Effects on Nonlinear Geometric and Material Properties on the Seismic Response of Fluid - Tank Systems

Liquid storage tanks are considered essential lifeline structures. Water storage tanks in particular, are important to the continued operation of water distribution systems in the event of earthquakes. Recent earthquakes have shown liquid storage tanks to be vulnerable to damage. Current knowledge about the behavior of liquid storage tanks is extensive, but many of the analytical and theoretical results are for tanks without roof systems, and include a number of simplifying assumptions, for example, small deformation and linear elastic material property assumptions. To explore the effects large deformation and nonlinear material properties have on the seismic response of fluid/tank systems, the computer program ANSYS was selected to develop a Finite Element Analysis (FEA) model of a ground level, cylindrical steel shell and roof tank structure with contained fluid under seismic load. The ANSYS program was selected for its ability to include shell and structural steel elements, contained fluid elements, fluid-structure interactions, material and geometric nonlinearities, and contact type elements. For purposes of this study, analysis results from a linear elastic, small deformation fixed base model are compared with an elasto-plastic material property model with large deformation assumptions. Results show the significant difference in results based on the assumptions used and indicate that current design code based values may not be conservative in resultant loading calculations.

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