Make also sure that the load forces you specify must be acting in the same plane. The direction of the load forces influences the result of the effective moment of inertia. In the final stage of the calculation, you specify the direction of the load forces. In the next step, the program draws the principal axes, 1 and 2, for the moment of inertia and performs the moment of inertia calculation. AutoCAD Mechanical toolset fills the area, and you are asked if the filled area is the area you want to be analyzed.
The cross section you select must be a closed contour. When you start the AMINERTIA command, you are prompted to select a cross section.