If you are using response spectrum module, you don't have to add displacements but you have to make sure you are using correct spectrum at 8m height. If you have some idea of how the rack is going to be and the dead load of pipes, you can ask a structural analyst to calculate amplified spectrum for you.

Alternatively, I don't know if this is correct but you can model the rack in CAESAR-II structural modeller and connect pipe with it using CNode. You will not need to amplify the spectrum in this case. But this method is a little complicated and requires some knowledge of structural analysis.