## 4 Discussion  
Our study shows the new isolation system plays an important role in decreasing the peak acceleration values of each story or the inter-story displacement in the story-isolated RC structure when the structure is subjected to seismic waves at PGA of either 0.30 g or 0.40 g (Figures 8, 10, 17 and 19), and thus reducing the seismic energy that is directly applied to the building. The displacement of the isolation layer is larger by one order of magnitude than other parts of the building, indicating the majority of seismic energy was absorbed by the isolation system. This high energy dissipation capacity of the new isolation system achieves the aim of seismic design of RC frame buildings. When the earthquake wave has a higher PGA, the peak acceleration values of superstructures in the story-isolated model increases much less than those of superstructures in the non-isolated model. This seismic damping effect is more significant when the story is above the isolation layer. For the substructure, it is rigidly connected with the base, thus its acceleration response is close to the acceleration response of the shaking table, and thus a negligible damping effect was present in the story-isolated model. This was further verified by our testing results shown in Table 10 and was also consistent with results from studies such as [34; 35]. The above results also suggest that the isolation performance of the superstructure could depend on the position of the isolation layer in an RC frame building, consistent with a comparison between the base isolation building and middle-story isolation building [13]. The detailed design and analysis of the inter-story isolation system can be found in references [36; 37]. Our study focuses on the first-story isolated RC frame building, where the isolation-protected superstructure has a relatively large story range (e.g., the second floor to the top floor in this study). In addition, compared to the base-story isolation [38; 39], the first-story isolation provides a more continuous connection between the ground and the above frame building for easier retrofitting, though the response mechanism is similar to the base-isolated structure. The horizontal displacement angles of the base-1 floor in the first-story isolation model are much less than those in the non-isolated model, as shown in Tables 8 and 11. In addition, the displacement angles of the base-1 floor in the first-story isolation model are also less than other inter-floors. According to the seismic design code of building [18], the safe capacity of the substructure is higher than that of the superstructure. In addition, the small displacement angles indicate the elastic deformation of substructure in the first-story isolation RC building with the coupled beam-column connections. Consistent with the acceleration results, the inter-story displacements in the isolation model significantly decreased by up to 92% compared to those in the non-isolated model, presenting good stability of structures. For inter-story between the ground and the first floor, the damping effect is the most significant as the isolation system starts to realize the damping effect on the first floor just above the position of the isolation layer. Once the isolation layer is starting to exert its effect, both the acceleration and displacement decreased sharply. Without the damping contribution to the base-first floor in the non-isolated model, the large inter-story displacement that was close to the structure collapsing limit was thus found when the earthquake level was high at a PGA of 0.40 g.  
For the non-isolated model, two different earthquake intensity levels at PGAs of 0.30 g and 0.40 g cause various high inter-story displacements and displacement angles, which are approaching to the code limit of \(1/50\) for the first story [18]. While, for the story-isolated model, the inter-story displacements and corresponding angles are hugely decreased, indicating that the isolation system could significantly improve the seismic safety of the RC frame building.