Effective weigth for Vertical Seismic Forces - Appendix E - API 650

[carlosbenavides]

Dear coelleagues,

I'm writing to find an answer about the meaning of a note in E.6.2.1.2 : "If the ratio of operating pressure to design pressure exceeds 0.4, the purchaser should consider specifying a higher factor on design".
I always thought that note refers to the factor 0.4 that multiplies Av (vertical seismic coefficient) in equations like E.6.2.1.2-1: "...(1 - 0.4 Av)". (See attached figure)
So, that factor could be greater depending on a particular situation. For example, if the Purchaser consider that the maximum design product level is a normal operation level, that factor becomes 1.0.
Am I alright?

Thank you very much for your answer.

Best regards,
Carlos Benavides G.

 

[JStephen]

I would interpret that statement to refer only to the 0.4 factor in load combination (f) in Appendix R which modifies the internal pressure. (Also to the similar equations combining wind and pressure uplift.)

I believe the 0.4 factor on vertical acceleration is based on the assumption that maximum lateral and vertical acceleration are not likely to happen at the same instant, and is unrelated to the design pressure issue or the fill height.

 

[carlosbenavides]

You have all the reason, dear JStephen.
0.4 factor comes from a simplification of SRSS method, which take in account what you said about maximum lateral and vertical acceleration are not likely to happen at the same instant:

Considering Cv = 0.7 Ch,

sqrt(Ch^2+Cv^2) = Ch sqrt(1 + 0.7^2)= Ch * 1.22 = ...
...= Ch + 0.22 Cv/0.7 = Ch + 0.32 Cv

...what is aproximately Ch + 0.4 Cv.

Thanks JStephen for your answer.