Study on shock absorption performance and damage of a new staggered story isolated system

Abstract

The new staggered story isolated system is developed according to the base isolated system and the mid-story isolated system. Non-linear finite element model of an eighteen stories new staggered story isolated structure is established. For a comparative analysis, the models of a base isolated structure, a mid-story isolated structure, and an aseismic structure are also established, and their shock absorption performances and damages are analyzed for comparison. The results indicate that the new staggered story isolated structure has a small seismic response, good shock absorption performance which is feasible for application. Besides, the shock absorption performance of the new staggered story isolated structure is a little worse than the base isolated structure but slightly better than the mid-story isolated structure. The bottom of core tube and the story below the frame isolated story have large acceleration response which needs to be paid more attention in design.

Keywords

new staggered story isolated system base isolated mid-story isolated finite element analysis shock absorption performance

Introduction

The staggered story isolated system is a new isolated system which is developed from the base isolated system and the mid-story isolated system. The characteristic of the base isolated structure is that the isolated story lies at the bottom of the structure. The characteristic of the mid-story isolated structure is the isolated story lies in the middle of the structure. Different from the above two kinds of isolated systems, the isolated story of the new staggered story isolated structure is divided into two parts, the isolated story of core tube part is located at the bottom of the core tube, forming a similar to base isolated mode, the isolated story of the frame part is located in the middle of the structure, forming a similar to mid-story isolated mode. The Huafa Project in Baotou, Inner Mongolia, China is a representative building of the new staggered story isolated structure. (Zhang, 2019). In addition, a hospital was reinforced by the new staggered story isolated system after the earthquake in Istanbul (Kubin et al., 2012). The diagram of a new staggered isolated structure, an aseismic structure, a base isolated structure, and a mid-story isolated structure is shown in Figure 1.

Figure 1.

Structure diagram: (a) aseismic structure, (b) base isolated structure, (c) mid-story isolated structure, (d) new staggered story isolated structure (The blue part represents the core tube, the red part represents the frame, and the shadow part represents the isolated bearings).

As earthquakes occur frequently all over the world, researches about isolated structure have been increasing. The base isolated system experienced three successive generations, each one was marked by the occurrence of significant event (Luca and Guidi, 2019). Base isolated system could effectively protect buildings under high intensity ground motion (Losanno et al., 2021), and reduce the seismic response of structure (Cancellara and Angelis, 2016). Researchers have studied many kinds of isolated bearings and found that most of them can not only reduce the seismic response of the structure, but also control the displacement of the isolated story (Castaldo and Tubaldi, 2018; Fu et al., 2019; Shan et al., 2020; Shang and Hu, 2020). For the application of base isolated, Japan has more experiences than other countries (Pan et al., 2005).

The base isolated structure has been studied for many years, and it has been applied in many countries all over the world. However, the mid-story isolated structure has a relatively short time and it is not mature enough. Kim et al. (2018); Kim and Kang (2018); Kim and Kang (2019) tried to find out the appropriate yield strength, horizontal stiffness and fuzzy soft-computing technique with a multi-objective optimization to control the isolated story displacement of a high-rise mid-story isolated structure. Zhang et al. (2009) found that reasonable stiffness and damping of the isolated story could significantly reduce the collapse probability of mid-story isolated structure under large earthquake. Chang and Zhu (2011) concluded that the superstructure and substructure has good isolated effect. The behavior of the mid-story isolated structures during earthquake was highly influenced by the characteristics of the isolated system (Loh et al., 2013). Besides, the mid-story isolated could be applied to the reinforcement of existing buildings, which could reduce the seismic response of the structure (Kawamura et al., 2000; Tasaka et al., 2008; Liu et al., 2012; Li et al., 2014; Zhang et al., 2020).

Miyata et al. (2004) developed a new staggered story isolated structure. Kubin et al. (2012) reinforced a hospital with a new staggered story isolated structure, by reinforcing the core tube with isolated bearings, which showed good seismic performance under ordinary ground motion. In addition, Liufu et al. (2020) introduced the new staggered story isolated structure into the study of structural scheme selection and design of the high-rise buildings with large chassis and multi towers, and found that the period of staggered story isolated structure increased significantly under ordinary ground motion, which was conducive to reduce the seismic response of the structure. Moreover, Qi et al. (2020) found that the new tensile device could reduce the bearings tensile stress of the high-rise new staggered story isolated structure under ordinary ground motion. The Terminal II of Meilan Airport in China was also used as a staggered story isolated structure (Liang et al., 2020).

