CalvinD82;
Plug welds can be treated like fillet welds. If you know the size of the weld, weld metal type you have all of the essential variables necessary to determine the allowable stress per unit length of weld under tension (based on weld metal ultimate tensile strength) and shear (50% of the ultimate tensile strength as and approximation).
Plug welds are there own unique item. As metengr has stated they can be treated like a fillet weld. But there is a lot to be said for the welding procedure. AWS D1.1 (06) C-5.26.5 describes a technique that is much better than the "swirl the electrode" technique, yet both are seen when working to AWS.
