Lateral-torsional buckling of stepped beams

Continuous multispan beams with stepped cross sections at the interior supports are common in construction of steel buildings and bridges. Design equations for lateral-torsional buckling (LTB) resistance in the American Institute of Steel Construction LRFD Specifications in 1998 account for only prismatic and web-tapered beams. A study of LTB capacity of stepped beams was performed and simple design equations are proposed. Finite-element method buckling analysis results for stepped beams under uniform moment were used to develop new design equations that account for the change in the cross section of stepped beams. Traditional moment gradient factors for prismatic beams were reviewed and were found to be sufficiently accurate for some stepped beam cases. A new moment gradient factor equation was developed from the finite-element analysis results and is proposed for the determination of the LTB resistance of stepped beams under general loading conditions.