# Structural Monitoring of a Drawbridge in Operation: Signal Analysis
## 1 Introduction
They also emphasize that a lack of measurements in the field is partly responsible for the limits on the understanding of many civil engineers regarding the operational behaviors of contemporary structures and highlight that the development of automated data-processing methods is of the utmost importance in the implementation of effective SHM strategies. Similar principles have also been applied to bascule bridges [10; 11; 12; 13], where the goals are often focused on monitoring their opening mechanism, but usually also include regular structural considerations, with the first reports of continuous remote monitoring systems for bascule bridges originating 25 years ago [11]. [PERSON] et al. [10] also performed a long-term monitoring campaign of a heel trunion buscule bridge, where strain gauges, accelerometers, and inclinometers were employed with the goal of analyzing fatigue damage to the structure. This work also highlighted problems such as sensor faults, which are extremely important in real-world measurements but are often not approached in the academic literature. Finally, the work also discussed the difficulty of reaching a root cause for any changes, proposing the use of cameras to provide further context to the measurements. However, they did not monitor the bascule bridge's control system, which could have provided further insights into those mechanical issues. [PERSON] et al. [13] developed an SHM system for a bridge that combines a tied arch span, two continuous three-span truss units, and a rolling lift buscule span. The system monitored strain, tilt, and temperature and also included a video feed synchronized with the sensor data, enabling the long-term identification and tracking of structural performance. While they included process variable data in the monitoring, no data processing is described. The same research group, [PERSON] and [PERSON] [12], also employed a similar approach to critical parts of a bridge's life; in this case, its construction and rehabilitation. This work also highlights the importance of monitoring in these stages because the magnitude of forces in these processes is often larger than during regular service. However, since the work was mainly focused on the rehabilitation and construction stages, it does not represent a long-term continuous monitoring solution. [PERSON] et al. [14] have employed robotic total stations, 3D terrestrial laser scanning, and distributed fiber optic sensing (DFOS) to perform measurements on a highway bridge. While this was not a permanent system, it enabled very-high-spatial-resolution measurements of deformation along the bridge span, highlighting the shape of the deflection curves, but with the significant drawback of generating a considerable amount of data. Additionally, this work highlighted that fiber optic sensing systems as a whole have the major advantage of a simplified cabling, in which a single fiber can contain multiple sensors [15]. [PERSON] and [PERSON] [16] bring attention to the importance of temperature compensation in static and dynamic analyses of bridges and to the difficulties of implementing wireless systems in comparison to wired ones. Overall, there has been significant development in bridge SHM systems, but nonetheless, as [PERSON] and [PERSON] [16] mention in their work, it is difficult to design an SHM approach that is usable for all bridges due to the significant variations among them, as they often have little in common with each other. The present work reports the development of a monitoring system for a drawbridge located in a port in northern Portugal ahead of its rehabilitation, as well as the signal-processing methods that were implemented for event detection, classification, and analysis. This is intended to be a permanent SHM system, which draws from fiber optics and conventional sensors, as well as process variables from the bascule bridge's operation, unlike many of the works in the literature.