The response of reinforced concrete (RC) elements subjected to in-plane shear and axial stresses is governed by various potential failure mechanisms. For structural panels subjected to uniform stress states, their ultimate strength can be governed by yielding of the reinforcement, crushing of the concrete struts, sliding along a failure plane or spalling of the concrete cover. Although some expressions have been proposed in the literature to address this issue, they normally are empirical or semi-empirical in nature and do not distinguish among the failure mechanisms and their interaction. The purpose of this paper is to derive a mechanically-based efficiency factor for concrete in compression that is able to predict the actual behaviour of structural cracked concrete. This approach is shown to suitably predict the various potential failure modes and the development of the secondary shear cracks as well as the interaction between the reinforcement and the concrete.