# Automatic Identification of Bridge Vortex-Induced Vibration Using Random Decrement Method
## 4 Application to the Xhiounen Bridge  
### Description of Bridge and SHM  
The Xhiounen (XHM) Bridge is a two-span continuous suspension bridge with a 1650 m central main span and a 578 m side span, as shown in Figure 10. It carries asix-lane roadway by separated twin steel box girders, which are employed to improve the aerodynamic performance of the bridge against flutter. It is located in the sea area affected by typhoon, as shown in Figure 11. During the construction period, in September and October 2007, the bridge was attacked by typhoonsWipha and Rosa, respectively. Since it was opened to traffic in 2009, it has withstood many severe typhoons with wind velocity up to 50 m/s. However, at the normal wind velocity of 6-10 m/s, the VIV is easy to occur at the stiffening girder of the Xhiounen Bridge. Figure 8: The identification result with different signal-to-noise ratio (SNR). Figure 10: Elevation of XHM Bridge. Figure 9: **(a)** The power spectrum with signal-to-noise ratio (SNR) of 0.015; **(b)** the power spectrum with signal-to-noise ratio (SNR) of 0.3. To measure the characteristics of the environmental loadings and the structural response of the bridge, a SHM system was implemented [32]. The environmental loadings monitored included the wind loads, vehicle loadings, the temperature, and humidity. The structural responses included displacement, acceleration, and strain of the bridge. There are various kinds of sensors, including ultrasonic anemometers, propeller anemometers, wind pressure transducers, GPS, and accelerometers. Those sensors were placed on the girder, towers, and some suspenders. In this paper, only the data from anemometers and accelerometers were used. A propeller anemometer (PA) was installed on the top of the east side of each tower. The propeller anemometer can measure the wind speed and direction with a sampling frequency of 1 Hz and a measuring range of 0-100 m/s. Six three-dimensional ultrasonic anemometers (UA), as shown in Figure 12, were installed on the bridge deck. Those anemometers can measure the three-dimensional wind velocity with a frequency of 32 Hz in the range of 0-65 m/s. Figure 13 showed the position of those two kinds of anemometers. Thirty uniaxial accelerometers were installed on the stiffening girder and pylons of the bridge. Figure 14 illustrates the measurement locations for the accelerometers, w  
Figure 11: Location of Xihoumen (XHM) Bridge. Figure 12: Anemometers in XHM Bridge: (**a**) Ultrasonic anemometers; (**b**) propeller anemometer. Figure 13: Location of anemometers. positioned at each location. At position of AC1, AC2, AC3, and AC4, the three sensors in each position were arranged to obtain the vertical, lateral, and torsional acceleration responses. The positions of AC5, AC6, AC7, and AC8 were located at anchorages and piers, and they were used to measure the acceleration responses in vertical, lateral, and longitudinal directions. Three accelerometers were installed at the AC9, AC10, respectively. Two of them were used to measure the acceleration along the axis of the bridge and another one was used to measure the lateral accelerometers. Figure 15 showed two kinds of data acquisition instruments. According to the description of bridge owners, the bridge experienced notable VIV by several dozen every year, which does cause discomfort to users. Based on the data recorded by the SHM of the Xhiounen Bridge in 2015, the proposed method was employed to identify the VIV events from massive long-term monitoring. The data in a whole year was up to 1 Terabyte. The acceleration data at the measuring stations of quarter and half span of the main span were used. The sampling frequency of the acceleration data was 50 Hz. The wind velocity measured at the two positions was also used, and the sampling frequency of wind was 32 Hz. The measured data in January, May, and August 2015 was used to identify the VIV events.