Enhanced seismic fragility analysis of unanchored steel storage tanks accounting for uncertain modeling parameters

Catastrophic failure of above ground storage tanks was observed during past earthquakes, which caused serious economic and environmental consequences. Many of the existing steel storage tanks were designed with outdated analysis methods and underestimated seismic loads. Therefore, the assessment of their seismic vulnerability is extremely important. Fragility functions are useful tools to quantify the seismic vulnerability of structures in the framework of probabilistic risk assessment. They give the probability that a seismic demand on a structural component exceeds its capacity. The objective of this study is to examine the seismic vulnerability of an unanchored steel storage tank based on the fragility analysis, considering both aleatoric and epistemic uncertainties. The significance of uncertain modeling parameters, attributed to the epistemic uncertainty, is first investigated with a screening study, which is based on nonlinear pushover analyses of the tank using the ABAQUS software. In this respect, a fractional factorial design and ANOVA technique have been adopted. The results indicated that the considered modeling parameters have a significant effect on the uplift behavior of the tank. The fragility curves are then developed based on a simplified model, where the uplift behavior is modeled based on static pushover analysis. Sources of uncertainty, associated with the significant parameters previously identified and the ground motion, are considered in the fragility analysis using a sampling procedure to generate statistically significant samples of the model. The relative importance of ground motion and modeling parameter uncertainties on the fragility curves of the tank is assessed and discussed in detail.