What is a Bridge?


A bridge is a construction used to cross a valley, road, body of water or other physical barrier. It grants passage over obstacles. Designs of bridges differ based on the function of the bridge and the nature of the land where the bridge is built. A bridge is built for trains, pedestrians or road traffic. A road-rail bridge can carry both road and rail traffic. Let's take a look at some of the different types of bridges.



The first bridges were created by nature and were as simple as logs fallen across streams. Humans made the first bridge with spans of wooden logs, planks and stones using a simple hold up and crossbeam arrangement.



The earliest bridges were simple structures and were built by spanning a gap with timber or rope. As the builders adopted new construction techniques and invented better materials, designs became more multifaceted. The stone arch was the first main advance in bridge design. Arch design was further perfected by making use of arches to build huge stone bridges all over the Roman Empire. They remained in style until the early 19th Century. After that designers started researching a wide variety of bridge types and materials to deal with the demand for greater heights, spans and strengths.



This type of bridge consists of a horizontal beam which is supported at every end by jetties. It is considered to be the simplest type of bridge. Modern beam bridges are large box steel girder bridges. One of the best examples of this type of bridge is the 647ft-long Poplar Street Bridge across the Mississippi, USA, which was built in 1967.



These bridges are suspended from cables. The earliest of them were made up of ropes or vines that were covered with pieces of bamboo. Modern suspension bridges have cables hanging from towers connected to the foundation. The best example of this kind of bridge is the Akashi Kaikyo Bridge in Japan which is 12,826ft long.



This is a bridge built using cantilevers - structures that project horizontally into space. For small footbridges, the cantilevers may be simple beams, however large cantilever bridges designed to handle road or rail traffic use frames built from structural steel. The Quebec Bridge in Canada is the largest cantilever bridge and has an overall length of 3,239ft and a width of 94ft.



Similar to suspension bridges, cable-stayed bridges too are held up by cables. These bridges require less cable and the towers holding the cables are comparably shorter. C T Loescher designed the first well-known cable-stayed bridge in 1784. The Sutong Bridge over the Yangtze River in China is the longest cablestayed bridge.



They are arch-shaped and have abutments (the point where two structures meet) at the ends. Greeks constructed the first arch bridges including the Arkadiko Bridge. Presently Dubai is constructing the Sheikh Rashid bin Saeed Crossing, listed for completion in 2012. It will be the worldʼs longest arch bridge.



This kind of a bridge is made of connected elements. Truss bridges have a hard deck and a web of pin-jointed or gusset-jointed iron beams for the sides. The earliest truss bridges were composed of wood and later ones were made of wood with iron tensile rods. The Bollman Truss Railroad Bridge at Savage, Maryland, USA is the only surviving example of a revolutionary design in the history of bridge engineering.