The only thing you will find in MERM is how to calculate the Inertia of the weld group (Ap. 51.A). (Please correct me if I'm wrong). Then you go to Shigley and use the weld joint shear stress equation: Shear Stress = Moment * r / (0.707 * h * Ju), where Ju is the inertia calulated from the Ap. 51.A based on the shape of the weld. The "r" will be the distance from the centroid of the weld joint to the far point on the weld.
Hope this help,
Rafa