Grillage Model for Elevated Ramp - Torsional Constant

I am modeling a grillage for an elevated ramp and have been following instructions from couple of textbooks. In order to define the transverse diaphragm members I need to define two section properties as below, second moment of area and torsional constant:

I = bd^3/12 & C=bd^3/6

When I define a rectangular concrete section the STAAD Torsioanl constant, i.e. Ix is not equal to C=bd^3/6 .

The first question is how Ix is being calculated in STAAD and also how can I define my torsioanl constant for my grillage model, i.e. C=bd^3/6.

many thanks