Equation (C-I3-4) of the Commentary to Section I3.1 of AISC 360-10 gives the value of I_equiv. Earlier in that section it indicates that I_eff should be taken as 0.75I_equiv. It is presented as an option in the Criteria - Design Defaults command, but is not recommended. The Commentary to that section in AISC 360-16 states that the necessity of that provision cannot be substantiated, and has removed that recommendation.