# Study on Solar Radiation and the Extreme Thermal Effect on Concrete Box Girder Bridges
## 1 Introduction  
Bridges in the natural environment are affected by solar radiation, air temperature variation, wind, and other factors. The temperature distribution in the bridge girder changes all the time. On the surfaces of an object, heat is transferred from the surrounding medium mainly in three ways as heat convection, heat conduction, and radiation. This heat transfer of the concrete bridge in the natural environment is related to solar radiation, air temperature, wind speed, material properties, and other factors [1, 2]. The amount of solar radiation on the surfaces of the box girder mainly varies with seasons, time, weather, air quality, and other factors; it is also closely related to the geographical location, topography, and orientation of different surfaces of the girder [3]. Due to the low thermal conductivity and higher specific heat of concrete material, there will be an apparent thermal effect produced under solar radiation, which will affect the durability and even safety of concrete structures [4]. Therefore, many researchers studied solar radiation and the thermal effect, and field observation is a common and effective method. [PERSON] studied the time-varying sunlit-shaded areas of small radius curved concrete box girder bridges and presented a ray-tracing algorithm to analyze the thermal stress distribution of small radius curved bridges [5]. [PERSON] studied the temperature distribution of a low-speed maglev concrete box girder and the solar effect of a high-speed rail bridge pier; he proposed a tail data-fittingmethod of temperature gradient based on the GPD distribution and obtained the bridge girder and pier's temperature gradient fitting model with a 100-year return period [6,7]. [PERSON] worked on temperature actions in composite bridges by measuring for 15 months, proposed an accurate shadow recognition method, and developed a simulation model to study the long-term thermal actions of composite bridges [8]. [PERSON] observed long-term weather data for the portal frame bridge simulations and gave suggested values for design based on the statistical analyses; he pointed out that there is a gradual temperature change between different parts of the bridge, which could influence the thermal stress [9]. [PERSON] investigated a concrete cable-stayed bridge cross-section temperature field based on a 17-month continuous experiment and studied the structural response under thermal loads [10]. [PERSON] performed thermal-induced behavior of a suspension bridge under solar action and investigated the relationship between thermal expansion and contraction cycles of cables and the temperature distribution [11]. The temperature field of the girder cross-section is usually simulated by the two-dimensional finite element models. Many studies focus on the characterization of the cross-section temperature using a comprehensive heat transfer method and transient thermal analyses [12, 13, 14, 15]. As a result of the regional difference in solar radiation, temperature, wind speed, and other factors, scholars also focus on the processes and coefficients of bridge heat transfer to improve the accuracy of the simulation. [PERSON] investigated thermal conductivity coefficients, specific heat, solar absorptivity, and their combined effect on bridge temperature [16]. [PERSON] studied the convective heat transfer coefficient between a concrete surface and ambient air [17]. [PERSON] proposed a formula of diurnal variation of the solar radiation to study the temperature stresses in composite box girder bridges [18]. [PERSON] measured temperature data through experiments and worked on concrete thermal conductivity and heat transfer [19]. [PERSON] presented a rapid computing method for absorbing solar energy on the earth's surface [20]. [PERSON] analyzed the joint actions of wind and temperature on bridges [21]. [PERSON] studied the relationship between the severe weather and the atmospheric transparency coefficient, the thickness of the deck covering layer, and the absorptivity of the concrete surface [22].