Measuring the dynamic displacements of bridges using geophone data: Application and validation on a lively footbridge

Dynamic displacement is a highly valuable information for the evaluation of bridge safety and performance, providing data on the dynamic behaviour of the structure under service loads. However, its measure is often challenging and costly: traditional techniques require a fixed support and a direct connection (or a clear line-of-sight) with the target point that is rarely present in large-scale infrastructures, making their deployment in long-term monitoring often not feasible. In this study, an indirect measuring approach deploying an electro-dynamic velocity sensor (geophone) and digital filtering is adopted to measure human-induced dynamic displacements on a lively footbridge. These sensors have an excellent cost to performance ratio, do not require any powering and, overall, are easy to install and deploy for a long time. Furthermore, geophones measure velocities and consequently only “one integration step” is needed to obtain displacements. The field test was performed with the bridge partially open to pedestrian passages, the geophone was placed in different positions directly on the deck, and a reference measure of the vertical displacement was obtained from a laser-based system placed nearby. Appropriate digital filtering is then applied on the geophone data before the integration. The calibration of digital filters and the validation of the results was carried out comparing the reconstructed vertical displacements from geophone data and the directly measured vertical displacements from the state-of-art measuring device. Under the assumption of pure dynamic loading, some promising results were obtained, reaching a tenth of a millimetre accuracy for the integrated signals.