Seismic response of a suspension bridge: Insights from long-term full-scale seismic monitoring system

This paper describes an assessment of seismic responses of Hakucho Suspension Bridge based on data obtained from long-term monitoring system. The bridge with a total span length of 1,380 m has a permanent monitoring system consisting of 27 channels of accelerometers, and seismic responses have been recorded from its opening in 1998. Between 1998 and 2006, the monitoring system has recorded about 213 earthquakes with various amplitude, including the major ones such as the 2003 Tokachi-Oki, the 2004 Kushiro-Oki, the 2004 Kushiro, and the 2005 Miyagi-Oki earthquakes. Studies by multi-input multi-ouput time-domain system identifications, time- frequency analysis, and inverse analysis were conducted to investigate seismic response characteristics. From the four largest recorded earthquakes, it was observed that girder responses are generally dominated by the modes with frequencies between 0.3 and 0.6 Hz corresponding to the higher order of vertical bending modes. Significant pylon vibration that corresponds to standalone pylon lateral frequencies at 0.6 Hz (pylon in phase) and 0.8 Hz (pylon out of phase) with low damping was also observed. Results of system identifications from 213 sets of seismic record show dependency of natural frequencies and damping ratios of girder-dominant modes on the amplitude of ground motion. The trends of increase in damping and decrease in natural frequency as the ground motion amplitude increases were observed. This behavior is explained by an inverse analysis method using identified modal parameters at different amplitude of excitation. Furthermore, effect of stiffness and damping variation caused by the friction force at the bearings was clarified.