# Numerical Assessment of the Seismic Vulnerability of Bridges within the Italian Road Network
## 5 Discussion
### Fragility Curves  
Before discussing the results obtained through the fragility curves, it has to be taken into account that this study is oriented toward a large-scale assessment of existing bridges, so some assumptions and simplifications had to be made. The present discussion refers only to the analyzed case studies and is not meant to have general validity. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Seismic Exposure Class**} \\ \hline
**Case Studies** & **Structural Exposure Class** & **Strategic Structure?** & **Exposure Class** \\ \hline Bridge 1 & High & Yes & High \\ Bridge 2 & High & Yes & High \\ Bridge 3 & High & Yes & High \\ Bridge 4 & High & Yes & High \\ Bridge 5 & Medium-Low & Yes & Medium \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Parameters for the assessment of Seismic Exposure class. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Seismic Class of Attention**} \\ \hline
**Case Studies** & **Hazard Class** & **Vulnerability Class** & **Exposure Class** & **Class of Attention** \\ \hline Bridge 1 & Medium-High & Medium-High & High & High \\ Bridge 2 & Medium-High & Medium-High & High & High \\ Bridge 3 & Medium & High & High & High \\ Bridge 4 & Medium-Low & Medium-High & High & Medium-High \\ Bridge 5 & Medium-High & High & Medium & High \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Assessment of Seismic Class of Attention. In regions with higher seismicity, it has been noted that the response of the bearings is critical. This can be explained considering that, at the time of construction, the design of bearings was carried out in a simplified manner, neglecting the fact that their behavior has a direct influence on the global seismic response of the structure. In regions with lower seismicity, the response of the bearings is not particularly critical, as expected, since the actions due to ground motions are not very high. The ductile response (chord rotation) of piers is generally good. The response in terms of brittle failures does not highlight excessive problems, except in the case of Bridge 3. The reasons that could explain this result are that the bridge is in a high seismicity region (which implies that the piers are subjected to large forces), the structure of the pier is not beneficial since a localized failure can have severe consequences, and also the material properties were defined on the basis of the results of experimental tests performed to investigate the degradation of the structure. For the connecting elements of frame piers, the main issues are related to the critical response in terms of brittle failures. This phenomenon could be explained considering that the spacing of the stirrups in these elements is quite large, which leads to issues when compared with shear forces. This is an aspect that should not be underestimated as failures in the connection elements can modify the flexural behavior of the frame piers, with direct consequences on the global response of the bridge. As expected, the seismic behavior of existing bridges can show some issues directly related to the seismicity of the region. These results are confirmed by the literature [53; 55] where it is reported that bearings and piers are between the most vulnerable parts of bridges under the effect of earthquakes. Some remarks have to be made on the methodology employed to develop the fragility curves. Using the MSA, the selection of ground motions is usually made by matching a Conditional Mean Spectrum [57], while in this work the matching operation was performed using a code-based spectrum specified by the NTC2018, which is based on the Uniform Hazard Spectrum. There are some specific reasons that motivated this choice. The Guidelines are strongly code-oriented so following the approach proposed by NTC2018 to select the ground motions has been considered consistent with the multilevel approach, in order to discuss the differences between Classes of Attention and Fragility Curves.