Abdelkader,
1. The value of force at the anchor is different because the anchor is not the only restraint in your system. You probably have other restraints that are sharing the load. For example, assuming that you are using untied expansion joints and assuming that you have modeled the equipment (I presume you did), then the load will probably go to the equipment's anchor at the bottom. The bottom line is that the external and internal forces acting on the system are balanced (i.e. in equilibrium).
2. The equation is the same for any type of expansion joint (regardless of material), because it is independent of material. It is a purely mechanical principle; that Force = Pressure x Area.
Please check 1 above and tell me if that is what happened.
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ASorour
Mechanical Engineer