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Overturning Moment

Design bending and overturning moments in the API standard [2] as applied to the bottom of the shell and to the foundation, respectively, are determined using the simplified procedure developed by Housner [7] for rigid tanks. A lateral force coefficient, equals to 0.24, is specified to represent the amplified tank acceleration as a ratio of the acceleration of gravity; it does not take into account explicitly the effects of shell flexibility, site conditions, or support conditions (anchored vs. unanchored). Each of these factor may amplify the tank acceleration, for example, soil tank interaction and associated rocking motion could, under horizontal excitation, amplify the tank acceleration as high as 200% or more for soft soil.

The New Zealand recommendations provide normalized response spectra for soil and rock, recommended geographic coefficient for the design according to tank location zone, and a recommended peak ground acceleration of 0.35 g. A mechanical analog [6], which takes into account the deformability of tank wall, is used to reduce the tank and its content to equivalent masses and springs at different heights. A correction formula is used to modify the frequencies of the mechanical analog and the corresponding damping ratios depending on the soil type and its mechanical properties. It should be noted that this mechanical analog was developed for tanks anchored at their base. However, due to the lack of readily available methods, the recommendations include this model for calculating the amplified overturning moments in unanchored tanks!

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