# Structural Monitoring of a Drawbridge in Operation: Signal Analysis
## 4 Results  
The event identification procedure takes two reference curves as templates, which are used to determine if a particular event is an opening or closing, and are shown in Figure 4. The regions for cross-correlation analysis are highlighted on each plot in yellow (\(t_{o}=\frac{N_{b}-1}{f_{s}}\), \(t_{c}=\frac{N_{c}-1}{f_{s}}\)), while the thresholds for the rough alignment procedure are shown as red lines (\(r_{o}\), \(r_{c}\)). After event identification, the average curve, as well as the maximum and minimum envelopes, can be calculated. The results obtained for the month of January 2024 can be seen in Figure 5, where it is possible to see that the curves follow the same trends in the shaded region, but deviate significantly later. Figure 4: Reference curves for the (**a**) open and (**b**) close events. The shaded region represents the part of the signal that is employed for matching, delimited by **(a)** 0 and \(t_{o}=\frac{N_{o}-1}{f_{s}}\) and (**b**) \(t_{c}=\frac{N_{o}-1}{f_{s}}\) and the total number of samples. Here, \(f_{s}\) is the sampling rate. The red lines correspond to the threshold levels, which are **(a)** \(r_{o}\) and **(b)** \(r_{c}\). Figure 5: Analysis of the maximum, minimum and average curves for January 2024: (**a**) average curve, and maximum and minimum curve envelopes; (**b**) maximum and minimum difference envelopes. Here, \(t_{o}\) is the time associated with \(N_{o}\). The calculated average curve can also be used to facilitate the extraction of certain parameters from individual events, such as oscillation frequencies, and to easily find deviating behaviour. For example, Figure 6a shows an outlier opening event, where it is possible to see significant oscillations that are either not present in the average signal or have significantly lower amplitudes. In parallel, the same analysis was performed for other sensor signals. For example, in the same outlier opening event, the principal strain measured on the structure, and shown in Figure 6b, also includes deviating behaviour, which manifests as an oscillation and a deviation from the average. To further analyse these deviations, the difference between this particular event and the average curve can be calculated, obtaining the curve shown in Figure 7. In this particular case, it is possible to clearly notice an oscillation in the middle of the shaded region at approximately 0.2 Hz, which is not present in the reference signal. Additionally, it is possible to see that the difference magnitude increases approximately linearly after the end of the cross-correlation matching region, showing that the underlying behaviour of the structure is still present, such as the final hump of Figure 6a, which is removed in the difference plot. Figure 6: Acquired data for an outlier opening event compared with the monthly average for (**a**) a pressure sensor; (**b**) the principal strain on a particular point of the structure. Notice the increased oscillations on both signals. Figure 7: Difference between a particular event and the reference. The shaded region is the area that was used for matching. Here, \(t_{o}\) is the time associated with \(N_{o}\). The proposed classification procedure is able to classify events into three categories: the previously mentioned opening and closing movements, as well as inconclusive ones. These often become the most interesting ones, as they do not fit the requirements for either an opening or closing movement and thus reflect an unexpected situation. For example, the curve shown in Figure 8a was classified as an inconclusive event. Even though it is possible to see that the event starts as a regular opening, it is also visible that, at some point, a mechanical issue caused a deviating behavior with significant oscillations. This is followed by the bridge operator's actions to return the system to working order. Complementing this, Figure 8b shows matching rosette data for the same event, where it is possible to see the same oscillation profile as with the pressure sensors.