Nonlinear Equation for Flexural Stiffness of Slender Composite Columns in Major Axis Bending

The ACI standard 318-02 permits the use of a moment magnifier approach for the design of slender composite steel-concrete columns. This approach is strongly influenced by the effective flexural stiffness (EI), which varies due to the nonlinearity of the concrete stress-strain curve and the cracking along the column length among other factors. The EI equations given in the ACI code are approximate when compared to the EI values computed from the axial load-bending moment-curvature relationships. This study was undertaken to determine the influence of a full range of variables on EI used for the design of slender, tied, composite columns in which steel shapes are encased in concrete, and also to examine the existing ACI EI equations. Approximately 12,000 isolated square composite columns, each with a different combination of specified properties of variables, were simulated and used to generate the stiffness data. The columns studied were subjected to short-term ultimate loads and equal and opposite end moments causing symmetrical single curvature bending about the major axis of the encased steel section. A new nonlinear equation for EI was then developed for use in design of slender composite columns subjected to major axis bending and is proposed as an alternative to the existing ACI EI equations.