Lateral-Torsional Buckling of Singly Symmetric I-Girders with Stepped Flanges

Guidance for the lateral-torsional buckling behavior of singly-symmetric, nonprismatic (stepped flanges) sections are often absent from design specifications or are overly simplified. This paper presents simplified design procedures that estimate the buckling capacity of these complex systems that commonly face designers. The results of a robust parametric finite-element study that included 14,040 unique prismatic and nonprismatic beam sections are detailed. The study specifically examines the effects of common span-to-depth ratios, intermediate bracing schemes, degrees of monosymmetry, variable flange transitions, and moment gradients on the buckling response. A proposed weighted-average section approach as well as traditional moment gradient expressions are evaluated based on their ability to accurately approximate the finite-element solutions of these singly-symmetric and/or nonprismatic beams. The computational study and proposed design expressions focus on elastic buckling behavior, midheight loads, and stiffened web elements, such that distortional effects that may limit the beam capacity are precluded. This approach matches the assumptions used to derive the classic analytical solutions adopted in most design specifications. For a broad range of conditions, the proposed methods are shown to produce reasonable and reliable estimates of the buckling capacity obtained from the finite-element models.