PSI - Issue 64

Antonio Bilotta et al. / Procedia Structural Integrity 64 (2024) 2109–2116 "Bilotta et al." / Structural Integrity Procedia 00 (2023) 000 – 000

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3. Structural health monitoring system 3.1. Type of sensors

Multiple sensors are used in monitoring day-by-day structural response due to traffic loads and environmental actions, such as seismic and wind loads, as well as thermal actions. For the latest, estimations of thermal effects are particularly needed in structural health monitoring system, mainly to identify environmental displacements/deformations varying the observation period during the seasonal changing. Indeed, since simply supported decks are mainly analyzed, the measure of deformations due to thermal actions (especially bearings displacements) can be clear and simple to be processed. The layout for the dynamic part of the SHM is made by accelerometers. These were mainly included to characterize the evolution of the bridge deck dynamic parameters, such as fundamental periods of at list four main vibration modal shapes. Also, position of accelerometers allows to identify the dynamic amplification bridge response due to heavy traffic loads, possibly catching arising structural damage due to anomalous behavior in terms of both accelerations and displacements (the latest eventually obtained by integrating the recorded signals). Additional accelerometers were also used to record vibrations of piers and/or pier caps, depending on the relative importance of these structural elements in the overall bridge context. Also, the presence of accelerometers along the pier allows to identify the pier characteristics (for estimating both seismic response and soil structure interaction). The static response of the structure is further monitored by inclinometer and displacement transducers. The inclinometer sensors are used both for monitoring the bridge deck than the piers. These sensors allow to identify deck tilting at the bearing location, eventually matching the bridge dynamic response due to traffic loads using static parameters. Displacement transducers are included into the generic SHM setup for two main reasons: (i) to evaluate evolution of bearing displacement during time; (ii) to characterize displacement and tilting of half joints when relevant; (iii) to measure the mean curvature in a middle span section. In all cases, the objective is the identification of structural defects that could arise during time due to natural degradations of materials and components, as well as damages caused by heavy loads passing through the bridge. 3.2. Main SHM features All the sensors have been connected each other with specific data acquisition systems, allowing synchronization of all the records. Even if the measurements are practically continuously, periodic acquisitions are set to minimize the recorder data and the dataset space for storage issues: therefore, four times per day (i.e., in 24 hours) a recording time of twenty minutes was fixed as the standard. The four defined dataset were chosen considering the probable different traffic level (minimum and maximum) during the day, i.e., having two datasets with low traffic condition and two with high traffic condition with respect to the mean traffic conditions estimated using data collected from the highway operator. Moreover, the SHM system is set to record data in two further relevant conditions: (i) when certain fixed threshold values of detected parameters are attained, for example a target acceleration value in a specific location, as well as a target displacement recorded in displacement transducers on bearing elements. When some thresholds are overcome the data acquisition system record data also including 15 mins registration before the event; (ii) as a specific request of the managing company, for example to monitor the structural response when some known special trucks are passing through the bridge. The sampling frequency could change as a function of the measured parameter velocity rate. Based on preliminary experimental and numerical analyses on the bridges, the sampling frequency for the accelerometers has been set at 200 Hz . For displacement and rotation (i.e., static parameters) the sampling frequency has been set at 100 Hz to account for the truck transit effect at any permissible velocity. Nevertheless, experimental evidence during first data acquisition will allow tuning such values to optimize both data storage and quality. At present, the acquisition control units have a self-diagnosis system based on checking the factory electrical parameters. The systems checks whether the sensor (accelerometers, transducers, etc.) is working according to the manufacturer's standards or, trivially, is not transmitting as expected. The platform provides warning in real time, in such a case: the sensor appears 'yellow' if it is operating abnormally and 'red' if it is offline. As future development, the functionality of sensors or transducers should be checked implementing a two-level system to ensure proper sensor functionality. At the first level, it is suggested to train an artificial intelligence to detect abnormal readings that could indicate sensor failures, with a focus on local problems on one or two sensors simultaneously. At the second level, it is proposed to conduct periodic tests (e.g. each 2 years) using a vehicle with known characteristics to verify the simultaneous operation of all sensors, considering decay and potential drift of sensors but also the possible decay of the structures. This approach is intended to identify problems involving the entire sensor system. In this case, it is suggested that