# Temperature Load Mode of Bridge in Permafrost Region of Qinghai-Tibet Railway
## 1 Introduction  
The Qinghai-Tibet railway is one of China's four major projects in the new century and the first railway to the hinterland of Tibet. It is world-famous for overcoming extreme conditions such as severe cold on the plateau, lack of oxygen, fragile ecology, and frozen soil (Figure 1). The Golmud-Lhasa section of the Qinghai-Tibet railway passes through Gobi deserts and marsles and across the Qinghai-Tibet Plateau, with a total mileage of 1142 km and about 550 km through continuous permafrost region. The average elevation of the whole line is about 4380 m, and the highest point of the railway crossing Tangsula Mountain is 5072 m above sea level. The natural conditions along the line are harsh, with many unfavorable factors such as severe cold, low atmospheric oxygen content, strong ultraviolet radiation, and large changes in daily temperature [1, 2]. Due to the influence of complex plateau environment, especially strong solar radiation and significant daily temperature difference, all manner of bridge damage has appeared on the Qinghai-Tibet railway. In the field equipment inspection, it was found that the bridge damage in the Golmud-Lhasa section of the Qinghai-Tibet railway mainly manifested in the following aspects: direct contact of adjacent beams (Figure 1(a)), fracture of bearing limitblock (Figure 2b), displacement of bearing, bending of bearing bolt, deflection of pier and abutment, etc. The damage and excessive deformation of the bridge structure will lead to the irregularity of the line, affect the driving stability and comfort, and even threaten the operational safety of the train. Therefore, it is urgent to study the temperature load mode of the concrete bridge in the permafrost region of the Qinghai-Tibet railway, explore the deformation and damage mechanism of the bridge-track structure, and provide a scientific basis for the design and maintenance of CWR (Continuously Welded Rail) on the bridge in the plateau area. In the design of CWR on the bridge in China, when calculating the expansion force of concrete bridges with ballasted track caused by temperature difference, the maximum daily temperature difference of the beam body is taken as 15 \({}^{\circ}\)C [3]. As the temperature in the permafrost region of the Qinghai-Tibet railway and the conditions for the bridge to receive sunshine are different from those in other areas, it is still unclear whether the value of the beam's daily temperature difference in the code is applicable. Domestic and foreign scholars have conducted plentiful research on the temperature field and temperature effect on the bridge structure under the action of solar radiation. The main research methods include the experimental research method [4; 5; 6; 7; 8; 9], theoretical analysis method [10; 11; 12; 13; 14; 15], finite element software analysis method [16; 17; 18; 19; 20; 21], and a research method combining the above methods [22; 23; 24; 25; 26; 27]. For instance, [PERSON] conducted long-term monitoring of a prestressed concrete box girder bridge, established the solar radiation boundary condition and transverse temperature gradient prediction model based on the measured data, and calculated the longitudinal and transverse tensile stress of the bridge structure [8]. [PERSON] proposed a new method to determine the effect of solar radiation based on the stationary binomial probability model of random process and obtained the design standard values of equivalent  
Figure 1: Permafrost region of Qinghai–Tibet railway. Figure 2: Bridge damage of Qinghai–Tibet railway. (**a**) Direct contact of adjacent beams. (**b**) Fracture of bearing limit block. linear temperature difference and cross-section temperature difference of high-performance concrete box girder in the design reference period [12]. [PERSON] studied the influence of solar temperature gradient on long-span concrete box girder bridge during construction by using ANSYS software [17].