Imperfection amplitudes for nonlinear analysis of open thin-walled steel cross-sections used in rack column uprights

When numerical models are used to predict behaviour of open thin-walled steel cross-sections, the nonlinear analysis results can be influenced by the magnitudes introduced as initial imperfection. At present, a wide range of values is considered in research concerning this topic. This paper explores up to what point the nonlinear analysis results are sensitive to the choice of imperfection magnitudes and attempt to refine the spectrum of magnitudes that should be used. The study focuses on rack uprights (with and without perforations) under compression and the numerical model has been validated with experimental tests conducted by the authors. Three different column lengths have been selected to reproduce a mainly local, distortional and global failure mode, so that coupled instabilities have not to be considered in this case. The results show that the ultimate load and collapse mode are both sensitive to imperfection amplitude, mainly in the case of distortional buckling.