Abstract This paper discusses applying a nonlinear analytical model to reinforced concrete frames for computing its response to lateral loading. The nonlinear analysis takes into effect the material and geometric nonlinearities. From lateral loading and lateral drift, lateral stiffness can be obtained. Comparisons between nonlinear lateral stiffness and linear lateral stiffness based on equivalent rigidity EI values suggested in the ACI-318 Code [Building code requirements for reinforced concrete (ACI-318M-89). American Concrete Institute, Detroit, MI (1989)] are given. The nonlinear analysis also gives the rigidity values along the members of the frames. Finally, the paper assesses the accuracy of the approximate formula for calculating lateral story stiffness given by Cheong-Siat-Moy.