Thin-wall metallic tanks are largely used to contain liquids in various engineering fields. For example, vertical cylindrical tanks, obtained by welding courses of different thickness, are typically used in wine industry for fermentation and conservation of wine. Structural integrity assessment usually requires a static analysis of tank subjected to gravity loading and hydrostatic liquid pressure. However, a seismic analysis may also be required in so far as tank location is characterized by earthquake risk. In Italy this issue has become even more important as a consequence of some recent earthquakes (e.g. Emilia, May 2012).
The European standard for tank seismic design is EN 1998-4:2006 Eurocode 8 – Part 4, which has to be completed by other codes to characterize seismic ground actions (for example, the Italian reference is the Ministerial Decree January 14th 2008 “Technical rules for constructions”). The Appendix A in Eurocode 8 gives information on the analysis procedures for tanks subjected to horizontal or vertical seismic action, having cylindrical geometry with vertical axis, rigid or flexible foundation, fully or partially anchored at the base.
This works presents a case study concerning the seismic analysis of a cylindrical tank used in wine industry, subjected to horizontal earthquake action. Two different approaches are followed: one based on Eurocode 8 recommendations (with either analytical or numerical methods), the other based on Finite Element response spectrum analysis. Finite element simulations are performed with general purpose ANSYS software. The analytical method of Eurocode 8 is based on a lumped dynamic model that represents the convective (sloshing) and impulsive response of liquid/tank system under seismic base acceleration. The second approach suggested in Eurocode 8 computes, instead, the hydrodynamic pressures for convective and impulsive actions, which are next applied to the tank Finite Element model.
The third approach proposed here discussed is a response spectrum analysis of liquid/tank system. A Finite Element model with shell and displacement-based fluid elements is used. Suitable node coupling is applied to simulate liquid motion relative to tank. A modal analysis with “matrix condensation” technique is first performed to extract natural frequencies and mode shapes. Then, the acceleration response spectrum is applied to tank base to simulate horizontal earthquake motion. The system dynamic response is estimated by modal combination techniques. An overall comparison of results by all methods addressed is discussed, showing a general quite good agreement among all approaches presented.