A temporary wooden footbridge leading toLuang Prabang in Laos
Abridge is a structure built tospan a physical obstacle (such as abody of water,valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually something that is otherwise difficult or impossible to cross. There are many different designs of bridges, each serving a particular purpose and applicable to different situations. Designs of bridges vary depending on factors such as the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it.
The Oxford English Dictionary also notes that there is some suggestion that the word can be traced directly back to Proto-Indo-European*bʰrēw-. However, they also note that "this poses semantic problems."[3]
The simplest and earliest types of bridges werestepping stones.
Neolithic people also built a form ofboardwalk acrossmarshes; examples of such bridges include theSweet Track and thePost Track in England, approximately 6000 years old.[6] Ancient people would also have usedlog bridges[7] consisting of logs that fell naturally or were intentionally felled or placed across streams. Some of the firsthuman-made bridges with significant span were probably intentionally felled trees.[8] Among the oldesttimber bridges is theHolzbrücke Rapperswil-Hurden bridge that crossed upperLake Zürich in Switzerland; prehistoric timber pilings discovered to the west of theSeedamm causeway date back to 1523 BC. The first wooden footbridge there led across Lake Zürich; it was reconstructed several times through the late 2nd century AD, when theRoman Empire built a 6-metre-wide (20 ft) wooden bridge to carry transport across the lake. Between 1358 and 1360,Rudolf IV, Duke of Austria, built a 'new' wooden bridge across the lake that was used until 1878; it was approximately 1,450 metres (4,760 ft) long and 4 metres (13 ft) wide. On 6 April 2001, a reconstruction of the original wooden footbridge was opened; it is also the longest wooden bridge in Switzerland.
TheArkadiko Bridge is one of four Mycenaeancorbel arch bridges part of a former network of roads, designed to accommodatechariots, between the fort of Tiryns and town of Epidauros in thePeloponnese, in southernGreece. Dating to the GreekBronze Age (13th century BC), it is one of the oldestarch bridges still in existence and use. Several intact, arched stone bridges from theHellenistic era can be found in the Peloponnese.[9]
The greatest bridge builders of antiquity were theancient Romans.[10] The Romans built arch bridges andaqueducts that could stand in conditions that would damage or destroy earlier designs, some of which still stand today.[11] An example is theAlcántara Bridge, built over the riverTagus, in Spain. The Romans also used cement, which reduced the variation of strength found in natural stone.[12] One type of cement, calledpozzolana, consisted of water,lime, sand, andvolcanic rock. Brick andmortar bridges were built after theRoman era, as the technology for cement was lost (then later rediscovered).
In India, theArthashastra treatise byKautilya mentions the construction of dams and bridges.[13] AMauryan bridge nearGirnar was surveyed byJames Princep.[14] The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperorChandragupta I.[14] The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century.[15] A number of bridges, both for military and commercial purposes, were constructed by theMughal administration in India.[16]
Although large bridges of wooden construction existed in China at the time of theWarring States period, the oldest surviving stone bridge in China is theZhaozhou 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. European segmental arch bridges date back to at least theAlconétar Bridge (approximately 2nd century AD), while the enormous Roman eraTrajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.[17]
Rope bridges, a simple type ofsuspension bridge, were used by theInca civilization in theAndes mountains of South America, just prior to European colonization in the 16th century.
TheAshanti built bridges overstreams andrivers.[18][19] 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.[19]
During the 18th century, there were many innovations in the design of timber bridges byHans Ulrich Grubenmann,Johannes Grubenmann, as well as others. The first book on bridge engineering was written byHubert Gautier in 1716.
A major breakthrough in bridge technology came with the erection ofthe Iron Bridge in Shropshire, England in 1779. It usedcast iron for the first time as arches to cross theriver Severn.[20] With theIndustrial Revolution in the 19th century,truss systems ofwrought iron were developed for larger bridges, but iron does not have thetensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas ofGustave Eiffel.[21]
In Canada and the United States, numerous timbercovered bridges were built in the late 1700s to the late 1800s, reminiscent of earlier designs in Germany and Switzerland. Some covered bridges were also built in Asia.[22] In later years, some were partly made of stone or metal but the trusses were usually still made of wood; in the United States, there were three styles of trusses, the Queen Post, the Burr Arch and the Town Lattice.[23] Hundreds of these structures still stand in North America. They were brought to the attention of the general public in the 1990s by the novel, movie and playThe Bridges of Madison County.[24][25]
Bridges can be categorized in several different ways. Common categories include the type of structural elements used, by what they carry, whether they are fixed or movable, and by the materials used.
