# Dynamic Stability Assessment of High-Speed Railway Bridges Using Numerical Model Updating
## 2 Experimental Setup for Ambient Vibration Test
### Experimental Setup  
The train speed (approximately 270 km/h to 300 km/h) and load on the bridges that connect Busan, Gyeongju, and Daegu on the Gyeongpu high-speed railway in Korea hardly change. Likewise, the northbound and southbound tracks for high-speed trains do not intersect on the target bridges. Consequently, measurement was taken when high-speed trains passed along the northbound track in order to acquire a response under similar \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Imgi 2 nd Bridge** & **Maeaji Bridge** & **Hwalchun Bridge** \\ \hline type & PSC box girder & PSC box girder & PSC box girder \\  & 40 m single span 11 & 25 m @2 spans two & 25 m @3 span one \\ span length & girders (440 m) & continuous girders & continuous girder (75 m) \\  & 14 m & 14 m & 14 m \\  & & second span & third and fourth spans & first to third spans \\ number of measurement points & 34 (four groups) & 32 (three groups) & 48 (five groups) \\ measurement time & 50 s & 50 s & 50 s \\ sampling rate & 200 Hz & 200 Hz & 200 Hz \\ high-speed train speed & 270–300 km/h & 270–300 km/h & 270–300 km/h \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the target bridges. Figure 1: Target bridges: (**a**) Imgi 2 nd Bridge; (**b**) Maeaji Bridge; (**c**) Hwalchun Bridge. load conditions. Limiting the load characteristics of a bridge according to the operation characteristics of high-speed trains is helpful in examining the numerical model. In order to estimate the accurate dynamic properties of the high-speed railway bridges in the field, the response measured at various points is required. A wireless measurement system was applied in order to measure the response at various points using a small number of sensors. The wireless measurement system is an excellent tool for measuring at various points because of its mobility. Figure 2 shows the wireless measurement system (NARADA system) used in the ambient vibration test [40,41]. Figure 2a shows the NARADA Wireless Sensing Unit (WSU) and accelerometer installed on the high-speed railway bridges, while Figure 2b shows the laptop on which the base station of the NARADA system was installed. Ambient vibration was measured in the lower part of each bridge because measuring the response on the tracks in the upper part may affect the operation of the trains. For the response measurement, the acceleration response sensitive to dynamic properties was measured for all frequency bands. This was measured in the lower part of the northbound and southbound tracks of each bridge at regular intervals for an accurate mode shape analysis. The measurement point roaming method was applied in order to acquire the response of each bridge at various points using a small number of sensors. The measurement point roaming method is applicable because the loads applied by high-speed trains are similar for high-speed railway bridges. In addition, it can be an excellent solution for accurate measurement with a limited number of sensors in the field test. Figure 3 shows the measurement positions and groups for the target bridges. All measurement points for the target bridges were classified into three to five groups, while measurement was performed by using up to 11 sensors. Following the measurement, the reference point for signal processing was selected in the lower part of the northbound track in the center of the bridge, and the response of the reference point was continuously measured in each group. Despite the time interval between each group, the correlation between them can be estimated by comparing the signals measured at the reference point. In order to apply the measurement point roaming method, the responses acquired from the reference points among the responses measured from each group are normalized. Other than the responses acquired from the reference point, the remaining responses are normalized using the coefficient values applied to the normalization for each group.