AbstractSteel-concrete composite columns are strong and efficient structural members. However, their strength is not consistently recognized within design standards. In particular, current provisions regarding the strength of composite columns under combined axial compression and biaxial bending are overly simplistic and conservative. This paper presents a study of the biaxial behavior of square and rectangular, filled and encased composite columns and methods of evaluating their strength. Analyses of composite cross sections confirm the conservative nature of current design provisions and form the basis for modified strength equations that better capture the shape of the three-dimensional interaction surface. Analyses of composite columns confirm that the proposed design equations are suitable for use within the direct analysis method for evaluating members with length effects. The proposed design equations are further validated against published experimental results. This paper provides key insights on the behavior of composite columns subjected to axial compression and biaxial bending and offers an improved yet practical method of evaluating strength under these conditions that will enable more efficient designs.