## 5 Parameters Study
### Influence of Interference Signal  
In the process of signal transmission, it is possible for the signals to be affected by electromagnetic environmental interference. The interference signals directly impact the signals themselves, resulting in disturbances. The distribution of these interference signals is similar to that of interference forces, as they appear randomly in time while maintaining a relatively stable power level. These interference signals can also be simulated using white noise signals. The original acceleration signal was based on the signal without interference forces applied. The time-domain and spectrogram of the vehicle acceleration under different signal-to-noise ratios (SNR) of the interference signals are shown in Figure 27. As the SNR gradually decreased, the power of the interference signals relative to the original signal increased, causing the original vibration signal of the vehicle to become increasingly blurred. When the interference signal became sufficiently strong, it completely masked the original vehicle vibration signal. Figure 27: _Cont._  
Figure 26: Acceleration and frequency responses of the test vehicle at different interference power values: (**a**) 20 dBW; (**b**) 30 dBW; (**c**) 40 dBW; (**d**) 50 dBW. as road unevenness, interference forces, and interference signals were discussed. The main conclusions are as follows:  
1. In the ideal condition without interference, the indirect method exhibited a high level of accuracy in identifying the natural frequencies of the composite beam bridge, especially the fundamental frequency. In this ideal condition, the vehicle speed had a significant impact on the identification of the composite beam frequencies. Due to the influence of the vehicle's driving frequency, the amplitude spectrum of the measured acceleration exhibited bifurcation peaks. Vehicle parameters also affected frequency identification, resulting in an increased amplitude around the vehicle's natural frequency in the spectrogram. Vehicle damping, on the other hand, had a suppressing effect on the amplitude of all frequency orders. The interlayer shear stiffness of the composite beam and the magnitude of the prestress of the tendon can cause variations in the bridge frequency. The shear stiffness and the prestress value corresponding to fundamental frequencies after the changes can be identified using this method. 2. The influence of road disturbances on frequency identification was such that the fundamental frequency can be identified when the road surface roughness is relatively low. Vehicles with larger masses contribute to improved frequency identification. As the vehicle speed increased, the amplitude of the acceleration spectrum also increased. However, the occurrence of bifurcation phenomena at the peak points became more pronounced, necessitating a comprehensive consideration when selecting an appropriate speed. Due to limitations in research funding and time, the method proposed in this paper has not yet been applied to practical engineering projects. Research will be conducted in the next phase of our study, with the proposed method being applied to practical engineering practices. Conceptualization, T.W. and J.-P.L. ; software, T.W. and B.C. ; methodology, Y.C. and J.-P.L. ; investigation, Y.C. and J.-P.L. ; writing--original draft preparation, T.W., B.C., Y.C. and J.-P.L. ; writing--review and editing, T.W., B.C., Y.C., B.H. and J.-P.L. ; resources and supervision, T.W. and J.-P.L. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Natural Science Foundation of China (No. 52378158), the Natural Science Foundation of Xiamen, China (No. 3502 Z20227200), the Natural Science Foundation of Fujian Province (No. 2023 J01106), the National Natural Science Foundation of China (No. 52008255), Natural Science Foundation of Guangdong province (No. 2021 A1515010474), Open Fund of State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology (No.