Fatigue analysis of unanchored steel liquid storage tank shell-to-base connections during earthquake induced uplift

During severe seismic events the base of unanchored steel liquid-storage tanks can uplift, causing large inelastic rotation demands at the shell-to-base connections. With repeated cycles of uplift, these shell-to-base connections are susceptible to low-cycle fatigue failure. Current code provisions (EC8 and NZSEE) limit uplift rotations to 0.2rad for an unspecified number of cycles based on an assumed base-plate strain of 5%; however, limited research actually exists on the rotation capacity of shell-to-base connections. The studies that have been conducted are limited to constant amplitude static demands, and neglect the presence of multi-axial stress states that can develop in the tank base-plate. Multi-axial stresses in the base-plates have the potential to reduce connection capacities through increased yielding. In this paper, the performance of tank shell-to-base connections are analyzed under uplift histories generated from realistic earthquake ground motions. Uplift histories from design-level earthquake motions are applied to two axi-symmetric finite element models representing two tank geometries (broad and slender tanks), and fatigue damage is then assessed using a calibrated stress modified critical strain (SMCS) low-cycle fatigue criterion. Multi-axial stress states in the connections are directly considered by using an axi-symmetric element formulation. Results from the analyses indicate excess capacity in the shell-to-base connections where the EC8 provisions indicate failure (exceedance of 0.2rad).