Evaluation of rocking motion of unanchored cylindrical tanks subjected to the ground acceleration is a main topic in the framework of the seismic vulnerability assessment of this particular kind of structures. Previous researchers basing on a mechanical model including rocking and bulging effects carried out non-linear seismic analyses of the tank-fluid system. However, in order to provide simple tools for the evaluation of the tank response, they neglected the influence of rocking on the tank bulging motion. In this paper, an interesting investigation on the role of inertial and centrifugal forces in the context of the interaction between rocking and translational motions has been conducted by solving the simultaneous dynamic equations of a 2DOF model through a numerical software and comparing results with those of experimental tests. Moreover, employing the dynamic properties governing the tank rocking-bulging motion into the simultaneous equations, a simplified method to determine the tank bulging response and the measure in which it is reduced by the rocking appearance is provided. Validation of the proposed analysis is conducted comparing its results with those computed through an Explicit Finite Element Analysis on a sample tank.

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