Effective Length Factor for Columns in Unbraced Frames

The present AISC (American Institute of Steel Construction), ACI (American Concrete Institute), and CAN‐S16.1‐M84 Specifications use alignment charts conveniently to determine effective length factor for columns in both braced and unbraced frames. In this method, the effective length factor K is determined directly by the relative joint bending stiffness ratio G's at each end of the column under consideration. For the development of alignment charts, the far ends of columns above and below the column being designed are assumed to be rigidly connected. In engineering practice, however, some columns may not be rigidly jointed. For an unbraced frame, the present chart always provides an unconservative result when the far end conditions of columns above or below the column being considered are different from the usual alignment chart case. The general equation for the effective length factor K for columns in unbraced frames is derived in the first part of this paper. Extensive numerical studies arecmade using the equation developed. Against the background of this information, a modified alignment chart procedure for the determination of a more accurate ^‐factor for the design of columns in unbraced frames is then proposed. The proposed approach is illustrated by several numerical examples. Similar development for columns in braced frames has been reported previously by the authors.