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Moment of Inertia using Autocad

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I think this came up last year but I don't remember much ...

I am designing a concrete building with several irregular shaped columns that sprouted from two types into about 7 or 8 types after the architect did some thinking.

Anyway, I tried using the MASSPROP command in autocad after creating a solid from the cross section of the columns. The answer didn't jive with my hand calc. Then I drew a rectangle 10"x20" to see if Autocad was working properly, made it a solid and did the MASSPROP. Again the result was incorrect for the moment of inertia about the centroid. The answer for Ixx should have been 6666.667 in^4 but was much higher. I believe it has something to do with the mass portion of this calculation and is dependent on the 3rd dimension (z dimension) or thickness of the solid created.

Anyone know what I'm doing wrong?

-Gerard

Gerard Madden, P.E.
Civil Engineer

Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105
Tel: 415.546.4900
Fax: 415.974.3680
Email: gmadden(--nospam--at)mplusl.com