Your question is not clear, but I will still try to answer.

What specifically you mean by rigid wt.? Typically rigid elements should be used to model non pipe elements like valves, Flanges, equipments etc.

If your question is about modelling dummy supports which I presume you want to model as rigid element,then my suggestion is : to get accurate results model the dummy as pipe element . Keep the temp. as ambient and pressure equal to zero for the dummy.The flexibility of the dummy can play a vital role to transfer of displacements on the nozzle , an effect which can be grossly ignored if it is modelled as rigid element.This is true if the nozzle loads are just qualifying.You can go through the different methods of dummy modelling in CAESAR II manuals.

If your question is otherwise, please be more specific, so that you can get the answer you want.

Anindya Bhattacharya