Keywords: tank, liquid, sloshing, uplift, near fault records, seismic analysis.

In this paper, the dynamic response of the anchored and unanchored flexible on-grade storage tanks under the action of near-fault input ground motions has been investigated. Such earthquake ground motions, especially having long-period directivity pulses, are important for flexible systems such as liquid storage tanks and their fluid-structure interaction. Furthermore, due to the importance of the all three components in this type of earthquake, their combined effects on the behavior of the storage tanks are worth considering.

For numerical analysis, an oil storage tank with its content has been modeled with the displacement-based elements using a general-purpose finite element program [1]. The fluid-structure interaction has been taken into account using the constraint equations. Non-linear behavior caused by the base uplift in unanchored tanks has been considered by implementing the gap elements between the tank base and the ground level. The compressibility and the viscosity of the fluid have been also taken into account. For the dynamic time history analyses, two free-field near fault ground motions recorded in the 1994 Northridge earthquake at Sylmar County Hospital and Rinaldi stations, were considered.

Performing modal analysis, it is observed that the fundamental sloshing frequency is only affected by the dimensions of storage tank. Also, the natural frequency of the first lateral mode of the coupled system is not sensitive to the compressibility of the liquid. Changing the density of the liquid contained or the wall thickness will have a direct effect on the effective mass and effective stiffness of the first lateral mode of the tank.

The results of the time history analyses indicate that the non-linear behavior caused by the base uplift, leads to remarkable changes in most of the tank's seismic response parameters. This clearly shows the importance of accurate modeling of the restraint conditions. Moreover, three directional excitation of the tank complicates the distribution of the surface waves, even though the axi-symmetric modes are still dominant in the liquid free surface displacements. Therefore, it will be difficult to properly estimate the maximum wave height in a three directional input analysis through the superposition of the maximum wave height obtained from the single input analyses alone. Also, as the results indicate, the change in the base conditions (anchored or unanchored) had no remarkable effect on the height of the free surface waves in both the single and the three directional input cases.

Comparison of the results from the numerical analyses with those obtained using the API-650 code [2], revealed that the API code predicts conservative results for the seismic response of the broad tanks (after considering the inherent force reduction factor used in this code.) But it may lead to less conservative or even non-conservative results for the slender tanks that are more flexible than broad tanks.

1

"User's manual for ANSYS revision 5.4", ANSYS Engineering Analysis System, SAS IP, Inc., 1997.

2

"Welded Steel Tanks for Oil Storage", API Standard 650, American Petroleum Institute, Washington DC, 1998.

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