Story-Based Effective Length Factors for Unbraced PR Frames

This paper proposes a practical method to evaluate the effective length factor K for compressive members in unbraced partially-restrained frames under elastic buckling. In light of story-based buckling and the introduction of the end-fixity factor to characterize the beam-to-column connections of partially-restrained frames, the lateral stiffness of columns with consideration of the effect of axial load is derived. The formulations and procedure of calculating story-based column effective length factors are presented. Numerical examples are then presented to illustrate the effectiveness of the proposed procedure. With the adoption of the end-fixity factor, different member end rotational conditions can be readily modeled by derived formulations. Therefore, the proposed approach is comprehensive and can be applied for both unbraced partially and fully restrained frames.