# Impact Evaluation of Track Girder Bearing on Yeongjong Grand Bridge
## 3 Field Measurements  
In this study, displacements were measured at the center and end of the track girders of the Yeongjong Grand Bridge to understand the effect of loads on the track girders. Vertical rail displacements were measured using displacement transducers such as linear variable differential transformers (LVDTs) mounted on a jig anchored on a side bridge inspection passage without influence of train load, as shown in Figure 8. LVDTs (CDP-25 M) has a sensitivity of \(500\times 10^{-6}\)/mm, a rated power of 6.25 mV/V \(\pm\) 0.3%, and a frequency response of 7 Hz. Displacements were measured and recorded automatically by a computer controlled data acquisition system. In addition, the level of the displacement that occurred at the end of the direct-fixation track of the Yeongjong Grand Bridge was identified and compared with the numerical result. Further, it was used as an input specification for the impact analysis of the track support bearing.  
To examine the upward displacement occurring at the track girder end of the direct-fixation track of the Yeongjong Grand Bridge, the field measurement was carried out by installing a displacement gauge for measuring the displacements of the rail and the track at the center and end of the track girders, as shown in Figure 8. Figure 9 shows an example of the measured waveform.  
Figure 8: View of sensor installation at the track girder center and end.  
The comparison of the deflection between the rail and the track girder reveals a slight difference in the generated displacement for the 12.5 m girder, as shown in Figure 10. At the end of the direct-fixation track, the rail displacements were 0.24 mm for KTX and 0.15 mm for Airport Express (AREX). At the center of the direct-fixation track, the track girder displacements were 0.31 mm for KTX and 0.18 mm for AREX. The effect of the flexural curvature deformation of the direct-fixation track on the rail and track girder displacements was negligible.  
In this study, field measurements were performed by mounting an accelerometer on top of the rail and track girder to analyze the effect of the vibration occurring in the direct-fixation track of the Yeongjong Grand Bridge on the rail and the track girder. An accelerometer (ARF-50 A) has a frequency response range of DC--130 Hz, a natural frequency of 240 Hz, and a gravimeter with an acceleration of 5.1 g. The accelerometer was used to measure the acceleration of structures subject to vibration. The natural frequency and the damping ratio were estimated using the data measured by the accelerometer.  
Figure 11 shows the views of the accelerometer installation for the rail and the track girder. Accelerometers were installed on the rail and the track girder in the same section by the measuring position.  
Figure 12 shows an example of the measured waveform of acceleration in the direct-fixation track of the Yeongjong Grand Bridge for track vibration impact evaluation.  
The reason for installing one accelerometer was confirmed whether the driving train at accelerations within 0.5 g was accelerated. The vibration level of the current track was analyzed to be up to five times higher than the ballastless track (acceleration less than 0.5 g) using the general elastic rail-fastening  
Figure 10: Comparison of vertical displacements between the rail and the track.  
Figure 9: Example of a measured waveform Airport Express (AREX) of an end track displacement.  
system. It was expected to be susceptible to vibrations and thus damage and impact at the support of the track. In this study, the structural safety and the durability of the line bearing of the track were analyzed using the upward displacement measurement results of the support.