On the analysis of vertical circular cylindrical tanks under earthquake excitation at its base

Based on two international well accepted design standards, Eurocode 8 Part 4—Tanks, Silos and Pipelines and API Standard 650—Seismic Design of Storage Tanks, the structural response of seismically excited vertical circular cylindrical tanks is analysed from a novel perspective. The common basic assumption, adopted from Haroun-Housner and Veletsos, that a circular cylindrical tank containing liquid behaves like a cantilever beam without deformation of its cross-section is obsolete. Instead the authors consider the shell modal forms in order to generate a refined model. Emphasis is laid on the analysis of the fundamental frequencies for the tank-liquid-system. They are calculated by a new method, based on Galerkin’s approximations for cylindrical shells. As the results differ significantly from those calculated by the proposed formulae in both EC8 and API Standard 650, the new results are compared with tank failures during recent earthquakes. This comparison is astonishing. It can be seen from recent examples of tank damage that most failures are caused by resonance effects, which are taken into account neither in EC8 nor in API Standard 650. And, therefore, we take into account a high safety risk. This leads to the conclusion that the basic assumptions for current design provisions are no longer tenable under the present knowledge of shell theory and shell design, and, therefore should be reconsidered.