## 4 Discussion
### The Corner Column at the Concave Corner Is Removed  
Figure 14 shows the structure displacement nephogram of Model 1 and Model 2 5 s after removing the D4 column from the first floor. The displacement of each strengthening model is shown in Figure 15. As can be seen from the figure, in the original irregular structure model, the vertical displacement increased significantly before 1.0 s and gradually stabilized to 57.6 mm after 2.4 s. The value is also much smaller than the failure criterion of the progressive collapse of the structure, and thus the structure model does not collapse. In Model 1, when a steel truss was added to the roof, the vertical displacement of the nodes above the failure column increased rapidly before 1.2 s and then fluctuated, and gradually stabilized to 35.6 mm after 4.6 s. Compared with the original structure, the vertical displacement is reduced by 37.8%. With the addition of the X-type tension-strengthening structure in Model 2, the vertical displacement increases rapidly before 0.98 s, and gradually becomes stable at 50.2 mm after 2.3 s. Compared with the original irregular structure, the vertical displacement is reduced by 12.8%. Compared with the original structure, the vertical displacement of Model 1 is reduced by 37.8%. The vertical displacement of Model 2 is reduced by 12.8%. This shows that both the steel truss and X-type tension reinforcement can form an alternative load transfer path and reduce the vertical displacement of the upper square column caused by the failure of the corner column. However, the effect of the steel truss is more obvious. Figures 16 and 17 show the bending moment diagram and axial force diagram of the D-axis in the modified Models 1 and 2 when the corner pillar D4 fails for 5 s at the concave corner of the first layer in working condition 2. In Model 1, the maximum bending moment of the D-axis is 335.2 KN\(\cdot\)m due to the tensile action of the steel truss in the roof. The maximum axial force of the adjacent column b is 7265 KN. The span of the beam above the failure column is increased two times, and the bending moment of the upper beam end changes from negative to positive, while the far beam end is still negative, and the bending moment of the beams above the failure column increases significantly. This indicates that in Model 1, the adjacent span of the failure column loses the support of the lower column, and the load transfer path changes. The adjacent columns of the failure column and the top steel truss jointly bear the load above the failure column, and a part of the load is transferred from the floor to the secondary beam. It is then transferred from the secondary beam to the main beam, and then distributed to the adjacent columns. A part of the load is transferred through the main beam to the upper square column of the failure column, and then to the top steel truss. \begin{table}
\begin{tabular}{c c c c} \hline
**Structure Type** & **Bending Moment** & **Column \(a\) Axial** & **Vertical** \\  & **(KN\(\cdot\)m)** & **Force (KN)** & **Displacement (mm)** \\ \hline Steel roof truss & 375.6 & 5365 & 53.4 \\ X-tie bar & 453.4 & 5136 & 76.3 \\ Original structure & 513.2 & 4966 & 96.6 \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of the results of each model under working condition 1. In Model 2, the load above the failure column is transferred to the adjacent column mainly through the main beam, and the load transfer process is similar to that in the original model. The axial force of each column above the failure column is small, and the axial compression is the main force being applied. The axial pressure of the adjacent column b increases significantly. The maximum axial force is 7185 KN, but its axial pressure is less than that for the same column in Model 1. The beam ends above the failure column, which is also changed from having negative to positive bending moments, and the maximum bending moment of the D-axis is 461.2 KN larger than that of Model 1.