The behavior of aboveground storage tanks subjected to seismic excitation was investigated using numerical methods by taking flexibility of foundation into account. The hydrostatic load due to stored liquid has an axisymmetric distribution on the tank shell and base. However, during seismic events, the hydrodynamic load originating from the seismic acceleration of liquid in the tank starts to act in the direction of the earthquake motion. This leads to a nonaxisymmetric loading distribution, which may result in buckling and uplifting of the tank structure. Finite element models were created having nonlinear material properties and large deformation capabilities. Three different tank geometries with liquid height to tank radius aspect ratios of 0.67, 1.0, and 3.0 were selected representing broad, nominal, and slender tanks. These tanks were subjected to two different hydrodynamic loading based on Housner's and Jacobsen–Veletsos' pressure distributions, which forms the basis of design provisions used in American Petroleum Institute API 650 and Eurocode 8, respectively. These pressure distributions were formulated under the assumption of rigid tank wall and base. Furthermore, each tank for a given geometry was subjected to two different foundations: (1) representing a rigid foundation and (2) representing a flexible foundation. The flexible foundation was created using a series of compression-only elastic springs attached to tank base having equivalent soil stiffness. Static analysis corresponding to maximum dynamic force was performed. The finite element results for circumferential and longitudinal stress in the shell were compared with the provisions of API 650. It was found that the effect of foundation flexibility from the practical design point of view may be neglected for broad tanks, but should be considered for nominal and slender tanks.

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A numerical analysis is conducted on several unanchored aboveground, open-top, steel, welded, liquid-containing storage tanks with imperfections subjected to seismic forces. Nonlinear material properties, nonlinear geometry deformations, and a flexible soil foundation idealized by a series of elastic springs are employed in order to simulate as-built field conditions of the tank at the time of the seismic event. A static pushover analysis was performed using impulsive and convective hydrodynamic effects of fluid contained in the tank subject to seismic motion modeled as an equivalent pressure distribution acting on the tank wall and base. Out-of-plumbness (OOP) imperfections, which is the deviation of the top of the tank shell with respect to the shell-to-base connection, and out-of-roundness (OOR) imperfections, which is a deviation in a given shell course with respect to the undeformed shell configuration, are implemented into the tanks to observe the comparative behavior when between perfect and imperfect tanks. The tolerances for construction prescribed by American Petroleum Institute (API) and recommended by the New Zealand Society of Earthquake Engineering (NZSEE) are used for the amplitude of the imperfections. In a worst case scenario, the OOP and OOR imperfections are combined. The analysis shows that the behavior of the tanks with imperfections using the API 650 tolerances show little deviation from the perfect tank conditions, and in some instances, even out-perform the perfect shell conditions, and thus shell imperfections when they are within the specified tolerances of relevant standard of construction pose little additional threat to the tank when experiencing a seismic event.

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