Abstract

Thin-walled beams are widely used in construction with various restraint arrangements and sizes. While lateral torsional buckling is a major design concern for thin-walled beams used as girders distortional effects may become important depending on restraint conditions as well as for beams with slender webs, stocky flanges, and/or shorter spans. This thesis introduces a novel beam-type nonlinear Finite Element formulation that is applicable for the analysis of I-sections that are prone to lateral-torsional buckling and distortion. A linearized buckling formulation has been derived as a special case of the nonlinear beam formulation under the assumption of no pre-buckling deformations. The material is assumed elastic. The formulation was also applied for composite laminated thin-walled beams. The effect of shear deformation on the flexural and lateral-torsional buckling predictions of composite-laminated thin-walled beams were illustrated without the distortional effects. For the analysis of composite-laminated thin-walled beams the finite element formulation has the flexibility to choose one of the alternative members of the Doyle-Ericksen family of strains. As such the developed formulation is an extension of both Engesser and Haringx column buckling formulas to shear deformable thin-walled beams with rigid webs. It is shown that alternative strain definitions lead to changes in the geometric stiffness matrices in finite element buckling analysis of thin-walled beams. The effect of the selected strain definition on the elastic buckling load predictions is illustrated in numerical tests including short sections and beams with low shear modulus.