Bridges may be classified by how the actions oftension,compression,bending,torsion andshear are distributed through their structure. Most bridges will employ all of these to some degree, but only a few will predominate. The separation of forces and moments may be quite clear. In asuspension orcable-stayed bridge, the elements in tension are distinct in shape and placement. In other cases the forces may be distributed among a large number of members, as in a truss.
Beam bridge
Beam bridges are horizontal beams supported at each end by substructure units and can be eithersimply supported when the beams only connect across a single span, orcontinuous when the beams are connected across two or more spans. When there are multiple spans, the intermediate supports are known aspiers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges can range from small, wooden beams to large, steel boxes. The vertical force on the bridge becomes ashear andflexural load on the beam which is transferred down its length to the substructures on either side[27] They are typically made of steel, concrete or wood.Girder bridges andplate girder bridges, usually made from steel, are types of beam bridges.Box girder bridges, made from steel, concrete, or both, are also beam bridges. Beam bridge spans rarely exceed 250 feet (76 m) long, as the flexural stresses increase proportionally to the square of the length (and deflection increases proportionally to the 4th power of the length).[28] However, the main span of theRio–Niteroi Bridge, a box girder bridge, is 300 metres (980 ft).[29]
The world's longest beam bridge isLake Pontchartrain Causeway in southernLouisiana in the United States, at 23.83 miles (38.35 km), with individual spans of 56 feet (17 m).[30] Beam bridges are the simplest and oldest type of bridge in use today,[31] and are a popular type.[32]
Truss bridge
Atruss bridge is a bridge whose load-bearing superstructure is composed of a truss. This truss is a structure of connected elements forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads.[33] Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by nineteenth and early twentieth-century engineers. A truss bridge is economical to construct owing to its efficient use of materials.
Cantilever bridge
Cantilever bridges are built usingcantilevers—horizontal beams supported on only one end. Most cantilever bridges use a pair ofcontinuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses. Cantilever bridges are constructed using much the same materials and techniques as beam bridges. The difference comes in the action of the forces through the bridge.
Some cantilever bridges also have a smaller beam connecting the two cantilevers, for extra strength.
The largest cantilever bridge is the 549-metre (1,801 ft)Quebec Bridge in Quebec, Canada.
Arch bridge
Arch bridges haveabutments at each end. The weight of the bridge is thrust into theabutments at either side. The earliest known arch bridges were built by the Greeks, and include theArkadiko Bridge.
With the span of 220 metres (720 ft), theSolkan Bridge over theSoča River atSolkan in Slovenia is the second-largest stone bridge in the world and the longest railroad stone bridge. It was completed in 1905. Its arch, which was constructed from over 5,000 tonnes (4,900 long tons; 5,500 short tons) of stone blocks in just 18 days, is the second-largest stone arch in the world, surpassed only by the Friedensbrücke (Syratalviadukt) inPlauen, and the largest railroad stone arch. The arch of the Friedensbrücke, which was built in the same year, has the span of 90 m (295 ft) and crosses the valley of theSyrabach River. The difference between the two is that the Solkan Bridge was built from stone blocks, whereas the Friedensbrücke was built from a mixture of crushed stone and cement mortar.[34]
The world's largest arch bridge is theChaotianmen Bridge over theYangtze River with a length of 1,741 m (5,712 ft) and a span of 552 m (1,811 ft). The bridge was opened 29 April 2009, inChongqing, China.[35]
Tied arch bridge
Tied-arch bridges have an arch-shaped superstructure, but differ from conventional arch bridges. Instead of transferring the weight of the bridge and traffic loads into thrust forces into the abutments, the ends of the arches are restrained by tension in the bottom chord of the structure.[36] They are also called bowstring arches.
The longest suspension bridge in the world is the 4,608 m (15,118 ft)1915 Çanakkale Bridge in Turkey.
