All this for a Materials Science Question?!
I just spent way too long trying to find a dedicated section for eccentrically loaded weld groups (the J formula, FMX, and FMY stuff) in the manual, but….there isn’t one…
You gotta pull from Statics & Mechanics of Materials.
1.) Treat the welds as lines, (width 1 unit), Use the Parallel Axis Theorem (I = I_c + Ld^2) to build your I_x and I_y piece by piece.
2.) Use the polar moment of inertia (J = I_x + I_y). The formula for the force components (f = My/J) is just the torsional shear stress formula (tau = Tr/J) in disguise.
Basically, this is a Structural problem "disguised" as a Statics/Materials Science hybrid. If you're looking for a shortcut in the Structural tables, you're going to waste a lot of time searching for a page that isn't there.