# Numerical Assessment of the Seismic Vulnerability of Bridges within the Italian Road Network
## 4 Bridge Assessment Guidelines
### Assessment of Structural and Seismic Classes of Attention  
The assessment of the Class of Attention for a bridge is fundamental since it is used to determine whether the bridge passes to more rigorous levels of evaluation. The Guidelines consider four different types of hazards:  
* Structural and Foundational hazard;  
Figure 11: Structure of the multilevel approach proposed by the Guidelines. * Seismic hazard;
* Geotechnical hazard;
* Hydraulic hazard. Each hazard is analyzed separately, defining four independent Classes of Attention, which are then combined to assess the overall Class of Attention of the bridge. The definition of the Structural and Foundational Class of Attention for the case studies is based on the determination of hazard, vulnerability, and exposure classes. In terms of hazard class, the main parameters are the load level of the bridge and the frequency of commercial vehicles. To estimate the vulnerability class, the level of defectiveness, structural scheme, span length, number of spans, materials, design code, and speed of degradation are considered. The exposure class is defined on the basis of mean daily traffic (MDT), the average length of spans, the presence of road alternatives, the type of entity under the bridge, and the transit of dangerous goods. These three classes, defined using decisional trees that combine the parameters introduced above, are then employed to derive the Structural and Foundational Class of Attention using tables in the Guidelines. In the definition of the Class of Attention, vulnerability is the most important factor because if it is high the Class of Attention will be High independently from the classes related to the other two factors. In fact, the vulnerability class is directly connected with the level of defectiveness of the structure. A bridge with a poor state of conservation has to be associated with a high Structural Class of Attention, which indicates a high priority. Tables 8-11 report the definition of the parameters introduced earlier and the assessment of the Structural and Foundational Class of Attention for the five case studies. For all the case studies, the Structural and Foundational Class of Attention is High. The definition of the Seismic Class of Attention follows the same approach presented for the Structural and Foundational Class of Attention. In terms of the hazard class, the main parameters are \(a_{g}\), topographic category, and subsoil type. The structural scheme, span length, materials, level of defectiveness, and design criteria are considered to derive the vulnerability class. The exposure class is derived from the level of MDT, medium length of the spans, presence of road alternatives, type of entity under the bridge, transit of dangerous goods, and strategic importance of the infrastructure. The complete definition of each of the listed parameters can be found in the Guidelines [26]. As in the case of the Structural and Foundational Class of Attention, these three classes are employed to derive the Seismic Class of Attention using tables in the Guidelines. Tables 12-15 report the definition of the parameters introduced earlier and the assessment of the Seismic Class of Attention for the five case studies. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{5}{c}{**Seismic Unnerability Class**} \\ \hline \multirow{2}{*}{**Case Studies**} & \multicolumn{1}{c}{
\begin{tabular}{c} **Support Condition,** \\ **Length, and Material** \\ \end{tabular} } & \multirow{2}{*}{**Design Criteria**} & \multirow{2}{*}{**Defect Level**} & \multirow{2}{*}{**Vulnerability Class**} \\ \hline Bridge 1 & High & Not seismic & Medium & Medium-High \\ Bridge 2 & High & Not seismic & Medium & Medium-High \\ Bridge 3 & High & Not seismic & High & High \\ Bridge 4 & High & Not seismic & Medium & Medium-High \\ Bridge 5 & Medium-High & Not seismic & Medium-High & High \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Parameters for the assessment of Structural Exposure class.