Imperfection-sensitivity of unanchored aboveground open-top steel welded liquid storage tanks subjected to seismic loads

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.