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A through arch bridge, also known as a half-through arch bridge or a through-type arch bridge, is a bridge that is made from materials such as steel or reinforced concrete, in which the base of an arch structure is below the deck but the top rises above it. Thus, the deck is within the arch, and cables or beams that are in tension suspend the central part of the deck from the arch.
For a specific construction method, especially for masonry arches, the proportions of the arch remain similar no matter what the size: wider arches are thus required to be taller arches.For a semi-circular arch, the height is half of the span. Bridges across deep, narrow gorges can have their arch placed entirely beneath a flat roadway, but bridges in flatter country rise above their road approaches. A wide bridge may require an arch so tall as to become a significant obstacle and incline for the roadway. Small bridges can be hump-backed, but larger bridges such as the Old Bridge, Pontypridd may become so steep as to require steps, making their use for wheeled traffic difficult. Railways also find arched bridges difficult as they are even less tolerant of inclines. Where simple arched bridges are used for railways on flat terrain the cost of building long approach embankments may be considerable.
Further issues are the foundations for the bridge. Arch bridges generate large side thrusts on their footings and so may require a solid bedrock foundation. Flattening the arch shape to avoid the humpback problem, such as for Brunel's Maidenhead bridge, increases this side thrust. It is often impossible to achieve a flat enough arch, simply owing to the limitations of the foundations – particularly in flat country. Historically, such bridges often became viaducts of multiple small arches.
With the availability of iron or concrete as structural materials, it became possible to construct a through arch bridge: a bridge where the deck does not have to be carried over the top of the arch.This requires a structure that can both support the deck from the arch by tension rods, chains or cables and allow a gap in the arch, so the deck can pass through it. The first of these in particular cannot be achieved with masonry construction and requires wrought iron or steel.
The use of a through arch does not change the proportions or size of the arch: a large span will still require a tall arch, although this can now reach any height above the deck without obstructing traffic. The arch may also reach downwards at its sides, to either reach strong foundations or to place the roadway at a convenient height for spanning a deep valley from a plateau above. The Tyne Bridge demonstrates both of these advantages.
A well-known example of this type is the Sydney Harbour Bridge in Australia, which is based on the Hell Gate Bridge in New York City.Other bridges include the Chaotianmen Bridge in China which is the world longest bridge; Tyne Bridge of Newcastle upon Tyne; the Bayonne Bridge that connects New York City to New Jersey, which is longer than the Sydney Harbour Bridge; the Bourne Bridge and Sagamore Bridge, smaller, near-twin bridges over the Cape Cod Canal; the Pennybacker Bridge in Austin, Texas and as the Hernando de Soto Bridge in Memphis, Tennessee. Wylam Railway Bridge is an early through arch bridge upstream of the Tyne Bridge.
The through arch bridge usually consists of two ribs,although there are examples like the Hulme Arch Bridge of through arches with a single rib. When the two arches are built in parallel planes, the structure is a parallel rib arch bridge. When the two arch ribs lean together and shorten the distance between the arches near the top, the span is a basket handle arch bridge.
Many tied-arch bridges are also through-arch bridges. As well as tying the side-loads of the arch, the tension member is also at a convenient height to form the bridge deck, as for a through-arch.
The converse is not true: through-arch bridges do not imply that they are tied-arch bridges, unless they also provide the deliberate tension member that is the key to a tied-arch. Although visually similar, tied- and untied- through-arch bridges are quite distinct structurally and are unrelated in how they distribute their loads. In particular, cast iron bridges such as the Stanley Ferry Aqueduct may resemble tied-arch bridges, but as cast iron is weak in tension they are not structurally a tied arch.
In some locations it is not practical to support the arch from beneath during construction.
In modern construction, temporary towers are erected and supported by cables anchored in the ground. Temporary cables fly from each side to support arch segments as they are constructed. When the arches are almost complete a jacking bridge is placed over or beneath the gap to force the arches apart, whence the final section is constructed in place or lifted into position.
In some cases, this type of arch has been created by constructing cantilevers from each side, with the shoreside ends bolted securely down into heavy piers. The incomplete channel ends are then constructed toward each other and either filled by construction or by lifting a prefabricated center section. This type of construction was used in the Sydney Harbour Bridge illustrated above, with the supporting cables to the higher side of the arches removed after completion.
The Sydney Harbour Bridge is an Australian heritage-listed steel through arch bridge across Sydney Harbour that carries rail, vehicular, bicycle, and pedestrian traffic between the Sydney central business district (CBD) and the North Shore. The view of the bridge, the harbour, and the nearby Sydney Opera House is widely regarded as an iconic image of Sydney, and of Australia itself. The bridge is nicknamed "The Coathanger" because of its arch-based design.
A suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 1800s. Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.
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A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements usually forming triangular units. The connected elements may be stressed from tension, compression, or sometimes both in response to dynamic loads. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by 19th and early 20th-century engineers. A truss bridge is economical to construct because it uses materials efficiently.
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The Mike O'Callaghan–Pat Tillman Memorial Bridge is an arch bridge in the United States that spans the Colorado River between the states of Arizona and Nevada. The bridge is located within the Lake Mead National Recreation Area approximately 30 miles (48 km) southeast of Las Vegas, and carries Interstate 11 and U.S. Route 93 over the Colorado River. Opened in 2010, it was the key component of the Hoover Dam Bypass project, which rerouted US 93 from its previous routing along the top of Hoover Dam and removed several hairpin turns and blind curves from the route. It is jointly named for Mike O'Callaghan, Governor of Nevada from 1971–1979, and Pat Tillman, an American football player who left his career with the Arizona Cardinals to enlist in the United States Army and was killed in Afghanistan in 2004 by friendly fire.
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