Moment of Inertia AutoCAD 2008 1

[Keystone1]

Hello
I am a newbe and I am have trouble getting correct moment of inertia values using AutoCAD 2008 (2D). I have done this several times on older AutoCAD versions with no problem. In 2008 once I create a region and then select mass properties I get both I and J values. However that are not relative to both x and y axis (1.0000, 0.0000) and (0.0000, 1.0000). I get values with non horrizontl and vertical axis ie ( 0.9848 0.1739) and (-0.1739 0.9848). Can someone tell me what I am doing wrong or tell me how to correct my problem?
Thanks
Keystone1

 

[ishvaaag]

It looks as if you are trying the area properties on a non symmetrical region. Place first your region on a well definite point and get the center of gravity of the section, then translate the shape through its center of gravity to lay at 0,0. Perform again the area properties and see that effecrively you have placed cog at 0,0. Then, if your shape is symmetrical will show principal axes along x and y axes, and not otherwise. Can this be the cause?

 

[Keystone1]

Thanks ishvaaag
I am not too sure. It is not symmetrical so maybe that is the problem. I have attached a file for your review.
Keystone1

 

[GregLocock]

The principal axes of a non symmetrical section are likely to be at an angle to the coordinate system's axes.

If you wish to transform them to MoI about the coordinate systemn axes you need to use Mohr's circle.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Keystone1]

OK Thanks I appreciate all the help.
Keystone1

 

[Kwan]

draw a line from the neutral axis to the points that autocad specified ( 0.9848 0.1739). This is a major or minor axis. Now rotate the section to align major/minor to x axis. Perform mass properties again.

 

[Keystone1]

Thanks Kwan
Is it possible for you to demo that on the attachment to post above?
Thanks
Keystone1