Massochy dos santos Ventura

I'm doing a modal analysis on a free-free steel plate in order to extract stiffness matrix, mass matrix and the vectors corresponding to the vibration modes for each natural frequency. in order to reduce the values ​​of the matrices so I intend to restrict some degrees of freedom all nodes since the matrix is ​​formed by the multiplication between numbers of nodes and the degrees of freedom (in this case they are 6)..