AutoCAD - Wrong moment of inertia for a complex assembly

[jmleecosmos]

I'm trying to use Autocad MASSPROP command to calculate the MOI (moment of inertia) for a 3d assembly of various parts but it keeps giving me an extremely large number.

I then went back to basics to do the same check on a single 100mm x 100mm Square hollow section cross section profile and it was able to give me a MOI result that is very close to the manufacturers data.

---------------- REGIONS ----------------
Area: 1886.2655
Perimeter: 753.1327
Bounding box: X: -50.0000 -- 50.0000
Y: -50.0000 -- 50.0000
Centroid: X: 0.0000
Y: 0.0000
Moments of inertia: X: 2832705.4895
Y: 2832705.4895
Product of inertia: XY: 0.0000
Radii of gyration: X: 38.7525
Y: 38.7525
Principal moments and X-Y directions about centroid:
I: 2832705.4895 along [0.9925 -0.1222]
J: 2832705.4895 along [0.1222 0.9925]

However, once I extruded the profile to the required 6m length, the MOI shot back up. 

---------------- SOLIDS ----------------
Mass: 11317592.8947
Volume: 11317592.8947
Bounding box: X: -50.0000 -- 50.0000
Y: -50.0000 -- 50.0000
Z: -3000.0000 -- 3000.0000
Centroid: X: 0.0000
Y: 0.0000
Z: 0.0000
Moments of inertia: X: 3.3970E+13
Y: 3.3970E+13
Z: xxx-xxxxxxxx.5833
Products of inertia: XY: 0.0148
YZ: 0.0217
ZX: 0.2419
Radii of gyration: X: 1732.4843
Y: 1732.4843
Z: 54.8043
Principal moments and X-Y-Z directions about centroid:
Press ENTER to continue:
I: 3.3970E+13 along [0.9940 0.1098 0.0000]
J: 3.3970E+13 along [-0.1098 0.9940 0.0000]
K: xxx-xxxxxxxx.5833 along [0.0000 0.0000 1.0000]

Could someone please advise? What am I missing here? Thank you in advance. 

[ The subject line of this post has been edited to include the product name by @handjonathan ]

[leeminardi]

The values seems correct to me.  The equation for the moment of inertia for a rectangle is"

MOI = 1/2 * b * h^3

for a rectangular hollow solid bar:

MOI = 1/12 * b * h * L^3

Here are the result from Excel

The results form AutoCAD

[dmfrazier]

From Wikipedia:

"The moment of inertia of a rigid composite system is the sum of the moments of inertia of its component subsystems (all taken about the same axis)."

So, an extruded 3D solid would naturally have a much larger MOI (depending on its length) than a 2D region (with no 3rd dimension), and the complex assembly even more so.

[jmleecosmos]

Usually when calculating the deflection of a beam, I usually use the cross sectional MI provided in the manufacturers data sheet. However, judging by how large the MI is once the item is extruded, it would imply the deflection for this beam would be little to none which doesn't make sense. Because of this I'm wary to use the generated MI from the extruded beam. 

[dmfrazier]

"...when calculating the deflection of a beam, I usually use the cross sectional MI provided in the manufacturers data sheet."

And you should continue to do that. (You can also use AutoCAD's value, because, as you stated, it "is very close to the manufacturers data.")

"...how large the MI is once the item is extruded..."

"...I'm wary to use the generated MI from the extruded beam."

Because the length is now (already) factored in, this would not be the correct value to use in your calculation for the beam, so you are right to be wary,

[dbroad]

Structural calculations require 2D MOI. 3D MOI are for calculating mass moment of inertia for rotational equilibrium/inertia when doing dynamic analysis (I believe).

A composite 2d moment of insert is possible to report for structural purposes (top and bottom chord and mid pipe) but not with extrusions.  Such composite moment of inertia would also be subject to how well the parts were connected together to resist shear flow. Create a composite 2d region and then apply the massprop to that.

[jmleecosmos]

I agree with your explanation and have followed your suggestion using the 2d cross section profile to work out the MI of the structure. However I had to skip the vertical members that are placed at certain intervals as they are not a continuously occurring member along the entire length. Do you have any suggestions for figuring out the MI for these? Thanks!

[dbroad]

Ignore them. They don't contribute to the bending strength per se.  Without diagonals in the system, the vertical members won't contribute to resisting horizontal shear flow either.  This will make the composite moment of inertia seem too large, even if you ignore them. If they were welded to the top and bottom members, then you have a Vierendeel truss.  

For frame and truss analysis, the top and bottom chords and the webs are only considered to carry compression or tension.