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. 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. Historically, such bridges often became viaducts of multiple small arches. It is often impossible to achieve a flat enough arch, simply owing to the limitations of the foundations – particularly in flat country. Flattening the arch shape to avoid the humpback problem, such as for Brunel's Maidenhead bridge, increases this side thrust. Arch bridges generate large side thrusts on their footings and so may require a solid bedrock foundation. Where simple arched bridges are used for railways on flat terrain the cost of building long approach embankments may be considerable.įurther issues are the foundations for the bridge. Railways also find arched bridges difficult as they are even less tolerant of inclines. 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. A wide bridge may require an arch so tall as to become a significant obstacle and incline for the roadway. 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. For a semi-circular arch, the height is half of the span. 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.įor 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. It can either be lower bearing or mid-bearing. The Bayonne Bridge, a mid-bearing through arch bridge spanning the Kill Van Kull, connecting Bayonne, New Jersey, with Staten Island, New York CityĪ through arch bridge, also known as 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.
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