Product moment of area = area product of inertia = polar inertia of a section in Autocad/Abqs

n3l3 · Jan 15, 2013

Hi all,

I am tryng to get that parameter ( //xydxdy (where // is the integral symbol) as defined here

http://en.wikipedia.org/wiki/Second_moment_of_area

for a plane section in autocad.

The section is drawn as a region, but I get a zero with the command fisprop. Furthermore I do not think that product of inertia is what I search for since:

Autocad help
"Product of inertia
Property used to determine the forces causing the motion of an object. It is always calculated with respect to two orthogonal planes. The formula for product of inertia for the YZ plane and XZ plane is
product_of_inertiaYZ,XZ = mass * distcentroid_to_YZ * distcentroid_to_XZ
This XY value is expressed in mass units times the length squared."

Someway to calculate the product moment area???

rb1957 · Jan 15, 2013

you're asking about Ixy, i think.

this is not polar moment of inertia.

you know (?) the basic expression for 2nd moment of inertia ... Iyy = integral(x^2dA); i won't go into the terms, you can find out about them just about anywhere ('cept here).

Ixy is a "mixed" product of inertia (i forget the correct term) = integral(x*ydA); ie uses both co-ordinates of your section elements. mostly used in determining principal moments of inertia, in fact i can't think where else it's used. if your section is symmetric then Ixy = 0.

professionals should know this; if you're student ... go away and ask your TA.

Cockroach · Jan 15, 2013

You are asking for the Moment of Inertia, the second moment of area. The polar moment is the addition of the second moment of area about two orthogonal axis, typically labelled the X and Y axis. The units are length^4. Asking for mass times length^2' is the mass moment of inertia.

Your question is very awkward, you want AutoCad to find the inertia for some plane section? Is this some type of homework assignment?

Regards,
Cockroach

EnergyMix · Jan 15, 2013

n3l3

I'm presuming you're not a student since you've been around here for a while. I think you might get better answers if you reformulated your question a bit using the information you've been given by rb1957 and Cockroach and then asking it over on forum555 where there are Autocad users.

If you do ask it over there, please red flag this one by clicking on the link in the bottom center of one of your thread that says "Inappropriate post? If so, Red Flag it!" and asking the site administrator to delete it because you've posted it in a new forum.

n3l3 · Jan 15, 2013

First of all thanks for your replys

In second case I must say that this may be just a language problem, and actually what I am asking for is a way to calculate it for a specific geometry with help of some software like acad or abaqus (those who I can use now).

what I actually want to calculate is the torsional inertia J, used to calculate the rotation in beams suffering torsional moments. This torsional inertia is equal to the product of inertia (//r^2 da) for tubular sections, as far as I know. Product of inertia is also involved in wing torsional problems, AS profesionals should know that.

cheers

rb1957 · Jan 15, 2013

if you have a section, AutoCad, Solidworks, etc can give you Ixx and Iyy which will give Ipolar (r^2dA = x^2dA+y^2dA).

maybe your section is a wing cross-section ? with a bunch of lumped areas (representing stringers). I would ('cause I have) consider your lumped areas at the CG of the lump (take a lump to be a stringer and effective skin). But I would base torsion properties on the enclosed area of the wing box [A] (the wing box would function as a closed cell, no?).

IFRs · Jan 17, 2013

In Autocad, are you looking for the command Massprop ? First you use the command Region on a closed polygon, then Massprop.

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Product moment of area = area product of inertia = polar inertia of a section in Autocad/Abqs

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