Arch bridge
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| Arch Bridge | |
|---|---|
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| Double stone arch bridge, Japan | |
| Ancestor | Clapper bridge |
| Related | None |
| Descendant | Truss arch bridge, moon bridge (masonry) |
| Carries | Pedestrians, vehicles, light rail, heavy rail |
| Span range | short, but often set end-to-end to form a large total span |
| Material | Stone, concrete, iron truss, steel truss |
| Movable | No |
| Design effort | Low |
| Falsework required | Yes |
An arch bridge (sometimes deck arch bridge to distinguish it from a through arch bridge) is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust against abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are now typically used. Multiple arch bridges are seen in ancient Roman aqueducts. Arch bridges were first invented by the ancient Greeks, who built them out of stone. Later, the Romans used cement in their arch bridges. The Romans only used semicircular arches, but longer and thinner bridges are possible with elliptical arches or inverted catenary arches. Some of these bridges are still standing today.
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[edit] Simple compression arch bridges
[edit] Advantage in use of simple materials
Stone and other such materials are strong in compression and somewhat so in shear, but cannot resist much force in tension so masonry arch bridges are designed to be constantly under compression. Each arch is constructed over a temporary falsework frame. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together and the mortar is allowed to set before the falsework is removed.
[edit] Construction sequence
- Where the arches are founded in a stream bed the water is diverted and the gravels excavated to a good footing. From this the foundation piers are raised to the base of the arches.
- Falsework arches are fabricated, typically from timbers and boards. Since each arch of a multi-arch bridge will impose a thrust upon its neighbors, it is necessary that all arches of the bridge be raised at the same time. The thrust from the end arches is taken into the earth by footings at the canyon walls, or by large inclined planes forming ramps to the bridge, which may also be formed of arches.
- The several arches are constructed simultaneously over the falsework. Once the basic arch is constructed the arches are stabilized with infill masonry between the arches, which may be laid in horizontal running bond courses, These may form two walls, which are then infilled with loose material and rubble.
- The road is paved and parapet walls protectively confine traffic to the bridge.
[edit] Aqueducts and canal viaducts
- In some locations it is necessary to span a wide gap at a relatively high elevation, such as when a canal or water supply must span a crossing stream valley. Rather than building extremely large arches, or very tall supporting columns (difficult using stone), a series of arched structures are built one atop another, with wider structures at the base. Roman civil engineers developed the design and constructed highly refined structures using only simple materials, equipment, and mathematics. 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 ellipses.
[edit] Use of modern materials
Most modern compression arch bridges are made from reinforced concrete. This type of bridge is suitable where a temporary supporting falseworks may be erected to support the forms, reinforcements, and uncured concrete. When the concrete is sufficiently set the forms and falseworks are then removed.
Many modern bridges, made of steel or reinforced concrete, often bear some of their load by tension within their structure. This reduces or eliminates the horizontal thrust against the abutments and allows their construction on weaker ground. Structurally and analytically they are not true arches but rather a beam with the shape of an arch. See truss arch bridge for more on this type.
A modern evolution of the arch bridge is the compression arch suspended-deck bridge (through arch bridge). This has been made possible by the use of light materials that are strong in tension such as steel, reinforced concrete, and post-tensioned concrete.
[edit] Headline text
[edit] See also
- List of arch bridges by length
- Truss arch bridge
- Compression arch suspended-deck bridge
- Tied arch bridge
