SELECTION AND USE OF THE REINFORCED CONCRETE BRIDGES AND THE ECONOMIC ANALYSIS.

Reinforced concrete is almost universally used for the bridge construction work both for Highways and the other types of road and the railway works , It's durability, rigidity, economy and ease with which good appearance can be obtained which makes it suitable for this purpose Different type of the RC bridges in common use are ,Solid slab bridges , Simple girder bridges , Continuous girder bridges, Cantilever bridges , Rigid frame bridges both single Span and the multi span ,Arch bridges ,Long trusses and Bow string girder bridges. Reinforced concrete bridges are generally used for highway bridges and their use for rail -road bridge is limited however for short span rail -road bridges reinforced concrete may be preferred .As the cost is generally the governing factor for the selection of the type of the bridge.The exact arrangement economically suitable for a particular case depends upon the natural conditions ie foundation and the specified requirements like width of the bridge , loading, appearance,water way ,time limit for construction, difficulties in form work etc etc.Experience is invaluable in deciding the type of bridge .The bridges having one span following arrangement should be considered, Simply supported deck , Right angle rigid frame, Right angle frame with concealed cantilever and simply supported girder with concealed cantilever.As for as the simply supported deck type is statically determined and is too simple for the design purposes however the cost of this structure is highest.When unyielding foundations are available rigid frame type is most economical and for long spans right angle rigid frame and simply supported girder with concealed cantilever may be considered Where unyielding foundations are available continuous girder , multiple rigid frames and arches are best suited for construction.Of these types ,cost of the multiple rigid frame is lowest .Where heavy piers are required or where bridge is to be supported on the already existing piers Right angle frame with concealed cantilever may be preferred and used .When the cost of pier is coming equal to the cost of the main girders and cross girders of the one span ,the total cost of the bridge will be the minimum.For the purpose of computing maximum stresses in any girder or member of a bridge the following loads shall ,where applicable,be taken into account:-Dead load ,Live load , Impact effect,Centrifugal forces ,Wind load ,Lateral loads such as nosing effect in the railway bridges and horizontal forces on parapets, longitudinal forces and the seismic forces . In addition to the stresses caused by above loading the following stresses shall also be taken in to account where applicable ie Secondary stresses , Temperature stresses and the Erection stresses .