## 6 Definition of Perception Index and Perception Scales for Glass Pedestrian Systems
### Perception Scales for Comfort Assessment  
In conclusion, following [37], the PI estimates for the vertical vibrations of the examined glass walkway were associated to a comfort level class that identifies five groups:  
- Very good VL-1 when \(1\leq\text{PI}\leq 1.5\)  
- Good VL-2 \(1.5<\text{PI}\leq 2.5\)  
- Normal VL-3 \(2.5<\text{PI}\leq 3.5\)  
- Bad VL-4 \(3.5<\text{PI}\leq 4.5\)  
- Intolerable VL-5 \(4.5<\text{PI}\leq 5\)  
It was shown in Figure 14 that--for a large number of the CN#\(n\) configurations--the in-service system should be classified with a global \"bad\" response (with a mean PI = 3.54, corresponding to VL-4). Even excluding the CN#1-to-#4 scenarios (\(p=1\), in-place jump), the scale perception would be only slightly modified (i.e., \"normal\" vibration performance (VL-3), with a mean PI = 3.35). As a result, the collected PI estimates would generally suggest the need of retrofit interventions, to ensure an optimal dynamic performance of the structure. The same subjective feelings, however, can provide further relevant feedback, as far as the collected data are compared with comfort levels and quantitative walking features. In Figure 15a, a possible analytical correlation of PI grades is hence proposed for the case-study system, as a function of the reference VL classes and walking frequencies \(f_{p}\). There, the average \(f_{p}\) is derived--for each CN#\(n\) configuration--from the corresponding experimental records. The mean walking frequency for \"normal\" activity is also emphasized in the figure (dashed lines). The second-order fitting curve of the experimental PI data shows that under \"normal\" activities the structure can be still associated to \"normal\" or even \"good\" performances. The comfort level decreases,in contrary, as far as \"unconventional\" walking frequencies are imposed to the system. For some CN#_n_ conditions, it is worth mentioning that a \"weak\" comfort level was perceived by the volunteers, even in the presence of \"normal\" walking activities, but random movement and high occupation density (VL-4 range in Figure 15a). Such an outcome reflects the class of use of the system, where it is rationally expected to have high occupation density and a combination of several movement features (especially frequencies) that do not match with \"normal\" parameters (i.e., when the visitors move by small groups, standing on the system, etc.). The advantage of Figure 15a is hence that the comfort performance of the in-service structure can be assessed for a given walking frequency \(f_{p}\), accounting also for the specific structural typology and destination. Otherwise, it does not consider other relevant movement features, like the imposed acceleration peak. It was in fact shown in [37] that PI-_a__max_f__p_ plots derived from experiments can offer a suitable tool for comfort estimates. A similar approach is hence proposed in Figure 15b, where fitting curves are obtained from the experimental data, including movement features (walking frequency, acceleration peak) and perception feelings from the volunteers. As shown, as far as a \"normal\" walking activity is taken into account, it is possible to notice that \"good\" or \"normal\" perception feelings of the volunteers are associated to relatively low acceleration peaks, up to a maximum of 0.07-0.08 g. When the walking frequency decreases and/or the acceleration peak increases, the perceived comfort rapidly  
Figure 15: Analytical correlation between (**a**) experimental walking frequencies \(f_{p}\) and corresponding vibration comfort levels VL (vertical direction) for the examined system, and (**b**) definition of possible perception curves for graphical comfort assessment. reduces. Such an outcome is mostly associated to relatively low acceleration amplitudes, compared to the threshold values in use by standards for \"maximum vibration comfort\" (see Section 5.3).