"Equilibrium of forces" is the principle used in the current New Zealand code requirements for shear reinforcement of beam column joint. This principle, however, yields irrational or empirical aspects of the current code requirements. The latter principle as well as a simplification of stress distribution along the periphery of the joint can yield a design equation to calculate the necessary amounts of shear reinforcement in inelastic interior joint. Normally, this design equation requires smaller amounts of shear reinforcement than the current New Zealand code, especially for the following cases: a) the joint shear stress is small, b) the axial force is small, c) the amount of top beam reinforcement is larger than at the bottom, and d) the amounts of vertical shear reinforcement is large.