# Evaluation of Bridge Maintenance Priorities in Megacities
## 3 Proposal for a Bridge Maintenance Priority Selection Model
### Risk Score Assessment Criteria  
For the bridge information, a total of four factors, that is, age, design load, presence or absence of seismic design, and safety grade, were selected to calculate the risk score. However, the effects on a bridge are all different depending on the characteristics of the selected factors, and it is difficult to establish standards by analyzing the effects of each factor on a bridge. Therefore, in this study, weights for risk factors were derived through an AHP, and the scores according to the evaluation criteria for each factor were calculated by multiplying the weights.  
Figure 2: Bridge maintenance priority selection model derivation process.  
The AHP consisted of a total of 35 experts, including 15 professors of civil engineering, 10 researchers in the area of bridges, and 10 maintenance practitioners. The 33 results with a consistency index (CI) evaluated below 0.1 were analyzed. The Consistency Index (CI) can be used to measure the consistency of questioners performing pairwise comparisons, as well as to identify any contradictions in the choices of the questioners [22]. In case the consistency index is high, it means that the questionnaire did not answer correctly or the respondent could not be trusted. It is considered unreliable if the consistency index is 0.1 (10%) or more. Generally, the higher the expertise of the respondent, the lower the consistency index. As a result of the AHP, the safety grade was highest among a total of four risk factors, the presence or absence of seismic design, age, and design load were high, and the final weight was selected in high order. The AHP analysis results are shown in Figure 3.  
The score for each factor that can evaluate the risk factor was calculated as follows. Age and design load were classified into four classes: less than 10 years, more than 10 years to less than 20 years, more than 20 years, less than 30 years, more than 30 years, DB-24, DB-18, DB-13.5, and unclear; and were classified into two classes according to the presence or absence of seismic design. In Korea, DB loads are virtual-designed vehicles in the form of semi-trailers, and they follow the same design procedures as AASHTO's LRFD  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Facilities Type** & & **Content** \\ \hline \multirow{4}{*}{Type 1 Facilities} & (1) & Bridge with suspension bridge, cable-stayed bridge, arch bridge, or truss bridge in the upper structure \\  & (2) & Bridge with a maximum span length of 50 m or more (excluding one-span bridges) \\  & (3) & Bridge over 500 m long \\  & (4) & Cover structure with a width of more than 12 m and an extension of more than 500 m \\ \hline \multirow{4}{*}{Type 2 Facilities} & (1) & Span bridge with a span length of more than 50 m \\  & (2) & Bridge not falling under Type 1 facilities and at least 100 m long \\  & (3) & Cover structure that does not correspond to a Type 1 facility and has a width of 6 m or more and an extension of 100 m long \\ \hline \multirow{4}{*}{Type 3 Facilities} & (1) & Bridge installed on a road under Article 10 of the Road Act with an extension of 20 m or more and less than 100 m \\  & (2) & Bridge of least 20 m in length installed on a road other than roads under Article 10 of the Road Act \\ \hline \hline \end{tabular}
\end{table}
Table 1: Classification of facilities in Korea.  
Figure 3: Factor ranking results following the analytical hierarchical process (AHP).  
bridge design specifications. Safety grades were classified into five grades from A to E according to the standards in Korea's Enforcement Decree of the Special Act on the Safety and Maintenance of Facilities [31], and these are displayed in Table 2. Table 3 shows the score composition according to the evaluation criteria for each risk factor through the information on the bridge and calculates the risk score using Equation (2).  
\[\text{Risk Score }=\Sigma(\text{Risk Factor}\times\text{Impact weight}) \tag{2}\]