A comprehensive and statistically significant analytical investigation is presented herein which evaluates the appropriate modal combination method for seismically induced hydrodynamic loading in liquid storage tanks. The study examines the Square Root Sum of Squares (SRSS) and Absolute Sum (ABS) modal combination methods for hydrodynamic loading due to the impulsive and convective (sloshing) mode responses. Six liquid storage tanks of varying geometry and dynamic characteristics are analyzed for a set of 173 earthquake events with 3,551 earthquake ground motion station recordings, each station recording has two directional components for a total of 42,612 analysis realizations. The maximum moment and shear at the base of the tank are calculated using SRSS and ABS modal combination solutions. The modal combination solutions for maximum tank response are then compared with the exact time history solution. It is concluded that SRSS modal combination predicts the mean maximum response with a 4% conservative bias and a coefficient of variation of 8%. The ABS modal combination is always conservative and overestimates the mean response by 21% with a coefficient of variation of 17%. The results were statistically similar for a subset of 91 pulse-like ground motion components.
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