The effects of the horizontal stiffness of a bracing frame on the effective length factors of a long columns in atrium are examined. Evaluation methods for the horizontal stiffness are presented, and the calculated effective lengths are compared with that obtained from a spring model. It is shown that 1) the effective length factor approaches that of the non-sway buckling mode as the horizontal stiffness becomes large, and then, 2) the effective length factor from G factor method is quite large and 3) an evaluation formulas of a flexural stiffness ratio for assuring a specific effective length factor are presented.
