Arch bridge
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.
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.
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.
 |
The Pont du Gard aqueduct showing the Roman technique of building multiple arch structures atop each other. |
*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 construction 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.
 |
Modern reinforced concrete arch bridges. The bridge in the background is a replacement to span a tributary stream of the Yangtze River (Chang Jiang) in the now flooded Three Gorges, China. |
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.
*
List of arch bridges by length*
Truss arch bridge*
Compression arch suspended-deck bridge*
Tied arch bridge*
NOVA Online - Super Bridge - Arch Bridges*
Matsuo Bridge Co. - Arch Bridges*
Historic Bridges of the Midwest
