Seismic isolation design of high-rise shear wall structures in high-intensity areas

1. Introduction

Shear wall structure, as a common and effective lateral force resisting system, is widely used in high-rise buildings. However, earthquakes, an unpredictable natural disaster, always pose a huge threat to building structures. Traditional shear wall design mainly focuses on enhancing the stiffness and strength of the structure to resist seismic forces, while isolation design introduces a new concept, which is to set isolation layers between the structure and the foundation, isolate and consume seismic energy through the flexible deformation of the isolation layers, thereby reducing the direct impact of earthquakes on the structure. In isolation design, multiple isolation lines are usually set up to increase the redundancy and ductility of the structure. Shear wall isolation, as an innovative structural seismic strategy, aims to effectively isolate the transmission of seismic energy by installing isolation devices, thereby significantly reducing the seismic effects on the upper structure [1-4]. The application of this technology, especially in high-intensity areas, not only provides new ideas and methods for seismic design of shear wall structures, but also has the potential to play a key role in ensuring the safety of people's lives and property and reducing earthquake disaster losses.

In recent years, scholars at home and abroad have conducted relevant research on shear wall isolation technology and achieved certain results. Calugaru et al. [5] compared the seismic response of a 20 story reinforced concrete shear wall structure located in a near fault area under isolation and non-isolation conditions, and the results showed that the isolation structure performed better than the non-isolation structure. Xing et al. [6] used a high-rise reinforced concrete shear wall structure in Xichang City as an example to conduct seismic isolation design and seismic performance analysis. The results showed that the maximum inter story shear force and overturning moment damping coefficient of the structure under moderate earthquake action was 0.24, which significantly reduced the seismic response of non-isolated structures. Under rare earthquakes, the surface pressure and horizontal displacement of the isolation bearings can meet the requirements of the code. In addition, a rare earthquake dynamic elastoplastic time history analysis was conducted, and the results showed that the structure can meet the seismic fortification goal of “not collapsing in large earthquakes”. Xu et al. [7] applied seismic isolation technology to a high-rise shear wall structure in Tangshan City, and established finite element models for both non isolated and isolated structures, and conducted nonlinear time history analysis. The results showed that the eccentricity of the isolation layer and the wind resistance design met the requirements of the specifications. Under frequent earthquake action, the horizontal seismic reduction coefficient of the isolation structure is not greater than 0.38, and the maximum inter story displacement angle of the upper structure is less than 1/800; Under rare earthquake conditions, the viscous damper effectively controls the deformation of the isolation layer, and no tension occurs in any bearings. The seismic performance of the isolation structure is good, and the isolation scheme is feasible.

For traditional seismic resistant structures, the larger the cross-sectional size of the component, the greater its stiffness, and the increased seismic absorption, which not only causes material waste but also is detrimental to the safety of the structure under rare earthquake conditions. After adopting seismic isolation technology, the seismic effect absorbed by the upper structure has been effectively reduced, and the building's usable space has been increased. In addition, the reasonable arrangement of shock-absorbing bearings can improve the seismic performance of the structure, and the upper structure can be designed with a reduction of one or half degrees. In order to better apply seismic isolation technology in high-intensity areas, this paper takes a high-rise reinforced concrete shear wall structure in high-intensity areas as the research object, analyzes the seismic performance of the high-rise shear wall system in high-intensity areas after adopting seismic isolation technology, and provides a basis and reference for seismic isolation of shear walls in high-intensity areas.

2. Overview of engineering cases

This project case is a shear wall bottom isolation structure. The building has 20 floors above ground, a height of 60.3 meters, a width of 27.3 meters, and a height to width ratio of 2.21. The isolation layer is set on the first floor, with a height of 3.60. The seismic grouping is designed as the third group, with a construction site category of Class II, ground roughness of Class B, and a site characteristic period of 0.45 s. The seismic fortification intensity is 9 degrees, and the designed basic seismic acceleration value is 0.4 g. The building has a service life of 50 years.

3. Layout plan of seismic isolation bearings

A seismic isolation layer is installed on the first floor of this building, using a total of 31 lead rubber bearings LRB1200. The bearing parameters are shown in Table 1, A seismic isolation layer is installed on the first floor of this building, using a total of 31 lead rubber bearings LRB1200. The bearing parameters are shown in Table 1, where the vertical stiffness is calculated according to Eq. (1) and the equivalent damping ratio is calculated according to Eq. (2):

where $E_{cb}$ is the modified elastic modulus obtained by considering the influence of volume change; $t_r$ is the thickness of each rubber layer:

where $W_e$ is the area of work done by the restoring force; $W_d$ is the area of damping energy dissipation.

