An analytical approximate model, leading to a closed form solution, is presented to account for buckling mode interaction in composite I section columns. Three buckling modes are considered in the analysis: a global mode (Euler mode about the weak axis); a primary local mode (rotation of the flanges and bending of the web); and a secondary local mode (bending of the flanges), which are modeled using analytical functions and four degrees of freedom. The fundamental state is shown to be linear and the three critical states for the isolated modes are found to be stable symmetric bifurcations. Mode interaction analysis in terms of the amplitudes of first order fields is carried out, for the first time, for prismatic sections of composite material. The tertiary (coupled) path involves coupling between the two local modes and it describes the sensitivity to imperfections of the buckling behavior of the composite column. A salient feature of the model presented is the closed form of the resulting solution, which enables the designer to easily perform parametric studies. Also, this is the first buckling mode interaction study for thin-walled composite columns. Numerical examples are presented to validate the results and to show the influence of the geometry and properties of the composite on the interaction phenomenon.
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