The current AISC Specification (AISC 360-10) specifies the design equations for estimating the axial strength of concrete-filled steel tube (CFT) columns subjected to ambient temperature. However, these equations are over-conservative for noncompact and slender CFT columns, and the effect of elevated temperature on the axial strength is not included. To address these, the authors conducted a two-stage research. In the first stage, the AISC design equations are modified to better predict the axial strength of noncompact and slender CFT columns under ambient temperature, using the effective steel and concrete stress-strain relationships developed and verified by the authors in a previous research. These effective stress-strain relationships implicitly account for the effects of geometric imperfections, steel tube local buckling, steel hoop stresses and concrete confinement. In the second stage, the modified design equations are further improved to include the effects of elevated temperature, using benchmarked heat transfer analysis model and nonlinear inelastic column buckling analysis (NICB) model.
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