From above analysis, it can be seen that researches on the new staggered story isolated structure are less and the staggered story isolated structure are still in the initial stage. In this paper, a finite element model is established for the new staggered story isolated structure. Dynamic time-history response of the new staggered story isolated structure, the base isolated structure, the mid-story isolated structure and the aseismic structure under earthquakes was analyzed and compared, and the shock absorption performance and damage of the new staggered story isolated system is explored. These studies hope to provide some references for application of the new staggered story isolated structure.

Finite element model of a new staggered story isolated structure

Project overview

An 18-story (including isolated story) new staggered story isolated structure is shown in Figure 2. Baotou, Inner Mongolia of China is located in high intensity areas, according to China’s code for seismic design of building (GB 50011-2010, 2016), the building site is defined as class II or III with the seismic fortification intensity of 7°, which designed basic seismic acceleration is 0.15 g. The construction is a 30 m × 15 m rectangle, and frame-core tube is adopted. The bottom story height is 4 m, and the standard story height is 3.6 m. The isolated story of the core tube is set at the bottom of core tube, and the isolated story of the frame is set at the eighth story of the structure. The concrete strength grade is defined as C40 and the protection thickness is 30 mm. All steel bars are adopted HRB400. The design of this structure meets the requirement of China’s code for seismic design of building. The information of the columns, beams, and core tube is shown in Tables 1 and 2.

Figure 2.

New staggered story isolated structure: (a) 3D, (b) elevation, (c) plan, and (d) information of column and beam.

Table 1.

Dimension of frame section.

Component type	Component location/story	Section size/mm	Steel bar information
Component type	Component location/story	Section size/mm	Net protective layer thickness/mm	Stirrup diameter/mm
Frame column	1–8	600×600	30	10
Frame column	9–18	500×500	30	10
Frame beam	1–8	600×350	50	10
Frame beam	9–18	500×300	50	10
Connecting beam	1–18	400×200	50	10

Table 2.

Section size of shear wall.

Section type	Section name	Unit type	Thickness of concrete layer/mm	Reinforcement ratio of vertically distributed reinforcement/%
Non-bottom reinforcement zone shear wall	Wall-200	Shell	200	—
Constrained edge member shear wall	Wall-edge-200	Stratified shell	200	5
Non-restrained edge member shear wall	Wall-200	Stratified shell	200	0.40

Model parameters

ETABS is used to build the finite element models of the new staggered story isolated structure. The number of isolated bearings are estimated based on the 2% of total yield force of the base vertical reaction force under the standard value of gravity load. All isolated bearings are LRB500 lead rubber bearings. As shown in Figure 3, the frame isolated story (eighth story) has 24 isolated bearings, the core tube isolated story (first story) has 12 isolated bearings. The information of the LRB500 lead rubber bearings are shown in Table 3. The non-linear material definition of C40 concrete is adopted the Takeda hysteresis type, and the HRB400 steel bars are adopted the Kinematic hysteresis type. PMM plastic hinges are set for the frame columns, and M3 plastic hinges are set for the frame beams and connecting beams. The two stories at the bottom of core tube are the strengthened area, while the rest are non-strengthened area. The multi-layer shell is used for simulation of the strengthened area, and the elastic thin shell is used for simulation of the non-strengthened area. For the purpose of comparative study, the finite element models of base isolated structure, mid-story isolated structure, and aseismic structure are also established.

Figure 3.

Layout of isolated bearings: (a)frame column and (b)core tube.

Table 3.

Parameters of isolated bearings.

