FUNDAMENTAL ON DESIGN OF SHEAR REINFORCEMENT.

Shear in a beam is caused by the variation of bending moment along the span .A beam when loaded have different stresses at adjacent sections in the same fibres because of the different bending moments.The variation of the stress intensity on each face of the portion,because unequal stresses in the same fibre ,there is tendency in each fibre to slide over adjacent fibre, which is prevented by the horizontal shear stress and these horizontal shear stresses five rise to vertical stresses .The variation is parabolic with maximum ordinate at N.A .,one and half time the value of average shear stress .As the concrete is very weak in tension when unreinforced , and cracks develop when tensile limit of concrete is exceeded .In order to avoid failure, it is necessary to reinforce the section . The purpose of providing the reinfocement is not to prevent cracking as that is not possible, but to prevent widening of the cracks and thus to avoid failure .To avoid failure of the beam against diagonal tension it is reinfoced with requisite amount of steel .The reinforcement of diagonal tension is provided in two forms ,vertical stirrups looped about the main reinforcement and bent up bars inclined at an angle across the region of diagonal tension .In the design of diagonal tension reinforcement assumption is made that diagonal tension is inclined at 45 degree and is equal to shear at the section .The use of inclined bars as shear reinfocement to form stirrup system at pitch is not always practicable, it is used in case of the large beams , bridge girders having considerable amount of tension reinforcement that sufficient bars can be bent up those enough to form a stirrup system . Generally a few bars are bent up near the support .The bars are generally bent up in pairs .The action of one pair of bars bent up to prevent cracking,the vertical component of the force in the bar resists the shear force.