This is where the magic happens. Besavilla uses the Whitney stress block to determine the nominal moment capacity ($M_n$).
He simplifies the interaction diagrams. For short columns under axial load and bending, he provides the famous "Besavilla Curve" approximations.
Singly and doubly reinforced rectangular beams, T-beams, and the Working Stress Design (WSD) method. Slabs & Staircases: