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, water |
| Span range | short, but often set end-to-end to form a large total length |
| Material | masonry, concrete, wrought iron, cast iron, timber, structural steel |
| 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 restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.
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[edit] History
Stone arches were first invented around 2500 BC in the Indus Valley Civilization, known by the ancient Greeks, but developed most fully for bridges by the ancient Romans, many of whose structures still survive.
Most Roman arch bridges were semicircular, although some were segmental. Generally, in any Roman bridge all the voussoirs were the same in size and shape. The Romans built both single spans and lengthy multiple arch aqueducts, such as the Pont du Gard and Segovia Aqueduct. Trajan's bridge over the Danube featured open-spandrel segmental arches made of wood (standing on concrete piers), making it the longest arch bridge for a thousand years both in terms of overall and individual span length.
In China, the oldest existing bridge is the Zhaozhou Bridge of 605 AD (although bridges were built since the ancient Zhou Dynasty), which combined a very low span-to-rise ratio with the use of weight-relieving spandrel arches (buttressed with iron brackets). The Zhaozhou Bridge is the world's first wholly-stone open-spandrel segmental arch bridge.
In medieval Europe, bridge builders improved on the Roman structures by using narrower piers, thinner arch barrels and lower span-rise ratios on bridges. Gothic pointed arches were also introduced, reducing lateral thrust, and spans increased. Bridge building reached new heights with constructions such as the Florentine Ponte Vecchio (1345), featuring an unprecedented span-to-rise ratio of over 6 to 1; the bridge at Trezzo sull'Adda, destroyed in the 15th century, with a world record span length of 72m, or the Renaissance Ponte Santa Trinità (1569), constituting the oldest elliptic arch bridge worldwide.
In more modern times, stone and brick arches continued to be built by many civil engineers, including Thomas Telford, Isambard Kingdom Brunel and John Rennie. A key pioneer was Jean-Rodolphe Perronet, who used much narrower piers, revised calculation methods and exceptionally low span-to-rise ratios. Different materials, such as cast iron, steel and concrete have been increasingly utilised in the construction of arch bridges.
[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. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary falsework frame, known as a centering. 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 material (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.
Traditional masonry arches are generally durable, and somewhat resistant to settlement or undermining. However, relative to modern alternatives, such bridges are very heavy, requiring extensive foundations. They are also expensive to build wherever labour costs are high.
[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, a point known as the springing.
- Falsework centering is fabricated, typically from timbers and boards. Since each arch of a multi-arch bridge will impose a thrust upon its neighbors, it is necessary either that all arches of the bridge be raised at the same time, or that very wide piers are used. 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 over the centering. Once the basic arch barrel 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, known as the spandrels, 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 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 centering may be erected to support the forms, reinforcing steel, and uncured concrete. When the concrete is sufficiently set the forms and falseworks are then removed. It is also possible to construct a reinforced concrete arch from precast concrete, where the arch is built in two halves which are then leaned against each other.
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] See also
- List of arch bridges by length
- Truss arch bridge
- Compression arch suspended-deck bridge
- Tied arch bridge
