With the same plan layout, the height of the structure is changed by increasing and decreasing the number of superstructure floors (the superstructure of each model differs by 9 floors), and three new staggered story isolated structure models with different aspect ratios are established. The plan dimensions are 33.6 m × 33.6 m, with a floor height of 4.5 m for the 1st to 3rd floors and a standard floor height of 3.9 m. The core tube isolated layer is located at the bottom of the core tube, and the frame isolated layer is located between the 8th and 9th floors of the frame, with a height of 1.6 m and a floor thickness of 160 mm. The thickness of the other floors is 120 mm. The seismic precautionary intensity is VIII, the design basic seismic acceleration value is 0.20 g, the seismic design grouping is the second group, the construction site classification is II, the characteristic period of the seismic response spectrum is 0.4 s. The aspect ratios of the three new staggered story isolated structures models are 2.122, 3.167, and 4.211, with total heights of 71.3 m, 106.4 m, and 141.5 m, and total floors of 17, 26, and 35, respectively. The 3D diagrams of the three models are shown in Figure 1 . The design of the three models meets the requirements of “Code for Seismic Design of Buildings” (GB 50011-2010) and “Technical Specification for Concrete Structures of Tall Building” (JGJ 3-2010), and complies with the requirement that the aspect ratio of a frame-core tube structure should not exceed 6 when the seismic fortification intensity is 8 degrees. Therefore, the design of the aspect ratio for the new staggered story isolated structure is reasonable. The concrete strength grade of the columns and beams is C40, and the grade of steel bars and stirrups is HRB400. The shear walls in the core tube are connected by coupling beams, and the frame element and layered shell element are connected by embedded beams. The section information of beams and columns is shown in Table 1 .

In this paper, the finite element software ETABS is used to establish the new staggered story isolated structure models. The bottom reinforcement area of the core tube is 1 to 3 floors, which is simulated by layered shell elements with a thickness of 400 mm; The non-bottom reinforcement area is simulated by elastic thin shell elements with a thickness of 300 mm. The embedded beams are placed between the frame and the layered shell elements. The Tekeda hysteresis model is used to simulate the nonlinear properties of C40 concrete, and the Kinematic hysteresis model is used to simulate the nonlinear properties of HRB400 steel bars. The frame columns are all equipped with fiber P-M2-M3 hinges, and the frame beams and connecting beams are equipped with M3 hinges. The number of seismic isolation bearings is determined according to the total horizontal yield force being 2% of the base vertical reaction force under the standard value of gravity load. A total of 78 isolated bearings with a diameter of 900 mm are set, of which 62 lead rubber isolated bearings are set in the frame isolated layer, and 16 lead rubber isolated bearings are set in the core tube isolated layer, which is simulated by nonlinear connection elements “Rubber Isolator”. The parameters of the isolated bearings are listed in Table 2 , and the layout of the isolated bearings is shown in Figure 2 . A schematic diagram of the lead rubber isolated bearing is shown in Figure 3 .

In accordance with the “Code for Seismic Design of Buildings” (GB50011-2010), based on the response spectrum, seismic intensity, seismic grouping, seismic frequency, site category, etc., two natural ground motions were selected from the Pacific Earthquake Engineering Research Center. According to the response spectrum, seismic parameters, and the first three natural periods of the structure, an artificial ground motion was generated using the GM-Tool software. The base shear force calculated using the time-history method under each ground motion is greater than 65% of the base shear force calculated using the mode-superposition response spectrum method. The average of the base shear forces calculated using the time-history method under the seven ground motions is greater than 80% of the base shear force calculated using the mode-superposition response spectrum method. Therefore, the selected ground motions are reasonable and effective, meeting the requirements of the code, and can be used for the analysis and calculation of the seismic response and overturning resistance of new staggered story isolated structures. The information on the two natural ground motions is listed in Table 3 , and the acceleration response spectra are shown in Figure 4 .

The envelope values of the base shear forces of the new staggered story isolated structures with aspect ratios of 2.122, 3.167, and 4.211 under rare earthquakes are 38193.949 kN, 36478.754 kN, and 42667.013 kN, respectively. With the increase of the aspect ratio, the base shear force increases. A larger base shear force requires larger isolated bearings to ensure the stability of the overall structure, thus reducing the risk of overturning the superstructure. As shown in Figure 5 , as the aspect ratio of the new staggered story isolated structure increases, the inter-layer shear force of the substructure increases. When the aspect ratio increases from 2.122 to 3.167, the average increase rate of the envelope value of the inter-layer shear force for each floor of the substructure is 6.036%. When the aspect ratio increases from 2.122 to 4.211, the average increase rate of the envelope value of the inter-layer shear force for each floor of the substructure is 43.166%. Therefore, as the aspect ratio of the new staggered story isolated structure increases, the substructure will bear a larger inter-layer shear force. Therefore, when designing new staggered story isolated structures with a larger aspect ratio, the substructure should be strengthened to avoid the substructure bearing excessive seismic forces, which could increase the risk of damage to the substructure and the risk of overturning the superstructure.

As shown in Figure 6 , the envelope values of the inter-layer displacements of the frame isolated layer and the core tube isolated layer in the new staggered story isolated structure with an aspect ratio of 2.122 are 130.648 mm and 81.368 mm, respectively; For the aspect ratio of 3.167, the envelope values are 229.99 mm and 124.905 mm, respectively; For the aspect ratio of 4.211, the envelope values are 310.543 mm and 235.013 mm, respectively. These values are less than the limit displacement of the isolated bearings specified in the “Code for Seismic Design of Buildings” (GB50011-2010), which is ${\mu }_i\le \left[0.55R;3H\right]$, that is, less than 495mm. As shown in Figure 7 , the inter-layer drift of the new staggered story isolated structures with different aspect ratios satisfies the requirement specified in the “Code for Seismic Design of Buildings” (2010) that the elastic-plastic inter-layer drift of the substructure should be less than 1/130 under rare earthquakes, and the elastic-plastic inter-layer drift of the superstructure should be less than 1/100 under rare earthquakes.

