Structural Behavior Of Tapered Steel Storage Tanks Under Measured Differential Settlement

Cylindrical steel tanks widely used for bulk and fluid storage generally consist of a thin bottom plate, a cylindrical shell, and a fixed or floating roof. Many practical tanks are tapered, which means the thickness of the tank wall is homogeneously decreases according to the height from the bottom to the top of the tanks. The differential settlement beneath the tank wall leads to most serious consequence for the tank structure and becomes the main cause for collapse of practical tanks.The method of harmonic settlements is used to research the behavior of steel tanks under differential settlement and the effects resulted from each component of differential settlement can be superimposed to obtain the complete solution but it is invalid for non-linear analysis of thin shells .It is necessary to research the structural behavior of tapered steel tanks under measured differential settlement.According to the measured settlements of eight large steel tanks, the differential settlements of eight tanks can be divided into two types by characteristic and consist of total differential settlement and local differential settlement. The structural behaviors of tapered steel tanks with fixed roofs and floating roofs under two types of differential settlement are studied systematically by geometrically nonlinear finite element analysis. The buckling models of tanks under the differential settlement are studied and the corresponding controlling conditions for designing are also given. It is found that the excessive axial deflection is mainly concentrated on the upper part of the tank wall. Compared with the equal thickness tanks, the tapered tanks are more sensitive to the differential settlement.The influence of circumferential single-axisymmetric weld depressions and multi-axisymmetric weld depressions to tapered steel tanks under differential settlement is studied in this paper. It is found that the influence of the axisymmetric weld depressions is approximated to the circular beam. The ultimate differential settlement is not reduced for the axisymmetric weld depressions. Compared with the equal wall thickness tanks, the maximum stress is mainly concentrated on the lower part of the tank wall because of the weld depressions. The negative influence of the variation of the tank wall thickness can be overcame in some degree by the axisymmetric weld depressions.Deformation of large steel storage tanks which are induced by differential settlement usually lead to initial geometric imperfection in the shell. This paper presents the results of the stability of axially compressed floating and fixed roof steel tanks with this geometric imperfection by using finite element method. It is found that the floating roof tanks are much more sensitive to the geometric imperfection than the fixed roof tanks.