I'm doing a CMS for a large model. I'm processing my results in Matlab for a target mode selection. My problem is, to solve the generalized eigenvalue problem in matlab my mass and stiffness matrices from NX should be the same size, and they are not.
In my SOL 103 real eigenvalues solution i obtain in a .pch file my original stiffness and mass matrices:
in the stiffness matrix all the nodes have 6 DOF
in the mass matrix all the nodes except those from RBE elements have 3 DOF, the former ones 6 DOF, so no rotational DOF for normal nodes.
as i understand i would require to make a static condensation of the matrix K, so Kred = Ktt - Ktr*(Krr)^-1*Krt. Where subscript t are translational DOF and r are rotational DOF.
As i do this in matlab and compare with the CMS (Craig-Bampton) from NX i obtain different results, specifically for the fixed interface modes. I'd like to know if there is a parameter or a easy way to make this static reduction. I have tried making a DOFset restraining the additional DOF of my stiffness matrix but this i would need to do it for all the nodes and it's no practical.
I would really much appreciate help,