Cable-stayed bridge
Cable-stayed bridges, like suspension bridges, are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately higher.[38] The first known cable-stayed bridge was designed in 1784 by C. T. (or C. J.) Löscher.[39][40]
Some Engineers sub-divide 'beam' bridges into slab, beam-and-slab and box girder on the basis of their cross-section.[42] A slab can be solid orvoided (though this is no longer favored for inspectability reasons) while beam-and-slab consists of concrete or steel girders connected by a concrete slab.[43] Abox-girder cross-section consists of a single-cell or multi-cellular box. In recent years,integral bridge construction has also become popular.
Alvis Unipower TBT (Tank Bridge Transporter) of the British army.
Most bridges are fixed bridges, meaning they have no moving parts and stay in one place until they fail or are demolished. Temporary bridges, such asBailey bridges, are designed to be assembled, taken apart, transported to a different site, and re-used. They are important in military engineering and are also used to carry traffic while an old bridge is being rebuilt.Movable bridges are designed to move out of the way of boats or other kinds of traffic, which would otherwise be too tall to fit. These are generally electrically powered.[44]
The Tank bridge transporter (TBT) has the same cross-country performance as a tank even when fully loaded. It can deploy, drop off and load bridges independently, but it cannot recover them.[45]
Double-decked (or double-decker) bridges have two levels, such as theGeorge Washington Bridge, connecting New York City toBergen County,New Jersey, US, as the world's busiest bridge, carrying 102 million vehicles annually;[46][47]truss work between the roadway levels provided stiffness to the roadways and reduced movement of the upper level when the lower level was installed three decades after the upper level. TheTsing Ma Bridge andKap Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their lower decks there are two lanes and a pair of tracks forMTR metro trains. Some double-decked bridges only use one level for street traffic; theWashington Avenue Bridge inMinneapolis reserves its lower level for automobile and light rail traffic and its upper level for pedestrian and bicycle traffic (predominantly students at theUniversity of Minnesota). Likewise, inToronto, thePrince Edward Viaduct has five lanes of motor traffic, bicycle lanes, and sidewalks on its upper deck; and a pair of tracks for theBloor–Danforthsubway line on its lower deck. The western span of theSan Francisco–Oakland Bay Bridge also has two levels.
A viaduct is made up of multiple bridges connected into one longer structure. The longest and some of the highest bridges are viaducts, such as theLake Pontchartrain Causeway andMillau Viaduct.
A multi-way bridge has three or more separate spans which meet near the center of the bridge. Multi-way bridges with only three spans appear as a "T" or "Y" when viewed from above. Multi-way bridges are extremely rare.The Tridge,Margaret Bridge, andZanesville Y-Bridge are examples.
A bridge can be categorized by what it is designed to carry, such as trains, pedestrian or road traffic (road bridge), a pipeline (Pipe bridge) or waterway for water transport or barge traffic. Anaqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. 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.[48]
Some bridges accommodate other purposes, such as the tower ofNový Most Bridge inBratislava, which features a restaurant, or abridge-restaurant which is a bridge built to serve as a restaurant. Other suspension bridge towers carry transmission antennas.[49]
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 assuicide bridges.[54][55]
The materials used to build the structure are also used to categorize bridges. Until the end of the 18th century, bridges were made out of timber, stone and masonry. Modern bridges are currently built in concrete, steel, fiber reinforced polymers (FRP), stainless steel or combinations of those materials.Living bridges have been constructed of live plants such asFicus elastica tree roots in India[56] andwisteria vines in Japan.[57]
Bridge type
Materials used
Cantilever
For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built fromstructural steel, or box girders built fromprestressed concrete.[58]
Stone, brick and other such materials that are strong in compression and somewhat so in shear.
