Lateral-torsional buckling of I-girders with discrete torsional bracings

Discrete torsional bracing systems are widely used in practice to increase the lateral-torsional buckling (LTB) strength of I-girders. However, only limited studies are available on the LTB strength of I-girders with mid-span torsional bracing. In addition, equivalent continuous brace stiffness concept is adopted for general discrete torsional bracing problems. This article presents an analytical solution for LTB strength and stiffness requirements of I-girders with discrete torsional bracings under a uniform bending condition. Firstly, the critical moment and torsional stiffness requirement are derived by using an energy method for an arbitrary number of bracing points. The proposed equations are then compared with the results of finite element analyses and those obtained by previous researchers. From the results, it is found that the proposed solutions agree well with the results of finite element analyses regardless of the number of bracing points, while the results for the equivalent continuous brace stiffness concept are not suitable for multiple discrete torsionally braced beams. Finally, reduced formula for the total stiffness requirement is proposed for the purpose of design, and effects of linear moment gradient loading and geometric imperfections on critical moments and stiffness requirement are also observed.