## 5 Conclusions  
A low-cost Structural Health Monitoring (SHM) system was designed and implemented in small- and full-scale tests. The Wireless Sensor Network (WSN), the base station, and the standard dynamical identification techniques enabled monitoring the structural responses to ambient, forced, and free-vibrations; and accomplished the dynamic characterization of civil structures in pseudo-real time. The vibration data and modal parameters estimated from the low-cost SHM system were compared to a reference SHM system to validate the performance of the WSN monitoring strategy with a deterministic approach. For the small-scale testing, the data vibration analysis and comparison in the frequency chirps from the shaking table tests were conducted based on ten different frequency levels (from 0.5 to 5.0 Hz) and four displacement levels (from 0.10 to 0.75 cm). It was observed that RMS errors decreased for the evaluation carried out on the resampled acceleration signals. The WSN monitoring system detected maximum accelerations associated with RMS error of up to 0.032 g, and the mean of the highest 100 resampled acceleration peaks displayed RMS error od up to 0.017 g. The dynamic characterization of the tested flexible structure showed that the low-cost SHM system effectively measured ambient vibrations greater or equal to 0.0008 g and identified precisely the first dominant natural frequencies within the frequency range between 0 and 10.0 Hz. Regarding the mode shape's correlation with the traditional SHM system, the modal assurance criterion (MAC) values were higher than 0.970. The estimated mean damping ratio was 2.339%, and its coefficient of variation was 36.8%. In the full-scale testing, the relevant natural frequencies of the lightweight RC slab were reliably estimated and monitored during the demolition work through the frequency analysis involving the power spectral densities and time-frequency distributions. The demolition and replacement of the original RC check-in beam did not result in structural disturbance to the prestressed concrete slab. From the dynamic characterization of the railway bridge, the low-cost SHM system appropriately measured ambient vibrations higher or equivalent to 0.0015 g, thereby permitting excellent identification of the first dominant vertical and transverse natural frequencies within the frequency range among 0 and 20.0 Hz. For the mode shape's correlation with the traditional SHM system, the MAC values ranged between 0.781 and 0.993. The estimated mean damping ratios were 1.514% and 3.630%, with coefficients of variation of 24.4% and 22.8%, respectively. The mean damping ratios of the small- and full-scale experimental tests represented low damping values, so the flexible structure and the railway bridge were recognized as lightly damped structures. Thus, besides the well-separated vibration modes obtained and white noise assumption for ambient vibrations, the frequency domain decomposition (FDD) technique was considered an accurate approach for the dynamic characterization performed in this study. To achieve higher accuracy with the modal identification technique employed, improving the vibration dataset synchronization of the WSN monitoring system would be ideal considering the notable performance of the low-cost accelerometers. The small investment required for the low-cost SHM system and its features support its implementation over existing WSN monitoring systems. This alternative WSN system supports a higher accelerometer resolution (20-bit) than the three previously mentioned WSN monitoring systems. The ADXL355 accelerometer covers a wide measurement range of \(\pm\)2.0 g for dynamic characterization from vibration data, as two of the monitoring proposals used in comparison do. However, the measuring node of the low-cost SHM system requires the highest cabled power supply (8.3 V) among these monitoring systems. Regarding the costs, the alternative WSN system has the lowest estimated cost per node of USD 73.5 compared to the rest of the wireless SHM systems.