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Bridge
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Bridge

This article is about the edifice. For other meanings, see Bridge (disambiguation).


A bridge is a structure built to span a gorge, valley, road, railroad track, river or other body of water, or any other physical obstacle.

The purpose of a bridge is to allow passage of people, cars, trains or ships over an obstacle.

A moveable bridge gives space for tall items such as ship's masts to pass through, or may be used to span distances that are variable in height or distance.

Table of contents
1 History
2 Etymology
3 Types of bridge
4 Works of art
5 Related topics
6 External link

History

The first bridges were simple wooden logs or planks spanning a stream or such; the next examples found use stone, but again as a simple support and crossbeam arrangement. The arch was first used by the Roman Empire for bridges, and many Roman bridges and aqueducts still exist today. The Romans also had cement, which reduced the variation of strength found in natural stone. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost.

With the advent of the Industrial Revolution of the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, with its high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel, which were first shown at the Eiffel Tower in Paris France.

Etymology

The Oxford English Dictionary traces the origin of the word bridge to an Old Norse word brygga, meaning "landing stage, gangway, or movable pier".
spanning the River Avon in Bristol, England]]

Types of bridge

Bridges may be classified by their use or by the structure employed in their design.

By use

A bridge is usually either designed for trains, pedestrian or road traffic or may carry a pipeline or waterway for water transport or barge traffic. In some cases there may be restrictions in use; for example, it may be a bridge carrying a highway and forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles. A bridge which has a series of spans, typically arches, is called a viaduct. An aqueduct is a kind of bridge that carries water, resembling a viaduct. Sometimes a bridge carries a pipeline only. When a bridge spans a road or railroad track, it is often called an overpass (US) or flyover (UK). A skyway is typically a fully-enclosed bridge between two buildings.

Decorative and ceremonial bridges

]] Bridges such as that shown at right are often built much taller than necessary for practical use, simply so that the reflection in the water will complete a circular image. This type, found in oriental gardens, is called a "Moon Bridge", evoking a rising full moon.

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 the Forbidden City in Beijing, China. The central bridge was reserved exclusively for the use of the Emperor, Empress, and their attendants.

By structure

Log bridge

The earliest bridges were opportunistic use of logs that fell naturally across streams. The first manmade bridges were probably intentially felled trees. The use of emplaced logs is now sometimes used in temporary bridges used for logging roads, where a forest tract is to be harvested and the road then abandoned. Such log bridges have a severely limited lifetime due to soil contact and subseqent rot and wood-eating insect infestation. Long lasting log bridges may be constructed by providing well drained footings of stone or concrete combined with regular maintenance to prevent soil infiltration.

Beam bridge

The direct descendant of the log bridge, this is now made from steel "I" beams, reenforced concrete, or post-tensioned concrete (concrete with tubes for cable tenons). It is frequently seen in pedestrian bridges and for highway overpasses and flyovers.

Plate girder bridge

Often seen supporting railroad roadbeds over short spans, this bridge may be a shallow u-shape in cross section or may be under the roadbed. Each side consists of steel plate shear members with a top and bottom chord.

Trestle

A trestle consists of a large number of short spans, supported by splayed vertical elements. Timber trestles were extensively used in the nineteenth century in mountainous areas. these were typically constructed using treated peeled logs as vertical elements with sawn timbers for bracing. Twentieth century construction has eliminated much of the need for trestles by using more extensive grading and tunneling. The trestle shown is a steel structure using relatively long span plate girder spans. Trestle structures are also used for the approaches to bridges where required by the local topography.

