Observational Seismic Fragility Curves For Atmospheric On-Grade Steel Storage Tanks Based On Damage States In Terms Of Structural Performance And Release Of Content

The evaluation of the seismic vulnerability of atmospheric on-grade steel storage tanks is a fundamental topic in the context of industrial safety. In process plants, areas assigned to the storage of liquid products are among the most critical because of the presence of huge volumes of dangerous substances. On the other hand, official post-earthquake reports reveal that thin steel shell structures of which usually liquid storage tanks are made have suffered several type of failures. Depending on the shell portion affected, on the extent of damage and on toxicity, flammability and reactivity of stored substances, liquid leakages can trigger hazardous chains of events whose consequences affect not only the plant but also the surrounding environment. In the light of that, the study proposed herein provides an analysis of the seismic fragility of cylindrical on-grade storage tanks, based on observational damage data. The first phase of this work has consisted in collecting a large empirical dataset of information on failures of atmospheric tanks during past earthquakes. Two sets of Damage States have then been used in order to characterize the severity of damage and the intensity of liquid releases. From a statistic analysis of the collected data, empirical fragility curves has been fitted by using Bayesian regression. The advantage of this approach is that it is well suited to treat direct and indirect information obtained from field observations and to incorporate subjective engineering judgement since it can take into account statistical and model uncertainties. The hazard parameter used for tank fragilities is the Peak Ground Acceleration. Seismic fragility curves obtained from the described procedure are compared to those available in the technical literature. Finally, the influence of tank size, filling level and presence of anchorage on the tank performance have been investigated.