Calculate massproperties self created hollow section

[gert.valk]

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

[leeminardi]

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

[gert.valk]

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?

[leeminardi]

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.

[dbroad]

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]

[leeminardi]

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

[gert.valk]

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

[leeminardi]

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.

[gert.valk]

Yes

[leeminardi]

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.

[dbroad]

The most obvious questions are:

1. Did you draw it accurately according to the dimensions?
2. To what standard are you comparing it?
3. What formulas are you using?
4. Could you have erred in your calculations?
5. 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.

[gert.valk]

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?

[dbroad]

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

[gert.valk]

Succes!

[gert.valk]

Succes!👍

[leeminardi]

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.

[dbroad]

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.

[gert.valk]

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.

[dbroad]

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.

[gert.valk]

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.