I work for a window manufacture and we have accomplished 1/2 or more of live load movement before but it is not as common as 1/4 or 3/8. The other thing to remember they are probably getting pressure to keep a small caulk joint at the head and depending how it is detailed and what caulk is used...
I don't know if this is quite what you are asking, but the polar moment of inertia of a circular weld is J=(Pi*d^3)/4. To end up with an actual strength/in I would use your Moment (or applied torque)*d/J With your allowable strength/in being .6*Fexx(7/16*0.707).
If your asking about a square...
