Area Moments

Hello, I found a probable bug in the calculation of area moments of inertia for a planar figure.
In the particular case of figure below (planar figure laying on plane XY): principal area moment I3 should be equal to Iz (and not zero). Moreover I1 (associated to direction 0,0,1) and I2 (associated to direction 0,1,0) seem to be swapped.
Using Rhino 6 SR34.
Best regards

Hi @tempadd9,

If you don’t mind, can you download a Rhino 7 evaluation and see if you seeing the same issue?

If not, then feel free to post your 3dm file and I’ll have a look.


– Dale

Hello Dale,
I just saw your reply. No, I don’t have rhino7.
I think I lost the example used in the first post, in any case the error is always reproducible with any planar figure.

For example, issuing the following command:
! _-Plane 0,0 10,2 _SelLast _AreaMoments

the result is:
Area Moments of Inertia about Centroid Coordinate Axes
Ix: 6.66666667 (+/- 1e-09)
Iy: 166.666667 (+/- 1e-07)
Iz: 173.333333 (+/- 1e-07)
Area Principal Moments of Inertia about Centroid and Principal Axes
I1: 166.666667 , Direction ( 1, 0 ,0)
I2: 6.66666667 , Direction ( 0, 1 ,0)
I3: 0 , Direction ( 0, 0 ,1)

The issues are the same:
I1 (along direction 1,0,0) should be equal to Ix and I2 should be equal to Iy: they are swapped.
I3 should be equal to Iz (in general for planar figures I3=I1+I2 by definition).
Best Regards

This has been fixed in Rhino 7.

– Dale

Thanks, I understand Rhino 6 is no more mantained, even for bugs.
That’s a pity, as I still have extensions that force me to adopt this version.

Rhino 6 plug-ins run in Rhino 7…