Equivalent Uniform Moment Diagram Factor for Composite Columns in Major Axis Bending

The ACI Building Code permits the use of the equivalent uniform bending moment diagram factor (Cm) for computing the effect of moment gradient, along the column height, caused by unequal column end moments. The concept of equivalent uniform moment diagram was introduced into design practice to eliminate the need for extensive calculations based on the solution to a differential equation. The expression currently used by the ACI Building Code is a simplified equation based on the elastic behavior of columns, and does not include the inelastic material behavior. This study was conducted to investigate the influence of different variables on Cm of slender, tied, rectangular composite columns in which steel shapes are encased in concrete and to examine existing expressions for Cm. Approximately, 11,000 square composite columns, each with a different combination of specified values of variables, were simulated. The columns studied were subjected to short-term ultimate loads and unequal end moments causing moment gradient in single curvature and double curvature bending about the major axis of the encased steel section. Two design Cm equations are proposed in this paper.