Theoretical study of extracting modal frequencies and hinge joint stiffness for thin-walled assembled multi-girder bridges from 3D vehicle.

• Closed-form CP responses for 3D vehicle passing over the AMGB are derived firstly. • A new two-step retrieving strategy of AMGB's modal frequencies is presented. • Hinge joint stiffness can be estimated briefly by retrieved bridge frequencies. • Reliability and robustness of proposed procedure is verified by various examples. This paper presents a theoretical framework for extracting the modal frequencies of thin-walled assembled multi-girder bridges (AMGBs), which are widely adopted in small- and medium-span highway bridges, using the dynamic response of a three-dimensional (3D) moving vehicle. For the first time, closed-form solutions for the responses of the bridge and contact point (CP) are derived when the test vehicle passes through the AMGB. Based on the derived CP response, a guiding procedure for retrieving the modal frequency and hinge joint stiffness of the bridge is proposed. The correctness of the analytical solutions for the CP response and the reliability of the bridge frequency identification are verified through numerical examples. Additionally, the robustness of the proposed technique is examined under various influence factors. The results show that maintaining a moderate vehicle speed can balance the accuracy and efficiency of the bridge frequency identification. The damping of vehicle tires has a small effect on frequency extraction using the CP response, while an increase in bridge damping ratio diminishes the visibility of its frequencies. Nevertheless, for the investigated bridge damping ratios, the effectiveness remains satisfactory, as the first four frequencies still being discernible. The residual contact response of the front and rear wheels can effectively eliminate the masking effect of pavement roughness, thereby enhancing the frequency extraction effect. Furthermore, the proposed method exhibits strong robustness against measurement noise. [ABSTRACT FROM AUTHOR]