Check out "Aboveground Storage Tanks" by Phillip Myers. his book has a good discussion of seismic design for both the tank and ringwall.
Jeffcoat Smitt
www.etank.com
www.fteinc.com
I didn't see the header. If you're asking about a 3/16" bottom, API-650 Appendix S will allow this only for stainless steel.
Jeffcoat Smitt
www.etank.com
www.fteinc.com
API-620 can be used for conical bottoms, but as Steve mentions, more than the bottom plate thickness needs to be checked to ensure the integrity of the tank. The bottom plate, compression ring area, supports, and nozzle reinforcement all need to be addressed. Etank2000 software can do most of...
As stated above API-650 Appendix A will allow for a 3/16" shell and 1/4" bottom with no radiograpy. Depending on roof loads and internal pressure a 3/16" or 1/4" Domed Roof would work. If a domed roof is too tall, a conical roof is possible. But an unsupported conical roof with a 2:12 slope...
Like you said the code IS very clear, but your argument does sound reasonable. There are a lot of 100' - 200' tanks in service and I'm sure their compression rings are bigger than the area of the shell that you are allowed to take credit for, so the API-650 committee must have it's reasons...