An Effective Method of Analyzing Rocking Motion for Unanchored Cylindrical Tanks Including Uplift

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.