We recalled that a transverse shear force V = ΔM/Δx; therefore, whenever V ≠ 0, the resultant forces acting on the 2 x 12 are unbalanced in the longitudinal direction. It is this unbalanced force acting on a portion of the cross section that we calculate with the shear flow equation.
Fa , Fb =
resultant forces of bending stresses acting
on the bottom 2 x 12 board
For the 2 x 12 board, the shear flow q is the unbalanced force per unit length of beam that must be supplied by the nails so that the 2 x 12 board satisfies equilibrium in the x direction.