Recent earthquakes have demonstrated the vulnerability of cable-stayed bridges to near-fault earthquake excitation. The end span connections of the Higashi-Kobe cable-stayed bridge fractured during the Hyogoken-Nanbu (Kobe) Earthquake in 1995 causing moderate damage to the overall system [1,2]. The end spans of the Chi-Lu cable-stayed bridge rose vertically out of their transverse shear keys during the 1999 Chi-Chi earthquake in Taiwan resulting in extensive damage to the non-ductile superstructure at the superstructure to central pylon connection [3]. This paper explores isolation for protecting these structural systems due to large magnitude, near source seismic events. Using isolation devices under the central pylon in conjunction with isolation devices at the end span connections, the complete system can be isolated and the superstructure can be protected. This paper focuses on issues related to isolating the bridge in the transverse direction. This form of isolation minimizes the demands to both the superstructure and substructure elements. Flexible bearings are provided connecting the end spans to the substructure and the pylon base to the foundation. These isolator groups are tuned such that a vibration mode is created where the superstructure is engaged primarily in translation. This paper develops the required relationship between the end span and pylon-foundation flexible connections to optimize the effectiveness of the isolation system. Furthermore, analytical results for a fixed base, fixed end, moderate span, single-tower cable-stayed bridge are compared with results from the pylon isolation system for varying isolation periods using simulations with large magnitude near-fault ground motions. Conclusions are drawn on the overall all effectiveness of this method of isolation as compared with the traditional fixed base system. Results will show that isolation of the entire system is a viable option for the reduction of seismic damage.
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