Dear FSB1,
The best way to get interface loads beteween any type of elements is to use the FREEBODY Diagrams in FEMAP, making sure to activate the FORCE BALANCE in the NASTRAN Ouput Request:
The following image is an example of the FREEBODY diagram feature of FEMAP extracting loads between RBE2 and 2-D Shell CQUAD4 elements:
The same example problem using RBE3 elements:
In the FREEBODY Diagram creation in FEMAP make sure to use the option INTERFACE LOAD:
To learn more about RBE2/RBE3 elements with FEMAP please visit my blog in the following address: