## 6 Conclusions and Discussions  
### Conclusions and Prospects  
Based on the Chongqing Nanjimen track bridge, this paper analyzes the internal temperature field distribution, transverse temperature distribution and vertical temperature gradient of the steel-concrete composite beam (SCCB), and studies the vertical deformation under the action of the temperature difference and temperature gradient of the system. Moreover, the measured temperature and deformation data of the structure in 2023 are collected and analyzed, and the following conclusions are drawn:  
1. The temperature field distribution of steel-concrete composite beams has obvious nonlinearity under the same ambient temperature change. The daily temperature difference is as high as 26 \({}^{\circ}\)C, and the internal temperature gradient is significant. The maximum daily temperature difference between the upper and lower edges of the concrete slab is 11.1 \({}^{\circ}\)C. Under the influence of solar radiation, the daily transverse temperature gradients of the upper, middle and lower edges of the concrete slab are 3.2 \({}^{\circ}\)C, 5.3 \({}^{\circ}\)C and 7.4 \({}^{\circ}\)C, respectively, due to the thermal insulation effect of the air inside the box girder. Because of its good thermal insulation, the steel beam can adjust the overall temperature change to a certain extent, enhance the adaptability of the structure to the temperature change, and effectively reduce the influence of temperature stress. 2. Under the action of temperature difference and temperature gradient of the system, the overall deformation of the bridge is significant, which affects the alignment of the bridge and the smoothness and comfort of the train. In the context of overall temperature rise, vertical deformation occurs on both sides of the main beam. The maximum deformation of the main beam on the high tower side and the low tower side is 23.9 mm and 92.3 mm, respectively, while the maximum deformation of the mid-span main beam is \(-\)63.1 mm. Under the overall cooling effect, the maximum deformation of the main beam at the high tower side and the low tower side is \(-\)8.6 mm and \(-\)33.3 mm, and the maximum deformation of the mid-span main beam is 22.8 mm. Under the action of positive temperature difference, the maximum deformation of the main beam is 132.1 mm, and the change in the main beam on the low tower side and the high tower side is \(-\)7.8 mm and \(-\)8.4 mm, respectively. The vertical deformation of the main beam is reversed for negative temperature differences. 3. Temperature has a significant influence on the vertical and lateral deformation of bridges. In the design stage, the impact of temperature changes on material properties should be fully considered, and enough space for expansion joints and deformation should be reserved. In the operation and maintenance stage, temperature monitoring should be strengthened to ensure that the temperature change in key parts is within a reasonable range to ensure the safety of the bridge and the stable operation of the train. Based on the structural temperature and deformation data in 2023, it was found that the temperature change has a significant impact on the bridge structure. In summer, the high temperature causes the thermal expansion of the material, and the transverse deformation of the main beam reaches the maximum value of 36.4 mm at 16:00, and the vertical deformation reaches the maximum value of \(-\)274.4 mm at 15:00. At low temperatures in winter, the transverse deformation changes little and mainly fluctuate around the base point, and the vertical deformation reaches the maximum value of \(-\)203.6 mm at 18:00, but the value is lower than that in summer. It is found that the deformation of the main beam with the same section under the action of temperature difference and temperature gradient is far greater than that of a single temperature change under the action of ambient temperature and solar radiation.