Finding inertia of two sections

Hi there

 

New to the forum. I have two sections both region-ed. What is the correct way to find the combined inertia. The sections kind of "fit" into each other. So far I have used two different methods and I don't know which is correct.

 

1. Centre each section individually, find the section properties of each sections and then add them together.

 

2. Combine the section, centre them together, and then find the inertia of the combined sections.

 

So which one is correct? The first one gives a much smaller I value then the second ones.

Hi there

 

New to the forum. I have two sections both region-ed. What is the correct way to find the combined inertia. The sections kind of "fit" into each other. So far I have used two different methods and I don't know which is correct.

 

1. Centre each section individually, find the section properties of each sections and then add them together.

 

2. Combine the section, centre them together, and then find the inertia of the combined sections.

 

So which one is correct? The first one gives a much smaller I value then the second ones.

I'm not sure what you mean by "centre" a section. If you are talking about the moment of inertia, look into the parallel axis theorem. Your moment of inertia is dependent on what axis you are referencing.

Also, note that if you build a composite section from overlapping sections, then the resultant sectional area will be smaller than the two separate areas simply added together. Coplanar figures don't stack in 3D.

I'm not sure what you mean by "centre" a section.

 

By Centre, I mean find the centre of the object, and move it so thats its centroid is sitting at 0,0. So I would have done this for two different sections (which fit together, no overlapping) find the inertia while each section is at 0,0, and then add them together. Which I assume now is the wrong way to go?

 

Thanks for your help

You're welcome, of course. To do it right, the two areas must maintain their relationship for the calculation. If they kind of fit but don't touch, you can still UNION the two regions, then use MASSPROP to get the new composite area.

 

NOTE: If you still have trouble after reading this, try posting your areas you want to analyze (preferably as a DWG so no one has to draw it).

Thanks for your help.

 

Have figured it out. I think it is ok to get the inertia of both sections individually as they are not properly "connected" so they will be rotating around there own axis, not there combined.

 

Thanks again

Thanks for your help.

 

Have figured it out. I think it is ok to get the inertia of both sections individually as they are not properly "connected" so they will be rotating around there own axis, not there combined.

 

Thanks again

Great that you've got it figured out. And, you're welcome again.