Another variation of the CMR of RS method is to use a mixed boundary reduction. In the standard solution, all the t-set DOF are restrained in the first eigenvalue solution. With mixed boundary conditions, the user can designate that some of the t-set DOF will be unrestrained in this solution. All t-set DOF are treated the same in the constraint mode solution.
The reason for using the mixed boundary approach is to add exterior DOF to the solution that are more appropriately considered as a free DOF rather than a connection DOF. For example if a component will have four nodes used as connection points in the MBD analysis, those points should be restrained exterior points. If there are other points on the component that will not be connections but may be a marker location to track response, or to apply a force, those nodes may best be treated as an unrestrained exterior point.
Additional bulk data cards for the mixed boundary approach are:
•CSET : define the exterior DOFs that will be unrestrained in the first eigenvalue solution.
•BSET : define the exterior DOFs that will be restrained in the first eigenvalue solution. BSET and ASET are treated the same in this case.