This paper presents effects of parameters, which are the input earthquake direction, hysteresis characteristics of steel members, actual strength of steel material, the composite action of floor slab, and the additional axial force of columns, on plastic deformation of steel members. Seismic responses of steel structures against bi-directional earthquake are obtained by using the proposed plane-frame analytical model. As a result, the following conclusions are reached; (1) strength of columns are larger than that of beams and panel in order to contain damage of the columns, (2) damage of the steel members with the actual strength decreases from the nominal strength, and (3) effects of the composite action of floor slab and the additional axial force on damage of steel members are quite little for object frames.
