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ITS on Doorsheet ReinforcementBy: ITS 
Date: Tuesday, January 17, 2017
Time: 3:59:55 PM
Well, there is no escaping the reinforcement of an opening 2 inches or greater. A door sheet is of course much larger. Regardless of the shell course, a large opening will weaken the area around the opening and would be evident in a 45 degree ripple, so it needs to be stiffened.
On the buckling issue of the shell I would refer to API Bulletin 2U, which I think was last updated in 2004. The root or basis of this rule of thumb on buckling stress is based on one-third or 1/3 of yield stress which means anything over that is fair game to a wrinkle or buckle in the shell.
Hope other have more insight...