## 3 Methodology
### Descriptions of the Tests
#### 3.2.2 Full-Scale Testing  
Two experiments were carried out on different large civil structures: The first experiment consisted of monitoring natural frequencies on a structural slab during demolition work, whereas the second experiment consisted of the dynamic characterization of a railway bridge during its operation. * _Beam Demolition Monitoring_: The reinforced concrete (RC) beam (later demolished), or so-called check-in beam, was a lateral support element of a lightweight RC slab. The  
Figure 5: Flexible structure monitoring: (**a**) Two-story flexible structure. (**b**) Experimental setup. (**c**) Location of measurement channel 2.  
prestressed concrete slab has five primary spans of 24-m joists placed in the lightweight RC slab transverse direction. The 49-m check-in beam supported 30 prestressed concrete joists. Prior to the demolition work, an RC girder was constructed to resist the lightweight RC slab laterally and replace the check-in beam. The prestressed concrete slab was monitored to evaluate whether the vibrations of the demolition would produce significant disturbances to the current service zone of the airport, and thus identify the stiffness changes that may compromise the stability of the structure. The SHM strategy was carried out on the central joist, for each span, through ambient and forced vibration records during the TE 1500-AVR Hilti demolition hammer operation, whose impact frequency was 1620 impacts/minute (27.0 Hz) [33]. A measuring node was installed at the base of the central joist mid-length to every span (Figure 6). The demolition work and structural monitoring were completed at once, span by span, as pointed in Figure 7. Thus, the base station moved continuously throughout the check-in beam demolition. The vibration data were resampled at 70 Hz, a frequency that includes the dominant vibration modes. From the lightweight RC slab response measured, natural frequencies were identified and monitored by power spectral densities and spectrograms before and during the demolition work. The frequency analysis implemented for the prestressed concrete slab considered just the vertical transverse direction of the joists from the slab using the low-cost SHM system. Figure 6: Beam demolition monitoring: (**a**) Joists of the slab and check-in beam. (**b**) Measuring node. Figure 7: Experimental setup of the beam demolition monitoring. * _Railway Bridge Monitoring_: The test bridge is a single-span steel structure composed of a tridimensional truss, connected along the top seam by a latticework and along the bottom seam by structural steel members. The steel structure has one 44.0-m span in the longitudinal direction, a deck width of 5.3 m, and a height of 9.0 m. Figure 8 presents a perspective image of the steel structure. The railway bridge was defined as a 6-DOF dynamic system for testing: 3 vertical and transverse DOF. The dynamic characterization of the structure was conducted for both vertical and transverse directions individually through ambient, forced, and free-vibration records from the structural responses elicited by diverse train traffic (classified by velocity, loading capacity, and type of train) on the steel bridge. The experimental setup consisted of three measurement channels composed of a vertical and transverse DOF. On this subject, the measurement channels were set to L/4, L/2, and 3/4L of the bridge length from the base station placement, positioning the measuring nodes between the railway sleepers and above the top flange of the bottom structural steel members as shown in Figure 9. The locations of the measuring nodes for channel 1 and the base station (located at the south side of the structure) are pointed to in Figure 10.  
The vibration data of the reference SHM system were resampled at 250 Hz to, as described above, process and compare digital signals from unbiased frequency ranges. As the 6-DOF dynamical system response was measured, it was possible to identify vertical and transverse dominant vibration modes of the railway bridge.