Towards a Rational Theory for RC Members in Shear

The diagonal compression field theory for symmetrically reinforced concrete beams monotonically loaded in shear at sections where flexure is unimportant, is presented. This theory assumes that after cracking the concrete can resist no tension and that the shear will be carried by a field of diagonal compression. As well as considering the equilibrium conditions that relate the average stresses in a beam to the applied loads the theory considers the compatibility conditions that exist between the average strains in the various directions. Expressions are given which enable the shear deformations (and thus the effective shear modulus) of diagonally cracked reinforced and prestressed beams to be calculated. Based on a suggested failure criterion for diagonally cracked concrete, equations for the shear capacity of reinforced and prestressed beams are derived. A design chart based on these equations is given.