## 5 The Pushover Analysis of the Center Column
### Analysis Results  
Before the pushover analysis of the center column, the axial pressure of the upper center column and lower center column need to be determined by static calculation, including the working conditions with only gravity force and with both gravity force and vertical seismic load. The calculation model under gravity conditions is consistent with the model shown in Figure 6. One of the calculation models only applies the gravity of the structure and soil, while the other model adds vertical inertial force. After preliminary calculations, the axial forces of the upper and lower center columns are about 5869 kN and 6669 kN, respectively, when the gravity load is considered. Considering the seismic condition, and with reference to the results of [PERSON] et al. [46], it is assumed here that the axial force of the center column increases by 50% when the vertical seismic action is considered. The lateral  
Figure 8: Pushover modes of center column. \begin{table}
\begin{tabular}{c c c c} \hline
**Case** & **Column** & **Cross Section** & **Pushover Mode** & **Horizontal Load** \\ \hline UT-A-G & upper & traditional & Mode A & Gravity \\ UT-A-GE & upper & traditional & Mode A & Gravity and earthquake load \\ UT-B-G & upper & traditional & Mode B & Gravity \\ UT-B-GE & upper & traditional & Mode B & Gravity and earthquake load \\ US-A-G & upper & split & Mode A & Gravity \\ US-A-GE & upper & split & Mode A & Gravity and earthquake load \\ US-B-G & upper & split & Mode B & Gravity \\ US-B-GE & upper & split & Mode B & Gravity and earthquake load \\ LT-A-G & lower & traditional & Mode A & Gravity \\ LT-A-GE & lower & traditional & Mode A & Gravity and earthquake load \\ LT-B-G & lower & traditional & Mode B & Gravity \\ LT-B-GE & lower & traditional & Mode B & Gravity and earthquake load \\ LS-A-G & lower & split & Mode A & Gravity and earthquake load \\ LS-A-GE & lower & split & Mode A & Gravity and earthquake load \\ LS-B-G & lower & split & Mode B & Gravity \\ LS-B-GE & lower & split & Mode B & Gravity and earthquake load \\ \hline \end{tabular}
\end{table} 
Table 5: The pushover analysis of the working conditions of the center column. force-drift ratio curves of the center column under different working conditions are shown in Figure 9.  
From the lateral force-drift ratio curves, it can be seen that for the various working conditions listed in Table 5, the lateral force shows a trend of increasing and then decreasing with the increase in the lateral drift ratio. When the horizontal drift increases to a certain degree, the lateral thrust of the column reaches the peak, which does not mean that the column fails at this time. However, if the column has sufficient ductility performance, the column still has the ability to bear the horizontal load after the peak lateral thrust. Therefore, when the lateral force falls to 85% of the peak value, it is defined as the damage state; that is, the column no longer has the bearing capacity. From Figure 9, it can be seen that the ultimate lateral drift ratio of the center column decreases when vertical inertia forces are considered for both traditional and split columns compared to the gravity load-only condition. For example, the ultimate lateral drift ratio of the lower center column is 0.0116 in the gravity load-only condition, and the ultimate lateral drift ratio of the lower center column is only 0.0089 after considering the vertical inertia force, which is about 23% lower. The vertical inertia effect of the overlying soil is one of the key factors affecting the seismic performance of the center column, which should be given sufficient attention in the seismic design process of underground structures, especially the shallow ones. When the split column is adopted, for the same level of axial load, the horizontal deformation capacity of both the upper and lower columns shows an increasing trend.