The larger inter-layer displacements indicate a greater influence of seismic action on the structure, and larger inter-layer displacements can lead to structural damage, reduce the overall stability of the structure, and increase the risk of overturning. With the increase of the aspect ratio of the new staggered story isolated structure, both the inter-layer displacements and inter-layer drift of the core tube and frame at each floor gradually increase, indicating that a larger aspect ratio will result in a greater seismic impact on the structure. The inter-layer displacements of the core tube isolated layer are greater than those of the frame isolated layer. Furthermore, as the substructure of the new staggered story isolated structure acts as the non-isolated structure, the inter-layer displacements and inter-layer drift of the core tube below the frame isolated layer are less than those of the frame at each floor.

As shown in Figure 8 , the compressive stress of the isolated bearings in the frame isolated layer and the core tube isolated layer of new staggered story isolated structure increases with the increase of aspect ratio, and both are less than the specified limit of 30 MPa. When the aspect ratio increases from 2.122 to 3.167, the average increase in compressive stress for the isolated bearings in the frame isolated layer and core tube isolated layer are 93.900% and 47.907% respectively; when the aspect ratio increases from 2.122 to 4.211, the average increase in compressive stress for the isolated bearings in the frame isolated layer and core tube isolated layer are 189.109% and 98.267% respectively. As shown in Figure 9 , the isolated bearings of the frame isolated layer in the new staggered story isolated structure do not experience tensile stress, only the isolated bearings of the core tube isolated layer experience tensile stress. The tensile stress and the number of isolated bearings experiencing tensile stress increase with the increase in aspect ratio. When the aspect ratio is 2.122 and 3.167, the tensile stress of the isolated bearings in the core tube isolated layer is less than the specified limit of 1 MPa, but when the aspect ratio increases to 4.211, some of the isolated bearings in the core tube isolated layer exceed the specified limit of 1 MPa for tensile stress.

As shown in Figure 10 and Table 4 , with the increase in aspect ratio, the height of the superstructure increases, which causes the overturning lever arm of the superstructure to increase, so the overturning moment of the superstructure increases with the increase of the aspect ratio. In addition, when the height of the superstructure increases, the number of floors increases, so the permanent load standard value of the superstructure also increases with the increase of the aspect ratio. Because the permanent load standard value of the superstructure is proportional to the overturning resistance moment of the superstructure, and the width of the structure remains constant, the overturning resistance moment of the superstructure also increases with the increase in aspect ratio as the overturning resistance lever arm remains constant. When the aspect ratio increases from 2.122 to 3.167, the overturning moment and overturning resistance moment of the superstructure increase by 239.565% and 87.817% respectively. When the aspect ratio increases from 2.122 to 4.211, the overturning moment and overturning resistance moment of the superstructure increase by 619.634% and 174.581% respectively. Although both the overturning moment and overturning resistance moment of the superstructure are increasing, the increase in overturning moment is much greater than the increase in overturning resistance moment. Therefore, as the aspect ratio of the new staggered story isolated structure increases, the overturning resistance ratio of the superstructure will decrease with the increase in aspect ratio. When the aspect ratio of the new staggered story isolated structure increases, the absolute horizontal acceleration of the substructure decreases. Since the absolute horizontal acceleration of the floors is proportional to the overturning moment, the overturning moment of the substructure decreases as the aspect ratio of the structure increases. In addition, when the aspect ratio of the structure increases, the horizontal displacement of the floor increases. Since the horizontal displacement of the floor is inversely proportional to the overturning resistance moment, the overturning resistance moment of the substructure decreases as the aspect ratio of the structure increases. However, the decrease in the overturning moment of the substructure is greater than the decrease in the overturning resistance moment, so the overturning resistance ratio of the substructure of the new staggered story isolated structure increases as the aspect ratio of the structure increases.

When the aspect ratio of the structure increases from 2.122 to 3.167 and 4.211, the overturning resistance ratio of the superstructure in the new staggered story isolated structure decreases by 45.323% and 62.773% respectively. The overturning resistance ratio of the substructure in the new staggered story isolated structure increases by 5.560% and 8.270% respectively. It is apparent that the decrease in the overturning resistance ratio of the superstructure is much greater than the increase in the overturning resistance ratio of the substructure. When the aspect ratio increases from 2.122 to 3.167 and 4.211, the overturning resistance ratio of the overall structure decreases by 33.618% and 53.496% respectively. Therefore, with the increase in the aspect ratio, the overturning resistance ratio of the superstructure and the overall structure decreases, leading to an increased risk of overturning for the superstructure and the overall structure. Therefore, in the design of the new staggered story isolated structure with a larger aspect ratio, particular attention should be paid to the superstructure. This can be achieved by altering the arrangement of the isolation layer to reduce the overturning lever arm of the superstructure, thus reducing the risk of collapse of the overall structure due to the overturning of the superstructure. In addition, special attention should be given to the state of the isolated bearings in the core tube isolated layer, and the tensile stress and the number of isolated bearings that generate tensile stress should be reduced as much as possible. By altering the arrangement of the isolated layer, the parameters of the isolated bearings, or utilizing combined seismic mitigation isolation methods, it is possible to reduce damage to the weak positions of the new staggered story isolated structure and consequently reduce the risk of overturning and collapse of the structure.