A modified AISC P-M interaction curve for square concrete filled tube beam-columns

Concrete filled tube (CFT) columns have been in the spotlight over the world for the last decades due to their excellent performance. Although CFT members have many advantages over other structural systems, they have not been widely used in the U.S., mainly because of the lack of knowledge about the system. The estimation for CFT beam-columns based on the current AISC design method provides too conservative results. The over-conservative estimation of the current AISC method is mainly due to the different behavior of the pure steel columns and the composite columns. Considering the contribution of the concrete, a method to simulate the strength of the CFT beam-columns with much improved accuracy is presented. The method idealizes the P-M interaction curve of CFT beam-columns with two lines intersecting at the maximum moment assuming a full composite action. As a first step to modify the current AISC design equations for composite members, square CFT members subjected to single axis bending are selected and studied. A simple equation to estimate the nominal moment strength at no axial load based on full composite action is presented. To analytically determine the moment strength of square CFT beam-columns for wide range of parameters, a parametric study is performed using fiber analysis which has been acknowledged as an excellent tool for estimating the sectional strength of CFT members. The axial load at the maximum moment is also evaluated from the fiber analysis. The maximum moment and the axial load at the maximum moment are normalized with the nominal moment strength at no axial load and the nominal axial strength, respectively. Then the normalized values are expressed with respect to the width-to-thickness ratio and the relative strength of the concrete to the yield strength of the steel. By embedding the relations of the normalized values and the main parameters into the current AISC equation, modified AISC beam-column design equations are proposed. It is observed that the modified equations provide a greatly improved agreement with the experimental results found in the literature.