Truss bridge

Compared to a log or wood beam bridge, a truss bridge can be much more economical in its use of materials, partcularly when wood truss members are combined with metal fittings to transfer tension through the structure. Trusses can carry heavier loads over longer spans using shorter elements than can log bridges. Conceptually, simple trusses are considered to be pin-joined triangulated structures, where the engineering analysis of the structure depends upon the assumption that there is no bending stress placed upon the truss members. Such structures may be analysed using simple methods. The assumption of bending stresses require a more sophisticated analysis, yet well within the capablilities of pre-computer engineering. Since most early bridges of this type were constructed largely of wood, it is typical that they are either formed from treated wood (typically using creosote) or are protected with a roof and enclosed in building-like siding, forming a covered bridge. There is a substantial variation in the design of trusses, but all use the principle of alternating tension and compression elements (some elements perform both tasks under dynamic loading). Where elements are known by engineering analysis to carry only tension without bending, compression, or shear, they may be made of slender steel rods. The upper chord of a truss is generally in compression, while the lower is in tension.

Bridges generally use a pair of trusses joined with diagonally braced horizontal elements to form a box structure. The roadbed may form part of the upper or lower elements or may be suspended somewhere in the middle. Where a bridge must span a canyon at an angle the side trusses may be offset. This is usually seen where a highway or railroad through a canyon is carried over a stream to take advantage of more favorable grading or construction opportunities presented by the opposite bank.

Truss bridges may be made from almost any material with sufficient rigidity and strength, including peeled logs, wood beams, reenforced concrete, and metal beam structural elements.

The concepts of the truss are used in other types of bridges and in components of bridges such as the deck structures of some suspension bridges.

, China crossing the Chang Jiang (Yangtze) river ]]

Compression arch bridge

This form was the earliest type of bridge that could form long spans using stone, brick, or simple concrete, materials which while strong in compression and somewhat so in shear, cannot resist much force in tension.

In most arch bridges he roadway rides on or above the supporting structure. Early roman aquiducts use a vertical stack of such bridges, progressing from long spans to shorter as elevation is added in order to obtain height while maintaining rigidity in the supporting structure by avoiding tall, thin vertical elements.

This type is still used in canal viaducts and roadways as it has a pleasing shape, particularly when spanning water, as the reflections of the arches form a visual impression of circles or elipses.

Most modern compression arch bridges are made from reenforced concrete. This type of bridge is suitable where a temporary supporting falseworks may be erected to support the forms, reenforcements, and uncured concrete. When the concrete as sufficiently set the forms and falseworks are then removed.

Compression arch suspended-deck bridge

Made from modern materials such as steel or reinforced concrete, the compression arch soars above the roadbed. Cables connect the roadbed to the arch. One of the most famous bridges of this type is found in Sydney, NSW, Austrailia. Where the arch cannot be supported from below it is brought out from each landing point using temporary cable stays supported from temporary towers, kept from tipping inward by stays downward to earth anchors.

Simple suspension bridge

Suspension bridges are another early bridge type are still formed from native materials, chiefly grass rope, in some areas of South America. These must of course be periodically renewed owing to the limited lifetime of the materials. A more permanant variation, suitable for pedestrian and sometimes equestrian use, may be made from simple wire rope. In such bridges, the roadbed of the bridge will follow the downward and upward arc of the load bearing cables, with additional light ropes at a higher level used to form handholds. Owing to practical limitation in the grade of the bridge deck this type is quite restricted in its load carrying capacity relative to its span. To walk on such a bridge at a reasonable pace requires a particular gliding step, as the more normal walking step will induce traveling waves that can cause the traveler to pitch (uncomfortably) up and down.

Suspended-deck suspension bridge

Modern vehicle-carrying suspension bridges use towers to support the main load bearing cables. These cables must be securely anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The roadway is supported by vertical suspender cables. This type of bridge is the only practical type usable for longer spans, where it is impractical or hazzardous to maritime traffic to add central supports. This kind of bridge is particularly pleasing to the visual senses, with the most beautiful of the type being generally acknowleged as the Golden Gate Bridge at the entrance to San Francisco Bay.

Cable-stayed bridge

Cable-stayed bridges are relatively modern. The design uses a number of individual cables from the tower to the roadway. These do not arc (except under their own weight) but proceed in straight lines to multiple attachment points along the roadway. Towers may hold a single set of cables that tie to the center of the roadway as shown at right, or may contain two sets tied to the edges of the roadway.

