Observations and modeling of flexural-mode Lamb waves in box girder bridges using pre-existing telecommunication fiber-optic cables

Bridge superstructures, such as slabs and box girders, behave as plate-like structures where Lamb waves propagate. Lamb wave characteristics are highly sensitive to structural properties and damage. Variations in Lamb wave characteristics can thus serve as indicators of structural changes. However, conventional methods struggle to capture their wavefields due to the high cost of deploying dense sensor networks. This study presents the successful measurement of wavefields of low-frequency (1-70 Hz) flexural-mode Lamb waves in bridge box girders using distributed acoustic sensing (DAS). DAS turns pre-existing telecommunication fiber-optic cables into ultra-dense dynamic-strain-sensing arrays, enabling continuous, high-resolution vibration and seismic monitoring. Through ambient noise interferometry, we reconstruct Virtual Shot Gathers from traffic-induced vibrations, eliminating the need for active seismic sources and minimizing disruptions to regular transportation. This method captures both standing waves (resonance vibrations) and propagating flexural waves, corresponding to the fundamental antisymmetric Lamb wave mode. To validate these observations, we model Lamb wave dispersion by approximating box girders as elastic isotropic plates and Bernoulli-Euler beams, incorporating material properties from design drawings. The strong agreement between modeled and measured dispersion curves confirms the reliability of DAS-recorded flexural waves in box girder bridges. These findings demonstrate the potential of DAS for capturing low-frequency Lamb waves, providing a scalable, real-time solution for bridge condition assessment.