Residual stresses and initial imperfections in non-linear analysis

When non-linear finite element analysis (FEA) of cold-formed thin-walled steel sections subject to compression is carried out, an initial perturbation must be introduced in the model in order to trigger the failure due to instability. The most usual way is to introduce an equivalent initial imperfection in the model (conventional models) that includes the effects of geometric imperfections and residual stresses due to the roll-forming manufacturing process, possible loading eccentricity, etc. It is thought that results would be more accurate if the actual measured geometric imperfections and the actual residual stresses induced from manufacturing were introduced. This paper presents the determination of residual strains and stresses generated in the cold roll-forming process of a typical rack section (without perforations) from steel sheet, using FE simulation. The obtained residual elastic and plastic strains have then been incorporated in the model, and used as an initial strain state in the non-linear finite element analysis. The results obtained agree well with the experimental results for short and intermediate length columns, where the failure mode is predominately local and distortional.