The layout plan of the seismic isolation bearings is shown in Fig. 1. The yield to weight ratio is 4.21 %, and the eccentricity between the rigid position of the seismic isolation layer in the $X$ and $Y$ directions and the overall structural center of gravity is 0.0102 % and 2 %, respectively, which is less than 3 %. The maximum compressive strength of the support under the representative value of gravity load is 9.2 MPa, which is lower than the compressive stress limit of Class C fortified buildings of 15 MPa and meets the requirements of the specifications.

Fig. 1Layout plan of seismic isolation layer bearings

Fig. 2Three dimensional model

To ensure the effectiveness of the wind resistance device and the good reset performance of the isolation device under multiple earthquakes, the isolation layer should have sufficient horizontal bearing capacity and elastic recovery capacity. The standard horizontal sheer force of the isolation layer under wind load is 988 kN in the $X$ direction and 1210 kN in the $Y$ direction. The design horizontal shear force is 1383 kN in the $X$ direction and 1694 kN in the $Y$ direction, which is less than the yield force of 7192 kN of each support of the isolation structure and meets the design requirements.

4. Structural seismic isolation analysis

4.2. Seismic response analysis under fortification intensity

Under seismic fortification intensity, the envelope diagrams of inter story shear force and overturning moment of the structure are shown in Fig. 3 and Fig. 4, respectively. Comparing the seismic responses before and after isolation, it can be seen that the average maximum floor shear force in the $X$ direction of the upper structure after isolation is only about 28 % of that of the non-isolated structure. Among them, the maximum inter story shear force ratio is 0.34, and the maximum floor shear force in the $Y$ direction is only about 25 % of that of the non-isolated structure. The maximum inter story shear force ratio is 0.31. The average maximum overturning moment of the upper structure in the $X$ direction after isolation is only about 30 % of that of the non-isolated structure, with a maximum overturning moment ratio of 0.36. The average maximum overturning moment of the upper structure in the $Y$ direction is only about 22 % of that of the non-isolated structure, with a maximum overturning moment ratio of 0.26. According to the specifications, the maximum horizontal seismic impact coefficient after isolation is 0.203. After comprehensive consideration, the maximum horizontal seismic influence coefficient of the upper structure after seismic isolation design is 0.203 or reduced by 0.5 degrees according to the seismic intensity.

Fig. 3Envelope diagram of interlayer shear force under seismic fortification intensity

a) Envelope diagram of interlayer shear force in $X$ direction

b) Envelope diagram of interlayer shear force in $Y$ direction

Fig. 4Envelope diagram of overturning moment under seismic fortification intensity

a) Envelope diagram of overturning moment in $X$ direction

b) Envelope diagram of overturning moment in $Y$ direction

4.3. Seismic response analysis under rare earthquakes

Under the rare earthquake of 9 degrees (0.40 g), the displacement angles between the $X$ and $Y$ layers of the structure are shown in Fig. 5, and the maximum horizontal displacement of the isolation layer of the isolation structure is shown in Table 3.

Fig. 5Inter story displacement angle of structures under rare earthquakes

a) Inter story displacement angle in the $X$ direction

b) Inter story displacement angle in the $Y$ direction

From Fig. 5, it can be seen that the maximum inter story displacement angles in the $X$ and $Y$ directions of each floor of the structure are 1/302 and 1/446, respectively, both of which are less than the inter story displacement angle limit of 1/120. The results indicate that the structure meets the requirements of the inter story elastic-plastic displacement angle limit and meets the seismic fortification goal of “not collapsing in large earthquakes”, which also means that the structure will not overturn. From Table 3, it can be seen that under rare earthquake conditions, the maximum displacement of the isolation layer meets the requirements of the maximum allowable displacement of the support.

Table 3Maximum horizontal displacement of isolation layer

Direction	Maximum horizontal displacement						Maximum allowable value
	Artificial wave 1	Artificial wave 2	Natural wave 1	Natural wave 2	Natural wave 3	Average value
$X$ direction	650	640	396	550	611	569	660
$Y$ direction	644	646	405	570	629	579	660

5. Conclusions

1) This project adopts seismic isolation design, and the damping coefficient can be controlled within 0.36. The maximum horizontal seismic influence coefficient of the upper structure is 0.203 or reduced by 0.5 degrees according to the seismic intensity.

2) The maximum inter story displacement angles in the $X$ and $Y$ directions after seismic isolation of the structure are 1/302 and 1/446, respectively, meeting the requirements of inter story elastic-plastic displacement angle limit. The maximum displacement of the isolation layer meets the requirements of the maximum allowable displacement of the support.

3) The eccentricity and wind resistance design of the isolation layer of the structure meet the requirements of the specifications. The maximum surface pressure value of the isolation bearings in rare earthquakes is 28.9 MPa, the minimum surface pressure value is 5.58 MPa, and the maximum displacement is 579 mm, which meets the requirements of the specifications. This indicates that the isolation structure design of this project is reasonable and the arrangement of the isolation bearings is reasonable.