## 2 Experimental
### Finite Element Analysis
#### 2.2.2 COMSOL Segment Model  
* Material parameters of the steel-concrete composite beam model. The steel-concrete composite beam (SCCB) is mainly composed of steel and concrete materials. This research mainly considers the influence of solar radiation on structural materials and temperature fields. Materials are set according to Equations (2)-(4). Materialparameters include thermal conductivity, specific heat capacity, absorptivity, emissivity, etc. The specific performance parameters are shown in Table 1. The short-wave absorptivity mainly considers solar radiation and the long-wave absorptivity mainly considers environmental radiation. * The model setting of COMSOL. * Climate parameters. Under the action of solar radiation, air temperature usually changes according to a certain law, and the sine and cosine function is generally used to describe the temperature change in actual calculation. According to the research in the literature [30; 31], the atmospheric temperature is expressed by the following cosine function:  
\[T_{a}(t)=T_{1}+\frac{T_{2}}{2}\text{cos}\,\frac{\pi(t-t_{m})}{12},0\leq t\leq 24 \tag{7}\]  
where \(T_{1}\) and \(T_{2}\) represents the daily mean value and daily fluctuation value of air temperature in the corresponding period. In this document, the values are 32.0 \({}^{\circ}\)C and 12.0 \({}^{\circ}\)C, respectively. \(t\) indicates the time of day, h. \(t_{m}\) is the time when the highest temperature appears, which in this paper is 15:00. 2. Solar radiation settings. The solar radiation interface of COMSOL 6.2 can automatically calculate the declination angle, solar hour angle, solar altitude angle, azimuth angle, and ray incidence angle based on the longitude, latitude, and time zone of the target point, thereby applying solar radiation loads. The Chongqing Nanjimen track bridge is located in Chongqing, China, latitude 29.55\({}^{\circ}\) N, longitude 106.59\({}^{\circ}\) E. The time zone is East 8 zone (UTC/GMT+08:00). The solar irradiance is set at 1000 W/m\({}^{2}\), and the horizontal scattering irradiance in the clear sky at noon is 150 W/m\({}^{2}\). * Establishment of a mixed steel segment model. COMSOL6.2 was used to establish the section model of the main beam of the Chongqing Nanjimen track bridge, simulate the temperature field change under solar radiation, select the section with the thickness of 260 mm concrete slab in the span, construct the superimposed beam model with a length of 7 m through Soilwork 2023, and introduce it into COMSOL6.2 to form a solid and automatically build a consortium. The model consists of 3 domains and 81 boundaries. Using physical field control to refine the grid, the complete grid contains 54,559 domain units, 35,672 boundary units, and 2236 edge units, and the number of degrees of freedom to solve is 145,148 (including 75,997 internal degrees of freedom), as shown in Figure 1. To deal with complex problems more effectively, concentrate research resources and ensure the practicability of the model, the following assumptions are made in this paper: (1) The model has good contact with air, and air is assumed to be a uniform medium. (2) The materials used in the model are assumed to be homogeneous materials, and there is no difference in the thermal performance of a single material. (3) The solar radiation uses COMSOL6.2 with solar radiation interface, convection heat transfer \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material Property** & **Steel (Q345 qD)** & **Concrete (C60)** \\ \hline Density (kg/m\({}^{3}\)) & 7850.0 & 2500.0 \\ Specific heat capacity (j/kg.\({}^{\circ}\)C) & 460.0 & 960.0 \\ Coefficient of heat conduction (w/m.\({}^{\circ}\)C) & 58.2 & 2.94 \\ Coefficient of thermal expansion (1/\({}^{\circ}\)C) & 12 \(\times\) 10\({}^{-6}\) & 10 \(\times\) 10\({}^{-6}\) \\ Shortwave absorption rate & 0.3 & 0.65 \\ Long wave absorption rate & 0.8 & 0.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermal performance parameters of materials.