Health monitoring of a cable-stayed bridge from traffic-induced vibrations

The dynamic characteristics of the Kao-Ping-Hsi cable-stayed bridge, located in between Kaohsiung and Pingtung county, Taiwan, is monitored under daily traffic flow for assessing its health condit ions. The bri dge is an as ymmetric single tower cable-stayed bridge with the main span of 330 m, and side span 180 m. The height of the inverse Y-shaped pylon is 183.5 m, measured from the foundation to the top of pylon. Experimental data were measured from servo-velocity sensors, with six channels, were placed at various locations on the bridge. Data sampling rate was 100 Hz, and system-identification techniques, such as the random decrement (RD) technique and Ibrahim time-domain (ITD) method, were adopted for data reduction. The vibrational modes of the bridge were identified for their natural frequencies and damping ratios under different traffic loading conditions. The loading conditions are classified by the root-mean-square (RMS) deck velocities. The magnitude of the torsion mode of the Kao-Ping-Hsi cable-stayed bridge is found to be one order-ofmagnitude less than that of the transverse mode, and two orders-of-magnitude less than that of the vertical modes. Conventionally, through monitoring the natural frequencies, vibrations induced by traffic flow ca n be used to reliably monitor the health of the bridge. However, we found that the damping ratios may serve as a more sensitive indicator to describe the condition of the bridge, due to its high sensitivity to the RMS velocities. It is expected that damping ratio may be more sensitive to local change in material properties or component connectivities. Therefore, use of both of the natural frequencies and damping ratios may provide a more accurate description of the health of cable-stayed bridges.