Effective flexural stiffness of slender structural concrete columns

The CSA A23.3 standard permits the use of a moment-magnifier approach for the design of slender reinforced concrete and 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 CSA standard are approximate when compared with the EI values computed from the axial load - bending moment - curvature relationships. This study was conducted to determine the influence of a full range of variables on EI used for the design of slender reinforced concrete and composite steel-concrete columns, and also to examine the existing CSA EI equations. Over 27 000 isolated concrete columns, each with a different combination of specified variables, in symmetrical single-curvature bending were simulated to generate the stiffness data. Two new design equations to compute EI of structural concrete columns were then developed from the simulated stiffness data and are proposed as an alternative to the existing CSA design equations for EI.