LYRA: a novel autonomous monitoring solution for bridge stays and hangers

Abstract

The determination of the tension in cable-stayed and tied-arched bridges has been a concern for authorities for decades in order to verify the integrity of the structures. The increasing age of the bridges and the recent collapse of several of them across Europe is making their monitoring more important than ever. Nowadays, the analyses usually rely on sparse on-site measurements of the strain or of the direct force when hangers are replaced (i.e., a few times in the lifetime of the structure) or on the use of accelerometers and the simple taut string theory, which ignores the bending stiffness of the cable and its boundary conditions, to measure the tension (usually at best once a year). As a result, damages can be detected late and the measured data depends on the environmental conditions during the measurement (traffic, temperature, wind,…) In this paper, we first describe LYRA, a novel autonomous bridge monitoring solution based on wireless accelerometers and on a robust mathematical model of the hangers. The accelerometers measure the response of the hangers to environmental solicitations six times a day and dispatch the data to a central station. Then, the station computes the tension of the cables from the time response of the sensors by means of a robust algorithm, which is an extension of the simple taut string theory for flexibles anchors and/or non-negligible bending stiffness. Finally, the measured tensions are uploaded on a webserver, where they can be monitored from any apparatus with an internet access, and alerts are automatically generated if tensions outside of pre-set boundaries are measured. Then, we demonstrate the capabilities of the system on the Lanaye bridge, located at the border between Belgium and the Netherlands, whose 30 hangers have been monitored since July 2021.