Seismic Fragility Evaluation of Metal Flat-Bottom Storage Tanks with Short Anchor Bolt Chairs

Seismic fragilities of anchored metal flat-bottom storage tanks are usually governed by tank overturning. Tank overturning capacity is typically influenced more by the hold-down strength of the anchors rather than the buckling strength of the tank shell. For tanks with short anchor bolt chairs, the anchor bolt hold-down strength may be limited by the ability of the tank shell near the bolt chairs to accommodate the local demands resulting from the eccentricity between the anchor bolts and the tank shell. The smaller the height of the bolt chairs, the lower the capacity of the tank shell to accommodate this eccentricity.

This paper presents a case study for an anchored tank with relatively short bolt chairs. An initial seismic fragility was developed for the tank following the standard approaches outlined in EPRI (1994), EPRI (1991a) and EPRI (1991b). The short bolt chairs controlled the hold-down strength of the anchors. The resulting overturning capacity computed was low, and the tank was a dominant risk contributor in initial seismic risk quantification runs for the plant seismic probabilistic risk assessment. Consequently, a static nonlinear analysis of the tank shell at and around an anchor bolt chair was performed to compute a more realistic anchor bolt hold-down strength and the associated maximum permissible uplift at the heel of the overturning tank. Material nonlinearity in the tank shell, bolt chair, and anchor bolt was included. Effects of fluid pressures were captured in the analysis.

The analysis results showed that the anchor force corresponding to the tank shell failure was nearly two times the force computed following the guidance in EPRI (1991a). Furthermore, the analysis justified the consideration of this failure mode as ductile, as opposed to the traditional guidance in EPRI (1994) to consider it as non-ductile. As such, a significantly higher permissible uplift was demonstrated by the nonlinear analysis. The substantial increases in the anchor bolt hold-down strength and the maximum permissible tank uplift significantly improved the tank seismic fragility. The improvement essentially eliminated any contribution to seismic risk from the tank.

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