Strength of Singly Symmetric I-Shaped Beam-Columns

The AISC-LRFD Specification provides a conservative prediction of the strength of singly symmetric I-shaped beam-columns bent about the axis of symmetry if the compression flange is larger than the tension flange. This paper presents a more economic method that bases the in-plane capacity on the attainment of the full plastic capacity of the cross-section, and the lateral-torsional strength on an inelastic modification of the elastic buckling solution. The proposed derivations are based on essentially the same philosophy as the ones for the doubly symmetric wide-flange design rules in the AISC Specification. However, for singly symmetric shapes it is not possible to arrive at simple approximate empirical interaction equations. The proposed methods are of necessity spreadsheet oriented, and an appendix to this paper provides a sample calculation scheme using the MATHCAD software as the vehicle of calculation.