## 4 Analysis of Structure Progressive Collapse Performance under Ordinary Ground Motion
### Failure Condition of Bottom Column  
Compared with the traditional seismic structure, because the horizontal stiffness of the isolation layer is much less than the lateral stiffness of the superstructure, the isolation structure shows good performance under an earthquake and possesses better ability to resist horizontal earthquake collapse. Moreover, due to weak horizontal restraint of the isolation bearing of the isolation structure, the catenary mechanism could not be formed, which results in different performance of the progressive collapse of the isolation structure after the failure of the bearing and the failure of the column. The seismic isolation structure C2K3 was used to analyze the dynamic response of the structure under the vertical unbalanced load and two-directional coupled dynamic excitation. To remove the bottom  
Figure 13: Axial force of span beam of base-isolated structure failure. (**a**) Seismic isolation structureC2K3-A (**b**) Seismic isolation structureC2K3-B (**c**) Flat ground isolation structure, Support bearing failure condition Support bearing failure condition A Support bearing failure condition. (**d**) Flat ground isolation structure (**e**) Flat ground isolation structure (**f**) C2K3 Seismic isolation structure, B Support bearing failure condition Diagram of beam element position Diagram of beam element position. grounding column A, the bottom grounding column B, the bottom of the adjacent column D, the bottom grounding column F, the edge of the adjacent seismic isolation bearing E for analysis, the load coefficient was 0.5. First, we adjusted the PGA of the ground motion to 0.7 g, and then removed the support at 1.5 s while inputting the horizontal seismic wave to the X-direction node of the seismic isolation support of the remaining structure. The duration of the seismic wave was 25 s.  
From Figure 14, it can be seen that the maximum displacement and the stability value of the failure point of the seismic isolation structure only considering the vertical unbalanced load effect (that is, only the bottom-layer column is removed) were less than the two-directional coupled dynamic excitation. Generally speaking, the remaining structure was stabilized in 4-5 s under the condition of only removing the bottom column, while the vertical displacement of the failure point of the remaining structure in the two-directional coupled dynamic excitation still increased to varying degrees due to the horizontal earthquake. It can be seen from Table 3 that under the two-directional coupled dynamic excitation, the displacement amplitude and the stability value of the failure point after the removal of the base column A of the seismic isolation structure C3K2 were higher than 3.4% and 2.6%, respectively, without considering the two-directional coupled dynamic excitation; under the two-directional coupled dynamic excitation, the displacement amplitude and stability value of column B of the demolition isolation bottom layer were higher than 1.3% and 1.2% of the cases where only the vertical unbalanced load was considered; for the bottom-layer columns D and E, under the two-directional coupled dynamic excitation, the displacement stability values were higher by 2.5% and 1.3%, respectively, while the displacement amplitudes of the two working conditions of the D and E columns were close; under the two-directional coupled dynamic excitation, the displacement amplitude and stability value of the column F of the C3K2 bottom layer of the seismic isolation structure were higher than 3.0% and 2.8% considering only the vertical unbalanced load; The displacement of the bottom-layer columns B, D, and E failure conditions of the seismic isolation structure C3K2 was significantly smaller than that of the corner columns A and F failure conditions. The displacement values of the corner columns at the bottom of A and F under failure conditions were similar.