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Analysis on Seismic Performance and Application Prospect of Isolated Buildings

1.Brief development of seismic building design method

Great disasters that the earthquake brought do harm to human production and living. That is the reason why how to reduce the earthquake disaster is a long-term topic all around the world. Before the 20th century, there is almost no special seismic design of the building. People began to realize the importance after they experienced several major earthquakes, and then seismic design theory and practice began to develop.
Early seismic thinking focused on fighting against inertia force so that the building will not be damaged due to the lack of strength. With this method of designing, most of the buildings are damaged and collapse in varying degrees during earthquakes. People are aware that the earthquake can not be overcome just against the power. The second stage of the seismic theory focused on reducing the seismic response of the building and enhancing the load and deformation capacity of the structure after yielding, that is, to improve the toughness of the building. For instance, seismic specification in China is “three standards and two stages” which causes huge losses that there are still a lot of houses damaged and collapsed in the earthquake.
The occurrence of Wenchuan earthquake in 2008 caused great losses of life and property, so that domestic scholars and design community began to re-examine the defects of existing seismic design methods and explore and practice new methods. Then seismic design method came into the third stage --- isolation and energy dissipation. The concept of building isolation first came up in 1891 by a Japanese scholar Kawaguchi Hoka. After him, some scholars have proposed other isolation model. Although the early concept is clear, the technical conditions at that time is limited in which the concept couldn’t get a good development and put it into practice. With the gradual development of seismic engineering theory and the accumulation of a large number of actual seismic data, people began to study and practice.
At present, the relatively mature building isolation technology includes: rubber laminated bearing isolation, friction pendulum isolation, base isolator with concrete filled and so on. Among them, the technology of rubber laminated bearing isolation is the most commonly used. In 1974 the first isolated building in the world was built in New Zealand; In 1983 Japan's first rubber laminated bearing isolation building --- Yachiyo Tokyo was built. In 1993 in Shantou, China built the first rubber isolation building.

2.Seismic response characteristics of the isolated structure and non - isolation structure

In order to explore the response characteristics of isolated buildings under the earthquake, two finite element models were established using the SAP2000 calculation program. (Model A is an isolation scheme and model B is an ordinary scheme.) The same analytical method was used to calculate and analyze the results.

2.1 Overview of structural model
Model A is an isolation scheme and model B is an ordinary scheme. The structure, dimensions and reinforcement of the two models are the same. It has two layers with height of 3m. X direction has three spans, while Y direction has 2 spans. The column span is 6mX6m; frame column size is 450mmX450mm. There are eight longitudinal reinforcements with length of 18mm each, eight stirrups @ 100/200(All steels belong to HRB400). The test use time history analysis method and CORRALIT wave. The acceleration peak of X direction is corrected to 247gal(Slightly larger than the acceleration of eight-degree seismic precautionary intensity).
Model A isolation bearing uses Rubber Isolator to simulate, with the stiffness of two horizontal directions is 1000KN / m. It adopts immediate integration for calculation. After calculation, select a vertex joint creating time-history curve of acceleration and a joint of isolation bearing creating time-history curve of displacement and then check the base shears. The structural responses are as follows: Maximum acceleration of vertex is 37.1gal with ratio to the peak of ground acceleration of 0.15. The maximum horizontal displacement of the bearing is 10.2cm, and the maximum value of the base shear in X direction is 138KN. Model B is a non-seismic design.
In order to facilitate the comparative analysis, the same time-history analysis is used to calculate as the Model A. After calculation, also select a vertex joint creating time-history curve of acceleration and a joint of isolation bearing creating time-history curve of displacement and then see the base shears as Model A. The structural responses are as follows: Maximum acceleration of vertex is 257.9gal with ratio to the peak of ground acceleration of 1.044. View frame deformation diagram: when it is shown at 0.85s, the maximum force of the base shear in X direction is 577KN, at this moment the middle of Nov.31 beam bearing has appeared plastic hinge. To 0.85s, the other end also appeared plastic hinge. The structural responses of the two models are shown in the following table:

Project Model A Model B Ratio
Vertex Acceleration (gal) 37.1 257.9 6.952
Relative Bearing Displacement(cm) 10.2 0 0
Base Shear( KN) 138 584 4.232

As shown in Table A, the acceleration and absolute inertial force of the isolation structure are greatly reduced compared with the non - isolation structure and the base shear of the non - isolated structure is 4.232 times that of the isolated structure. The above also explains why the isolated structure is without any damage under strong earthquakes. Some cases show that the residents in this kind of buildings can not even feel the earthquake occurred in some of the less intense sites. It is worth noting that the horizontal displacement of the rubber bearing in the isolation structure is very large (10.2cm in this case), which requires a very high horizontal shear deformation.

Model perspective of finite element structure

3.Limitations and future prospects of Isolation building

Although the seismic building has a great advantage in seismic performance, it doesn’t have promoted the application. The main problem lies in the following two aspects: Firstly, It needs to set up an isolation layer which is both to increase the cost and influence the effect of construction program. Secondly, the application of the isolation bearing in the high-rise buildings and low-intensity area is not widely, and the most current residential projects are to build high-level residential housing. Therefore, the current isolation buildings are used in high intensity areas, such as schools, hospitals and other public constructions, and the large-scale residential projects are difficult to promote the application.
At present, China's construction industry is vigorously promoting the industrial model. In view that the seismic performance of the prefabricated structure is generally poor (especially the concrete structure), and there is a certain gap between the domestic technology of construction industry and developed countries’. The author believes that it is possible to improve the seismic performance of such structures by setting isolation bearings, which is beneficial for mitigating earthquakes disasters.

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