Seismic Fragility Analysis of Liquid Storage Tanks

摘要

Elevated spherical and cylindrical tanks are commonly used to store liquids either under pressure or at atmospheric pressure in various industrial installations, e.g., nuclear power plants, chemical plants, etc. If the stored liquid is dangerously toxic or inflammable, failure of these tanks is catastrophic. Therefore, safety of these liquid storage tanks is of paramount importance in the event of extreme natural loading such as earthquakes. A number of approaches are available for seismic fragility analysis of structures but most of these consider safe shutdown earthquake (SSE) having a particular peak ground acceleration (PGA). Even for a particular site, earthquakes characteristics vary not only in amplitude but also in frequency, frequency band and duration. To arrive at a rational estimate of seismic fragility of a structure, it must be evaluated over a reasonable range of all the parameters to define a realistic seismic excitation for the particular site. Though, lower and upper limit of the parameters such as frequency, damping, etc., can be decided for a particular site on the basis of geological conditions, the worst possible seismic excitation depends not only on the site but also on the structure. In this study, an analytical and numerical methodology has been proposed for seismic fragility analysis of elevated spherical liquid storage tank. Though, the earthquakes are non-stationary, for development of present approach, input excitation has been represented by a stationary Kanai-Tazimi spectrum. Parameters (frequency and damping of the Kanai-Tazimi spectrum) corresponding to critical (worst possible) excitation have been obtained for the spherical liquid storage tank under consideration. The obtained critical power spectral density (psd) has been used to obtain conditional probability of failure of the structure for different PGA. The proposed methodology has been demonstrated for an elevated spherical liquid storage tank for two different sites one is having stiff soil and another is having soft soil. A significant difference has been observed in the results obtained by proposed approach in comparison to existing approach for these two types of sites.