Dear Linus,

I do not comment your expectations on version 3.10.
However, just looking to the release date of version 3.10 (09/2008) your Client would ask you whether you comply or not with the contractual API 650 edition.

About your Tank 3.30 calculation, first I understand is based on thickness 0.355in and you've considered tuned seismic spectrum as giving Ai, Ac values near the values found in example #6 from EC/ API 650 Addendum 2 . In addition, you've considered AV -vertical acceleration parameter 0.12488g as in Example #6.
I understand your question is related to the fact in example #6 the total hoop stress is 15,449 psi @thickness of the bottom shell course of 0.5625 in.
Your claim is that when thickness of the bottom shell course is 0.355in in the same seismic calculation, the hoop stress should be 15,449*0.5625/0.355=24,479 psi,
instead you can see other value in your 3.30 Tank report based on thickness 0.355in.
I hope I understood your question.

My opinion: as a Tank user involved in this discussion or as an engineer looking to your Tank posted results, I have no evidence which is the thickness considered by your seismic Tank calculations and I have no chance to understand why a particular value of thickness has been considered in your seismic calculation. So I can say nothing about your claim.

I cannot see other solution than sending your calculation to Coade; for sure they would find an explanation based on an analysis of your Tank file.

And just as a personal remark, for me is hard to understand why you are following a complicated procedure to check Tank results; I think it is enough to input 0.5625in as thickness with CA=0 and to check the Tank 3.30 hoop stress is or is not 15,449 psi (or nearby value).

My best regards.