Design Perspectives of the Structural Modes for Ground Liquid Storage Steel Tanks

Liquid retaining tank probably the only structure where liquid mass is considered as modal masses i.e., convective, in which portion of the liquid sloshes on the top and the impulsive which moves along with the tank body. Various researchers made investigations on seismic analysis of liquid storage tanks by including two distinct modes. Based on the different outcomes of their research, various codes like API 650, NZSEE etc. have set the procedures to compute different quantities like time periods, base shears, base moments, hydrodynamic pressures with respect to the two modes. Present paper deals with parametric and comparative study of such quantities with the different tanks as per the API 650 and NZSEE guidelines. Also, study of tank seismic parameters of design spectra of both the types of modes is discussed. The purpose of the study is to get overall idea of the weightage of each mode (Convective and sloshing) in view of analysis part of the tanks. Further, some of the identified tank damages believed to occurred because of the convective mass also been discussed. Authors have observed that the percentage of convective mass contribution in different quantities is very less. So, the analysis of the tanks can be made curtailed by merging the convective mass to the impulsive and thus single mode analysis can be adopted at great extent at least in the seismically less prone zones. In addition of the above, the design spectrum can further be reduced to the single coefficient value on higher side if single merged impulsive mode is adopted in the analysis. It is to note, literature survey clears that investigation on analysis with shell flexibility, out of roundness and soil structure interaction not affecting the convective mass anymore. The observations discussed in the paper may helpful to structural designer for making simplified i.e. single merged mass mode analysis for the design of tanks. Authors’ view is thus inclined towards consideration of merged impulsive/single mass along with shell flexibility for finding more precise structural properties like time period, base shear, base moment or in any analytical study will be authentic in the tank analysis.