*2.3. Numerical Method*

In this section, a three-dimensional finite element method is adopted to analyze the steel assembly bracing via ABAQUS software. The calculation results can be used as a reference to compare with the results from the real-time monitoring [21]. The finite element analysis involves modal analysis, steady-state thermal analysis and dynamic analysis. The vertical natural frequency and mode of the steel assembly bracing are obtained by modal analysis; the steady-state thermal analysis is used to analyze the temperature effect on the assembly bracing; the dynamic analysis is used to analyze the vibration effect of transport vehicles. Finally, the most unfavorable combination of temperature and vibration is considered to analyze the steel assembly bracing.

It should be emphasized that the highest temperature and the maximum vibration response of the vehicle driving during the monitoring period are simultaneously considered in the numerical simulation which cannot occur at the same time in practice. Therefore, the numerical simulation results in the paper will be conservative.

#### 2.3.1. Numerical Simulation of Soil Layer

In order to consider the influence of the surrounding soil layer on foundation pit structure, the length and width of the soil layer are taken as four times that of the foundation pit size, and the thickness of the soil layer is taken as twice that of the foundation pit depth. The total size of the ABAQUS finite element model is set as 576 m × 860 m × 30 m [22]. An eight-node C3D8R volumetric element is adopted for the soil layer and the number of elements is about 334,000. Considering that there is a concrete trestle bridge, concrete crown beam and diaphragm wall around the steel assembly bracing to be considered, a linear elastic model is adopted to simplify the calculation of soil layer parameters, which are shown in Table 1. In the process of calculation and analysis, the soil layer adopts C3D8R element type with hourglass control. At the same time, in order to improve the calculation accuracy, the soil elements inside and around the foundation pit are encrypted, and the

size of the elements far away from the foundation pit is gradually increased to improve the calculation speed. The bottom boundary of soil is constrained by three-dimensional displacement, the side of soil is constrained by normal displacement, and the upper surface is free. Because the mechanical properties of soil are usually little affected by environmental temperature, the temperature of the soil is set at a normal temperature of 25 ◦C.


**Table 1.** Physical and mechanical parameters of soil layer.
