Buckling of beams, in a general sense, can be described as the behaviour in which a structure suddenly deforms in a plane different to the original plane of loading. The usual types of buckling of steel beams assumed in design are local and lateral-torsional buckling. In local buckling, an element of the beam, such as the compression flange or the web, buckles over a short length with local changes in the cross-sectional shape. Lateral buckling, on the other hand, involves lateral deflections and twist along the member without any change in the cross-sectional shape. The mode of buckling considered in this thesis, however, is one in which lateral deflection and twists are combined with changes in the cross-sectional shape.
Coupled local-lateral buckling, also referred to as distortional buckling, may occur in I-section beam-columns with slender webs and stocky flanges. A computationally efficient method is presented in this
thesis to study the distortional buckling of I-beams. Previous studies on distortional buckling have been on the use of 3rdand 5th order polynomials to model the displacements. The present study provides an alternative way, using Fourier Series, to model the behaviour. Beams of different cross-sectional dimensions, load cases and restraint conditions are examined and compared. The accuracy and versatility of the method are verified by calibrating against the results of other published studies. The present method is believed to be a simple and efficient way of determining the buckling load and mode shapes of I-section beam-columns that are susceptible to distortional buckling modes.
Based on the formulation of the model, an attempt has been made to develop a simple closed-form formula, similar to the familiar formula of lateral-torsional buckling, to aid designers in making a quick estimation of the distortional