Elastic Global Lateral-Torsional Buckling of Straight I-Shaped Girder Systems

Narrow width I-girder systems with relatively long lengths are susceptible to global lateral-torsional buckling, which is a mode that is not generally sensitive to the spacing between cross frames or diaphragms. This buckling mode can control the capacity of girder systems in a variety of structural systems ranging from bridge to building applications that use torsional bracing. Because of global instability, these systems can experience excessive deformation during construction, which severely compromises the safety or constructability of the steel girder systems. This paper documents an investigation into the elastic buckling behavior of these systems and considers many variables that impact this behavior during construction. This study included eigenvalue buckling analyses to determine critical buckling loads and large-displacement analyses on girders with initial imperfections. Load-deflection analyses indicated that the second-order amplification of both lateral and torsional displacements of the girder system is highly dependent on the shape and distribution of the imperfection. Although the “critical shape” imperfection is identified, the study also considers the likelihood of this shape occurring in practice by simulating the installation of torsional braces during erection. Although the girder system may initially possess an imperfection close to the “critical shape,” the installation of braces generally reduces the severity of the initial imperfection with respect to second-order amplification associated with global lateral-torsional buckling. Design recommendations are presented based on the inclusion of moment gradient modification factors and limits on the elastic global lateral-torsional buckling expression to control second-order effects in narrow girder systems.