"Question: Is it permissible to install stiffening rings on cone roof tanks to maintain the roundness, peaking, and banding requirements set forth in Section 5?
"Answer: No."
This is an interpretation from November 4, 1994, to the Ninth Edition of API-650.
I would say that question and interpretation could be taken different ways, though. There's nothing that says you can't put additional stiffeners not required by design on a tank. But the point is, you shouldn't be using stiffeners to force an out-of-tolerance shell into the tolerance.

Nice memory !!!

thank you all for your reply .

JStephen,
Where can i find that interpretation? i found the document "technical inquiries for API standard 650" online from the api committee website but when i search for that one it doesn't show up there.