When I use 'Section Properties' to calculate the polar moment of inertia (i.e. the object's ability to resist torsion) of a shape that is NOT circular the sold gives me an answer the doesn't mach the textbook calculation.

For example when I calculate the MOI of a square (10x10mm) the solid says it's 1666.6mm^4 and according to the textbooks it's 10^4 X 0.141 = 1410mm^4

Why does it happens and how can see the object's ability to resist torsion?

Eyal,

Jim asked me to help you out here. In Simulation we do compute the torsion constant when we do weldment analysis (treated as beams) for any cross-section. See attached picture showing you the values for these calculations for a square tube. So if you have SOLIDWORKS Premium you should be able to find the section properties of any cross section. A nice enhancement would be to leverage this piece of code inside section properties dialog box so that you do not have to go through simulation to get these values. Can you please go through your VAR and file this enhancement. The more customers request for this helps SOLIDWORKS gets a sense of priorities for features to be implemented in each release.