# Development of a Structural Monitoring System for Cable Bridges by Using Seismic Accelerometers
## 3 Measured Data and Condition Assessment  
The analysis procedures and usability of the developed system are presented through the reference application for a cable-stayed concrete girder bridge (Figure 5). Locations of the seismic accelerometers are given in Figure 6. A three axes accelerometer is located at the base of one of the pylon. Two axes accelerometers are located at three locations; center of the deck, top of the pylon and deck height of the pylon. The bridge is located at the south coast of Korea peninsula. The height of the pylon and the length of the main span are 90 m and 230 m, respectively. A set of acceleration records were obtained from the reference bridge during an earthquake with magnitude of 5.0, which occurred off the coast of Ulsan, South Korea, on 5 July 2016. Locations of the bridge and the epicenter are shown in Figure 7. Since the reference bridge is 229 km away from the epicenter, the intensity of ground shaking was minor. The measured peak ground acceleration is 1.4 gal, which is only 0.93% of its design level earthquake intensity. As shown in Figure 2, all of the measured acceleration time histories are converted from the binary format of the miniSEED to the ASCII (American Standard Code for Information Interchange) format and this is followed by the synchronization process. Since some of the response indices in this system are based on the relative displacements synchronization of the acceleration time histories is necessary. After the format conversion and synchronization, noise filtering and base-line correction are carried  
Figure 5: Overview of the reference bridge. Figure 6: Location of the installed seismic accelerometers on the reference bridge. Figure 7: Location of the epicenter of the example earthquake. out. The latter stage is followed by the calculation of the response indices for the condition assessment (Table 1). Details of the calculation procedure of the response indices are as follows:  
1. A peak ground acceleration was obtained by calculating the maximum absolute value of the horizontal acceleration time histories, which is the vector sum of the two horizontal components (North-South and East-West) of the accelerometer at the free field (Figure 8a). In this calculation, the vertical component was not accounted for. Figure 8b shows that the accelerations at the top of the pylon were much larger than those at the ground because the accelerations were amplified by the structural responses. 2. A frequency change of the pylon was calculated by the percentage ratio of the change in the natural frequencies to the original of natural frequency. The natural frequency was obtained by identifying the location of the peak amplitude of the transfer function, which is the ratio of the Fourier amplitudes [37] of the top to the bottom. The results of the Fourier transformation performed on the measured data from the example earthquake are given in Figure 10. Details on the calculation of the transfer function are given elsewhere [38]. 3. A frequency change of the cable was calculated by the percentage ratio of the change in the natural frequencies to the original of natural frequency. The natural frequency was obtained by identifying the location of the peak of the Fourier amplitude, which is derived from the acceleration time histories at the center of the cable span in the vertical direction. 4. A maximum vertical displacement of the deck is the peak value of the absolute of the vertical displacement time histories at the center of the deck (Figure 9b). Since there were two accelerometers at the center of the deck as shown in Figure 6, mean value of the vertical displacement of the two devices was used as the vertical displacement at the center of the deck. It is worth noting that the natural frequencies of the pylon and the deck could be implicitly estimated based on the observation on the distances between the peaks of the waves in the displacement time histories in Figure 9.
5. A maximum torsion of the deck is the maximum value of the angle of torsion of the deck.