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Seismic evaluation of unanchored cylindrical tanks

Martin G. Koller, Praveen K. Malhotra

Proceedings of 13th World Conference on Earthquake Engineering


The seismic resistance of seven existing unanchored cylindrical oil storage tanks was calculated according to Eurocode (EC) 8, part 4, Appendix A [1]. Five of these tanks, with H/R ratios (H = height of liquid, R = radius of tank) of 1.12, 1.14, 1.75, 2.16 and 3.48, were also calculated with the non-linear pushover analysis proposed by Malhotra [2]. All tanks are located in Switzerland. None of them was designed to resist earthquakes.

EC 8 limits the cyclic plastic rotation in the base plate to a maximum value of 0.2 radians (11.5 degrees). In all cases, this constraint turned out to be more stringent than elephant-footing or elastic buckling of the shell.

Very different results were obtained by the two methods of calculation. The EC 8 results show a strong correlation between H/R ratio and plastic rotation and nearly no correlation between tank volume and plastic rotation. Only a very weak H/R influence on plastic rotation can be seen in the results from the pushover analysis. However, plastic rotation increases linearly with tank volume, irrespective of the H/R ratio (with the exception of the very slender tank with H/R = 3.48).

For moderate to high H/R ratios (H/R ≥ 1. 75), EC 8 leads to plastic rotations at least twice as high as those obtained by the non-linear pushover analysis. The opposite is true for the low H/R ratios (H/R = 1.12 and 1.14). Here, pushover analysis leads to about 1.5 times the plastic rotation of the EC 8 calculation. Possible reasons for these discrepancies are discussed and tentative recommendations for practitioners are given.

Vancouver, Canada

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