topic Re: Calculate massproperties self created hollow section in AutoCAD Forum

Calculate massproperties self created hollow section

gert.valk — Wed, 26 Aug 2020 13:51:03 GMT

How to calculate massproperties of self created 2D hollow sections in autocad 2020?

Re: Calculate massproperties self created hollow section

leeminardi — Wed, 26 Aug 2020 19:28:08 GMT

Convert the 2D shape to a region with the region or boundary command then use the massprop command.

Re: Calculate massproperties self created hollow section

gert.valk — Thu, 27 Aug 2020 06:39:48 GMT

If I do it your way the result are wrong. For example, if I want to calculate the moment of inertia of a pipe, the values that autocad has calculated do not match with the values in the table at all, if I calculate it manually the results are matching the table. What am I doing wrong in autocad?

Re: Calculate massproperties self created hollow section

leeminardi — Thu, 27 Aug 2020 11:38:54 GMT

The table is giving you the mass moment about the shape's centroid while AutoCAD is giving it to you about 0,0,0.

Use massprop to find the centroid then create a UCS at the centroid and execute massprop again.

Re: Calculate massproperties self created hollow section

dbroad — Thu, 27 Aug 2020 12:37:39 GMT

Massprop should actually be giving both the moments of inertia about the x and y centroidal axes through 0,0 AND the principal moments of inertia about its own centroid. It also gives the directions of the principle axes. It just depends on which numbers to pay attention to. For non-circular shapes, the process you described (moving centroid to 0,0 or temporarily moving the ucs to the centroid) would be necessary since the directions of the principal axes wouldn't be parallel to the x and y axis. See below:

Command: MASSPROP Select objects: 1 found Select objects: ---------------- REGIONS ---------------- Area: 12.56637 Perimeter: 12.56637 Bounding box: X: -2.00000 -- 2.00000 Y: -2.00000 -- 2.00000 Centroid: X: 0.00000 Y: 0.00000 Moments of inertia: X: 12.56637 Y: 12.56637 Product of inertia: XY: 0.00000 Radii of gyration: X: 1.00000 Y: 1.00000 Principal moments and X-Y directions about centroid: I: 12.56637 along [1.00000 0.00000] J: 12.56637 along [0.00000 1.00000]

Re: Calculate massproperties self created hollow section

leeminardi — Thu, 27 Aug 2020 13:19:56 GMT

@dbroad You are correct. I relied on my failing memory when I thought I had to move the axes to the centroid.

Re: Calculate massproperties self created hollow section

gert.valk — Thu, 27 Aug 2020 13:37:40 GMT

See the results calculated according your advice, it's not correct.

Re: Calculate massproperties self created hollow section

leeminardi — Thu, 27 Aug 2020 14:12:30 GMT

Are you sure you have a region that includes the hole? Change your visualstyle to conceptual to check. The results I get do agree with hand calculation.

Re: Calculate massproperties self created hollow section

gert.valk — Thu, 27 Aug 2020 14:28:34 GMT

Yes

Re: Calculate massproperties self created hollow section

leeminardi — Thu, 27 Aug 2020 16:24:42 GMT

At 314016, note that the area you have calculated is the sum of the two circles. The area should be 14042. You have not subtracted the smaller circle from the larger one.

Re: Calculate massproperties self created hollow section

dbroad — Thu, 27 Aug 2020 16:44:11 GMT

The most obvious questions are:

Did you draw it accurately according to the dimensions?
To what standard are you comparing it?
What formulas are you using?
Could you have erred in your calculations?
Did you subtract the inner region from the outer region to make the pipe before using massprop?

This is my result, which agrees with @leeminardi

---------------- REGIONS ---------------- Area: 13414.60063 Perimeter: 2682.92013 Bounding box: X: -218.50000 -- 218.50000 Y: -218.50000 -- 218.50000 Centroid: X: 0.00000 Y: 0.00000 Moments of inertia: X: 305901522.31017 Y: 305901522.31017 Product of inertia: XY: 0.00000 Radii of gyration: X: 151.00869 Y: 151.00869 Principal moments and X-Y directions about centroid: I: 305901522.31017 along [1.00000 0.00000] J: 305901522.31017 along [0.00000 1.00000]

This is the formula for the moment of inertia for a pipe that I use:

So pi*(437^4-417^4)/64= 305901522.310175, which agrees with above.

Re: Calculate massproperties self created hollow section

gert.valk — Fri, 28 Aug 2020 06:01:29 GMT

Sorry, I have tried several times as you explained but I keep getting the same wrong result. Maybe it has to do with a variable?

Re: Calculate massproperties self created hollow section

dbroad — Fri, 28 Aug 2020 12:11:10 GMT

Post your drawing. Anything else is a waste of time.

Re: Calculate massproperties self created hollow section

gert.valk — Fri, 28 Aug 2020 12:23:29 GMT

Succes!

Re: Calculate massproperties self created hollow section

gert.valk — Fri, 28 Aug 2020 12:27:08 GMT

Succes!👍

Re: Calculate massproperties self created hollow section

leeminardi — Fri, 28 Aug 2020 13:39:38 GMT

See post #10 then change the visualstyle as noted in post #8! To change the visual style give the visualstyles command then select the conceptual option. You should see a ring not a solid disk. USe subtract to subtract the smaller disk from the larger one. It is a bit tricky selecting the smaller disk. Click it at its edge.

Re: Calculate massproperties self created hollow section

dbroad — Fri, 28 Aug 2020 14:36:35 GMT

Good. Just as @leeminardi explained, subtract the inner region from the outer region. Then use massprop. For my calculations, I had used the outside circle diameter as 437 and inside circle as 417 so my numbers will be off from yours anyway. But AutoCAD's massprop will do the calculation correctly given correct input.

Re: Calculate massproperties self created hollow section

gert.valk — Mon, 31 Aug 2020 05:52:34 GMT

many thanks for your efforts but it doesn't work. I am getting a bit tired of it because I can't subtract the inner circle.

Re: Calculate massproperties self created hollow section

dbroad — Mon, 31 Aug 2020 12:41:59 GMT

First both the outer circle and the inner circle must be regions (which they are in your test file). Second, you should always read the help files concerning whatever command you're having trouble with. Then you should practice with the visual style set to on of the 3d types (conceptual, x-ray, and realistic have the best shading). Practice on something that doesn't matter. For example, draw a box and a cylinder and overlap them. Then practice the subtract command, which is a two step command, requiring two sets of selections. You must select the set of objects to subtract from first. Then you must select the objects representing voids. After you've mastered subtracting, then apply it to the problem at hand.

Re: Calculate massproperties self created hollow section

gert.valk — Mon, 31 Aug 2020 14:09:54 GMT

Thank you very much to everyone for their information, but I am probably too dumb to understand this command. It takes too much time so I will do the calculation manually.