Modelling Shear Design for double web plate girder

[Syahzada]

Im conducting an analysis of a steel frame, with a double plate girder beam as one of its element. In the first analysis results on the unstiffened beam, the calculated shear stress is 17.97 kg/mm^2 and the shear area is 1520.00 cm^2, resulting an unity ratio of about 1.55 based on AISC allowable stress with an increase of 1/3. Correct me if im wrong, stiffeners usually calculated based on the failure modes of the flange yielding and the web buckling or crippling, but if i want to stiffened the beam as per shear design, how do i design the dimensions of the stiffener that'll increase the shear capacity of the beam (stiffened)?

And also, if the structural analysis software im using (SACS) do not have any stiffener option in modelling of the beam, does modelling the cross-section length of the stiffener plate as the new web thickness of the Double Plate Girder acceptable?