When a seismic analysis was conducted for infinitely long elevated bridge, generally, the boundary conditions for joint of a unit bridge have been considered as free. It has been also considered that the results of such analysis would be on safe side. In this research, the transmitting boundary for discrete system was formulated by using beam element model of 3 D. O. F, and its verifications were carried out. Furthermore, parametric analyses were carried out, which are dynamic analyses for structures applied harmonic excitation with various frequencies. As results, we have verified the possibility that responses considering the transmitting boundary could be greater than that of free boundary.