Biaxial Bending of Steel Angle Section Beams

The loads applied to angle beams usually act out of the principal planes so that they cause simultaneous biaxial bending about both principal axes. The general practice for designing unbraced beams against biaxial bending is to consider the separate failure modes of either in-plane bending or lateral buckling under bending about the major principal axis, and in-plane bending about the minor principal axis, and then to use an interaction equation to combine the two principal axis strengths. However, the interaction equations provided by many codes for designing against biaxial bending are largely derived from research on doubly symmetric I-section beams, which may be inappropriate for angle section beams, while the lateral buckling rules of these codes appear to be in error for nonuniform bending with loads acting away from the shear center. This paper investigates the biaxial bending of unbraced steel angle beams. The biaxial bending of compact equal angles in uniform bending is considered first, and a simple interaction equation is developed for their design which utilizes recent proposals for lateral buckling design. A corresponding interaction equation is developed for the design of semicompact and slender equal angles. Suggestions are then made for extending these to the biaxial bending of unequal angles under general shear center loading, and finally, a worked example illustrating the method is provided.