Abstract This paper presents a model for determining the shear capacity of RC members without shear reinforcement. The model is a modification of the so-called Crack Sliding Model (CSM) and is developed on the basis of the upper bound theorem of the theory of plasticity. The modification includes the derivation of a new expression for the sliding resistance of cracked concrete and a new expression for the contribution from dowel action. The derivation of the sliding resistance of cracked concrete is established using the theory of plasticity and rests upon the influence of crack width and maximum aggregate size. The modified model enables a prediction of the shear capacity, critical crack width, and the location of the critical diagonal crack. The proposed model is compared to measured crack kinematics of selected tests from the literature and two large shear databases for non-slender and slender beams. The model shows a satisfactory agreement with tested capacities, even for shear span-to-depth ratios less than a / d 2.5 .