## 6 Result and Discussion
### Safety Assessment of Aged-CGD  
The safety check has been carried out here through the design IM for different earthquake levels. To give a guideline for the dam operator, Operational (OP) and Life Safety (LS) [68] is selected to determine the reliability index for earthquake levels. In the safety check, the procedure discussed in Section 2.2.4 is applied to determine the target reliability index of the CGD. The fragility curve from the failure probability for different year consideration has been multiplied by the integration of the slope of the site seismic hazard curve. Figure 19a-d shows the reliability index curve for the example CGD for each considered year, showing the time-variant seismic safety evaluation. Figure 19 shows the reliability curve for mean, 16% and 84% fractile for both the serviceability and safety stage. For both levels of the target PGA (serviceability and safety), the HCLPF point from the fragility curve has been determined and indicated as target (serv.) and target (safety) as shown in Figure 19. The target reliability index then is calculated by the identification of the structural capacity. Table 3 shows the list of HCLPF points from the fragility curve for each year. From Figure 20, the regression analysis is summarized paired with their annual frequency of exceedance for the \(T_{R}=73\) years (OP) and \(T_{R}=975\) years (LS) in hazard levels. To compare with the target value, the reliability index for different earthquake levels has also been taken for two of these performance levels (OP and LS). From the average curve, as shown in Figure 20, the design earthquake PGA is 0.075 g and 0.221 g, respectively, for the taken CGD site. Using a peak ground acceleration value of 0.302 g, which is equal to that estimated for the records having a 2% in 50 years hazard level, it can be extracted directly from the mean fractile of the fragility curve. From Figure 19, the design PGA, as shown in Figure 20, two, 0.075 g and 0.221 g, were taken to measure the reliability index. \begin{table}
\begin{tabular}{c c c} \hline \hline
\begin{tabular}{c} **Aging-Effect** \\ **(year)** \\ \end{tabular} & \multicolumn{2}{c}{**HCLPF Point**} \\ \cline{2-3} \multicolumn{1}{c}{**Serviceability**} & **Safety** \\ \hline
0 years & 0.176 & 0.284 \\ \hline
25 years & 0.127 & 0.256 \\ \hline
50 years & 0.100 & 0.237 \\ \hline
75 years & 0.093 & 0.229 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: High confidence of low probability of failure (HCLPF) point from the CGD response of each year. Figure 19: Reliability index curve for mean, median, 16% and 84% fractile curve; (**a**) 0 years, (**b**) 25 years, (**c**) 50 years, (**d**) 75 years. To visualize the trend of reliability index of the prescribed aged-CGD target (capacity), response with time can be compared with demand (EQ level) as shown in Figure 21. Figure 21,b shows the comparison between the capacity and demand reliability index for all fractile cases in serviceability and safety level, respectively. With increasing time this CGD shows a safe index in serviceability level even each year, wherein safety level the CGD looks unsafe after 45 years. Therefore, adopting the proposed methodology described in this study is useful to check the safety index in a specific location. The reliability index for the performance level of serviceability (check for OP earthquake level) and safety (check for LS earthquake level) for each fractile (16%, 50% and 84%) is listed in Table 4. The target reliability index was determined from the HCLPF capacity of the dam and compared with each earthquake level to check the safety assurance. Moreover, to check the safety, a 16% to 84% fractile range has been taken in this study. Table 4 shows that the dam for serviceability performance level is safe for the whole year and for all fractile cases. Nevertheless, for safety performance level, the dam fails to satisfy for 50 and 75 years with 84% fractile cases.