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RE: Moment of Inertia using Autocad

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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