Beam
Beam bridges can use pre-stressed concrete, an inexpensive building material, which is then embedded withrebar. The resulting bridge can resist both compression and tension forces.[60]
Truss
The triangular pieces of truss bridges are manufactured from straight and steel bars, according to the truss bridge designs.[61]
Unlike buildings whose design is led by architects, bridges are usually designed by engineers. This follows from the importance of the engineering requirements; namely spanning the obstacle and having the durability to survive, with minimal maintenance, in an aggressive outdoor environment.[43] Bridges are first analysed; the bending moment and shear force distributions are calculated due to the applied loads. For this, thefinite element method is the most popular. The analysis can be one-, two-, or three-dimensional. For the majority of bridges, a two-dimensional plate model (often with stiffening beams) is sufficient or an upstand finite element model.[62] On completion of the analysis, the bridge is designed to resist the applied bending moments and shear forces, section sizes are selected with sufficient capacity to resist the stresses. Many bridges are made ofprestressed concrete which has good durability properties, either by pre-tensioning of beams prior to installation or post-tensioning on site.
In most countries, bridges, like other structures, are designed according toLoad and Resistance Factor Design (LRFD) principles. In simple terms, this means that the load is factored up by a factor greater than unity, while the resistance or capacity of the structure is factored down, by a factor less than unity. The effect of the factored load (stress, bending moment) should be less than the factored resistance to that effect. Both of these factors allow for uncertainty and are greater when the uncertainty is greater.
Most bridges are utilitarian in appearance, but in some cases, the appearance of the bridge can have great importance.[63] Often, this is the case with a large bridge that serves as an entrance to a city, or crosses over a main harbor entrance. These are sometimes known as signature bridges. Designers of bridges in parks and along parkways often place more importance on aesthetics, as well. Examples include the stone-faced bridges along theTaconic State Parkway in New York.
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.[64] To create a beautiful image, some bridges are built much taller than necessary. This type, often found in east-Asian style gardens, is called aMoon bridge, evoking a rising full moon. Other garden bridges may cross only a dry bed of stream-washed pebbles, intended only to convey an impression of a stream. Often in palaces, a bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind. A set of five bridges cross a sinuous waterway in an important courtyard of theForbidden City in Beijing, China. The central bridge was reserved exclusively for the use of the Emperor and Empress, with their attendants.
The estimated life of bridges varies between 25 and 80 years depending on location and material.[65][66]
Bridges may age hundred years with proper maintenance and rehabilitation. Bridge maintenance consisting of a combination of structural health monitoring and testing. This is regulated in country-specific engineer standards and includes an ongoing monitoring every three to six months, a simple test or inspection every two to three years and a major inspection every six to ten years. In Europe, the cost of maintenance is considerable[42] and is higher in some countries than spending on new bridges. The lifetime of welded steel bridges can be significantly extended byaftertreatment of the weld transitions. This results in a potential high benefit, using existing bridges far beyond the planned lifetime.
While the response of a bridge to the applied loading is well understood, the applied traffic loading itself is still the subject of research.[67] This is a statistical problem as loading is highly variable, particularly for road bridges. Load Effects in bridges (stresses, bending moments) are designed for using the principles ofLoad and Resistance Factor Design. Before factoring to allow for uncertainty, the load effect is generally considered to be the maximum characteristic value in a specifiedreturn period. Notably, in Europe, it is the maximum value expected in 1000 years.
Bridge standards generally include a load model, deemed to represent the characteristic maximum load to be expected in the return period. In the past, these load models were agreed by standard drafting committees of experts but today, this situation is changing. It is now possible to measure the components of bridge traffic load, to weigh trucks, usingweigh-in-motion (WIM) technologies. With extensive WIM databases, it is possible to calculate the maximum expected load effect in the specified return period. This is an active area of research, addressing issues of opposing direction lanes,[68][69] side-by-side (same direction) lanes,[70][71] traffic growth,[72] permit/non-permit vehicles[73] and long-span bridges (see below). Rather than repeat this complex process every time a bridge is to be designed, standards authorities specify simplified notional load models, notably HL-93,[74][75] intended to give the same load effects as the characteristic maximum values. TheEurocode is an example of a standard for bridge traffic loading that was developed in this way.[76]
Most bridge standards are only applicable for short and medium spans[77] - for example, the Eurocode is only applicable for loaded lengths up to 200 m. Longer spans are dealt with on a case-by-case basis. It is generally accepted that the intensity of load reduces as span increases because the probability of many trucks being closely spaced and extremely heavy reduces as the number of trucks involved increases. It is also generally assumed that short spans are governed by a small number of trucks traveling at high speed, with an allowance for dynamics. Longer spans on the other hand, are governed by congested traffic and no allowance for dynamics is needed.
