**3. Results and Discussion**

*3.1. Strain of the Steel Assembly Bracing*

According to the change of steel assembly bracing with temperature for more than 100 days in Figure 9, the surface temperature changes of section steel collected in typical weather environments of low temperature, medium temperature and high temperature are selected and input into the finite element model, and the temperature influence analysis of static strain at the SSTS-01 measuring point of T01 steel assembly bracing is carried out. SSTS-01 steel surface temperature, monitoring strain, and model in-situ calculated strain

are shown in Figure 16, and monitoring strain and calculated strain information are listed in Table 3.

**Figure 16.** Influence of typical ambient temperature change on steel brace strain.

**Table 3.** Strain of steel brace at different ambient temperatures.


According to the data listed in Figure 16 and Table 3, it can be concluded that the calculated strain is larger than the actual monitored strain, and such a difference at high temperature is larger. In addition, the temperature of steel in the steel assembly bracing is affected by environmental temperature and sunlight radiation, and the surface temperature of each member varies with the degree of sunlight exposure. The temperature of steel used in calculation and simulation comes from SSTS-01, which is located on the upper surface of the first support of the steel assembly bracing. During the monitoring period, it receives more sunlight radiation, which belongs to the support area most affected by temperature and is the most unfavorable position affected by temperature. When the input steel temperature measuring point is located in the position that can accept the full radiation of the sun, the calculation and analysis results of temperature action according to the high temperature environment are too large, but it is beneficial to the structural safety from the engineering point of view.

Then, the finite element dynamic analysis is carried out under the vibration action of the earthwork truck after amplitude modulation. The total time of model calculation is 6 s, the step size is 0.005 s, and the sampling frequency of the field vibration sensor is 200 Hz. It is assumed that the temperature of steel is approximately in a stable state of 50.6 ◦C during the period when the earthwork truck passes through the trestle bridge. As shown in Figures 17–20 below, the monitoring data of the dynamic strain of typical position number DSS-06 (the same position as static strain SSTS-01), DSS-10, DSS12 and DSS-14 of T01 are compared with the calculation results at the same point of the model (the monitoring data of dynamic strain of other positions are in Supplementary Materials).

The following conclusions can be drawn from the data analysis of Figures 17–20: (1) Dynamic strain responses monitored, except for DSS-10, are less than the calculated results, including the dynamic strains of other measuring points not listed in this paper; (2) The maximum monitoring value of DSS-10 is 0.76 με, which is slightly larger than the calculated result of 0.46 με. Considering the influence of a tower crane near the measured support point, the larger monitoring value may be acceptable; (3) The maximum dynamic

strain of measuring points is 1.4 με, which accounts for about 1% of the static strain of 146.44 με of steel assembly bracing under a high temperature of 50.6 ◦C. The sum of the stress caused by temperature rise and the maximum stress caused by vibration is 30.46 MPa, which is larger than that of the first (initial pre-axial force *P*<sup>01</sup> = 3200 kN, compressive stress 18.22 MPa) and the second (initial pre-axial force *P*<sup>01</sup> = 8000 kN, compressive stress 22.78 MPa) bracing initial prestress of T01 and less than the yield strength of steel.

**Figure 17.** Comparison of dynamic strain between measurement and simulation results of DSS-06.

**Figure 18.** Comparison of dynamic strain between measurement and simulation results of DSS-10.

**Figure 19.** Comparison of dynamic strain between measurement and simulation results of DSS-12.

**Figure 20.** Comparison of dynamic strain between measurement and simulation results of DSS-14.

#### *3.2. Acceleration of the Steel Assembly Bracing*

Figure 21 shows the vertical calculated acceleration response and monitored acceleration response of measuring point 3AS-02 on steel assembly bracing. The acceleration response is less than the calculated results. It should be noted that the vertical vibration of T01 steel assembly bracing causes complaints from on-site construction technicians, which can be clearly felt during the construction. In order to understand the influence of the vibration of steel assembly bracing on the walking comfort of technicians, according to ISO 2631-1:1997, the vertical frequency-weighted vibration acceleration is calculated as *a*w = 79.30 dB for Figure 21. Obviously, the vertical frequency weighted acceleration *a*w is really large and it is reasonable for technicians to complain about the insecurity of bracing vibration [24].

**Figure 21.** Comparison of vertical acceleration between measurement and simulation results of 3AS-02.

#### **4. Conclusions**

As a new type of bracing system for the construction of deep, large foundation pits, steel assembly bracing has been gradually applied in the civil engineering field. In this paper, the effects of vibration and temperature of the construction site on steel assembly bracing of the foundation pit is studied by virtue of the safety real-time monitoring system and FEM technology based on an engineering case. The following conclusions can be drawn:

(1) The influence of environmental temperature on steel assembly bracing is significant and cannot be ignored. The axial compressive stress of bracing caused by environmental temperature even exceeds the prestress. Under the most unfavorable conditions, the stress caused is less than the yield strength of material Q355b, and hence the steel assembly bracing is in a safe state.

(2) The steel assembly bracing is made up of hundreds of H-beams of different specifications connected by high-strength bolts, and the bolt joints are easy to loosen due to construction vibration. Therefore, the working state of key joints of the steel assembly bracing should be checked regularly during construction. Meanwhile, the steel assembly bracing is under axial compression, and the out-of-plane vertical deformation has great influence on its stability. Accordingly, it is necessary to regularly monitor the out-of-plane deformation of brace.

(3) The vertical frequency-weighted vibration acceleration *a*w is an important indicator to reflect the comfort related with vibration. In this engineering case, this value of the steel assembly bracing *a*<sup>w</sup> = 79.30 dB, which is large. The reality is that on-site technicians complained about the insecurity of bracing vibration which is consistent with our theoretical analysis.

(4) The safety real-time monitoring system can be used as an effective way to analyze the steel assembly bracing under complex working conditions. The results of the paper can provide guidance for the systematic design and further application of steel assembly bracing in engineering.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/buildings13020450/s1, Figures S1–S11 about Dynamic strain comparison between simulation and monitoring of DSS-01 to DSS-16.

**Author Contributions:** Conceptualization, Y.Y., B.Y. and Y.L.; methodology, Y.Y. and Y.L.; software, B.Y., H.Z. and X.L.; validation, Y.L.; formal analysis, Y.Y. and H.Z.; investigation, Y.Y. and H.Z.; resources, Y.L.; data curation, Y.Y. and H.Z.; writing—original draft preparation, Y.Y. and H.Z.; writing—review and editing, B.Y. and X.L.; visualization, X.L. and B.Y.; supervision, X.L. and B.Y.; project administration, Y.Y., B.Y. and Y.L.; funding acquisition, Y.Y. and Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Science Foundation of Zhejiang Province, grant number LGG20E080002.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
