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Seismic Evaluation of Petroleum Storage Tank

jstello's picture

I´m doing a plastic collapse evaluation as per API 579-1 Appendix B1, and one of the load cases is Earthquake loads. 579 refers to ASCE 7, which for tanks refers to API 650 Appendix E. The approach in 650 is a static equivalent lateral force seismic analysis where you find equivalent lateral forces and moments according to some likely earthquake spectrum. It seems that what you do is apply these at the top of the tank shell, which mimics the overturning and shear behavior, assuming the tank doesnt slide from inertia. 

My question is whether this analysis is valid as far as evaluating tank roofs under earthqake loads, as it seems from intuition that it would only be valid for the tank shell. 

 

Any insight is welcome! Thanks a lot!

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Alejandro Ortiz-Bernardin's picture

Jstello,

The static procedure described in the API 650 standard involves the calculation of an impulsive and a convective seismic force. The location of these two seismic forces is also calculated using the procedure provided in this standard. They are not necesarilly located at the top of the shell and usually they are located at different levels.

The base of the roof is located at a distance of at least equivalent to the free board. The free board is at least equal to the height of the wave of the sloshing mode. Therefore, the seismic force that is applied on the roof is determined by the impulsive mode. This calculation is also provided in this standard and so you can use it for the roof.

Best.

 

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