Depending on the situation, you have to decide which one does not. The issue is related to b beam height, a related to the width of the column on which the beam is supported, and the longitudinal reinforcement diameter of the beam. 1) To achieve a+b>lb, you must increase the column width and/or beam height. In general, it is sufficient to increase the column width. 2) You have to increase the column width to get a>0.4lb. 3) To achieve b>12fi, you must reduce the rebar diameter.