Vu is the nominal ultimate shear strength of the member. Vc is the component of strength contributed by the concrete. Vs (if shear reinforcing is provided)is the component of strength contributed by steel reinforcing.
If shear reinforcing is provided:
Vu = Vc + Vs
If no shear reinforcing:
Vu = Vc
In design, the nominal ultimate strength is modified by a reduction factor to account for variability in material properties, workmanship quality, etc.
