The 1995 Kobe earthquake caused severe damage to cylindrical oil storage tanks. The extent of the damage sustained by such tanks in this earthquake indicates that hte crrent seismic design code for the tanks is not suitable. One factor not taken into consideration i nthe current code is initial stress. Initial stress means the stress caused by normal operating conditions. The circumferential tension stress due to static liquid pressure and the meridional copression stress due to the tank's own weight acting on the shell plate are the major initial stresses. This paper describes the seismic response characteristics of the cylindrical oil storage tanks, and the effect of initial stress on natural frequency is investigated. Free vibration analysis of the fluid-filled tanks is performed using axisymmetric finite element analysis. Results show that initial stress has a significant effect on the natural frequency of high-order circumferential wave numbers, and the antural frequency is more sensitive to the initial circumferential stress than the initial meridional stress.
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