## 6 Definition of Perception Index and Perception Scales for Glass Pedestrian Systems  
### Acceleration Peaks and Perception Index (PI) Values  
The \"perception index\" (PI) value, as also shown in [37; 55; 58; 59], etc., allows us to quantify the feeling of walking/standing occupants over a given pedestrian structure. It hence represents a parameter of interest for structural assessment considerations, especially for existing footbridges or for new structures including innovative technologies and materials. In the case of the glass system discussed herein, an intrinsic limit of the PI estimates could be represented by the size of the independent panels (and thus, by the number of occupants that can be involved simultaneously). On the other side, the collected PI values can support the development of dedicated comfort analysis methods, specifically calibrated for glass structures. The human perception of vibrations (even at low amplitudes) can be in fact magnified by psychological effects, hence resulting in possible additional unfavorable feelings. Structural glass in buildings, in this regard, can be sometimes recognized by the occupants as an \"unsafe\" material [60; 61], and such an unpleasant perception is reasonably expected to further increase in presence of transparent walking surfaces (see Figure 3). During the experimental campaign, the perception of vibrations from the involved volunteers was collected for each test scenario of Table 2. Based on [37], the PI values were calculated as:  
\[PI=\frac{1}{N}\sum_{i=1}^{5}W_{i}\cdot n_{i} \tag{11}\]  
with \(n_{i}\) denoting the number of volunteers choosing the \(i\)-th \(W_{i}\) grade and \(N\) the total number of involved occupants. Five perception grades were hence detected, i.e., \"imperceptible\" (\(W_{i}\) = 1), \"just perceptible\", \"obviously perceptible\", \"unpleasant/annoying\" and \"intolerable\" (\(W_{i}\) = 5). In Table 5 and Figure 14, the obtained perception estimates are shown. It is important to notice that the number of occupants reflects the actual size of the examined walking surface. Given the class of use of the structure (monumental/religious building), in addition, the CN#\(n\) configurations can be considered well representative of the typical operational condition for the examined system. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{Occupants} & \multicolumn{2}{c}{Vibration} & \multicolumn{3}{c}{Perception Grade} \\ \cline{2-9}
**Test Scenario** & \multicolumn{1}{c}{**Total**} & **Density** & \(a_{max}\) & **Imperceptible** & **Just Perceptible** & **Obviously Perceptible** & **Unpleasant/Annoying** & **Intolerable** \\ \cline{2-9}  & \(P\) & \(p\)/m\({}^{2}\) & **(\%g)** & **(\(W_{i}\) = 1)** & **(\(W_{i}\) = 2)** & **(\(W_{i}\) =  
* the occupation density \(d\), see Figure 14c, seems a poor parameter for the detection of the PI trend. As shown, the (PI-\(d\)) data are mostly scattered and a clear correlation between PI estimates from different walking scenarios can be hardly perceived;
* the added mass ratio \(R_{M}\), which is partly related to \(d\), could provide a more accurate description of the walking setup and of the overall dynamic response of the examined structure. As shown in Figure 14d, for small \(R_{M}\) values (i.e., 0.15-0.20), the corresponding PI data are still sensitive to the movement features. In contrary, the PI values tend to rapidly stabilize with the increase of \(R_{M}\);
* the PI values, finally, generally increase with \(\xi_{os}\) (see Figure 14e). This effect is implicitly related to the imposed amplitude of vibrations (increasing with PI) and to the effects of added mass from the occupants.