Thank you Artur,
One more question about Rlink elements (I don’t know if it’s needed to make a new post with another title about that).
What the help topic for RLINK type elements says that: “..provides the ability to use a concentrated mass matrix for dynamic problems” does it mean that when using RLINK elements the mass that correspond to all slave nodes is transferred and concentrated to the master node? In such case are we obliged to select Lumped without rotations for the mass matrix in the Modal Analysis Parameters in order to have only diagonal non zero values in the mass matrix (when provided that all nodes of each storey of the structure are slave ones to a master node)?
No, this only means that you have to use RLinks when you want to use lumped (concentrated) mass matrix for a model with rigid links.