Seismic fragility analysis of large unanchored steel tanks considering local instability and fatigue damage

When unanchored steel liquid storage tanks are subjected to strong ground motions, the contained liquid may cause uplift of the tank base resulting in compressive buckling of the tank shell and or ultra low-cycle fatigue (ULCF) damage to the shell-to-base welded connections. This paper presents a method for investigating the shell buckling and connection ULCF vulnerability for unanchored steel tank geometries using simplified mass-spring models and incremental dynamic analysis. The presented simplified mass-spring models simulate resistance to local base uplift, tank rocking, and provide equilibrium-based approaches for determining tank shell compressive forces resulting from dynamic earthquake loadings. In this study, four existing tanks (considering both broad and slender geometries) are analyzed under 20 recorded ground motions, scaled at 30 different levels (from 0.05 to 1.5 g) for a total of 2400 dynamic analyses. Results from the incremental dynamic analyses are used to create fragility curves, wherein shell buckling and connection ULCF probabilities are related to the level of seismic excitation. Results from the analyses on the four existing tank geometries highlight shell buckling vulnerabilities for both broad and slender tank geometries under moderate seismic excitation.