topic Re: Calculation for Product of Inertia in AutoCAD using MassProp in AutoCAD Forum

Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Wed, 05 Apr 2017 18:39:56 GMT

Please see attached png file, pay special attention to the value of "Ixy", how come the sign is different between AutoCad 2010 and 2017?!

Btw, AC2010 value is the correct one!!!.

Aero1

@Anonymous,

john.vellek has edited your subject line for clarity: Product Of Inerti,...

Re: Calculation for Product of Inertia in AutoCAD using MassProp

imadHabash — Wed, 05 Apr 2017 18:40:33 GMT

Hi,

would you please attache the CAD file for checking?

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Wed, 05 Apr 2017 18:40:50 GMT

You can simply sketch your own, 3 rectangular shapes! (a simple "Z" or "S" section), any dimensions or scale would give you the same results,.... you will notice that the sign of Ixy or Iyz,.... will be different between 2010 and 2017,... this is very important when you are dealing with combined bending in analysis, it would change all the tension values to compression and vise versa throughout the sections,....!.

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Wed, 05 Apr 2017 18:41:08 GMT

Here you go, it seems that the Principal Axis are rotating!!!

Re: Calculation for Product of Inertia in AutoCAD using MassProp

john.vellek — Sat, 08 Apr 2017 14:26:28 GMT

Hi @Anonymous,

The oldest version of AutoCAD that I have installed is 2014. It shows the value to be the same as version 2010. In 2015 and newer products it shows the opposite value.

[EDIT] Thanks for your patience.

Thank you for posting about this issue in the forums. I have logged this issue with the development team for further investigation. Please do not hesitate to reply to this thread with any additional questions you may have in regard to this issue or any message that you want me to share with development that we haven’t already discussed.

You are also welcome to contact the product team directly. If you have additional comments specific to the software or its functionality for future consideration, please use the Product Feedback page. How to provide feedback on Autodesk products goes into more detail.

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Tue, 11 Apr 2017 17:31:44 GMT

I think this is an important issue, I believe that many consulting firms "all over the world" have performed their analysis using this feature "since 2015"!?

Re: Calculation for Product of Inertia in AutoCAD using MassProp

john.vellek — Tue, 11 Apr 2017 17:37:25 GMT

Hi @Anonymous,

This feature is apparently working as designed.

It was explained to me that the product of inertia is a measurement of resistance to rotation around an axis (or intertia to rotate around an axis). Positive is (conventionally speaking) that the axis goes left to right and negative is right to left. Because you are measuring rotational inertia around that axis both positive and negative values have the same meaning.

I hope this explains the change. Thanks again for your patience.

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Wed, 12 Apr 2017 08:36:21 GMT

Hi John,

Thanks for your reply, you are right about both positive and negative have the same meaning,... but do they have the same value!?

That negative sign would make a huge difference!, it would basically flipping the coordinate system!,...

Please have a look at the attached example.

Aero1

Re: Calculation for Product of Inertia in AutoCAD using MassProp

john.vellek — Wed, 12 Apr 2017 13:55:10 GMT

Hi @Anonymous,

I am still investigating this issue with @Victoria.Studley to see if we can come up with more information and evidence on this issue. Thank you for your patience.

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Wed, 28 Jun 2017 11:36:41 GMT

I stumbled upon this problem myself.

Massprop seems to flip the sign of the product of inertia, or as someone else have pointed out, it rotates the reference system by 90°

You can try also with this minimal working example:

If you draw a square of length one (or any bi-symmetric shape) , the formula for the product of inertia can be simplified as follows:

Jxy = xG * yG *A

where:

Jxy: product of inertia
xG: x-coordinate of the square's centroid, i.e. distance from y-axis
yG: y-coordinate of the square's centroid, i.e. distance from x-axis
A: area of the square (=1 in this case)

Now, assuming the axes have origin in (0,0), put the square in each quadrant (counterclockwise) at this coordinates:

Quadrant I:   (x,y) = (1,1)   -> Jxy = 1 * 1 = 1
Quadrant II:  (x,y) = (-1,1)  -> Jxy =-1 * 1 = -1
Quadrant III: (x,y) = (-1,-1) -> Jxy =-1 *-1 = 1
Quadrant IV:  (x,y) = (1,-1)  -> Jxy = 1 *-1 = -1

Instead the Massprop command return the exact opposite for each quadrant.

Re: Calculation for Product of Inertia in AutoCAD using MassProp

Anonymous — Sun, 02 May 2021 16:50:00 GMT

Version 2021 and we still have the same problem ... anybody found the way to config the MASS properties to solve this issue at least locally?? Thanks!