We measured the natural frequencies of the fluid-elastic-coupled shell plate vibration excited in a large-sized, flat-bottomed, cylindrical steel tank by observing microtremors at three points of the tank. By comparing the appearance frequencies and the values of the five peaks seen in the observed microtremor spectral ratios of the top or mid-height of the shell plate to the bottom on the tank foundation with the solutions obtained from a fixed-base FEM eigenvalue analysis, we identified the five peaks as belonging to modes (m, n) = (1, 1–5), with m specifying the vertical order and n the circumferential wave number. The measured non-soil-coupled natural frequencies from the spectral ratio agreed fairly well with those obtained from theory using the FEM analysis. The measured frequencies of the fundamental mode (m = n = 1) were also in good agreements with those estimates using a simplified equation assuming a fixed base adopted in seismic codes of the Japanese Fire Service Act. This equation is expected to provide a reliable soil-coupled fundamental-mode natural frequency for a tank resting on firm ground; the storage-soil-coupled effects are presumed to be weak. Without using an FEM analysis, we present a simple approach to determine the non-soil-coupled fundamental-mode natural frequency solely from the observed microtremor spectral ratios. This simplified procedure works very well for the tank examined.

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