shearing stress of parallel plates

huangyhg

shearing stress of parallel plates
i am trying to analyze shearing stress of (2) parallel plates and need confirmation in applying theory.
visualize the joint as (2) parallel plates projecting out of a base plate, there is a cross pin (located opposite end to the base plate) through the parallel plates of which a torsional load is applied.
i am considering using polar moment of inertia for weld calculation theory, j=bt(3a^2+b^2)/6.
any thoughts?
rick