Hi Dylan

Check flexible joint you are suppose to use is Expansion bellow or hose.
1) If it is expansion bellow check material ( metallic/rubber) & bellow type ( tied/untied). Check caesar's application guide for modelling procedure of bellow. Ask vendor for stiffness & effective ID of bellow.
For technical know how about bellow refer EJMA ( Expansion joint manufacturing association)

2) If it is hose leave stress calculation before hose. Hose do not transmit force/moment from one point to other as piping.


Regards

Habib