A recently developed analytical model for a perfect I-section strut experiencing a nonlinear interaction between local buckling and global buckling about the strong axis is enhanced and subsequently extended. The initial enhancement is achieved by devising a simplified and calibrated model that provides an improved prediction of the local buckling load. A purely numerical model is then constructed within the commercial finite element (FE) software Abaqus for validation purposes and excellent comparisons are observed, demonstrating that the analytical model is considerably improved on previous work. The model for interactive buckling is then developed subsequently to include the effects of global and local geometric imperfections, which are introduced individually and in combination. The strut is found to be sensitive to all considered shapes of imperfection and the combined imperfection case correlates excellently with an equivalent FE model, particularly in the neighbourhood of the secondary instability that leads to mode interaction. This demonstrates that the enhanced analytical model predicts the actual load carrying capacity and the structural mechanics accurately.