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RE: Autocad Section Property Calculation

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The results from executing the Acad MASSPROP command should give you the
moments of inertia about the origin (wherever you choose to locate it) and
ALSO the principal moments about the members centroid.

For expample, here is the text screen for the MASSPROP command run on a
12x24 rectangle
with the UCS origin set at the lower left corner of the shape ...

Command: _massprop
Select objects: 1 found

Select objects:

 ----------------   REGIONS   ----------------

Area:                    288.00000
Perimeter:               72.00000
Bounding box:         X: 0.00000  --  12.00000
                      Y: 0.00000  --  24.00000
Centroid:             X: 6.00000
                      Y: 12.00000
Moments of inertia:   X: 55296.00000
                      Y: 13824.00000
Product of inertia:  XY: 20736.00000
Radii of gyration:    X: 13.85641
                      Y: 6.92820
Principal moments and X-Y directions about centroid:
                      I: 13824.00000 along [1.00000 0.00000]
                      J: 3456.00000 along [0.00000 1.00000]



Here it is again with the UCS origin set at the center of the rectangle....

Command: _massprop
Select objects: 1 found

Select objects:

 ----------------   REGIONS   ----------------

Area:                    288.00000
Perimeter:               72.00000
Bounding box:         X: -6.00000  --  6.00000
                      Y: -12.00000  --  12.00000
Centroid:             X: 0.00000
                      Y: 0.00000
Moments of inertia:   X: 13824.00000
                      Y: 3456.00000
Product of inertia:  XY: 0.00000
Radii of gyration:    X: 6.92820
                      Y: 3.46410
Principal moments and X-Y directions about centroid:
                      I: 3456.00000 along [0.00000 -1.00000]
                      J: 13824.00000 along [1.00000 0.00000]


As you can see, the first set of results lists the centroid at (6,12) from
the lower left hand corner origin. Right where we expect it. The second set
of results lists the centroid at (0,0) from the center of the rectangle.

As expected, for the second case, the calculated monents of inertia and the
principal momoents of inertia are one and the same since we located our
coordinated system origin at the centroid of the section.

Hope this helped.

Brent R. Koch, P.E.
Koch Engineering, Inc.
Livermore, CA
mailto:kochengr(--nospam--at)pacbell.net