Structural Behavior Of Steel Storage Tanks Under Measured Differential Settlement

Steel cylindrical tanks for the storage of fluids such as oil or water have either floating or fixed roofs which consist of a cylindrical sheik fixed or floating roof and a thin bottom plate.Various types of settlement displacement may occur for large storage tanks constructed on soft ground. The different 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. The effects resulting 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 steel tanks under measured differential settlement.The measured settlements of eight large steel tanks are collected and summarized and the components and characteristics of the differential settlement beneath the tank wall are analyzed using fourier decomposition analysis.The differential settlements of eight tanks are divided two types by characteristic and consist of total differential settlement and local differential settlementThe structural behaviors of steel tanks with a fixed or floating roof under two types of differential settlement are studied systematically by geometrically nonlinear finite element analyses .It is found that two types of tanks are fairly sensitive to differential settlement. The post-buckling behaviors of wind grider of floating-roof tanks under total differential settlement are stable and general buckling of shell lead to failure of structure finally.The snap-through buckling occur on shell of floating-roof tanks under local differential settlement when the degree of locality is high and the stability of post-buckling behaviors can be utilized in designing.The structural behaviors of fixed roof tanks are influenced by characteristic of differential settlement and the nonlinear buckling modes consist of shearing buckling and the elephant's foot failure.However the influence of differential settlement to fixed roof tank is more than floating roof tank.The influence of axisymmetric weld depressions to steel tanks under differential settlement is studied in this paper.It is found that the effect of axisymmetric weld depressions which bear the circumferential bending and prevent the transfer of differential settlement to top of tank is taken as the wind grider approximately,it leads to the increaseing of vertical stresses at the tank bottom, the decreaseing of top radial displacements and influence the buckling behaviors.Geometrical misfits of large steel storage tanks which are induced by differential settlement usually lead to initial geometric imperfection in the shell and this paper presents the results of the stability ofaxially compressed floating roof steel tanks with this geometric imperfection using FEM. It is found that the stability is fairly sensitive to this geometric imperfectionIt is reasonable to consider the rigidity of tapered tanks when designing.Linear finite element analyses are carried out in this paper on floating-roof tapered tanks under harmonic settlement. Deformations and stresses of tapered tanks are discussed and structural behavior of tapered tanks and uniform tanks under harmonic settlement is also compared.