New approach to AISC PP–MM interaction curve for square concrete filled tube (CFT) beam–columns

This paper presents a new method to design concrete filled tube (CFT) beam–columns based on the current American Institute of Steel Construction’s Load and Resistance Factor Design (AISC/LRFD) method that is known to provide an over-conservative estimation of strength. The over-conservativeness of the current AISC/LRFD method comes mainly from neglecting the contribution of the concrete in flexure and the different behavior of composite beam–columns from pure steel beam–columns. The new method reported herein assumes full composite action and idealizes the PP–MM interaction curve with two lines, as in the current AISC method; however, the lines intersect at a point where a maximum moment occurs. A complete parametric study is performed using fiber analysis to determine the maximum moment (normalized by the nominal moment strength) and the axial load ratio at the maximum moment with respect to tube width-to-thickness ratio (b/tb/t) and relative concrete compressive strength to yield strength of the steel tube (fc′/Fy). The strengths based on the modified AISC PP–MM interaction equations for square CFT columns subjected to axial load and single axis bending are compared with a wide range of experimental data, and they show greatly improved results when compared with current AISC/LRFD design equations.