# Perspectives on the Structural Health Monitoring of Bridges by Synthetic Aperture Radar
## 3 Analyses of Structural Integrity of Bridges from SAR
### Study Case 3  
The \"Bisantis\" bridge is an arched road and pedestrian bridge, consisting of a single carriageway and three lanes, two located in the south west direction and one existing in the north east direction, both built on a single reinforced concrete arch. The \"Bisantis\" bridge connects the city of Catanzaro with the other cities of the Calabria region. The SLC SAR magnitude representation of the scene is reported in Figure 16a. The vibration map is visible in Figure 17. Looking at this figure, several energy  
Figure 14: Temporal trend of vibrations.  
Figure 15: Vibration spectrum (DFT of Figure 14): (**a**) Linear scale, (**b**) Logarithmic scale.  
concentration points are clearly visible at one end of the bridge, in the middle and along the arch and the vertical supports. The energy group indicated by the yellow arrow A, are false alarms because the plots are generated by tree canopies, detected by SAR on the street surface due to the layover effect. Energy vibration accumulations 1, 2, 3 and 4 are instead generated by cracks and real deformations that are present on the structure. In particular, vibration anomaly 1 is generated by the strong degradation of the physical characteristics of reinforced concrete visible in Figures 18a and 19a. vibration anomaly 2 is generated by the lesion visible in Figure 19b. The vibrational anomalies indicated by arrows 3 and 4 could be associated with hypothetical damage to the structure. Coincidentally, as in case where significant damage can be observed in Figure 19c,d, every time vibrational anomalies to structures are measured, a visual feedback in terms of joints or material degradation is achieved. The vibration anomaly 5 corresponds to the thermal dilatation joint visible in Figure 18b while vibration anomaly 6 corresponds to the thermal dilatation joint visible in Figure 18c.  
Figure 16: (a): “Bisantis” bridge SLC SAR ROI, (b): “Italia” bridge SLC SAR ROI.  
Figure 17: “Bisantis” bridge vibration map.  
Figure 20 represents the trend over time of the oscillations on point 1 of Figure 17. Observing this trend, it can be noticed that this point has persistent components at low frequencies and others less marked at higher frequencies. The results of the spectral analysis are shown in Figure 21. Figure 21a represents the overall spectrum, using the linear scale, whereas, Figure 21b represents the overall spectrum in the logarithmic scale. This result brings out a main mode of oscillation on the frequency of about 5 Hz. Numerous other oscillation modes are visible at frequencies near 80 Hz, and from 180 Hz to 380 Hz. Finally, an abnormal peak around 580 Hz is observed which is probably due to the bridge's cracks (As reported in [18; 19; 20] when energy discontinuities are observed, especially at high frequency, an alarm bell should be triggered in order to preserve the health of the infrastructure.)  
Also in this case, the general trend of the frequency response of the infrastructure exhibits a low-pass nature.  
Figure 19: Optical particulars of the “Bismuth” bridge structure: **(a)** Particular 1 of Figure 17, **(b)** Particular 2 of Figure 17, **(c)** Particular 3 of Figure 17, **(d)** Particular 4 of Figure 17.  
Figure 18: Optical particulars of the “Bismuth” bridge structure: **(a)** Particular 1 of Figure 17, **(b)** Particular 5 of Figure 17, **(c)** Particular 6 of Figure 17.