Local plate buckling type imperfections for NSS and HSS welded box-section columns

The finite element model-based design of steel structures is getting increasing attention by designers, especially in the case of buckling problems of welded plated structures and by the static check of the interacting stability behaviour. The FEM-based design approach is introduced and standardised in the EN 1993–1-5 Annex C providing the ability to calculate the buckling resistance based on direct resistance check by geometrical and material non-linear analysis using imperfections (GMNIA). By this design process, the application of imperfections (shape and magnitude) has a large impact on the computed buckling resistance. It is especially true if the interacting stability phenomenon (interaction of plate and column-like behaviour) is to be analysed. The general way to consider imperfections in FE models is the application of equivalent geometric imperfections covering the effect of residual stresses and geometric imperfections. Several previous investigations prove the application of equivalent geometric imperfections can lead to conservative resistance, especially in the case of the interacting stability problem because the application of two different equivalent geometric imperfections leads to duplication of the residual stresses. Therefore, it is highly important to determine the appropriate imperfection shape and size for buckling problems. The current paper deals with the determination of local plate buckling-type imperfection, which can be further applied in numerical models used for interacting stability problems, which is the next step of the planned research program. Within the current paper, the necessary imperfection magnitudes are determined for welded square box-section columns for various steel grades (S235 – S960) to perfectly match the local buckling resistance proved by test results and gives design proposals for FEM-based design approaches.