This paper presents a new theory of failure for materials subjected to a combined state of stress. The theory is developed for brittle materials, such as concrete, in which the compressive and tensile strengths are of different magnitudes. It is a semi‐empirical theory which has some experimental support. Based on it working stresses are defined for various loading conditions. The use of these proposed working stresses in some cases makes considerable difference in design as compared to the present specifications requirements. These working stress values are conveniently represented by a diagram to facilitate the computations in design. The application of this information can be made simply in most cases where there are stresses acting in two right‐angled directions—such as slabs in buildings, roads and bridges, retaining walls, dams and other hydraulic structures of concrete and reinforced concrete.