Effective Length Factor for Columns in Braced Frames

Abstract

The alignment chart used in the present AISC and CAN3‐M84 specifications defines the column effective length factor K in terms of the relative joint bending stiffness ratio Gs at each end of the column. In the development of the alignment chart, the far ends of columns above and below the column being considered are assumed to be rigidly jointed. In practice, some columns are not rigidly connected. A direct use of the alignment chart without any modifications results in an effective length factor that can be either too conservative or even unsafe depending on the boundary conditions of the restraining columns. This paper derives the general effective length factor equations for columns in braced frames, corresponding to five different boundary conditions of top and bottom columns. A simple modification of the present alignment chart for the determination of a more accurate K‐value for the design of columns in braced frames is proposed. The procedure is illustrated by several numerical examples.