## 3 Analysis of Monitoring Results
### Stress Analysis of Supporting Structure at Different Temperatures
#### 3.2.3 Stress Analysis of Crown Beam and Waist Beams at Different Temperatures  
The crown beam and the waist beams were tied to the retaining piles to form a supporting system together. If the supporting structure is damaged by concentrated stress, the retaining piles cannot form a whole, leading to the collapse of the foundation pit. Therefore, it was necessary to study the forms of the stress on the crown beam and the waist beams. The strain monitoring points were located at a position of 0, 0.75, 1.5, 2.25, and 3 m on the crown beam and the waist beams, and the monitoring points on the crown beam, waist beam 1, and waist beam 2 were indicated by D1-D5, E1-E5, and F1-F5, respectively, as depicted in Figure 14. The supporting structure was axisymmetric, and the overall stress on the crown beam and the waist beams can be obtained by a mirror image. The stress-temperature relationship curves of the crown beam, waist beam 1, and waist beam 2 are plotted in Figure 15a-c, respectively.  
Figure 14: The layout of the strain gauges installed on the crown beam and the waist beams.  
It can be seen in Figure 15a that when the temperature rose, the stress on points D1 and D2 gradually decreased, while the stress on points D3-D5 gradually increased. The analysis shows that when the temperature rose, the crown beam expanded, which led to further deformation out of the foundation pit and enlarged the stress on the crown beam. The deformation of the adjacent crown beam outside the foundation pit led to the release of the stress on the side crown beam. As a result, the stress on the end of the crown beam decreased with the temperature rise, while the stress on the middle of the crown beam increased. When the temperature rose from 10 to 25 \({}^{\circ}\)C, the maximum change in the stress on point D1 was 2.00 MPa. Hence, it can be concluded that the temperature rise slightly influences the stress on the crown beam.  
Figure 15b demonstrates that after the excavation of the foundation pit, waist beam 1 was deformed into the pit. As the temperature rose, the waist beam was heated, expanded, and further deformed toward the inside of the foundation pit, which resulted in increased stress on waist beam 1. When the temperature rose from 10 to 25 \({}^{\circ}\)C, the increment in the measured stress on points E1-E5 was 10.57, 2.17, 6.33, 5.23, and 4.52 MPa, respectively. It can be seen that the stress on point E1 was significantly affected by the temperature and increased by 0.70 MPa/\({}^{\circ}\)C. After the temperature rose by 15 \({}^{\circ}\)C, the measured stress on point E1 reached 44.25 MPa, which was far smaller than the allowable stress on the HW175 steel section.  
The stress variation law of each measuring point in Figure 15c is basically similar to that in Figure 15b. When the temperature rose from 10 to 25 \({}^{\circ}\)C, the stress values on points F1-F5 increased by 5.79, 2.89, 4.93, 4.51, and 2.37 MPa, respectively, which were all smaller than the corresponding increments in stress on waist beam 1. Hence, waist beam 2 was not so severely affected by the temperature as waist beam 1. The analysis confirmed that waist beam 2 was deeply buried and shortly had a short-time exposure to sunlight, so it was less affected by the temperature compared to waist beam 1.  
Figure 15: The stress–temperature relationship of (**a**) the crown beam, (**b**) waist beam 1, and (**c**) waist beam 2.