Manual Estimation of Elastic Lateral Torsional Buckling Resistance of I-Section Members with Cross-Section Transitions

Abstract

This paper presents the evaluation of three methods for manually estimating the elastic lateral-torsional buckling (LTB) resistance of general doubly and singly symmetric I-section members having any combination of transitions (steps) in the cross-section plate dimensions along the unbraced length. The study focuses on common unbraced lengths having idealized torsionally simply supported (fork) end conditions. Method 1 is a simple procedure specified in the AASHTO LRFD Specifications. Method 2 is a recently published procedure that seeks to estimate the LTB resistance via a length-weighted average cross-section approach. Method 3 is an extension of an approach that has been shown to be accurate and efficient for doubly and singly symmetric prismatic I-section members. Method 3 addresses a wide range of nonprismatic I-section member geometries and is recommended in AISC-MBMA Design Guide 25 2nd edition. Results from all three methods are evaluated by an extensive parametric study of I-girder unbraced lengths having various single or multiple cross-section transitions. On average, Method 2 provides the best predictions for single-curvature bending cases; however, certain cases are shown to exhibit undesirable unconservative errors. Method 3 provides the best predictions of the three methods for reverse-curvature bending cases, although the predictions tend to be highly conservative. This method gives slightly more conservative results than Method 2 for single-curvature bending cases, and it limits the worst-case unconservative errors to 7%.