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# Mass Element Moment of Inertia

## Mass Element Moment of Inertia

(OP)
Hi All,

I understand that we always use 1E-6 for the moment of inertia when using mass element but I wonder why using 1E-6 or not to calculate an exact value?

Secondly, is there any cases that we will use RBE2 to transfer the load? As in to simulate a non-deformable load distribution? Thank you!

### RE: Mass Element Moment of Inertia

sure, calculate your specific MMI. I think the idea behind using 1e-6 is that the MMI of small components is insignificant compared to the MMI of the total structure.

### RE: Mass Element Moment of Inertia

oh, yes, I use an RBE2 for a structure I consider to be rigid, like the upper part on a console (when I'm modelling the lower 1/2).

### RE: Mass Element Moment of Inertia

(OP)
Thanks rb1957!

If we are transferring a component weight to the structure, generally using rbe3 but I do see people using rbe2 for non-deformable behavior. I wonder under what circumstances we should use rbe2?

### RE: Mass Element Moment of Inertia

an RBE2 models rigid structure, in my case the upper console is a pretty rigid structure, constraining the lower console (a pretty flexible structure) so that at the interface "plane surfaces remain plane" and the interface translates and rotates as one (rigid) structure. So I modelled the lower console and the upper console was an RBE2.

More typical is to use RBE3s to distribute load around the section, because you don't want to constrain your model.

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