Global Lateral Buckling of I-Shaped Girder Systems

A closed form solution for elastic global buckling of twin girder systems interconnected with cross frames is derived. Current design specifications for such systems only consider individual girder buckling between cross frames. The solution, which is suitable for design specifications, was developed for a uniform moment loading condition. Finite-element analyses (FEAs) were used to verify the closed form solution and extend it to more practical loading conditions. FEA showed that the load height condition had only a minor effect for twin girders compared to the published effects on single girders. Both singly and doubly symmetric sections were studied and showed that the girder spacing and the in-plane moment of inertia of the girders are the principal variables controlling global buckling of twin girders. The number and size of the intermediate cross frames had little effect. A method for improving the global buckling capacity through the use of a partial top flange lateral bracing system is presented along with a design example.