## 3 On-Site Test Results and Analysis
### Analysis of the Midspan Section Acceleration for the Largest Span  
A comparison of the measured horizontal and vertical acceleration data of the track structure at Section C is shown in Figure 6. The rail is directly subjected to train load, and its vertical vibration is strong. In addition to individual test results, the vertical acceleration amplitude of the rail is basically above 400 m/s\({}^{2}\), while the maximum value at the bridge deck does not exceed 1 m/s\({}^{2}\). According to Figure 6, it can be seen that in terms of the dispersion of the data, the dispersion of the maximum amplitude of the acceleration is stronger than the effective value within the travel time of the train due to the random vibration effect. As can be seen from the measurements, (1) the average amplitudes of the vertical acceleration of rails and the bridge deck are 589.3 m/s\({}^{2}\) and 0.485 m/s\({}^{2}\), with maximum amplitudes of 74.8% and 93.2% being greater than the average amplitude. The \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multicolumn{2}{c}{**Range of Effective Values of Vertical Acceleration (m/s\({}^{2}\))**} \\  & **Section A** & **Section B** & **Section C** \\ \hline Rail & 18.87–25.42 & 26.51–37.64 & 35.26–46.58 \\ Concrete slab & 0.614–0.978 & 1.029–1.344 & 1.576–2.024 \\ Deck & 0.032–0.059 & 0.030–0.033 & 0.068–0.072 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristic statistics of the effective value of vertical acceleration of bridges along the traveling direction. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multicolumn{2}{c}{**Range of Effective Values of Lateral Acceleration (m/s\({}^{2}\))**} \\  & **Section A** & **Section B** & **Section C** \\ \hline Rail & 7.86–11.39 & 9.93–13.53 & 16.73–17.36 \\ Concrete slab & 0.627–1.488 & 0.669–1.185 & 1.722–2.215 \\ Deck & 0.023–0.032 & 0.023–0.025 & 0.043–0.048 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristic statistics of the effective value of lateral acceleration of bridges along the traveling direction. Figure 8: FFT time – frequency diagram of vertical acceleration during the free vibration phase of the concrete slab in Section C: (**a**) waveform diagram; (**b**) single peak spectrum. Figure 7: FFT time – frequency diagram of vertical acceleration during the vehicle-induced vibration phase of the concrete slab in Section C: (**a**) waveform diagram; (**b**) single peak spectrum. Figure 9: \(Cont.\)  
Figures 7 and 9 demonstrate that during the vehicle-induced vibration stage of the concrete slab, a high-frequency bandwidth ranging from 2200 to 2500 Hz is observed, along with two distinct high-frequency eigenvalues at 2210 Hz and 2440 Hz within this range. In contrast, during the forced vibration stage of the bridge deck, high-frequency signals above 1000 Hz were nearly completely attenuated to 0, resulting in the concentration of all signal energy within the frequency band of 500 Hz and peak eigenfrequency of approximately 200 Hz. By analyzing Figures 8 and 10, it is obvious that the free vibration bandwidth of the slab is down to 150-650 Hz, and the main characteristic frequencies are 130 Hz, 230 Hz, 295 Hz, 330 Hz, 430 Hz, 465 Hz, 565 Hz, etc. The free vibration bandwidth of the bridge deck is within 0-200 Hz, and the main characteristic frequencies are 2.5 Hz, 6.5 Hz, 10 Hz, 22.5 Hz, 28.5 Hz, 32.5 Hz, 65.5 Hz, 130 Hz, etc. It is evident that both forced and free vibrations exhibit significant differences in the vibration signal frequencies of the deck and slab. This discrepancy arises from their intrinsic physical properties--such as mass, stiffness, and damping--as well as structural  
Figure 10: FFT time\(-\)frequency diagram of vertical acceleration during the free vibration phase of the bridge deck in Section C: (**a**) waveform diagram; (**b**) single peak spectrum.