# Study on Solar Radiation and the Extreme Thermal Effect on Concrete Box Girder Bridges
## 6 Conclusions  
This paper investigated the impact of solar radiation and the extreme thermal effect on concrete box girder bridges. The solar radiation intensity and temperature distribution of the box girder were obtained by field observing and calculation. The impact parameters such as solar intensity, wind speed, air temperature, and shadow occlusion on temperature distribution were considered. With solar radiation and the estimated extreme temperature with a return period of 100 years as the load condition, the thermal effect of the concrete box girder is studied using the finite element method, and the thermal stress is compared with the design values. The main conclusions are as follows.  
1. The solar radiation changes obviously with time on different surfaces of the box girder. It arrives at the maximum value and keeps stable from 10:00 to 16:00 on the top deck. For the surfaces without direct sunlight, it changes slowly due to scattering and reflection radiation.
2. The effective depth due to solar radiation is about 30 cm, and the sunlight side web overall temperature is higher than that of the sheltered web. The impact of the temperature difference factors shows that direct solar radiation has the most significant effect on the temperature difference. The maximum value of the top slab is 19.8 \({}^{\circ}\)C, which appears at 16:00.
3. A method of estimating the extreme thermal condition is presented. The extreme temperature is fitted by GEV distribution based on the historical meteorological data. The estimated highest air temperature with a return period of 100 years at the bridge site is 33.1 \({}^{\circ}\)C, the most significant daily temperature difference is 14.7 \({}^{\circ}\)C, and a minimum daily average wind speed with a 10-year return period is 0.5 m/s. Then, the extreme thermal condition is obtained by combining the above meteorological data and solar radiation.
4. Under solar radiation and extreme thermal condition, the maximum temperature difference of the girder slab is 22.2% larger than that on the measured day. With it as the most unfavorable condition, the thermal stress of the cross-section was calculated. The maximum transversal tensile stress is 3.0 MPa, and the minimum compressive stress is \(-\)7.6 MPa; the maximum vertical tensile stress and the minimum compressive stress are 0.9 MPa and \(-\)2.4 MPa, respectively. The tensile stress has exceeded the specified design value of the code and could cause cracks, and the maximum tensile stress is 50% larger the design strength in the code. Therefore, in practical engineering, the structural reinforcements should be added on the inner surface of the top deck and the lower edge of the web plate according to the obtained tensile stress distribution.  
Conceptualization, Y.L. and D.L.; methodology, Y.L.; software, Y.L. and K.W.; validation, D.L.; formal analysis, Y.L. and S.J.; investigation, Y.L. and D.L.; resources, D.L.; data curation, Y.L. and S.J.; writing--original draft preparation, Y.L.; writing--review andediting, Y.L.; supervision, D.L.; project administration, D.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.