Rigid Body Elements

Question

I am currently developing structural FEM solver in FORTRAN. My question is about Rigid Body Elements (Multi Point Constraints).

In NASTRAN there is RBE2 element defined by one independent and one or more dependent nodes. Given degrees of freedom of the independent nodes, defined by the user, are rigidly transferred to the dependent nodes. During the solution of the system of equations, degrees of freedom of dependent nodes are eliminated since they can be recovered from the defined Multi Point Constraint Equations. If there is a node which is in RBE2 definition defined as independent, and if there are no applied loads or constraints or finite element attached to that node, what degrees of freedom will be transferred to dependent node? Since there are no associated coefficients of stiffness matrix to these degrees of freedom, they can not be defined.

Is there first check of finite elements association to the independent node and if there is no association, or specific degree of freedom as Constraint, RBE2 will not be considered?

Thanks, Vladimir

Answer 1

I'm not sure I understand exactly what is being asked here. Different FEM packages use different methods to enforce constraints, e.g. Lagrange multipliers versus condensation at assembly time. If you have an element contribution involving slave dofs, you can condense (transform) them to be in terms of the master dofs before adding the element matrix to a load vector or sparse matrix.

Feel free to update the question with more details.