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Seismic evaluation of unanchored cylindrical tanks

Martin G. Koller, Praveen K. Malhotra

Proceedings of 13th World Conference on Earthquake Engineering

2004/08/01

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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