# Analysis of Measured Temperature Field of Unpaved Steel Box Girder
## 1 Introduction  
Climate models predict continued global warming in the future [1]. This phenomenon affects all areas of people's lives. With the development of the tourism industry, the Monozail Tour Transit System (MTTS) has been increasingly applied and promoted. The entire route of the monozail tourism system is elevated, and consists of foundations, steel columns, supports, beam-column nodes, steel track beams, and rail-holding trains [2]. Under the direct influence of climate change, the nonlinear temperature gradient generated by some structures will cause nonlinear section deformation, resulting in self-equilibrium stress [3, 4, 5, 6, 7, 8]. Temperature is an essential factor affecting the structural design and damage, and the temperature field of steel beams' structure is changed over time by environmental factors. The track beam of a monozail tourism system is exposed to factors in the external environment, such as the atmospheric temperature and solar radiation, for a long time. The solar temperature field produces a nonlinear temperature distribution (i.e., gradient temperature load) inside the structure, which has a large impact on its internal strength and linearity. The corresponding temperature stress and deformation are important elements that should be considered in the structural design. Deformation has a great impact on the running safety and the running comfort of the monozail train travel system. In severe cases, it may cause disease or even damage to the beam-column joints. Relevant studies have shown that seasonal temperature differences can cause a maximum axial stress change of 20 MPa in some types of structures, such as long-span steel-mesh domes [9]. Therefore, it is necessary to conduct in-depth research on the distribution of the temperature field of sunlight in unpaved steel beams in monorail tourism systems, and to derive a temperature-gradient model suitable for pure steel beams, which can be used as a reference for engineering design and a useful supplement to relevant industry codes and standards. Many scholars have recently researched the sunshine's temperature field in concrete box girders and steel box girders. [PERSON] et al. [10] proposed a monitoring scheme and established a thermal field model for steel box girders. Through field monitoring and numerical simulation of the Tongling Yangtze River Bridge (TL Bridge), they studied the thermal-field characteristics of the steel bridge deck, when the asphalt pavement was paved. [PERSON] et al. [11] recorded experimental data on the test beam section for six months in hot and cold seasons using actual measurements, and discussed the influence of air temperature and solar radiation on the distribution of the temperature gradient in concrete-wrapped composite beams (I-shaped beams). [PERSON] et al. [12] analyzed the thermal stress response and variation trend of simple temperature and continuous constrained boundary conditions under different temperature loads through an experimental study on the effect of a 1:4 ratio temperature gradient. [PERSON] et al. [13] studied the law of variation of the temperature field in the long-span cable-stayed bridge, based on the monitoring data of the temperature of the Nanjing Third Yangtze River Bridge. They used the generalized Pareto model to predict the extreme temperature value of the 100-year return period. [PERSON] et al. [14; 15] studied the thermal response of the standard steel section itself under different geometric parameters, based on a measured test of the steel components combined with a finite element thermal analysis. [PERSON] et al. [16] conducted temperature tests on two steel structure specimens with different cross-sections under open-air sunshine conditions, and then summarized and analyzed the laws of temperature variation in the components. However, the object of this study is the unpaved steel box girder, which is characterized by no pavement and a narrow beam width.