# Structural Health Monitoring for Prefabricated Building Envelope under Stress Tests
## 4 Discussion  
The results demonstrate the efficacy of the developed technique, which is capable of monitoring various parameters. However, it is important to mention that during the test, the facade passed all the assessing criteria defined by the standard. Even though the facade did not undergo damage or deformation, the installed kit demonstrated its potential to be applied in SHM. Indeed, it is sensitive to minor structural variations, suggesting its potential for early damage detection. Several key findings regarding the performance of different sensor types for building envelope health monitoring under various loading conditions were highlighted by the activities to validate sensing technologies:  
* Accelerometers monitor dynamic evidence and impulsive effects acting on the structure and can highlight any critical effects and whether the structure has, therefore, been compromised. Structural variation can be highlighted by performing a comparison between different modal analyses or spectral response variations. From the accelerometers, several considerations can be made from the data collected. From the modal analyses performed before and after the monitoring of the phenomena acting on the glass facade, it was possible to notice a decrease in the modal frequencies of the structure after the exemption of the tests on the wall. This is evidenced by the results obtained in Section 3.2. Regarding the analysis of the structural vibration monitored with the accelerometers attached to the structure, two types of observations can be made: The first one is based on the RMS level of the acceleration that allowed for sensing a significant signal increase when the building was subjected to rain (dynamic), impacts, wind, building movements, and fan excitation. Also, a static rain and air permeability test was sensed by the accelerometers thanks to their high sensitivity (100.5 mV/g), even though the dynamic effects of those loads are limited due to their static nature. * The second observation is the monitoring of the dynamic behavior of the building, which was monitored by the power spectra estimated from the time histories measured by the accelerometers. The spectra show different signatures depending on the type of test that the building is subjected to. For example, in the fan excitation, the typical harmonic pattern is visible, consisting of the fan blade passing frequency and its high-order harmonics. Another piece of evidence is the deviation detected between the power spectra acquired on the first day of static rain and the same test realized on the second day, see Figure 24. Also, in this case, a decrease in the resonance frequencies of the building was observed, which confirms what is evidenced by the modal analysis. * Conversely, force washers effectively monitored structural health under static and dynamic loads. They accurately measured applied loads, providing valuable insights into the facade's stress response. Interestingly, washers with embedded sensors revealed normal behavior under typical service loads (600 Pa), regaining initial torque after  
Figure 32: Day 2 FBG for strain—Channel 2.  
stress as demonstrated at the end of tests in the 3 days. However, wind pressure tests (2625 Pa positive, \(-\)3000 Pa negative) showed permanent torque changes, indicating high-stress conditions. These sensors detected changes in the washer tightening torque, signifying critical stress levels requiring additional maintenance. This demonstrates the potential of integrating such sensors into facades for continuous monitoring of wind-vulnerable areas, confirming the force loads' suitability for SHM applications. This type of sensor made it possible to visualize the changes during all three days of the tests, and it enabled the capture of static phenomena more prominently. On the second day, it was possible to see that the tests tended to degrade the tension on the screws of the structure from the initial state.