Abstract: MDOF system is equivalent to SDOF system with bilinear force-deformation relations, representing the first mode of structures. It is assumed that yield displacement is the function of the geometry of components and the yield strain of the reinforcing steel, and that structural stiffness is dependent on strength. Overstrength factors of reinforced concrete frames are studied under different design parameters based on seismic design code. The practical strength is calculated through design strength multiplied by the overtstrength factor. The effective period of equivalent SDOF is determined by yield displacement and practical strength. When the effective period is longer than site character period, then the maximum inelastic deformation is determined by equal displacement approximation. When the effective period is shorter than site character period, then the maximum inelastic deformation is determined by equal energy approximation. This method avoids inelastic dynamic analysis, so it pushes over the analytical speed, and there is no necessity for iterations. Also, it is specially employed by quick evaluation of existing buildings