Anky,
The 'rigid' structural supports will result in a conservative value of the piping expansion loads. If structural flexibility is taken into account, then the actual piping expansion loads can be smaller by some proportion of the structural stiffness versus the piping stiffness.
The piping sustained loads will remain unchanged with flexible supports, since gravity weight requires same supporting force regardless of stiffness of support.
A structural engineer once told me figures of 1E6 lb/in for vertical stiffness, and 1E5 lb/inch lateral stiffness or less for industrial structural steel designs. These would be significantly less than the default 1E12 stiffness in CAESARII.
Otherwise you will need to model the structural steel to get the 'exact' stiffness of the support points, as suggested by Habib.
I would suggest you use the default stiffness (no entry in the 'Stif' field) unless there is a known problem with a flexible support point.
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R Yee