Capacity of rectangular cross sections under biaxially eccentric loads

This paper deals with the calculation of design interaction diagrams for rectangular reinforced concrete cross sections subjected to combined axial load and biaxial bending. The developed procedure is based on an analytical approximation of the moment contours, which represent horizontal sections of the failure surface for assigned axial loads. The analytical approximation is obtained through a function of the type suggested by Bresler, in which, however the power exponent is not fixed but calculated on the basis of the interaction diagrams for load eccentricities along axes parallel to the axes of symmetry and to a diagonal of the rectangular cross section. Calculation of these interaction diagrams is made easy by the equivalence between their dimensionless expressions and the interaction diagrams of an equivalent square cross section of unit-length side. Analytical proof of this equivalence is provided. The method is checked to be very accurate and conservative by comparison with interaction diagrams obtained through direct numerical integration. Dimensionless analytical expressions for the interaction diagrams calculated according to the ACI Code specifications are also reported.