Design of RC Sections with Generic Shape under Biaxial Bending

A design method for the reinforcement-steel area in a concrete section with a generic shape is proposed. The section is subjected to axial load and biaxial bending. The materials are assumed to be nonlienar elastic. The method is based on the failure condition prescribed by the ACI Building code (maximum compressive strain in concrete ${\phi }_{\mathrm{cf}}=0.003$). It permits finding the amount of reinforcement and the location of the neutral axis in a rapid manner, avoiding the usual trial-and-error procedures based on the failure of surfaces and strength-interaction diagrams. The solution is obtained by resolving a system of nonlinear equations by means of the Newton-Raphson method. The method was run using a computer program, and provided fast convergence in all the many tests carried out. Computational effort is not considerable, and a personal computer can be used for the design. A numerical example is reported.