Defining new steel section

I want a built up column consisting of 4 plates on all 4 sides. Therefore I tried to add the section in the section database. But while adding the section it is asking moment of inertia for all three axis (i.e) Ix, Iy & Iz. How to calculate moment of inertia for the centroidal axils, that is through the length of the column?

Eagerly waiting for response, Please help.

Thank you