This paper describes an experimental study to examine the behavior of steel beam-columns subjected to sidesway. A total 42 beam-columns are tested under a monotonic loading conditions; with the axial force, slenderness, geometrical, and material properities as major test variables. The results, maximum strength, deflection at critical points, and the interaction curves with the Axial force ratio versus moments(1st, 2nd order), are presented. Also, In this paper, the effectiveness of strength provisions stipulated by the Japan Plastic Design Code. AISC-Specification, and linear interaction equations of many researchers are calibrated against the data obtained. But the above beam-columns design formula may result in an overly conservative design for double curvature bending and the P-${Delta}$ effects by sidesway is taken into account in an indirect manner by incorporating two modifications(K-factor & $C_m$), into the beam-columns formulas. In this paper, a design interaction equation is derived; taking the P-${Delta}$ effects from sidesway into account, in a direct manner for unbraced steel beam-columns. This derived design equation was made and then compared with the results of 42 full scale beam-columns experiments conducted by the author.