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Re: Moment of Inertia using Autocad

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My only guess is that you're looking at the wrong moment. The MASSPROP function lists several values that are taken about the 0,0,0 axis, and then several that are taken about the centroid of the object. If the object's centroid is placed at 0,0,0, then these sets of number will be the same, but if you drew the object out in space (which is what everone always does), the numbers will not agree with each other and you'll have to look at the group that is taken about the centroid.

That's the only thing I've noticed while using MASSPROP.

Mike Perakis, P.E.

>>> GMadden(--nospam--at)mplusl.com 01/30/01 07:42PM >>>
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