The hinge yield moment is determined from the rebar assumed to be in the top and bottom of the beam, Astop and AsBot.
If the beam has been designed then AsTop and AsBot or taken from design, otherwise they are taken from the reinforcement specified for the beam (if any).
If AsTop is zero it is set equal to 0.0033 times the area of the beam. Similarly, if AsBot is zero it is set equal to 0.0033 times the area of the beam.