Type	Effective diameter/mm	Total rubber thickness/mm	Stiffness before yield/kN·m⁻¹	Equivalent stiffness		Vertical stiffness/kN·mm⁻¹	Yield force/kN	Ultimate displacement/mm
Type	Effective diameter/mm	Total rubber thickness/mm	Stiffness before yield/kN·m⁻¹	100% horizontal shear deformation/kN·m⁻¹	250% horizontal shear deformation/kN·m⁻¹	Vertical stiffness/kN·mm⁻¹	Yield force/kN	Ultimate displacement/mm
LRB500	500	92	10910	1270	1010	2400	40.0	275

Seismic wave information

According to the provisions of China’s code for seismic design of building, the building site is defined as class II or III with the seismic fortification intensity of 7°. As the ground motion is selected by building site classification, therefore, Nanjing wave which is suitable for class II site soils, Tianjin wave and Lanzhou wave which are suitable for class III site soils are selected. The basic information of the ground motion is shown in Table 4 and the acceleration response spectra of the ground motions is shown in Figure 4.

Table 4.

Information of ground motion.

Earthquake	Magnitude	Epicentral distance/km	PGA/cm·s⁻²
Nanjing wave	6.9	25	42.8
Tianjin wave	6.9	65	145.8
Lanzhou wave	6.8	32	196.2

Figure 4.

Acceleration response spectrum of ground motions.

Seismic response of the new staggered story isolated structure

According to the bearings specifications in Table 3, the effective stiffness in Z direction linear attribute is set as 24×10⁵ kN·m⁻¹, the effective stiffness in X and Y direction linear attribute is set as 1270 kN·m⁻¹, in non-linear attribute, the stiffness is set as 10910 kN·m⁻¹, the yield strength is set as 40 Kn, and the stiffness ratio after yield is 0.1.

Modal period

The first to sixth modal periods of the new staggered story isolated structure, the base isolated structure, the mid-story isolated structure and the aseismic structure are shown in Table 5.

Table 5.

First to sixth modal periods of structure.

Structure type	Staggered story isolated structure	Base story isolated structure	Mid-story isolated structure	Aseismic structure
T1/s	3.196	3.185	3.154	2.018
T2/s	2.972	2.953	2.927	1.787
T3/s	2.798	2.872	2.763	1.607
T4/s	1.001	1.067	0.911	0.818
T5/s	0.956	1.002	0.857	0.763
T6/s	0.918	0.997	0.848	0.757

As can be seen from the Table 5, compared with the aseismic structure, the first to sixth modal periods of the other three structures increases significantly.

Shock absorption performance

The main difference between the new staggered story isolated structure and the base isolated structure, the mid-story isolated structure is that the displacement and acceleration of the core tube and frame of the base isolated structure and mid-story isolated structure is consistent, but the new staggered story isolated structure has two isolated stories, the displacement and acceleration of the core tubes and frames above the eighth story is the same, while as the core tubes and frames below the eighth story are separated, their displacement and acceleration are different.

The acceleration comparison of the four structures is shown in Figure 5.

Figure 5.

Acceleration comparison of the new staggered story isolated, base isolated, mid-story isolated, and aseismic structure: (a) acceleration comparison diagram of core tube and frame of the new staggered story isolated structure (b) acceleration comparison diagram of new staggered story isolated structure and aseismic structure, (c)acceleration comparison diagram of new staggered story isolated structure and base isolated structure, and (d) acceleration comparison diagram of new staggered story isolated structure and mid-story isolated structure.

From the above analysis, it can be concluded that the structure acceleration of the new staggered story isolated structure, the base isolated structure and the mid-story isolated structure all reduce.

As can be seen from Figure 5(a), the acceleration of the core tube and frame above the eighth story of the new staggered story isolated structure is the same, while below eighth story is different, and the acceleration at the bottom of the core tube is the biggest.

As can be seen from Figure 5(b), compared with the aseismic structure, the acceleration of the staggered story isolated structure above the eighth story is slower, while some stories of the core tube below the eighth story are slightly more rapid. Compared with the aseismic structure stories, most stories of the frame have a slower acceleration, but the acceleration of the stories below the isolated story of the frame suddenly increases.

As can be seen from Figure 5(c), the acceleration of the new staggered story isolated structure and the base isolated structure above the eighth story is similar, and the latter is slightly slower. Below the eighth story, the acceleration of the frame and core tube of the new staggered story isolated structure is more rapid than that of the base isolated structure.

