Theoretical And Experimental Investigation On Mid-Story Seismic Isolation And Reduction Structures

Base-isolated building is an effective, economic and readily available technology and a new system for earthquake resistance. Many earthquake experiences at home and abroad have indicated that isolation technique, which sets an isolation layer between superstructure and base ,can avoid the earthquake damage to building structure and its contains. Since the first isolation building with laminated rubber bearings was built in 1993 in China, several hundred isolation buildings have been built all over the country.Isolation technique has been seriously acceded to the "Code for Seismic Design of Buildings" in China. There are also many isolation codes in other countries, such as the United States, Japan, European Economic Community etc. But in these codes, the limits are very strict for isolated buildings, such as superstructure must be regular, the isolation layer must be located on the top of base (base-isolated structure) or under the top floor of the building (TMD). Because of the needs of architecture and use function or the feasibility of technique, some limits have to be broken in practical projects. Sometimes isolation layer is set on the intermediate story, to form the "mid-story seismic isolation and reduction structure system".For mid-story seismic isolation and reduction structure system, according to the characteristic of structure, in general, isolation layer is set in the part where the structure's vertical stiffness is sudden changed. It can be set on the top of the first layer, middle layer, conversion layer of the structure, and so on. Laminated rubber bearings can be adopted. Because the isolation layer is set in intermediate story, the whole structure is divided into superstructure and substructure, the structure's dynamic characteristics are changed. The mechanism of mid-story isolated structure appears new characteristic in contrast with base isolation. The aim of mid-story isolation not only requires to reduce seismic responses of superstructure, but also to reduce seismic responses of the whole structure, meanwhile reducing or at least not increasing the seismic responses of substructure. Therefore, special research on this structure system is required.A series of shaking table tests of the 6-story steel frame mid-story isolated structures are carried out in Chapter 2. Based on the tests, some physical feature indexes, such as stiffness, deformation, damping and hysteresis curve are investigated. Effects of these main factors on the response of mid-story seismic isolation and reduction structure are researched in detail.Subsequent chapters extend the procedure to theoretical study on mid-story seismic isolation and reduction structures. In Chapter 3, the mechanical properties of rubber bearings and the force-deformation relation of isolation system are reviewed. The Bouc-Wen model is used to simulate the hysteretic curve of isolation layer for mid-story seismic isolation and reduction structures.Furthermore, In Chapter 4, the simplifying calculation model, the kinematical equation and time history analysis program of mid-story seismic isolation and reduction structure are developed. Earthquake response analysis of mid-story seismic isolation and reduction structure is carried out.The calculated results are considered to be certainly accurate by comparison with shaking table tests. Therefore, the calculation model and analysis program are reliable and can be used for further analysis.Chapter 5 deals with the analysis of main influencing parameters. For mid-story isolated structure, the key problems are force status and interaction of the structure above and below the isolation layer. Many factors, such as the number of stories, the stiffness ratio, frequency ratio, mass ratio of superstructure and substructure, parameters of the isolation layer, have influence on the seismic behavior of the mid-story isolated structure. In this chapter, comparing with base-isolated structure and base-fixed structure, different cases of mid-story isolation, such as isolated layer is set on the base of building or any middle story are researched respectively.According to the results of theoretical analysis, it is indicated that structure design of mid-story isolation building not only requires to control the first period of structure, but also requires to control the second period of structure. For superstructure, the target for design is to reach "seismic isolation" through designing the stiffness, damping of isolation layer reasonably to extend the first period of structure and to control the displacement of isolation layer moderately. For substructure, the target for design is to reach "seismic reduction" through selecting the location of isolation layer reasonably, adjusting the stiffness ratio, mass ratio of superstructure and substructure, and changing the dynamic properties of structure. The aim is to decrease or not increase the earthquake action of substructure, to decrease the earthquake force of whole structure simultaneously.In Chapter 6, parameter optimizations of base-isolation structure, structure with TMD and mid- story isolation and reduction structure are analyzed comparatively. Based on the optimum results, mid-story isolation and reduction structures are divided into three cases: tune frequency to reduction, part reduction and full reduction. According to the difference of design control target for mid-story isolation and reduction structure, the optimizations of main influencing parameters are carried out.The optimum combination relationship of these factors, the optimum frequency ratioα_opt and optimum damping ratioζ_opt are presented.As an example, Chapter 7 gives design and application of mid-isolation system to apartment buildings on subway platform. The design and construction process of mid-story isolation and reduction structure are presented. The isolation effect for superstructure and reduction effect for substructure are obvious, so the aim of structure design is reached.