A dynamic load combination method proposed in previous papers for seismic analysis of a liquid storage tank has been developed for response calculations of piping systems subjected to multiple support excitations. This method has an advantage in which cross·correlation among support excitations are properly taken into account. The correlation factor can be semi-automatically evaluated by using a stationary random vibration approach. Investigation is mainly devoted to a maximum acceleration response calculation of a simple piping model to two-input excitation. Results for the system corresponded fairly well with the exact numerical results through analytical simulation. On the other hand, the SRSS procedure and the absolute value summation which neglect cross-correlation among excitations can lead to large errors. Finally, the authors have also proposed the idea of generating a ""multi-excitation floor response spectrum"" using the results.