Shell Buckling Evaluation of Thin-Walled Steel Tanks Filled at Low Liquid Level According to Current Design Codes

The structural stability of thin-walled steel tanks becomes a major safety issue when these operate at low levels of contained liquid. Despite numerous tank failures due to buckling of their circumferential shell, provisions in current codes do not provide cost-efficient or high-safety level solutions regarding this phenomenon. For example, the American standard API 650, which has worldwide applications, proposes only an empirical design method for stiffening the tank shell based on its thickness, height and the design wind velocity. More recent codes, such as the European standard EN1993-1-6, provide analytical relationships for evaluating the buckling resistance of shells, with stability being verified by relevant checks against appropriate design stresses. However, their provisions have not yet seen many field applications and results raise, in certain cases, doubts regarding the efficiency of the design. This paper presents a direct comparison between these two standards by attempting to evaluate the buckling resistance of two existing thin-walled steel tanks, filled at a low liquid level. Both tanks have large diameters (47m and 88m approximately), variable wall thickness, are self-supported (unanchored) and one of them supports a conical roof. The design stresses required by EN1993-1-6 were obtained from finite element simulations of the tanks (linear elastic analyses were performed) which included application of the actions specified in the provisions of the Eurocode. Results from both standards are discussed in detail, comparisons are made and discrepancies between the two standards are highlighted. Comments and conclusions to be drawn will help improve current specifications and resolve issues related to the safe design of liquid storage tanks filled at low liquid levels.

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