As can be seen from Figure 5(d), the acceleration of the frame and core tube of the new staggered story isolated structure is smaller than that of the mid-story isolated structure, and that below eighth story is basically smaller than the mid-story isolated structure, while some stories are larger than that of the mid-story isolated structure.

To sum up, above eighth story, the shock absorption performance of the new staggered story isolated structure and the base isolated structure is similar and slightly better than that of the mid-story isolated structure. Below eighth story, the shock absorption performance of the base isolated structure is the best, and that of the new staggered story isolated structure and the mid-story isolated structure is similar. The bottom of core tube and the story below the frame isolated story have large acceleration response which needs to be paid more attention in design.

Comparison of the inter-story displacement angle is shown in Figure 6.

Figure 6.

Comparison of inter-story displacement angle between new staggered story isolated structure, base isolated structure, mid-story isolated structure and aseismic structure:(a) comparison of inter-story displacement angle between core tube and frame of new staggered story isolated structure, (b) comparison of inter-story displacement angle between new staggered story isolated structure and aseismic structure, (c) comparison of inter-story displacement angle between new staggered story isolated structure and base isolated structure, and (d) comparison of inter-story displacement angle between new staggered story isolated structure and mid-story isolated structure.

As can be seen from Figure 6, the inter-story displacement angle of the new staggered story isolated structure is generally in a D shape, and the eighth story increases abruptly because it is the isolated story. Above eighth story, the inter-story displacement angles of the core tube and the frame are consistent, while below eighth story, they are inconsistent and the inter-story displacement angles of the core tube are smaller. Generally, the inter-story displacement angles are all within 0.004 and within the limit of the regulations of China (GB 50011-2010).

Above eighth story, the inter-story displacement angles of the new staggered story isolated structure are larger than the base isolated structure and smaller than the mid-story isolated structure. Below eighth story, the inter-story displacement angles at the core tube are smaller than that of the base isolated structure and the mid-story isolated structure, the inter-story displacement angles at the frame of the new staggered story isolated structure are slightly larger than that of the base isolated structure and are similar to that of the mid-story isolated structure. Generally, the inter-story displacement angles are all within 0.0015 and within the limit of the regulations of China (GB 50011-2010).

Comparison of the isolated bearings displacement of the new staggered isolated structure, the base isolated structure and the mid-story isolated structure is shown in Figure 7.

Figure 7.

Displacement comparison of isolated bearings of different isolated structures.

As can be seen from Figure 7, the isolated bearings displacement at the two isolated stories of the new staggered story isolated structure is different, and isolated bearings displacement of the core tube isolated story is significantly larger than the frame isolated story. The maximum displacement of the core tube isolated bearing reaches 100 mm, while that of the frame isolated bearing is only 40 mm. The maximum displacement of the isolated bearing of the base isolated structure is 120 mm, while that of the isolated bearing of the mid-story isolated structure is 50 mm. The isolated bearings displacement of all isolated structures all satisfy the limit of the regulations of China.

Comparison of the base shear force and the shear force of the eighth story among the four structures are shown in Figure 8.

Figure 8.

Structural shear force: (a) comparison of base shear force of aseismic, base isolated, mid-story isolated, and staggered story isolated structure, (b) comparison of shear force of the eighth story of different structures.

As can be seen in Figure 8(a), the base shear force of aseismic structure is larger than the other three structures. The base shear force of the base isolated structure reduced most significantly and therefore has the best shock absorption effect, which is followed by the new staggered story isolated structure and the mid-story isolated structure, while the base shear force of the mid-story isolated structure reduced slightly and its shock absorption is worse than the base isolated structure and the new staggered story isolated structure.

As can be seen from Figure 8(b), the eighth story shear force of the aseismic structure is larger than other three structures, and that of the base isolated structure is significantly smaller than the other three structures, resulting in a good shock absorption effect, while that of the new staggered story isolated structure and the mid-story isolated structure is similar and therefore their shock absorption effect is also similar.

The comparison of the frame and core tube shear force of the four structures under Tianjin wave are shown in Figure 9.

Figure 9.

Comparison of frame shear force under Tianjin wave: (a) shear force of aseismic structure, (b) shear force of base isolated structure, (c) shear force of mid-story isolated structure, and (d)shear force of new staggered story isolated structure.