An advantage of this type over the suspension types outlined above is that strong anchorages to resist the sideways pull of the cables are not required. This makes this type of bridge applicable to locations with poor soil conditions, provided that the towers can be securely anchored. The bridge shown at the right is a three tower span over the Chang Jiang (Yangtze) river just below the Gezhouba Dam. In this bridge the central tower is taller than the flanking towers and the complete bridge forms a particularly dramatic impression.

Perhaps the most aesthetically pleasing single tower bridge is the recently completed (opened July 4, 2004) Sundial Bridge, a glass decked pedestrian bridge crossing the Sacramento River at Redding, California

Self-supporting suspension bridge

A bridge type combining elements of a suspended-deck suspension bridge and a cable stayed bridge has been proposed as a replacement for the eastern span of the San Francisco-Oakland Bay Bridge. Similar to a cable stayed bridge this type does not impose horizontal tension forces upon its anchorages - it is considered to be an unanchored bridge. As with a suspension bridge, the deck loads are transfered to the primary cables by suspender cables. See the Bay Bridge article for additional information.

Cantilever bridge

This is employed mostly to overcome construction difficulties where it is not practical to temporarily support the bridge from below during construction. An advantage of the cantilever is that it can be constructed by working only from the support caissons - this is done by building each side of the cantilever in sychronization to ensure the balance of the structure. Most bridge cantilevers uses balanced structures, with two cantilevers extending, one to each side, from a central tower. Typically, two such double cantilevers upon completion will be securely anchored to massive supports at their outer spans to resist the inward tipping of the cantilevers, with a substantial gap between the two cantilevers. Between each pair of cantilevers a prefabricated central span will be lifted into place using cables. One of the most famous of such bridges is the Forth Rail Bridge over the Firth of Forth in Scotland, notable for its innovative use of tubular structural elements and its use of three double cantelevers. The eastern span of the San Francisco-Oakland Bay Bridge uses this basic structure but with built-up lattice beams and plate rod tension elements. This portion of the Bay Bridge is soon to be replaced by a more modern (and suposedly more earthquake resistant) span.

The cantilever principle is used during the construction of compression arch bridges. In most modern construction, temporary vertical towers are used with cable stays to hold up the arch, which is built in sections, each attached to a new stay. Substantial backstays are anchored in the ground to support the loads. Upon near completion a jack bridge is placed across the final gap and the arches pushed apart to receive the final arch segments. Upon completion the stays, backstays jack bridge, and temporary towers are removed. The use of the temporary tower greatly reduces the amount of material required and simplifies the design. Compression arch bridges may be built using self supporting cantilevers during construction but as the loads on elements can vary in tension and compression between construction and final use this may lead to a less efficient use of materials.

Pontoon bridge

Pontoon bridges consists of a deck supported by tank- or boat-like floats. To allow passage of boats or ships requires either a conventional bridge at some point or a means of removing a section for passage. This type of bridge is typically used for temporary military use, but a number of permanent bridges of this type have been built in the state of Washington, USA.

Moveable bridges

(1885), London, with a central bascule bridge and suspension elements]]
To allow ships to pass which can not pass under it, a bridge may be constructed such that it (or part of it) can be turned up (drawbridge; either one part or two, also called a
bascule bridge) or sideways (swing bridge). A third method is that the bridge deck is lifted while staying horizontal (lift bridge or lifting bridge). (Alternatively, if road traffic is very light, a transporter bridge may be used.)

The tilting Gateshead Millennium Bridge spanning the river Tyne between Gateshead on the south bank and Newcastle upon Tyne on the north (see image below), is a pedestrian bridge with two huge hydraulic rams at each side that tilt the structure back allowing small boats, etc. through.

For small bridges these movements may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridge-person, sometimes remotely using video-cameras and loudspeakers.

There are often traffic lights for the road and water traffic, and moving barriers for the road traffic.

Smaller moveable bridges, called jetways, are used in airports to allow passengers to cross the variable distances and heights between the terminal building and aircraft of varying sizes.

See also

Works of art

featuring bridges or using a bridge metaphor

Related topics

External link