# Structural Monitoring of a Drawbridge in Operation: Signal Analysis
## 2 Monitoring System  
As mentioned before, monitoring large critical infrastructures is a highly complex and costly task, albeit essential and crucial. Among those, bridges are vital structures for both road- and water-based transportation networks.  
With this in mind, a monitoring system was designed and implemented with the goal of reducing bridge downtime and improving maintenance practices by monitoring the structural condition of a double-leaf bascule bridge, as shown in Figure 1.  
Figure 1: Port of Leixões bascule bridge during an opening maneuver, in Matosinhos, Porto, Portugal.  
In addition, it provides the ability to analyze the evolution of key structural parameters, allowing the maintenance team to identify potential anomalies before they can lead to failure, at an early stage.  
The monitoring system consists of a combination of fiber optics and traditional sensors, combined with the respective acquisition systems and computer software specifically developed for this purpose (Figure 2).  
Considering the structure type, its hydraulic actuation and its past behavior, the choice was made to monitor as many relevant parameters as possible with this system. Then, prior to installation, a study was performed to determine the ideal location and type of sensors, and the following were selected:  
* Strains/stresses;
* Triaxial accelerations;
* Temperatures;
* Inclinations;
* Displacements;
* Hydraulic pressures.  
The first four are fiber optics-based, using an interrogator from HBK FiberSensing, while the last two use current transmitters, using National Instruments' USB-6008 board-only DAQs with custom conditioning electronics. The acquisition was managed using Linux-based computers and, as previously mentioned, custom software.  
The system continuously monitors the sensors' values, storing them in a ring buffer, and whenever an event is detected, all the sensor data related to that event are saved, including data from before the detection. Events, in this case, include any opening or closing of the bridge as well as any anomalous events. All the saved data can later be consulted and exported alongside the identification of the events to which they refer.  
The software that was developed for the monitoring system of the bridge has two main objectives: acquiring and saving the data recorded by the installed sensors. Thus, two main software components were developed: an acquisition software, which is responsible for communicating with each sensor and collecting its data, and a processing software, with the main function of checking for operational or diagnostic anomalies, sending alarms if any are detected, and storing the acquired data. Data were stored on a NoSQL database, in binary form, to reduce storage requirements.  
Figure 2: Monitoring System Overview.  
For maintenance purposes, it is also possible to check the state of any of the sensors, i.e., check if coherent data have been acquired from it recently, and to visualize the data from any events, plotting the sensors' values in a graph.  
The alarm logic includes two main categories: diagnostic and operational. The first type includes problems such as sensor failures, different types of communication issues, and database problems. Among the latter, one can find every other alarm type, such as, for example, excessive accelerations or strains. When the alarms are triggered, an email is sent to a configurable list of recipients, depending on its type.