## 3 Results and Discussion
### Stress Analysis of BRT Asphalt Pavement Structure
#### 3.2.3 Shear Stress  
As shown in Figure 13, the variation in the shear stress over time was similar to that in the vertical stress. Notably, as the load approached the analysis point, the shear stress increased rapidly and then decreased rapidly and reversed after a period of time near the peak value. Under driving loads of 10, 20, 30 and 40 km/h, the duration of the shear stress on the pavement was 0.28, 0.14, 0.11 and 0.09 s, respectively. The slower the initial load speed, the longer the shear stress acted on the pavement structure, which was more likely to lead to shear failure. Although the whole pavement structure was subjected to shear stress, as shown in Figure 14, it can be seen from Figure 13 that the shear stresses of the upper, intermediate and lower surface layers were obviously greater than those of the base and subbase, which is an important reason for the shear failure of asphalt pavement at high temperatures. In the practical engineering of BRT asphalt pavement, special attention should be given to improving the shear resistance of pavement materials and strengthening the adhesion between surface layers.  
The peaks of the shear stress of each surface layer were compared. As shown in Figure 15, the change in the shear stress peak with the vehicle speed was similar to that in the tensile stress peak. When the vehicle speed was increased from 10 to 30 km/h, the shear stress peak at each depth of the surface layer gradually decreased. However, when the vehicle speed was increased from 30 to 40 km/h, the shear stress peak increased. With increasing surface depth, the curve of the shear stress peak with the vehicle speed tended to become horizontal.  
According to the analysis of Figures 7-15, the duration and peak value of the mechanical response of the BRT asphalt pavement varied differently with vehicle speed. As the load speed increased from 10 to 40 km/h, the duration of the pavement stress became shorter, which was beneficial for the BRT asphalt pavement structure. Notably, regarding the duration of the mechanical response, 40 km/h was the optimal velocity for the Lanzhou BRT asphalt pavement. Moreover, the influence of the load speed on the stress peak was mainly reflected in the tensile stress and shear stress, which were the lowest at a speed of 30 km/h. This suggests that 30 km/h is the optimal velocity for the Lanzhou BRT asphalt pavement regarding the peak value of the mechanical response. Therefore, it is necessary to comprehensively consider the duration and peak value of mechanical response when researching the influence of driving speed on BRT asphalt pavement damage. In the high-temperature season, BRT operating speeds in the 30-40 km/h range can effectively delay the occurrence and development of fatigue cracking and shear failure of the base and subbase. Because the vertical compressive stress and shear stress of BRT asphalt pavement are relatively high, the permanent deformation of BRT pavement should be the key issue in the high-temperature season. The next analysis focuses on the permanent deformation of BRT asphalt pavement structure based on repeated loading.