Stone Arch

An 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, and partially into a vertical load on the arch supports. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

History

Possibly the oldest existing arch bridge is the Mycenaean Arkadiko Bridge in Greece from about 1300 BC. The stone corbel arch bridge is still used by the local populace. The well-preserved Hellenistic Eleutherna Bridge has a triangular corbel arch. The 4th century BC Rhodes Footbridge rests on an early voussoir arch.

Although true arches were already known by the Etruscans and ancient Greeks, the Romans were – as with the vault and the dome – the first to fully realize the potential of arches for bridge construction. A list of Roman bridges compiled by the engineer Colin O'Connor features 330 Roman stone bridges for traffic, 34 Roman timber bridges and 54 Roman aqueduct bridges, a substantial part still standing and even used to carry vehicles. A more complete survey by the Italian scholar Vittorio Galliazzo found 931 Roman bridges, mostly of stone, in as many as 26 countries (including former Yugoslavia).

Roman arch bridges were usually semicircular, although a number were segmental arch bridges (such as Alconétar Bridge), a bridge which has a curved arch that is less than a semicircle. The advantages of the segmental arch bridge were that it allowed great amounts of flood water to pass under it, which would prevent the bridge from being swept away during floods and the bridge itself could be more lightweight. Generally, Roman bridges featured wedge-shaped primary arch stones (voussoirs) of 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. Their bridges featured from an early time onwards flood openings in the piers, e.g. in the Pons Fabricius in Rome (62 BC), one of the world's oldest major bridges still standing.

Roman engineers were the first and until the Industrial Revolution the only ones to construct bridges with concrete, which they called Opus caementicium. The outside was usually covered with brick or ashlar, as in the Alcántara Bridge.

The Romans also introduced segmental arch bridges into bridge construction. The Limyra Bridge in southwestern Turkey features 26 segmental arches with an average span-to-rise ratio of 5.3:1, giving the bridge an unusually flat profile unsurpassed for more than a millennium. Trajan's bridge over the Danube featured open-spandrel segmental arches made of wood (standing on concrete piers). This was to be the longest arch bridge for a thousand years both in terms of overall and individual span length, while the longest extant Roman bridge is the long Puente Romano at Mérida. The late Roman Karamagara Bridge in Cappadocia may represent the earliest surviving bridge featuring a pointed arch.

In medieval Europe, bridge builders improved on the Roman structures by using narrower piers, thinner arch barrels and higher span-to-rise ratios on bridges. Gothic pointed arches were also introduced, reducing lateral thrust, and spans increased as with the eccentric Puente del Diablo (1282). With more advanced design and bridge-building techniques, the alternative informal name of Devil's Bridge became more widely used across Europe, because many people could not believe that these were man-made and capable of carrying the size of loads that they did.

The 14th century in particular saw bridge building reaching new heights. Span lengths of , previously unheard of in the history of masonry arch construction, were now reached in places as diverse as Spain ( Puente de San Martín), Italy (Castelvecchio Bridge) and France (Devil's bridge and Pont Grand) and with arch types as different as semi-circular, pointed and segmental arches. The bridge at Trezzo sull'Adda, destroyed in the 15th century, even featured a span length of , not matched until 1796.

Constructions such as the acclaimed Florentine segmental arch bridge Ponte Vecchio (1345) combined sound engineering (span-to-rise ratio of over 5.3 to 1) with aesthetical appeal. The three elegant arches of the Renaissance Ponte Santa Trinita (1569) constitute the oldest elliptic arch bridge worldwide. Such low rising structures required massive abutments, which at the Venetian Rialto Bridge (1591) and the Pegnitz or Fleischbrücke (1598) in Nuremberg (span-to-rise ratio 6.4:1) were founded on thousands of wooden piles, partly rammed obliquely into the grounds to counteract more effectively the lateral thrust.

In China, the oldest extant arch bridge is the Zhaozhou Bridge of 605 CE, which combined a very low span-to-rise ratio of 5.2:1, with the use of spandrel arches (buttressed with iron brackets). The Zhaozhou Bridge, with a length of and span of , is the world's first wholly stone open-spandrel segmental arch bridge, allowing a greater passage for flood waters.Needham, Joseph. ''The Shorter Science and Civilisation in China''. Cambridge University Press, 1994. . pp. 145–147. Bridges with perforated spandrels can be found worldwide, such as the Bridge of Arta, (17th century) Greece and Cenarth Bridge, (18th century) in Wales.

With the coming of the Industrial Revolution, in the 18th and 19th centuries, stone and brick arches continued to be built by many prominent British civil engineers, including Thomas Telford, John Rennie, and latterly Isambard Kingdom Brunel. They also started the modern usage of different materials, such as cast iron — Telford designed the first bridge built of metal, completed in 1781, the Iron Bridge with a single arch of sections of cast iron constructed in traditional woodworking techniques — and then steel and concrete, which have been increasingly used in the construction of arch bridges, to almost the exclusion of other materials. A key pioneer was Jean-Rodolphe Perronet, who used much narrower piers, revised calculation methods, and exceptionally low span-to-rise ratios.

Simple compression arch bridges

Advantages of simple materials

Stone, brick 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 centring. 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 labor costs are high.

Construction sequence

* Where the arches are founded in a watercourse bed (on piers or banks) the water is diverted so the gravel can first be excavated and replaced with a good footing (of strong material). From these, the foundation piers are erected/raised to the height of the intended base of the arches, a point known as the springing.
* Falsework centering (in British English: arch frame) 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 be used. The thrust from the end arches is taken into the earth by substantial (vertical) footings at the canyon walls, or by large inclined planes forming in a sense ramps to the bridge, which may also be formed of arches.
* The several arches are (or single arch is) constructed over the centering. Once each basic arch barrel is constructed, the arches are (or arch is) stabilized with infill masonry above, which may be laid in horizontal running bond courses (layers). These may form two outer walls, known as the spandrels, which are then infilled with appropriate loose material and rubble.
* The road is paved and parapet walls protectively confine traffic to the bridge.

Types of arch bridge

Corbel arch bridge

File:arkadiko2.jpg, Corbel arch built from Cyclopean masonry, in the Greek Arkadiko bridge
File:Eleutherna Bridge, Crete, Greece. Pic 03.jpg, Corbel arch in the shape of an isosceles triangle, supporting the Greek Eleutherna Bridge
File:Corbelledarch.png, A corbelled arch with the masonry untrimmed
File:Trabeate Arch in New Delhi India.jpg, A corbel arch with the masonry cut into an arch shape

The corbel arch bridge is a masonry, or stone, bridge where each successively higher course (layer) cantilevers slightly more than the previous course. The steps of the masonry may be trimmed to make the arch have a rounded shape. The corbel arch does not produce thrust, or outward pressure at the bottom of the arch, and is not considered a true arch. It is more stable than a true arch because it does not have this thrust. The disadvantage is that this type of arch is not suitable for large spans.

Aqueducts

Aqueducts