You don't make it a solid !
You should make it a 'region'.
From: Gerard Madden [mailto:GMadden(--nospam--at)mplusl.com]
Sent: Tuesday, January 30, 2001 19:42
Subject: Moment of Inertia using Autocad
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
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 Madden, P.E.
Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105