In order to calculate the rocking response of the unanchored cylindrical tank, including the bottom uplift due to seismic loading, a simple but effective method is developed. Firstly, an analysis model for the rocking response is presented; tank wall and liquid content are replaced with a mass-spring system, and the nonlinearity associated with the partial uplift of bottom plate is considered as the rotational spring of a bilinear type. The reliability of this analysis was illustrated by comparing the calculation results of resonance frequencies and rotation angles of tank models with the results of vibration tests. Secondly, a simple calculation method for obtaining the rocking responses of tanks from a few graphs without troublesome calculations is proposed. And lastly, some examples of applying the calculation for full-scale tanks are shown. It is clarified that the bottom plates of tall tanks with a capacity of a few thousand kiloliters are most likely to be uplifted by seismic loading.

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