Bridge spatial displacement monitoring with 100 fiber optic sensors deformations : sensors network and preliminary results

In 1996, our laboratory fitted an highway bridge near Geneva (Switzerland) with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30 cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the behaviour between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor its long term deformations. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The curvature of the bridge is measured locally at multiple locations along the bridge span by installing sensors at different distances from the neutral axis. By taking the double integral of the curvature and respecting the boundary conditions, it is than possible to retrieve the deformation of the bridge. The choice of the optimal emplacement of the sensors and the sensor network are also presented.