Beam Torsional Stiffness by STAAD


Can you help me this out?

There is a simple beam resisting a torque as shown in the file attached. The beam is fixed at both ends. Calculated by STAAD, the torsional angle at the loading point is 0.121 rad. But when I do the hand calculation (using formulas in AISC Steel Design Guideline Series 9), the angle is 0.067 rad (about 50% for this calculation case).

I checked the sectional properties of the beam given in STAAD, everyone looks good except the warping constant Cw is not given.  Coicidently, when I ignore Cw in the calculation the rotational angle is close to the value from STAAD. Does STAAD consider Cw?