ZPA option in force time history analysis is approximate. AutoPIPE applies the maximum dynamic forces at every elbow at the same time as static loads and envelopes the static result with dynamic results. It is to be noted that ZPA option for earthquake ground time history and in response spectrum analysis is accurate. For slug loads which are relatively slow this approximate ZPA option could be very conservative since each elbow load can act independent of other elbows. I suggest that you define separate USER cases for each elbow and apply static ZPA loads individually. You can then envelope these USER cases with time history dynamic loads.
Regards,
Karim Rinawi