## 1 Introduction  
The natural frequencies of bridges play a crucial role in health monitoring. Continuous monitoring and analysis of these frequencies provide valuable insights for maintenance, repair, and ensuring the safety of bridges [1; 2]. Damping ratio [3] and mode shape [4] are also used as indicators of structural health and damage. Conversely, the damage degree of a bridge structure can be evaluated by the dynamic characteristics of the bridge. The conventional approach for measuring the dynamic response of a bridge involves installing dynamic sensors on the bridge structure [5; 6]. The damage of a bridge will lead to the change of its dynamic characteristics [7]. Vibration-based nondestructive testing has also been studied for many years [7; 8; 9; 10]. Researchers have increasingly focused on measuring devices, analysis methods, and system identification techniques for bridge vibration modal characteristics [11; 2; 12].  
To enhance work efficiency and reduce cost, an indirect method was proposed [13]. The vibration information of bridge can be extracted by analyzing the vibration information of the vehicle [14], and the influences of parameters, such as initial acceleration ratio, frequency ratio, and velocity, on bridge frequency identification were discussed. The authors in [1; 12] took the square of acceleration as the information carrier of structural dynamic characteristics. [PERSON] et al. [15] proposed a band-pass filter singular spectrum analysis method to filter vehicle frequency. [PERSON] and [PERSON] [16] utilized short-term frequency domain decomposition to estimate the mode shape of the bridge based on the dynamic response of the vehicle. They introduced external excitation to the bridge while subtracting the signal from the continuous trailer axle of the vehicle's traction, thereby mitigating the influence of roughness. Regarding parameter analysis, [PERSON] et al. [17] highlighted that improved accuracy in bridge frequency estimation can be achieved at lower speeds and on flat pavement conditions. Studies have shown that the initial vehicle-bridge acceleration amplitude ratio plays a crucial role in identifying the frequency of a bridge [15; 17]. Specifically, a smaller ratio of vehicle-to-bridge acceleration amplitude is conducive to the successful identification of the bridge frequency.  
To the best knowledge of authors, the applications of the indirect method for composite beam bridges have not been reported. Further investigation is needed to evaluate the applicability of the indirect method to composite beams. To study the dynamic interaction between the moving vehicle and the composite bridge, the vehicle system can be simulated by moving load, moving mass, moving spring-mass, and moving spring-mass-damping models [14; 18; 19]. There are also some complex vehicle models [1; 20; 21]. Composite beam bridge sections are usually composed of two or more materials (usually steel and concrete), which can make full use of the mechanical properties of different materials. However, the influences of the shear connection and interfacial slips between different subcomponents on the identification of bridge frequency have not been investigated [22; 23; 24]. This study focused on the application of the indirect method in composite beams. The vibration frequency of composite beams was obtained through the utilization of the indirect method. In order to determine the optimal configuration of vehicle parameters, an analysis was conducted to assess the influence of various parameters, including vehicle mass, vehicle stiffness, vehicle damping, vehicle speed, shear connection stiffness, and prestress of the tendon. Furthermore, the influences of environmental disturbances were also discussed.