How long have bridges been around

The first man-made bridges were tree trunks laid across streams in girder fashion, flat stones, and festoons of vegetation, twisted or braided and hung in suspension. These three types - beam, arch, and suspension - have been known and built since ancient times and are the origins from which engineers and builders derived various combinations such as the truss, cantilever, cable-stayed, tied-arch, and moveable spans Bridges of twisted vines and creepers were found in many parts of India.

Wooden bridges are some of the most ancient. Suspension bridges had been known in China as early as BC. Chinese built big bridges of wooden construction, and later stone bridges, and the oldest surviving stone bridge in China is the Zhaozhou Bridge built around AD during the Sui Dynasty. This bridge is also historically significant as it is the world's oldest open- stone segmental arch bridge.

The ancient Romans were the greatest bridge builders of antiquity. They used cement, - called pozzolana consisting of water, lime, sand, and volcanic rock, which reduced the variation of strength found in natural stone. Though extremely versatile, wood has one obvious disadvantage and during the 18th century there were many innovations in the design and a major breakthrough in bridge technology came with the erection of the Iron Bridge in Coalbrookdale in England during , using cast iron for the first time as arches to cross the river Severn.

With the Industrial Revolution, steel, which has a high tensile strength, replaced wrought iron for the construction of larger bridges to support large loads, and later welded structural bridges of various designs were constructed.

Bridges are classified as Beam bridges, Cantilever bridges, Arch bridges, Suspension bridges, Cable stayed bridges and Truss bridges. Beam Bridge: A horizontal beam supported at its ends comprises the structure of a beam bridge. The construction of a beam bridge is the simplest of all the types of bridges. Cantilever bridges are built using cantilevers—horizontal beams that are supported on only one end.

Most cantilever bridges use two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. The Arch Bridge is arch-shaped and has supports at both its ends. The weight of an arch-shaped bridge is forced into the supports at either end. The suspension bridge is suspended from cables. The suspension cables are anchored at each end of the bridge. The load that the bridge bears converts into the tension in the cables.

The Cable-stayed Bridges like suspension bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The longest cable-stayed bridge is the Sutong bridge over the Yangtze River in China. In walled cities, where accomodation is strictly limited, any firm foundation for a building is valuable; and with water mills now a common source of power, a bridge with a mill upon it serves two useful purposes.

Inhabited bridges are built in considerable numbers. France is known to have had as many as thirty-five. The most famous bridge with houses is also one of the earliest and the longest lasting. London Bridge is built between and , with the work apparently entrusted to Peter, chaplain of St Mary Colechurch. His task is formidable. This is the world's first stone bridge to be constructed in a tidal waterway, with a large rise and fall of level every twelve hours.

The stone foundations of the nineteen pointed arches are placed within timber cofferdams, in the technique pioneered by the Romans.

Water surges between them like a mill race - with the useful side effect of keeping the water level artificially high upstream of the bridge. Old London Bridge, with its tall and picturesque rows of houses and shops, lasts for more than six centuries until finally replaced in Of surviving medieval bridges with houses on, the Ponte Vecchio in Florence is probably the best known.

When built in it replaces one on the same site also known as the Ponte Vecchio, so the present bridge is first known as the Nuovo Ponte Vecchio 'new old bridge'. The covered way which forms a top storey above the shops is added in to enable the Medici to walk from the Uffizi to the Palazzo Pitti on the other side of the river without descending to street level.

In the space of a few months in the world's first iron bridge, with a single span of over feet, is erected for Abraham Darby the third of that name over the Severn just downstream from Coalbrookdale. It is also it the world's oldest stone segmental arch bridge built with open spandrels.

Between 12th and 16th century many bridges were built with houses on them. They were solution for limited accommodation in walled cities and only France had as many as Inca civilization used rope bridges, a simple type of suspension bridge, in the 16th century.

Hans Ulrich, Johannes Grubenmann, and others improved bridge-building in the 18th century. At the same time Hubert Gautier wrote a book on bridge engineering. The Iron Bridge, built in Coalbrookdale, England in , was one of the engineering marvels of the time because it used cast iron for the first time. Industrial Revolution in the 19th century brings truss systems of wrought iron an iron alloy with a very low carbon but it did not have the tensile strength to carry the large weights.

Enters steel, with its higher tensile strength, which replaces the iron and allows for much larger bridges.