Collapse of a high storage rack

Abstract A high storage rack collapsed during operation resulting in one fatality. The investigation had to prove whether an overloading or an incorrect design, manufacture or installation of the rack lead to the failure. A failure as a result of excessive external forces could have had two possible reasons: static failure due to local or global overloading with stored goods, or a dynamic failure due to an impact load (e.g., collision with forklift), vibrations or instable loads. At the start of the investigation, a detailed inventory taking of the scene of the collapse was performed, recording the broken rack components and reconstructing the loads carried at the time of failure. Based on the collected information, the utilisation of each rack bay had to be determined. The load capacity and the stability under load of the rack were investigated by finite element methods (FEM). Buckling analyses of the installed columns were performed and a sensitivity study comprising several geometric and load imperfections such as asymmetric loading of the column, local imperfections (dents) and bearing type (support) showed the impact of these parameters on the stability. The choice of an appropriate model cutaway and valid boundary conditions presented a major challenge. The recalculated actual load on the middle frame at the time of accident amounted to 68 kN per column. According to the rack manufacturer, the intermediate upright supports had a design capacity of 50 kN (comprising an unknown safety factor). The load capacity calculated by linear FE analysis amounted to 86 kN for the ideal case and less than 60 kN for the assumed imperfect case. Considering imperfections in a nonlinear static analysis, the ultimate load resulted between 67.3 and 70.3 kN. Thus, the failure could be attributed to a static overload of the intermediate frame without the need of any additional external perturbation. Only the assessment of the effective load capacity proved the causality between the non-compliance of the manufacturer instructions and the collapse.