Stiffened self supported roof under vaccuum loads

[Modeo]

HI everyone,

I'm designing a storage tank under vaccuum considerations according to the API 650 and I'm on the the roof calculation step. My roof must be stiffened.

According to the 5.10.6's NOTE which says :
"Self-supporting roofs whose roof plates are stiffened by sections welded to the plates need not conform to the minimum thickness requirements, but the thickness of the roof plates shall not be less than 4.8 mm (3/16 in.) when so designed by the Manufacturer, subject to the approval of the Purchaser."

The Annex V gives formula for self-supported roof minimum required thickness. My question is my stiffened roof plate thickness must respect minimum required thickness calculated by the annex V ? Or I have to align my design with the note ?

Regards,
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Modeste Silué
Mechanical Engineer

 

[JStephen]

I would interpret it that stiffened roofs do not need to comply with the self-supporting roof thickness requirements in Annex V, either. You DO need to check the thickness of the plate and the design of the stiffeners for the vacuum load, but that is not covered by the standard for a stiffened roof.

 

[IFRs]

I agree with the honorable JStephen but in addition think that the standard requires a minimum thickness of 3/16" in any case.

 

[Modeo]

Thanks JStephen and IFRS for your answers.

I re-checked the Annex V of the API 650 and I saw a note in bracket written before the formula. I pasted it below :
Note that design in accordance with API 620 is permitted for dished dome roofs meeting the requirements of API 620, 5.10.5.1.).

I forgot to mention that my roof is a dished one.

Have you any previous knowledge about the 620's method ?