## 2 Literature Review
### Effects of Time-Dependent Thermal Loads on Steel Bridge Structures  
The steel girder might be exposed to time-dependent thermal loads for a long time during the construction phase. The thermal loads, including solar radiation, air temperature and wind load, constantly fluctuate. The daily and seasonal fluctuations of thermal loads result in unfavorable thermally induced stresses and deformations, which severely cause damage to structures [8; 9; 10]. The thermal stresses induced by the fluctuation of the thermal loads can be significant and comparable to the dead load-induced stresses. For large-span reticulated domes with ETFE membrane roofs, solar radiation induced strong non-uniform temperatures, resulting in significant thermal stresses and displacements [11]. An axial stress of approximately 20 MPa can be induced due to changes in the thermal loads [11]. Steel truss bridge performance may be affected by variations in average and differential temperatures over time, presenting additional challenges for their design, construction and maintenance [12]. The impact of the thermal-induced stresses is not limited to steel members and could impose distinguished cracks in concrete structures [13]. Stress-induced cracks impact mechanical performance and durability, reducing the structure's lifespan and serviceability [14; 15]. The broader difference in daily temperature could induce vertical and lateral temperature gradients for some sections, which might produce undesired flexural thermal-induced stresses [16; 17]. On the other hand, the seasonal temperature variation leads to higher restrained moments and larger displacement in the continuous spans and simply supported superstructure, respectively [18; 19]. Experimental and Numerical Thermal Analyses of Temperature Distribution and Gradient and Their Thermally Induced Responses in Steel Structures  
The literature review revealed that concrete and composite structures received more attention than steel structures considering experimental and numerical research to predict temperature distributions and gradients [20; 21; 22; 23; 24]. [PERSON] et al. conducted a thermal analysis using the finite element (FE) method to investigate the effect of steel girder configurations on the thermal gradient and temperature stress induced by thermal loads. It was found that the geometrical parameters significantly affect temperature gradients and stresses [25]. [PERSON] et al. [26] found that the nonlinear thermal gradient developed due to time-dependent thermal loads resulted in thermal self-equilibrating stresses, which led to cracking in concrete members with higher depths. [PERSON] et al. [27] conducted experimental work on three concrete I-girders with different top flange dimensions to investigate the temperature gradients. They found that the developed vertical thermal gradient differs from the code thermal gradient and that the lateral gradients need to be modeled more accurately [27]. [PERSON] et al. [9] experimentally and numerically studied the effect of solar radiation variations on the non-uniform temperature distributions of a rectangular steel tube, I-shaped steel tube and circular steel tube by conducting parametrical studies. The steel members were instrumented with thermocouples to collect the thermal load data and accordingly to develop temperature gradients. It was found that the high thermal conductivity and small thickness of steel plates lead to approximately no difference in temperature distributions along the plate thickness and steel section span. The authors found that the section's size is considered the most prominent parameter affecting the nonlinear temperature gradients in the studied sections. Increasing the section's size leads to higher temperature gradients [9]. Based on the numerical method, a simplified calculation approach for the temperature field was presented, which provides practical and effective suggestions for thermally based design, monitoring, and controlling steel structures [9].