Design and Development of a Distributed Monitoring System for Bridges

The state of Italy's bridges is alarming: more than half were built before the 1970s, making them more than 50 years old and approaching the typical lifespan of reinforced concrete structures built after the Second World War. Inadequate maintenance has led to numerous closures and collapses, with the cost of extraordinary repairs often exceeding that of demolition and reconstruction. In addition, only 60000 of Italy's 1.5 million bridges are monitored, often using outdated visual inspection methods. In this context, this thesis aims to provide an alternative solution, in the hope that tragic events such as the collapse of the Morandi Bridge (Genoa, 2018) will never happen again. The thesis presents the design and development of a low-cost Structural Health Monitoring (SHM) system for bridges. The system is based on the postulate that a structural change due to damage results in a more or less pronounced change of its dynamic behaviour (natural frequency, damping ratio, mode shapes). The proposed solution consists of a wired sensor network to be applied to the structure under test for continuous and real-time monitoring. It is based on two different PCBs. The first PCB (called BridgeWatch) integrates an accelerometer, an inclinometer, and a gyroscope to capture bridge movements. In the project, four of these boards have been included, which are to be placed in strategic locations to capture accelerations useful for our purposes. The second PCB (called EnvironMonitor) collects environmental data, including rainfall, wind speed, temperature, and bridge-water level. Only one of this kind of board is sufficient. The integration of multiple sensors allows for a comprehensive understanding of both structural and environmental conditions affecting the bridge's integrity, and facilitates the identification of correlations between them. A on-site Raspberry Pi acts as the overall coordinator: it communicates with the BridgeWatch microcontrollers via an RS485 bus and with the EnvironMonitor microcontroller via an RS232 bus to receive data from all the boards, process the data and create an output file for subsequent data analysis. Furthermore, the system is powered by a 12V battery that is recharged by photovoltaic panels, eliminating the need for an electrical outlet. The steps taken to create the system described above began with an analysis of the requirements and a study of the state of the art to keep up to date with the technologies currently in use. All the individual sensors to be used and the hardware components necessary for the overall operation were then selected and the schematics and layouts of the PCBs were created using Altium Designer. Once the PCBs were printed, all the components were soldered by hand and the drafting of the firmware began. The main program is the one that runs on the Raspberry and manages all communication and synchronisation between the boards. On the other hand, each microcontroller on each board runs a firmware created with the IDE provided by STMicroelectronics, which manages communication with the sensors present and sends data to the Raspberry on request. The final phase was laboratory validation. One of the several tests carried out involved using a shaker to reproduce vibrations at different frequencies and analysing the data from a BridgeWatch board attached to the instrument using FFT. For all the frequencies used in the test, the peak obtained after applying the FFT was consistent.