## 3 Required Performance and Resistance Capacity Considering Environmental Factors  
### Required Performance Based on Traffic Conditions  
The required performance of a bridge is defined according to the characteristics of the loads applied to the bridge. Many types of loads are applied to a bridge in service, and it is virtually impossible to consider all of them. This section mainly considers the flexural moment generated by the dead load and live load applied to bridges for analysis. The dead load refers to the self-weight of a structure. When the self-weight of a bridge is calculated, the unit weights of the materials constituting the bridge and the specifications of members from the design plan are used. The Korea Highway Bridge Specifications [6] suggest unit masses of the general construction materials for bridges for calculating the dead load (Table 3). The maximum flexural moment occurs at the center of the girder from the dead load. This is calculated from the given unit mass and member specifications from the design plan, as given in Table 4. According to the calculation, the outer girder in the sample bridge was most vulnerable to the flexural moment produced by the dead load. \begin{table}
\begin{tabular}{c c c} \hline
**Environment** & **Strength Correction Factor \(a_{d}\) (\(\boldsymbol{\hat{\
u}}_{c}\); MPa)** & **Carbonation Rate Coefficient \(\boldsymbol{\hat{\mu}_{d}}\) (\(\boldsymbol{\text{cm}}/\sqrt{\boldsymbol{\hat{\eta}}}\))** \\ \hline Mild & \(-0.0463f^{\prime}_{c}+1.7048\) & \(\frac{w}{c}>0.6\)\(\frac{\frac{\pi}{\sqrt{0.3}}}{\sqrt{0.3}(1.15+\frac{w}{c})}\frac{1}{d_{d}}\) \\ Normal & \(-0.0265f^{\prime}_{c}+1.2538\) & \(\frac{w}{c}\leq 0.6\)\(\frac{4.6\times\frac{\pi}{\sqrt{2}}-1.76}{\sqrt{2}}\frac{1}{d_{d}}\) \\ Severe & \(-0.0184f^{\prime}_{c}+0.8556\) & \(\frac{w}{c}\leq 0.6\)\(\frac{4.6\times\frac{\pi}{\sqrt{2}}-1.76}{\sqrt{2}}\frac{1}{d_{d}}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Carbonation rate coefficient. In this study, the uncertainty of the dead load was modeled by applying the statistical characteristics of the dead load suggested by the Federal Highway Administration (FHWA) [10]. For the calibration of the LRFD bridge design code, the statistical characteristics of the dead load were classified into three types: precast members, cast-in-place members, and wearing surface according to the members and construction conditions (Table 5). This case had an annual average daily traffic (AADT) of 20,000 and heavy vehicle proportion of 25%, which slightly exceeds the average traffic characteristics of one-way, two-lane roads in Korea. This was set as the normal traffic conditions for the sample bridge. The case with an AADT of 10,000 and heavy vehicle proportion of 15% was set as the light traffic condition, and that with an AADT of 40,000 and heavy vehicle proportion of 35% was set as the heavy traffic condition. In current bridge designs, the DB-24 load and DL-24 loads are applied [6]. These do not reflect the actual traffic characteristics of the bridge once it is in service. In this study, in order to determine the required performance, an annual extreme load that reflects the actual traffic characteristics was referenced, as listed in [12; 13]. In these studies, the effect of the annual extreme load on bridges was calculated by considering various traffic characteristics, including the AADT, heavy vehicle proportion, vehicle running pattern, bridge, and lateral load distribution of the bridge. Table 6 presents the probabilistic distributions of passing vehicles: mobile cars (P), buses (B), mid-size trucks (T), heavy trucks (TT), and semi-trailers (ST). The first mode indicates the empty or lightly loaded condition, and the second mode represents the heavily loaded condition. In order to reflect the running patterns of vehicles passing the bridge, the running speed ratio of vehicle per AADT, the inter-vehicle distance models, and the vehicle consecutive models were measured (Table 7).