Coupled Instabilities of Lateral Torsional Buckling and Shear Buckling of Singly Symmetric Steel I-Girders: Experimental and Numerical Investigations

This paper investigates the potential to extend current design limits for transverse stiffener spacing and effective lateral unbraced lengths through experimental and numerical studies. The focus here is on noncompact and slender-web singly symmetric hybrid I-sections subjected to simultaneous high moment and high shear. The research presented in this paper is particularly pertinent to long-span bridge construction, where higher-grade flanges (hybrid sections) and singly symmetric girders with smaller compression flanges are becoming more common to optimize both material use and cost. Despite such optimization, a significant cost of long-span bridges is associated with the fabrication of intermediate transverse stiffeners necessary for shear strength and cross frames essential for the lateral stability of girders. Reducing the number of stiffeners or cross frames, especially in regions of high moment and high shear, could result in combined shear buckling and lateral torsional buckling. While moment–shear interaction is typically considered in cross-sectional capacities, few studies have explored the efficacy of the compression flange in anchoring the postbuckling tension field-induced stresses with the advent of lateral torsional buckling. Experimental findings and numerical analyses presented in this paper suggest that despite these conjoined instabilities, the current flexural and shear strength equations in the design codes are sufficiently conservative to warrant increasing the current allowances for unstiffened lengths for straight steel I-girders without any strength reduction. Finally, the authors show that the design shear strengths in codes can be enhanced by using a modified elastic shear buckling coefficient considering the flange and transverse stiffener contribution to the shear stability of steel I-girders.