Research On An Innovative Distributed Seismic Isolation System Based On SSI And Nonlinear Soil Response

China is a highly seismic country where severe damage caused by earthquakes takes place. As statistics state, about 35% of the mainland earthquakes with magnitude above seven in the world had occurred in China, causing casualties reached up to 590,000 among the 1.2 million victims killed by earthquakes worldwide in the twentieth century. In addition, one third area of the country is located in high seismic intensity zone (with seismic intensity above VII). From year 1990 to 2007, seventy events with magnitudes 7.0-7.9, six events with magnitudes above 8.0 had leveled Chinese Mainland, causing devastating disasters covering twenty-eight provinces, left 590,000 civilians killed, 760,000 injured and millions homeless. At 14:28 on May 12th, 2008, an unprecedented earthquake with magnitude 8.0 leveled Wenchuan, Sichuan Province, resulting in 69,188 victims and direct economic loss up to 845 billion yuan. For these reasons, the prevention and mitigation of earthquake damage as to minimize the seismically induced losses should be one of our country's fundamental national policies. This research put emphasis on the evaluation and investigation on one of the inherent earthquake mechanism, i.e. the soil-structure interaction phenomenon, through a series of laboratory dynamic cyclic simple shear test, in situ shear wave velocity assessment and prescribed POT, ambient vibration experiment on a seven-story, 1/4 scaled steel frame structure model constructed in the Soil-Structure Laboratory, College of Civil Engineering, Hunan University. The author anticipated to obtain an improved insight in the soil-structure interaction problem on the basis of organized theoretical analyses and experimental survey, or take one step further, to incorporate the natural mechanism of soil-structure interaction and nonlinear soil response into the mitigation and isolation of seismically induced damage to the building structures, so that the casualties and economical loss caused by earthquakes should be reduced to some extent. This dissertation is divided into six chapters; the main content of each chapter corresponds to:1. Through the summarization and review of numerous related literature, the state-of-the-art and state-of-the-practice concerning soil-structure interaction problem were observed and discussed, the methods for seismic analysis incorporating base flexibility were also collected and generalized, and the research significance and drawbacks of existing studies were analyzed. In addition, an innovative seismic isolation system based on soil-structure interaction and nonlinear soil response was introduced. Works related to this innovate isolation system were reviewed and the basic frame for further inspection was then established.2. The dynamic characteristics of the in situ soil and sand-rubber mixture were obtained by cyclic simple shear testing on the soil specimens. The effect of different saturation on dynamic soil behavior during oscillatory shear is studied. The maximum shear modulus Gm ax, maximum damping ratio Dm ax, together with shear modulus normalized curve G /Gm ax?γand damping normalized curve D /Dm ax?γwere obtained in order to lay down the foundation for further numerical simulation analysis.3. The influence of prestraining on soil dynamic shear modulus G and damping ratio D was examined, and the control effect of pre-vibration consolidation time, the amplitude of cyclic shear strain, as well as number of cycles on G and D were determined. A significant prestraining threshold symbol was found, and this symbol could be employed as a sign that soil had experienced vibration history. Soil damping ratio D was found to be more influenced by this symbol than modulus G . In situ bore-hole shear wave velocity survey and laboratory vibration shear test on soil were performed and small-strain shear moduli Gm ax representing each soil layer obtained by bore-hole testing were compared with those got from cyclic shear experiment. The work of this chapter provides supplementary data for the following investigation on SSI effect.4. A series of prescribed POT tests and ambient vibration tests were performed on a seven-story, 1/4 scaled steel-frame structure prototype constructed in the soil-structure laboratory, Hunan University. POT tests on the steel-frame structure covering 17 different scenarios with varied stiffness and mass of the superstructure, and with different base fixities, were conducted. At the same time, ambient vibration survey was also performed on the building structure to investigate SSI effect on this prototype during micro tremor. The experimental data was then gathered for future analysis.5. The testing data collected during the aforementioned experiment was analyzed. The impact of superstructure-to-soil stiffness ratio, structure-to-soil mass ratio, along with interstory height on SSI effect was studied. In addition, the fundamental parameters of structures with flexible base, pseudo-flexible base, and fixed base were derived, some beneficial conclusions were made. 6. Finite-Element Method (FEM) numerical simulation was established on the steel-frame building using SAP 2000 analytical tool. Considering different base conditions such as fixed base and rubber-soil seismic isolation system with different configurations, the steel-frame structure was subjected to selected earthquake excitations, i.e., El Centro, Kobe, Northridge, and Wenchuan accelerograms. The result obtained by numerical simulations show that this innovative seismic isolation system can effectively reduce the acceleration response and interstory shear of the superstructure when experienced strong-motion shaking. A series of prescribed seismic shaking table testing was then conducted to investigate the'real'dynamic performance of the newly introduced isolation system under strong excitations. By varying the controlling parameters such as the content of rubber, layer height, together with the mass of the superstructure, a thorough parametric study was performed. The experimental results resonably demonstrated the marked performance of this isolation system for seismic mitigation purposes. At last, topics relevant to the transition of the hypothesis of this seismic isolation system to applicable engineering practice were discussed.