to calculate if we need intermediate wind girder or not according to API650

5.9.7 Intermediate Wind Girders
5.9.7.1 to calculate the maximum height of the unstiffened shell

t = as ordered thickness, unless otherwise specified, of the top shell course (mm)

5.9.7.2 to calculate the height of the transformed shell

t= as ordered thickness

I think that we have to calculate according to corroded thickness as it is the worst case.

Please advise.

Yes, corroded thickness is the worst case.

In case Purchaser wants have the tank withstand to the designed hurricane in the end of tank's life, he must activate option "Check Buckling in Corroded Cond.? Yes No" in datasheet.

Best regards.

Thank you for your reply.
I have to revise the calculations from the contractor and the engineer prepared the data sheet didn't say anything about "Check Buckling in Corroded Cond."
and also the software use the un corroded condition

please advise

What I try to explain is the API 650 opinion.

I would add that in API there is an explicit note in 5.9.7.1.
NOTE 1 The structural stability check of wind girder stiffened shells in accordance with 5.9.6 and 5.9.7, shall be based upon nominal dimensions of the shell course and the wind girders irrespective of specified corrosion allowances whenever the "No" option is selected for "Check Buckling in Corroded Cond.?" on the Data Sheet, Line 9. Whenever the "Yes" option is selected, the check must be based upon the nominal dimensions minus the specified corrosion allowance.

IMO, you have to discuss with Client asking for Purchaser option.

Personally I don't agree with practice to say "No" i.e to use nominal thickness; the reason is that the formulas are based on the modified U.S. Model Basin formula for the critical uniform external pressure on thin-wall, where no safety coefficient is considered.
Just in brackets, also appendix V calculates shell under vacuum and wind and their procedure utilizes the nominal thickness of thinnest shell course and the transformed shell method to establish intermediate stiffener number and locations. However in appendix V a "ψ" has been considered ; ψ is the stability factor and acts as a safety factor.

thank you for your reply
now everything is clear