Improved stiffness distribution factors for evaluation of effective buckling lengths in multi-story sway frames

The objective of this paper is to propose improved expressions for the effective buckling length coefficient of columns in multi-story sway frames. To that effect, a complete set of rotational stiffness coefficients are derived, which are then used for the replacement of members converging at the bottom and top ends of the column in question by equivalent springs. All possible rotational and translational boundary conditions at the far end of these members, as well as the eventual presence of axial force, are considered. An analytical expression and a corresponding graph similar to those used by Eurocode 3 are then proposed for the calculation of the effective buckling length coefficient. Examples of three-story sway frames are presented, where the proposed approach is found to be in excellent agreement with finite element results, while applications of Eurocode 3 and LRFD lead to significant inaccuracies.