ABSTRACT
According to multi directional effects of earthquakes, box sections experience biaxial bending in the presence of axial forces. The resultant vector of biaxial moment usually has considerable inclination with respect to the column principal axes. In the interaction equations of AISC for combined flexure and axial forces, biaxial flexure of box-column are considered as separate projected moment about the box section principal axes, although the design based upon a single resultant moment is permitted for biaxial flexure of round HSS (AISC 2000). It should be noted that, while a section have same modulus about any axis, the AISC interaction equation lead to inconsistent results when compared with design for resultant moment (AISC 2000).
