Master of Engineering Science
Civil and Environmental Engineering
El Ansary, Ayman M.
The geometric imperfection in elevated steel conical water tanks is a key factor that influences the buckling capacity of the tank. Current considerations of imperfections in the design of conical tanks are based on theoretical analysis, whereby the imperfection shapes and locations are assumed to have the most critical impact on the capacity. This thesis investigates the initial imperfections of an actual stiffened liquid-filled steel conical tank (LFCT) based on high-resolution laser scan measurement data of the tank geometry.
In the first part of this study, detailed analyses of the laser scan data were carried out to extract the global and local initial imperfections of the tank. The global imperfection represents the ovalization of the tank circumferences at difference elevations and shift of the tank central axis from the nominal central axis position. The local imperfection is the difference between the overall and global imperfections. As part of the evaluation of the tank’s structural integrity the imperfections extracted from the laser scan data are compared with specified tolerances recorded in design standards (AWWA D100-11; EN 1993-1-6: 2007, etc.) and with theoretical expressions available in the literature. Analysis results have shown that local & total imperfections exceed the tolerances specified in the design standards at several locations on the tank and the discrepancy between the imperfection wavelengths specified in the standards and observed from the data.
In the second part of this study, three-dimensional finite element models of the stiffened conical steel water tank were established. Initial imperfections of various shapes have been incorporated into the models, including patterns extracted from the laser scan data and assumptions from previous studies. Their impacts on the buckling capacity were analyzed by a series of elastoplastic analyses and compared with each other. Conservativeness of assumed imperfection shapes have been verified with more impact than components of field measured imperfections of higher amplitude.
Summary for Lay Audience
Research on imperfect steel water tanks is important since the buckling capacity if these tanks can be significantly reduced with very small imperfections. Previous research and design codes assumed series of critical imperfection shapes which is close to the first buckling mode. However, the effect of imperfections is sensitive to their shapes, which requires deeper study on imperfections measured from real structures. Yet, the study on the geometric imperfections measured from a real liquid-filled conical tank (LFCT) is still lacking with only few investigations on cylindrical oil tanks and silos have been carried out. Geometric imperfections are considered as the critical shape in design standards which leads to conservative approach. To make an improvement, this study provided an investigation on the effect of geometric imperfections on a real LFCT.
The shape and amplitude of imperfections are evaluated in the first part. Geometric imperfections introduced in this study are measured on the exterior surface of a tank, which provided a set of high-resolution laser scan data. According to the consideration of imperfection components in design standards, the entire imperfections can be classified as global imperfections and local imperfections. A series of data analysis including least-square (LSQ) method was employed to decompose the laser scan data into global & local imperfections. Their shape and amplitudes were compared with tolerances in design codes to evaluate the quality of this imperfect tank based on various standards.
The second part employed elastoplastic finite element analyses (FEA) to evaluate the impact on buckling capacity of the tank by various imperfection shapes. This was realized by modelling a full-scale stiffened conical steel water tank with initial imperfections. Several imperfection shapes were applied on the tank including patterns extracted from laser scan data and conservative assumptions from previous studies. It was shown from the results that the conservativeness of assumptions reported in the literature have been confirmed by this experiment, while the effects of global, local and total imperfections from real measurements are complicated.
Zhang, Haoyi, "Finite Element Analysis of Buckling Capacity of Conical Steel Tanks Considering Field-measured Initiation Imperfections – A Case Study" (2021). Electronic Thesis and Dissertation Repository. 7636.
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