## 4 Results and Validation
### Full-Scale Testing
#### 4.2.1 Beam Demolition Monitoring  
The assessment of the dynamic response of the lightweight RC slab is presented for the span 1-2. The results reached for the rest of the spans from the prestressed concrete slab are extremely similar to the following analysis. Further information about the considerations and outcomes of other spans can be found in [36]. The ambient and forced vibration datasets during the hammer impact are shown in Figure 23. In these plots, the ambient vibration amplitude reached values up to 0.0022 g, and the peak detected for the forced vibration was 0.0040 g.  
Figure 23: Vibration response from the span 1–2: (**a**) Ambient vibration record. (**b**) Forced vibration record during demolition work.  
Figure 22: Free vibration response of the two-story structure for DOF 2: (**a**) Exponential free-decay window. (**b**) Representative acceleration peaks from the filtered free-vibration signal of the ADXL355 RBP3 accelerometer.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{4}{c}{**Vibration Mode 1**} \\ \hline \multicolumn{2}{c}{**Reference System**} & \multicolumn{2}{c}{**Low-Cost System**} \\ \hline
**Mean** & **Coefficient of** & **Mean** & **Coefficient of** \\
**Damping** & **Variation (\%)** & **Damping** & **Variation (\%)** \\
**Ratio (\%)** & & **Ratio (\%)** & \\ \hline
2.022 & 46.8 & 2.339 & 36.8 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Mean damping ratios and coefficients of variation calculated for both monitoring systems.  
The dynamic response of the lightweight RC slab in the frequency domain, resulting from the ambient and forced vibration, was analyzed by the power spectral densities indicated in Figure 24. The relevant frequency content included a first frequency component between 9.84 and 11.5 Hz and a second frequency component located within a range of 21.9 and 22.7 Hz for both power spectral densities. In particular, as revealed by the power spectral density of forced vibration, the frequency impact of the Hilti demolition hammer on the check-in beam was detected by a sharp peak at 26.8 Hz.  
For the time-frequency distribution analysis, the energy content of the mentioned dominant frequencies is presented by the spectrograms in Figure 25. The natural frequency identification of the lightweight RC slab from the power spectral densities was verified by the first frequency lines detected in both spectrograms; moreover, the natural frequency ranges did not change significantly during the forced vibration record. Likewise, the impact frequency of the demolition hammer was observed notably at a frequency line very close to 27.0 Hz, as shown in the spectrogram of the forced vibration.  
The first frequency component located between 9.84 and 11.5 Hz includes the fundamental natural frequency (10.0 Hz) of the same prestressed concrete slab reported in a previous official technical report, demonstrating an accurate estimation carried out from the low-cost SHM system. The demolition hammer was operating in a frequency bandwidth considerably separated from the dynamic response of the prestressed concrete slab. Therefore, the fundamental vibration mode did not experience resonance effects. It was observed that predominant vibration modes remained undamaged after demolition work. Thus, the stiffness and support conditions of the lightweight RC slab were preserved.  
Figure 24: Dynamic response of the lightweight RC slab corresponding to the span 1–2: (**a**) Power spectral density of the ambient vibration. (**b**) Power spectral density of the forced vibration.  
Figure 25: Time–frequency distribution of the response from the lightweight RC slab corresponding to the span 1–2: (**a**) Spectrogram of the ambient vibration. (**b**) Spectrogram of the forced vibration.  
The wireless Internet connection and remote communication of the low-cost SHM system functioned permanently during the fieldwork under the ambient conditions and experimental setup design.