Stability Behavior Of Large Tapered Steel Tanks Under Differential Settlement

Large vertical cylindrical steel tanks employed in various industries for bulk and fluid storage normally consist of a thin bottom plate, a cylindrical shell and a fixed or float roof. As a typical kind of thin-wall structure, these tanks frequently constructed in coastal regions are sensitive to the foundation settlements. The differential settlement beneath the tank wall is the principal potential source of overstress and distortion resulting in significant damage to the practical tanks. Most previous studies are based on idealized harmonic settlement and the tanks with uniform shell thickness. Nevertheless, real circumferential settlements do not occur in a single harmonic form, and practical tanks usually show a tapered wall thickness.In this paper, real circumferential settlements are obtained using Fourier decomposition, based on a mass of measured data for settlements of 8 in-site tanks. It is found that measured settlement of large steel tanks can be grouped into two types-global differential settlement and localised differential settlement. Then this paper describes the specific case studied of floating-roof tanks and fixed-roof tanks with different geometries, which are representatives of many parctical tanks.Nonlinear responses and stability behavior of the case tanks under above two type settlements are studied systematically by finite element analyses, and some suggestions of structural design are proposed. The studies in the thesis are mainly of such relevance as:(1) behavior of tanks under global differential settlement; (2) behavior of tanks under localised differential settlement; (3) A comparison of post-buckling behavior of floating-roof tanks with different geometries; (4) A comparison of the nonlinear responses of fixed-roof tank under different localized settlements; (5) the effect of the circumferential span of the local settlement; (6) the influence of weld depression to the stability of steel tanks under differential settlement.In practical project, the patterns of the measured settlemts are various and complex, so "Equivalent differential settlement" is derived by the superimposition fo harmonic settlements. The validity of the "Equivalent differential settlement" is guaranteed by investigating the localization of the peak-settlement segment and the structural behavior. "Most unfavorable equivalent differential settlement" is obtained by massive numerical analysis, which is the most disadvantage settlement for the structural stability, and can be applied in the preliminary design.