Abstract The nonlinear behavior of reinforced concrete frames with sidesway is governed by two effects: first, the nonlinearity of materials due to cracking and the plastic behavior of materials, and second, the nonlinearity of geometry caused by the second‐order deformation. These two effects may interact, and the whole phenomenon is known as the nonlinearity of geometry and materials. Reinforced concrete frames are frequently used to resist wind or earthquake forces. These forces will accentuate the complexity of the frame behavior because of the continuous change of the shape of the bending moment diagram. The change of moment diagram will in turn affect the magnitude of the cumulative plastic rotations. A trial and error method was devised to predict theoretically the moment‐load curves of reinforced concrete frames throughout the loading history. This method utilized the trilinear moment‐curvature curve for critical sections suggested by Macchi [8], as well as the principle of virtual work to express...