in static seismic analysis Sa/g value corresponding to rigid range is used. Attached find a typical seismic response spectra derived from IS code ( which is similar to many other codes like Euro code etc.). Rigid range corresponds to high frequency zone. A typical piping system is normally having much low frequency. But for low frequency Sa/g is not low , rather high. As we are not aware about the system frequency unless modal is run , a user is not having any idea about system frequency. Also contribution of which mode is higher also can be known if dynamic response spectrum seismic analysis is carried out. In the absence of above what would be the reasonable value of Sa/g to be considered in static seismic ?

One can consider that the maximum on spectrum is usually 2.5*PGA and dividing by R=2.5 we find out PGA as equivalent static. For regular piping one can assume I=1.