SEISMIC FRAGILITY CURVES FOR THIN-WALLED STEEL STRUCTURES USING KOITER’S MULTIMODAL ANALYSIS

Geometrically nonlinear phenomena and stability issues can significantly affect the seismic behaviour of a wide range of structures. Pallet rack systems, for instance, are prone to buckle when the material is still within the elastic range, undergoing sudden and catastrophic collapse mechanisms, as documented after several earthquakes. This work presents a vulnerability analysis methodology for cold-formed steel pallet rack systems affected by the presence of geometrical imperfections. The vulnerability analysis is composed of two steps. In the first step, a local imperfection sensitivity analysis is conducted for each structural element, modelled through a detailed shell finite element discretisation and Koiter’s multimodal analyses. The local analysis provides probabilistic distributions of the limit load fitted by the Gumbel Max distribution. The second step regards the global seismic analysis performed on a one-dimensional truss frame. For each truss, the limit load is assumed as an uncertain value, according to the distribution found on the local analysis. Its propagation is conducted through Monte Carlo simulations that eventually provide lognormal fragility curves. Then, for the pallet rack systems under consideration, insights on the influence that geometrical imperfections have on their fragility curves are presented. The proposed approach is highly efficient, and the local-global analysis time is well suited to large-scale seismic vulnerability assessments.