# Risk and Resilience Assessment of Lisbon's School Buildings Based on Seismic Scenarios
## 5 Risk Indicators and Intervention Prioritization
### Mitigation Simulation  
A conceptual strengthening intervention is implemented in the most vulnerable school buildings in order to mitigate the seismic risk of the school building portfolio. The structural systems of the four schools with estimated losses above 20% in 50 years are considered to be strengthened so that negligible losses and recovery times are obtained in 50 years. The estimated losses and recovery times associated with the entire school building portfolio are then reassessed based on the improved results achieved by these four most vulnerable schools.  
With this mitigation intervention, the highest losses are now concentrated in the group of three schools that record losses of approximately 12% (see Figure 12). Recall that, without mitigation, the highest losses were above 20% (in the four schools that were strengthened in the test mitigation intervention).  
Table 8 shows the expected losses for each seismic scenario, obtained considering these mitigation interventions, as well as the relative difference of the estimated losses in the current situation. It can be seen that, for seismic scenarios 4 to 7, i.e., scenarios with magnitudes larger than 6.6, an average loss reduction of 47.5% is obtained. For lower magnitude scenarios, the loss reduction is not as effective. This observation is due to the fact that a small number of schools do not comply with the most demanding limit states (limit states 3 and 4), thus concentrating the losses due to high intensity earthquakes on this relatively small number of schools. As a consequence, an intervention for these schools results in an effective reduction in the expected losses. On the other hand, for lower intensity seismic scenarios, the expected losses are spread through a larger number of schools, since the performance targets associated with functionality (limit states 1 and 2) are not met for a much larger number of buildings. As a consequence, intervention in just four of the buildings leads to a less effective loss reduction.  
With this test mitigation intervention, and for an earthquake with a magnitude of 6.6 and a return period of 275 years, the expected loss is now approximately 5.0% (9.1% without strengthening), while for an earthquake with a magnitude of 7.1 (return period of 2200 years) the expected loss is now 10.6% (17.7% without strengthening).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Scenario**} & **Return Period** & **Magnitude** & \multirow{2}{*}{**Expected Loss (\%)**} \\  & **(Years)** & **(M\({}_{\textbf{w}}\))** & \\ \hline
1 & 20 & 5.3 & 0.3\% \\
2 & 50 & 5.8 & 1.9\% \\
3 & 100 & 6.1 & 4.4\% \\
4 & 275 & 6.6 & 9.1\% \\
5 & 475 & 6.8 & 12.1\% \\
6 & 1100 & 7.0 & 13.5\% \\
7 & 2200 & 7.1 & 17.7\% \\ \hline \hline \end{tabular}
\end{table}
Table 7: Expected losses for the considered seismic scenarios.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Scenario**} & **Expected Loss w/o** & **Expected Loss w/** & \multirow{2}{*}{**Loss Reduction (\%)**} \\  & **Strengthening (\%)** & **Strengthening (\%)** & \\ \hline
1 & 0.3\% & 0.3\% & 0\% \\
2 & 1.9\% & 1.2\% & 36\% \\
3 & 4.4\% & 3.6\% & 19\% \\
4 & 9.1\% & 5.0\% & 45\% \\
5 & 12.1\% & 5.8\% & 52\% \\
6 & 13.5\% & 6.4\% & 53\% \\
7 & 17.7\% & 10.6\% & 40\% \\ \hline \hline \end{tabular}
\end{table}
Table 8: Expected losses and loss reduction with and without a test strengthening intervention in four schools for the considered seismic scenarios.  
This hypothetical mitigation intervention shows that, by strengthening a relatively low number of schools, it is possible to effectively reduce the expected losses and, thus, to mitigate the seismic risk for the school building portfolio.