Lion-Head River Bridge is a parallel twin bridge. The East Bridge was designed and constructed as a traditional prestressed concrete box of bridge (Pot Bearings); and the West Bridge used seismic isolation devices (Lead Rubber Bearings). The behaviors of these two bridges are compared with each other through several field tests including the ambient vibration test, the force vibration test induced by shakers, and the free vibration test induced by push and fast release devices. During the period of this construction, the East and West bridges were strongly damaged by Chi-Chi and Chi-I earthquakes and had been retrofitted. The damage changed the natural frequencies of the bridges, which was explored from the ambient vibration test. The models of the two bridges are simulated by finite element method according to the original design and the soil-structure interaction was put into consideration in the models. Simulations were modified based on the results of the every test in the field. The dynamic parameters of the bridges are identified and compared with those from the theoretical simulation. The efficiency of isolated bridge is observed and verified.

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