Seismic response of uplifting liquid storage tanks

Abstract

A systematic study is carried out on several aspects of the dynamic response of liquid containing, cylindrical steel tanks that may experience partial uplifting of their base during intense ground shaking. The tanks are presumed to rest on a rigid base and to be subjected to unidirectional horizontal ground motion. The objective is to formulate a method of analysis with which the dynamic response of such systems may be evaluated reliably and cost-effectively. The study consists of two parts: the first deals with the static uplifting resistance of the flexible base plate, and the second deals with the dynamic response of the uplifting system. In Chapters 2 and 3, an insight into the behavior of the uplifting plate is gained with the help of a prismatic beam solution. In Chapter 2, the solution is implemented 'exactly' whereas, in Chapter 3 it is implemented approximately by use of the Ritz energy procedure with a set of judiciously selected deflection shape functions. In Chapter 4, the 'exact' solution of the axisymmetrically uplifted plate is obtained. A detailed comparison is then made between the plate and the beam solutions in order to ascertain the suitability of the beam model for the plate analysis. The insight gained from this comparison is then utilized in the formulation of an approximate method of analysis for the solution of an asymmetrically uplifted plate. In obtaining the beam and plate solutions, due consideration is given to the effects of membrane forces associated with large deflections, plastic yielding in the base plate, and the restraining action of the tank wall at the plate boundary. In Chapter 5, an approximate method of analysis is formulated for efficiently computing the seismic response of uplifting tanks. Both unanchored tanks and partially anchored tanks for which the number of anchor bolts at the base is insufficient to ensure full fixity are considered. The responses examined include the hydrodynamic pressures, the base uplift at critical locations, the plastic rotation at the plate-shell junction, and the maximum compressive stress in the tank wall. The parameters that are varied include the intensity of ground shaking, the geometric and material properties of the tank, and the degree of base fixity.