*2.2. Monitoring Points Locations*

Steel tube internal bracing is a common form of the foundation pit, which has been widely used in a narrow and regular foundation pit. In order to monitor the safety of soil, buildings and structures, an underground pipe network and facilities around the foundation pit during excavation and construction of a deep and large foundation pit or special foundation pit, the designers must determine the safety monitoring points and monitoring contents of the steel pipe support in the design documents according to national standards, design documents and engineering experience [19]. In this study, the monitoring experience of reinforced concrete support and steel tube support is used for reference, to set up the monitoring contents and layout of various measuring points for steel assembly bracing.

According to the site investigation, the vertical vibration of T01 reflected by engineers and construction workers can be obviously felt, especially when several earthwork trucks pass through the trestle bridge one after another. In order to master the relationship between the bracing vibration and running vehicle, and study its influence on the safety of the bracing system, the resistive dynamic strain gauges (precision ±0.1 με, range ±3000 με, type XKKJ-DTA100) and vibration acceleration sensors (sensitivity 0.3 V\*s/m, range of scale 20 m/s2, range of frequency 0.25~80 Hz, type BY-S07) have been arranged on the first track (initial pre-axial force *P*<sup>01</sup> = 3200 kN, compressive stress 18.22 MPa) and the second track (initial pre-axial force *P*<sup>01</sup> = 8000 kN, compressive stress 22.78 MPa) of T01 which is the closest to the trestle passage and has obvious vibration sense to monitor the dynamic response. Vibrating wire static strain gauges (strain: precision ±7 με, range ±1500 με, type T-900, temperature: precision ±0.2 ◦C (0~50 ◦C), range −40~60 ◦C, type CG-01) have been arranged on the surface of an H-beam because they are widely used in monitoring for foundation pit support and can be utilized to measure the steel surface temperature and surface strain of an H-beam at the same time [20]. In addition, in order to analyze the correlation between vehicle and the response of structural dynamics, video surveillance is used to monitor earthwork truck transportation. A 5G network is used for wireless networking and remotely transmits data to the cloud database. Figure 3 shows the locations and numbers of various types of sensors on the T01 steel assembly internal bracing.

As can be seen in Figure 3, the vibration acceleration sensors are fixed on the beam for the reinforced concrete trestle, while all kinds of measuring points are installed on the upper flange of the H-beam for the steel assembly bracing. Ten dynamic strain measuring points (DSS-01~DSS-10) are arranged on the first steel assembly bracing, and six dynamic strain measuring points (DSS-11~DSS16) are arranged on the second steel assembly bracing. Two static strain measuring points (SSTS-01, SSTS-02) are arranged to master the static strain and steel temperature of steel assembly bracing. At the same time, acceleration sensors, numbered with 3AS-01 and 3AS-02, are arranged on the trestle bridge and steel assembly bracing, respectively. It is noted that the sampling interval of static strain and temperature data is 5 min; the sampling frequency of dynamic strain is 50 Hz; and the sampling frequency of vibration acceleration is 200 Hz.

**Figure 3.** Layout of monitoring points in steel assembly bracing.

According to the monitoring and analysis of real-time data for more than one month, the responses of vibration acceleration of the reinforced concrete trestle bridge and the steel assembly bracing are significantly larger when the earthwork truck passes through the trestle bridge, and the amplitude of vertical vibration acceleration on the first steel assembly bracing is larger than that of the concrete trestle bridge. In addition, the steel assembly bracing is obviously affected by environmental temperature. In order to analyze the influence of vehicle vibration and temperature on the safety of steel assembly bracing, the finite element method is adopted to analyze the structural responses under the combined action of vibration and temperature.
