Fragility Assessment of Unbraced Steel Pallet Rack System

The increasing frequency and intensity of seismic events pose significant risks to the integrity of storage structures, particularly steel pallet racks that are critical infrastructure in warehouses and retail spaces. These structures carry heavy live loads, higher than dead loads. Yet traditional civil engineering codes struggle to predict their response under seismic loading. Unlike standard buildings, unbraced pallet racks rely on the moment transfer capacity of their semi-rigid connections for lateral stability. This dependence creates a major technical bottleneck leading to struggles in modelling complex hysteretic behaviour of connections, which experience severe pinching and stiffness loss during cyclic loading. The objective of the thesis is to assess the seismic fragility of steel pallet racks using nonlinear time history analysis. A detailed two-dimensional OpenSeesPy model of a three-storey, two-bay steel pallet rack frames with semi-rigid connections is developed and calibrated, and fragility functions are derived for four damage states and four hazard levels defined by peak ground velocity. The study applies Multiple Stripe Analysis for the high seismic hazard site of Udine, Italy, using peak ground velocity as the intensity measure. The analyses produce peak inter-storey drift, peak base plate rotation, peak beam to column joint rotation, and an overall upright strain parameter, ε∗ based on normalised axial force and bending moment in the uprights. For drift, base rotation, and joint rotation, the work fits lognormal fragility curves for four damage states using stripe exceedance rates, and it treats collapse cases through consistent exceedance handling. For ε∗, the work estimates ordered median capacities with a common dispersion using likelihood-based fitting. The resulting median capacities, dispersions, and safety tables show higher reliability at lower damage states and decreasing margin as the limit state approaches collapse prevention. The results indicate that connection deformation demand, especially at the base plate, governs global response in the 2D model, which supports the use of connection-level fragility measures alongside drift.