Structural Behavior Of Large Steel Tanks Under Harmonic Settlement

Large vertical 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. Large steel tanks constructed in soft foundations are susceptible to various types of settlement deflections. The differential settlement beneath the tank wall leads to the most serious consequence for the tank, and becomes the main cause for the collapse of practical tanks.The differential settlements beneath the tank wall can be expressed as a Fourier series in harmonics; the effects resulting from each component of differential settlement can be superimposed to obtain the complete solution. A great deal of measured data for the settlements of various types of in-site large tanks are collected and summarized, and the components and characteristics of the differential settlement beneath the tank wall are analyzed using Fourier decomposition. The derived harmonic settlements are then introduced in finite element analyses of the tank structure. It has been shown that the stresses and deformations in the tank under differential settlements are dependent on its geometric parameters. Thus current empirical criteria for allowable deformation of foundation that is independent of geometric parameters seems unreasonable viewed from the structural perspective. A reasonable criterion should be based on the structural response.Linear static behaviors of the floating-roof tanks under harmonic settlement are studied systematically by finite element analyses. The studies in this thesis are mainly of such relevance as: a) the maximum top radial displacements and the vertical stresses at the tank bottom and other various most effects in the tank wall; b) The influences of various geometric parameters are expounded in detail; c) Comparisons between the circumferential stress at the top and the vertical stress at the bottom are made for large wave numbers; d) Tank behaviors along height are analyzed in detail, and the concept of height division of the tank is proposed; e) Design equations estimating the radial displacement at the top and the vertical stress at the bottom are developed by regression based on the FE results; f) New criteria based on harmonic settlement analyses are given.Eigenvalue buckling analyses and geometrically nonlinear buckling analyses are carried out for fixed-roof tanks under harmonic settlement. The influences of the harmonic wavenumber, the radius-to-thickness ratio, the height-to-radius ratio, the internal pressure and the axial pressure on the ultimate harmonic settlement are investigated, and the effects of initial geometric imperfections are also investigated. The nonlinear buckling modes of the tank under harmonic settlement agree well with the lowest linear buckling modes, and takes mainly two types of forms: shearing buckling extending throughout the entire height and the elephant's foot failure caused bythe local axial stresses. The nonlinear buckling modes of the tank are mainly dependent on the wavenumber, the radius-to-thickness ratio and the height-to-radius ratio. The ultimate nonlinear harmonic settlement decreases as the wavenumber or the radius-to-thickness ratio increases. The ultimate harmonic settlement decreases as the internal pressure or the axial pressure increases, and such trend is more obvious for tanks of small radius-to-thickness ratios than those of large ratios.