Calculating the loading due to congested traffic remains a challenge as there is a paucity of data on inter-vehicle gaps, both within-lane and inter-lane, in congested conditions.Weigh-in-Motion (WIM) systems provide data on inter-vehicle gaps but only operate well in free flowing traffic conditions. Some authors have used cameras to measure gaps and vehicle lengths in jammed situations and have inferred weights from lengths using WIM data.[78] Others have usedmicrosimulation to generate typical clusters of vehicles on the bridge.[79][80][81]
Bridges vibrate under load and this contributes, to a greater or lesser extent, to the stresses.[43] Vibration and dynamics are generally more significant for slender structures such as pedestrian bridges and long-span road or rail bridges. One of the most famous examples is theTacoma Narrows Bridge that collapsed shortly after being constructed due to excessive vibration. More recently, theMillennium Bridge in London vibrated excessively under pedestrian loading and was closed and retrofitted with a system of dampers. For smaller bridges, dynamics is not catastrophic but can contribute an added amplification to the stresses due to static effects. For example, the Eurocode for bridge loading specifies amplifications of between 10% and 70%, depending on the span, the number of traffic lanes and the type of stress (bending moment or shear force).[82]
There have been many studies of the dynamic interaction between vehicles and bridges during vehicle crossing events. Fryba[83] did pioneering work on the interaction of a moving load and an Euler-Bernoulli beam. With increased computing power, vehicle-bridge interaction (VBI) models have become ever more sophisticated.[84][85][86][87] The concern is that one of the many natural frequencies associated with the vehicle will resonate with the bridge's first natural frequency.[88] The vehicle-related frequencies include body bounce and axle hop but there are also pseudo-frequencies associated with the vehicle's speed of crossing[89] and there are many frequencies associated with the surface profile.[67] Given the wide variety of heavy vehicles on road bridges, a statistical approach has been suggested, with VBI analyses carried out for many statically extreme loading events.[90]
The failure of bridges is of special concern forstructural engineers in trying to learn lessons vital to bridge design, construction and maintenance.
The failure of bridges first assumed national interest in Britain during theVictorian era when many new designs were being built, often using new materials, with some of them failing catastrophically.
In the United States, theNational Bridge Inventory tracks the structural evaluations of all bridges, including designations such as "structurally deficient" and "functionally obsolete".
There are several methods used to monitor the condition of large structures, like bridges. Many long-span bridges are now routinely monitored with a range of sensors, including strain transducers,accelerometers,[91] tiltmeters, and GPS. Accelerometers have the advantage that they are inertial, i.e., they do not require a reference point to measure from. This is often a problem for distance or deflection measurement, especially if the bridge is over water.[92]Crowdsourcing bridge conditions by accessing data passively captured by cell phones, which routinely include accelerometers and GPS sensors, has been suggested as an alternative to including sensors during bridge construction and an augment for professional examinations.[93]
An option for structural-integrity monitoring is "non-contact monitoring", which uses theDoppler effect (Doppler shift). Alaser beam from aLaser 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.[94] The advantage of this method is that the setup time for the equipment is faster and, unlike an accelerometer, this makes measurements possible on multiple structures in as short a time as possible. Additionally, this method can measure specific points on a bridge that might be difficult to access. However, vibrometers are relatively expensive and have the disadvantage that a reference point is needed to measure from.
Snapshots in time of the external condition of a bridge can be recorded usingLidar to aid bridge inspection.[95] This can provide measurement of the bridge geometry (to facilitate the building of a computer model) but the accuracy is generally insufficient to measure bridge deflections under load.
While larger modern bridges are routinely monitored electronically, smaller bridges are generally inspected visually by trained inspectors. There is considerable research interest in the challenge of smaller bridges as they are often remote and do not have electrical power on site. Possible solutions are the installation of sensors on a specialist inspection vehicle and the use of its measurements as it drives over the bridge to infer information about the bridge condition.[96][97][98] These vehicles can be equipped with accelerometers, gyrometers, Laser Doppler Vibrometers[99][100] and some even have the capability to apply a resonant force to the road surface to dynamically excite the bridge at its resonant frequency.
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