Dynamic load monitoring of a concrete bridge using a fiber optic Distributed Acoustic Sensing (DAS) system

In the present study, the impact of dynamic traffic loading in the concrete structure of an existing bridge has been investigated using a Distributed Acoustic Sensing (DAS) system. The dynamic load results in excitation of Eigen Modes (EM) which have been investigated using dynamic strain measurements at various locations simultaneously. The results of the load tests on Amsterdam bridge 705 make an important contribution to the understanding of its structural behavior. The evaluated data can be used to support and verify Finite Element Models (FEM) contributing to ensure the safety of the ageing concrete structure of the bridge. In the face of increasing traffic density and severe traffic loading on existing infrastructure, a safety evaluation based on accurate distributed fiber optic measurements can be an important means to estimate remaining lifetime of the bridge. The concept of the dynamic loading was based on the use of a tram passing by and a well-defined movement of a 36-ton truck. The load applied in this way led to location-dependent small deflection effects recorded as longitudinal strain of the sensing fiber embedded at the underside of the bridge. The laying of the 93-m long sensing fiber in one piece across and along the bridge serves to obtain distributed two-dimensional dynamic strain data. The achieved results demonstrate the capability of the applied system and method to detect dynamic strain signals in the range of a few μm/m at the spatial resolution of 1 to 10 meter along the 93-m long sensing fiber installed on different trajectory of the bridge. The measurements were performed with an inhouse developed DAS system based on Rayleigh scattering.