spandrel-to-beam shear plate connection

huangyhg

spandrel-to-beam shear plate connection
aisc manul part 10 has a discussion entitled "girder-web supports"
they show the condition of a "spandrel" which has a beam framing into one side only.  the beam is connected to a full depth shear plate which is welded to the web and flanges of the supporting girder.  
my question is, why is the full depth shear plate necessary? what does it accomplish? i have read the discussion but its not clear to me.
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the shear plate (tab) has a tendency to rotate and introduces high concentrated tension & compression stresses on the web. by using a full depth plate, the connected flanges will take some, if not all, of the force away to minimize stress concentration on one part of the beam. i don't think it is mandatory, but suggestion of a good practice.