The horizontal stiffness method (the HS method) is proposed as an accurate approach to evaluate the effective length factor of columns when a frame with long columns buckles in a sidesway mode, utilizing the horizontal stiffness derived by the first-order analysis. A parametric study was conducted to investigate the accuracy of the method. Based on the numerical results, the HS method can evaluate the effective length factor of a long column in a frame with an accuracy of 5~6%. The Ai distribution (the Japanese design shear force distribution) is recommended for the story shear force in the first-order analysis.
