Showing results for 
Search instead for 
Do you mean 
Reply

SOL103 - Removing rigid body modes in a Craig Bampton reduction

Hello,

 

When doing a Craig Bampton reduction of a free body (ie no restraint is applied to the body), I would like to generate the K and M matrices of the reduced system but without the rigid body modes.

For example, if the modes of my models are 0 Hz, 15 Hz, 45Hz, 55Hz, I would like my reduced model to contain only the 15 and 45 Hz.

For experimentation purpose, I'm working on a very simple mass spring 1D model. I tried to set a minimal frequency value in the EIGRL command but I always end up with a 0 Hz in my reduced model. Is there a way to tell Nastran not to take into account the rigid body mode when reducing my model?

 

Thanks,

Damien

 

PS: I've joined my input data 

2 REPLIES

Re: SOL103 - Removing rigid body modes in a Craig Bampton reduction

Before the run the SOL103 Real Eigenvalue solution, change the lowest frequency to 0.1 Hertz. That will eliminate the "zero Hertz" or free-body modes.

Re: SOL103 - Removing rigid body modes in a Craig Bampton reduction

That's what (I thought) I did by specifying a minimal frequency of 0.5 Hz in my input file (line 28). However, if the rigid body mode is not present in my f06 file anymore, it is still here when I compute the eigen values given by the K and M matrices of my super element in the punch file. And I'd would like to have a super element without rigid body mode. Is this possible?

 

Thanks,

Damien