Hi.
some question.
I am calculating effect of deck deflections on a piping system. The piping is connected to a equipment that is standing on lower deck and the pipe supports are connected to the upper deck. the two decks have differential deflection of 6mm. I tried to check two cases.

Case A: considering lower deck fixed (without any displacements) and upper deck with -6mm displacement(downward)
Case B: considering -2mm displacements on lower deck and -8mm on upper deck (differential being 6mm)

I get two different sets of forces/moments.
Can anybody think about what could be the reason for this difference?