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RE: Moment of Inertia using Autocad
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Moment of Inertia using Autocad
- From: "Yang, Yi" <yhy(--nospam--at)hwhaep.com>
- Date: Mon, 26 Feb 2001 10:09:03 -0500
You can just convert the cross section of the column into a "region" using the "region" command on a closed polyline, then run the "massprop" command on the newly created region. Once you find out the coordinates of the centroid for the region, you can use those coordinates as the origin, and move the region to 0, 0, 0. If you run the massprop command again, on the moved region which has it's centroid located at 0,0, it should give you some more sensible answers. You can do this with multiple objects, even hollow objects, with some manipulation of the region objects. I made a little VBA utility to do all that, and some extra calc such as section modulus for you. Let me know if anyone is interested, it's pretty small and I can e-mail it to you. Yi Yang, P.E. Cleveland, Ohio -----Original Message----- From: Mike Perakis [mailto:PerakiM(--nospam--at)trusjoist.com] Sent: Tuesday, January 30, 2001 9:29 PM To: seaint(--nospam--at)seaint.org Subject: Re: Moment of Inertia using Autocad 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
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