As can be seen from Figure 9, for the new staggered story isolated structure, the frame shear force is small, and the shear force below the frame isolated story increases slightly, while the connecting beams shear force at the core tube is small. It can also be seen that the connecting beams shear force at the core tubes of the new staggered story isolated structure and the base isolated structure is similar and both smaller than that of the aseismic structure. The shock absorption effect at the core tube of the new staggered story isolated structure is better than that of the mid-story isolated structure, and the connecting beams shear force around the isolated story of the mid-story isolated structure is relatively large and larger than that of the aseismic structure. While the bottom connecting beams shear force of the mid-story isolated structure is smaller than that of the aseismic structure.

Damage research

The articulation situations of the new staggered story isolated structure, the mid-story isolated structure, and the aseismic structure under Tianjin wave are shown in Figure 10.

Figure 10.

Location comparison of structural plastic hinges under Tianjin wave (green dot in the figure): (a) aseismic structure, (b) base isolated structure, (c) mid-story isolated structure, and (d) new staggered story isolated structure.

As can be seen from Figure 10, plastic hinges appear at the frame beams of the 12^th and 13^th story and the core tube connecting beams of the 2^th story of the aseismic structure under Tianjin wave, but there is no plastic hinge in the base isolated structure. Plastic hinges appear at the connecting beams at the 2^th story of core tube of the mid-story isolated structure under Tianjin wave, and there are no plastic hinges at the frame. Plastic hinges only appear at the frame beams of the 13^th and 14^th story for the new staggered story isolated structure under Tianjin wave.

Based on the above analysis of the plastic hinges, the base isolated structure has a good shock absorption performance under the Tianjin wave, and there is few or no plastic hinges. Some connecting beams of the new staggered story isolated structure and aseismic structure is damaged under Tianjin wave. The frame of the mid-story isolated structure is relatively slightly damaged under Tianjin wave.

The comparison of the core tube damage of the four structures under Tianjin wave is shown in Figure 11.

Figure 11.

Comparison of shell stress and damage of structural core tube under Tianjin wave: (a) aseismic structure, (b) base isolated structure, (c)mid-story isolated structure, and (d) new staggered isolated structure.

As can be seen from Figure 11, the stress damage of the core tube shell of the new staggered story isolated structure and the base isolated structure is similar and that of the other two structures is also similar under Tianjin wave. The stress damage of the new staggered story isolated structure is relatively slight, and its most severely damaged area is the three stories above the isolated story of the core tube, especially the first and second story, while that of the mid-story isolated structure and the aseismic structure is both the three bottom stories, especially the second story.

In general, the core tube stress damage of the new staggered story isolated structure and the base isolated structure are not as serious as the mid-story isolated structure and the aseismic structure, the core tube stress damage of the mid-story isolated structure is less serious than that of the aseismic structure. Attention should be paid to the bottom of core tube for all isolated structures.

Conclusion

In this paper, non-linear finite element models are established for dynamic time-history analysis, the shock absorption performances and damages of the new staggered story isolated structure are studied and compared with the base isolated structure, mid-story isolated structure, and aseismic structure. The conclusions are as follows:

1. Compared with the aseismic structure, the first to sixth modal periods of the isolated structure increase significantly, especially the first to third modal periods. At the same time, the first to sixth modal periods of different isolated structures have certain differences. The first to third modal periods of the new staggered story isolated structure are larger than the other three structures.

2. The seismic response of the new staggered story isolated structure is small; the damage is slight. Therefore, this kind of structure is feasible for application.

3. The shock absorption performance of the new staggered story isolated structure is slightly worse than that of the base isolated structure and slightly better than that of the mid-story isolated structure.

4. The response of the core tube of the new staggered story isolated structure is relatively big, and the response of the story below the isolated story of frame is slightly bigger, which needs to be paid more special attention in design.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The writers gratefully acknowledge the financial support of National Natural Science Foundation of China (No.52168072, No.51808467), High-level Talents Support Plan for “Ten Thousand Talents” of Yunnan Province, China (2020).

ORCID iD

Yafei Zhang

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