## 3 Experimental Study on In-Service LG Beams
### Specimens and Test Methods  
The experimental study was carried out in May 2019, on a selection of 32 LG beams composed of two, \(h_{1}=h_{3}=10\) mm thick fully tempered glass layers and a middle PVB foil (\(h_{2}=0.76\) mm its thickness). The width of LG beams was fixed (\(b=0.14\) m), while the variations were represented by the total span \(L\). For the majority of them, \(L\) was in the range of 2.4 m and 2.7 m. The minimum span--even with identical nominal section properties and restraints--was in the order of 1.45 m. Such a  
Figure 5: Example of LG beams with end mechanical restraints: (**a**) real system and (**b**) corresponding mathematical model.  
Figure 6: Examples of severe delamination in LG member, in the region of point mechanical restraints. Reproduced with permission from (**a**) [36] and (**b**) [37].  
marked variability in the span was required--at the design stage--to accommodate some geometrical irregularities of the primary structure hosting the specimens. The examined LG beams are, in fact, currently part of an in-service glass walkway (in the form of handrails, see Figure 7), being constructed in the early 2000 in the context of a Roman age Basilica monument in Aquileia, Italy (see also [15; 16] for further details on the pedestrian system).  
For all of the LG members, the end restraints were realized in the form of stainless steel point-fixings according to Figure 8a, with holes having 42 mm nominal diameter and positioned at a distance \(d=b/2=70\) mm from the edges (see Figure 8b).  
At the time of the on-site experiments, the selected LG beams were subjected to induced vibrations and their acceleration in time was monitored via a single tri-axial sensor [42], glued on the top surface of the mid-span section (Figure 8b). All of the tests were carried out with a mean temperature of 23 \({}^{\circ}\)C and a 62% relative humidity.  
For each specimen, multiple measurements were collected (minimum three test repetitions), and then post-processed to predict the corresponding (mean) frequency.  
In this regard, the advantage and potential of Operational Modal Analysis techniques is represented by the possibility to derive even relevant mechanical parameters for in-service structures that cannot be subjected to destructive (or laboratory) experiments. Otherwise, for the LG structure object of analysis, the serviceability of the religious monument did not allow for performing massive experimental  
Figure 8: Experimental LG specimens: (**a**) detail of the typical restraint (photo by [PERSON], courtesy of So.Co.Ba.) and (**b**) schematic representation of the test setup test setup.  
Figure 7: Example of service loads for the tested LG beams (photos by [PERSON], courtesy of So.Co.Ba.).  
measurements, and required the use of a minimum number of instruments, thus involving some further uncertainties for the interpretation of test results.  
A key influencing parameter, for example, was represented by a certain degradation of the PVB interlayers (due to repeated non-controlled ambient conditions and time), with visible delaminated regions, close to the restraints and along the edges of each beam (detail views are proposed in Figure 9 for some of the tested specimens).  
Another major issue in the experimental study and vibration serviceability assessment consisted in the actual life-time of the selected LG specimens, thus in additional difficulties for the reliable estimate of PVB mechanical properties. Most of them were characterized by a mean service life of \(\approx\)15 years at the time of the research study. However, some of them have been replaced during the years, without any track of maintenance/replacement interventions.  
Finally, a further uncertainty was represented by the actual stiffness contribution of the steel point-fixings in use (Figure 8), thus its effects on the overall dynamic response of the LG members.