# Intelligent Safety Assessment of Prestressed Steel Structures Based on Digital Twins
## 5 Case Study
#### Construction of the virtual model
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Key Node** & **Theoretical Coordinates** & **Measured Coordinates** & **Coordinate Difference** \\ \hline Upper radial cable head hole 1 & \((-2110.4,0,-172.6)\) & \((-2111.3,0.3,-171.4)\) & \((-0.85,0.3,1.25)\) \\ Upper radial cable head hole 2 & \((-2116.4,0,-179.3)\) & \((-2117.4,0,3,-178.3)\) & \((-0.92,0.3,1.00)\) \\ Upper radial cable head hole 3 & \((-2110.5,0,-184.5)\) & \((-2111.5,0.3,-183.7)\) & \((-0.91,0.3,0.84)\) \\ Upper radial cable head hole 4 & \((-2104.5,0,-177.8)\) & \((-2105.5,0,-176.7)\) & \((-0.94,0.3,1.19)\) \\ Upper radial cable head hole 5 & \((-2110.4,20,-172.6)\) & \((-2111.3,20.3,-171.4)\) & \((-0.85,0.3,1.25)\) \\ Upper radial cable head hole 6 & \((-2116.4,20,-179.3)\) & \((-2117.4,20.4,-178.5)\) & \((-0.92,0.4,0.80)\) \\ Upper radial cable head hole 7 & \((-2110.5,20,-184.5)\) & \((-2111.5,20.3,-183.7)\) & \((-0.91,0.3,0.84)\) \\ Upper radial cable head hole 8 & \((-2104.5,20,-177.8)\) & \((-2105.7,20.3,-176.7)\) & \((-1.14,0.3,1.19)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The comparison between the theoretical coordinates and the measured coordinates of the key nodes. Figure 14: The correction process of the geometric model. (**a**) Field structural scanning, (**b**) Structure point cloud model, (**c**) Node coordinate correction, (**d**) Modified geometric model. In the finite element model, the command flow of the construction process is compiled. By changing the constraint conditions of components and other parameters, each construction step of the structure is intuitively reflected. At the same time, the temperature sensor, wind speed sensor, and RFID equipment collect the construction environment and the size change of the components in real-time. The collection of information provides the basis for the working condition setting of the physical model. The behavior change of the structure was simulated in the physical model. In this process, the sensor as a data connection device collects the construction information at the end of each construction step. The construction information provides the basis for the simulation of the virtual model and provides data support for the analysis of each stage of the construction process. The tests were conducted indoors, so the temperature and wind speed were relatively uniform. According to the structural specification, the threshold values of cable force, strain, and other parameters were set to ensure the effectiveness of the simulation. The construction of the virtual model can realize the sending and receiving of field data, the visualization of simulation results, the synchronization of structural state, and the acquisition of standard images. In the test process, the establishment of the virtual model for construction safety assessment is shown in Figure 15. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Member Type** & **Location** & **Cross Sectional Area (mm2)** \\ \hline Radial cable & Upper level & 24.6 \\ Lower level & 33.3 \\ \hline Ring cable & Upper circle & 24.6 \\ Lower circle & 49.1 \\ \hline \multirow{3}{*}{Strut} & Outer circle & 62.8 \\ \cline{1-1}  & Mid circle & 62.8 \\ \cline{1-1}  & Inner circle & 62.8 \\ \hline Ring beam & Outer circle & 4300 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Adjustment of cable section area. Figure 15: Establishment of the virtual model for construction safety assessment. After the structural virtual model was completed, the information was fused by the principle of Markov chain. The structural safety performance of the next construction step is predicted according to the probability of the occurrence of risk factors and the degree of changes in structural mechanical parameters. In the process of structural safety analysis, the theoretical method of Section 3.3 was used. In this study, the safety performance of the structure was measured by the cable force. Comparing the calculated cable force with the design value in each construction step, the safety performance was judged.