The current seismic design codes of aboveground storage tanks (ASTs) for oil storage are conducted based on the beam-type vibration with circumferential wave number m = 1. The oval-type vibration with m ≥ 2 in which shell plate vibrates like a petal shape occurs in ASTs. The causes are local mass/stiffness, initial imperfections or nonlinear strong vibration. The initial stress due to hydrostatic pressure was reported to increase the natural frequency in high m range by several studies. The circumferential tensile stress on shell plates due to hydrostatic pressure increases the stiffness and as a result, the natural frequency is also increased. Wind girders are installed to the shell plates of ASTs for the prevention of buckling due to wind pressure. The wind girder which is a ring stiffener increases the stiffness of the shell plate. In this paper, the natural frequency of the oval-type vibration in a large-sized AST is analyzed and the effect of the wind girder on natural frequency is studied. The method is the free vibration analysis of axisymmetric shell finite element method considering vibration coupled with fluid motion.

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