings, David (2000). "Composite Construction". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.407–448. ISBN0727727745. Retrieved 1 September 2025.
Jones, Vardiman (2000). "Suspension Bridges". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.595–662. ISBN0727727745. Retrieved 1 September 2025.
Mulheron, Mike (2000). "Protection". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.805–848. ISBN0727727745. Retrieved 1 September 2025.
Ryall, Michael (2000). "Loads and Load Distribution". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.43–94. ISBN0727727745. Retrieved 1 September 2025.
Shanmugam, N. E. (2000). "Structural Analysis". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.95–224. ISBN0727727745. Retrieved 1 September 2025.
Tytler, I.F.B. (1985). Vehicles and Bridging. Battlefield Weapons Systems and Technology. Brasey's Defense Publishers. ISBN0080283225. Retrieved 13 September 2025.
Vassie, Perry (2000). "Bridge management". In Ryall, Michael (ed.). The Manual of Bridge Engineering. Thomas Telford. pp.849–882. ISBN0727727745. Retrieved 1 September 2025.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
