Vol. 2, No. 4- Fall 2000

Effects of Foundation Geometry on the Natural Periods of Cylindrical Tank-Liquid-Soil Systems
Mahmood Hosseini and Mehdi Mohajer

Liquid storage tanks are essential structures in water, oil and gas industries, and their seismic safety is of great importance. On the other hand, modifying the dynamic characteristics of tank systems can be very useful for improving their seismic behavior. In this paper a study has been performed on the effect of the geometry of the tank foundation on the modal properties of the tank-liquid-soil system, in which both fluid structure and soil-structure interactions have been considered. For this purpose a set of cylindrical steel tanks with various height over radius (H/R) and thickness over radius (t/R) ratios have been considered. The tank foundations have been assumed to have two main different geometries, namely square and circular in plan with different thicknesses, as well as various dimensions and/or diameters. Various conditions have been considered for the subsoil varying from very soft to very stiff based on the value of shear wave velocity (vs). The first three modes of the tank system have been taken into account for modal characteristics calculations. The numerical results show that the natural periods of the system are quite sensitive to the foundation geometry. This sensitivity is much higher in the case of circular foundations, especially for lower H/R ratios and lower vs values. By choosing appropriate values for foundation dimensions, it is possible to make the period values a few times longer. Therefore, using a specific foundation geometry can be a good tool for modifying the period of the whole tank-liquid-foundation system in earthquake prone regions to make it far from the dominant frequency of the site.

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