I think that the standard states that for when the half-apex exceeds 30 deg. a reinforcement ring can be included to stiffener the cone structure.
Read all the rules in UG-32
A reinforcing ring shall be provided when required by the rule in 1-5(d) and (e).
Conical heads or sections having a half apex-angle a greater than 30 deg. without a transition knuckle shall comply with Formula (4) and 1-5(g).