The Institution of Civil Engineers of Great Britain (ICE) makes reference to four principal bridge forms. These are the beam bridge, the suspension bridge, the arch bridge and the truss bridge. With the help of modern technology, bridge designs and construction methods have expanded. Bridge construction projects that were once considered impossible, such as the Akashi Kaikyo Bridge, which has a main span of 1.23 miles, are now proving viable.
A beam bridge is the simplest and cheapest bridge to construct, consisting of a horizontal beam across two vertical piers. However, beam bridges are only practical over short distances, or where there will not be a great load placed upon the beam. The construction of a beam bridge usually begins with digging out holes for the piers. Sinking the piers in the ground gives the structure added strength. The beam is then placed across and secured to the piers.
Born in forests and jungles, the suspension bridge was originally made from vine and creeper. A modern suspension bridge is one where the deck, or roadway, hangs on cables between towers. Sitting on massive, reinforced concrete slabs, the hollow concrete towers of England's Humber Bridge, once the world's longest single span suspension bridge, were built with 48 hydraulic jacks. One of the anchorage points for the steel cables is over 38 yards deep. A box girder super structure was used for the roadway.
One of man's greatest discoveries, the arch bridge is a miracle of engineering, since it can be designed so that no part of it needs to endure tension, according to Joseph F. MacDonnell, S.J., former Professor of Mathematics at Fairfield University. The strength of the arch lies in its ability to spread the load along its curves towards its supports. MacDonnell claims that using reinforced concrete to build an arch bridge can result in lower construction costs.
A truss bridge has a deck, or roadway, supported by a truss, which is a structure of struts and bracing. Triangles are often used in the truss design, as they convey strength to the structure. North American bridge builders constructed many truss bridges in the 19th century to accommodate the expanding railway network. Truss bridges can be made of wood or metal, although there are several examples of older wooden truss bridges collapsing as trains passed over them.
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