Finite Element Analysis of Buckling Capacity of Conical Steel Tanks Considering Field-measured Initiation Imperfections – A Case Study
Abstract
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.