Thesis or Dissertation A RHEOLOGY MODEL OF HIGH DAMPING RUBBER BEARINGS FOR SEISMIC ANALYSIS AT ROOM AND LOW TEMPERATURES

NGUYEN ANH DUNG

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主指導教員 : 奥井義昭
ACKNOWLEDGEMENT ......................................................................................................... iiiABSTRACT ............................................................................................................................... vTABLE OF CONTENTS ......................................................................................................... viiLIST OF FIGURES ................................................................................................................... ixLIST OF TABLES .................................................................................................................. xiiiChapter 1: INTRODUCTION .................................................................................................... 11.1 General.................................................................................................................... 11.2 High damping rubber bearings ............................................................................... 21.3 Review on previous work for high damping rubber bearings ................................ 31.4 Scopes and objectives ............................................................................................. 61.5 Contents of dissertation .......................................................................................... 6Chapter 2: STRAIN-RATE DEPENDENT BEHAVIOR OF HIGH DAMPING RUBBER BEARINGs UNDER CYCLIC LOADING ............................................................................... 92.1 General.................................................................................................................... 92.2 Specimens and experimental set-up ....................................................................... 92.3 Loading conditions .............................................................................................. 132.4 Equilibrium hysteresis behavior from MSR tests ................................................. 162.5 Instantaneous state from CS tests ......................................................................... 212.6 Stress relaxation behavior from SR tests .............................................................. 242.7 Summary ............................................................................................................... 27Chapter 3: AN IMPROVED RHEOLOGY MODEL .............................................................. 283.1. General.................................................................................................................. 283.2. Layout of the improved rheology model ............................................................. 283.3. Rate-independent equilibrium stress part ............................................................ 303.4. Rate-dependent overstress part ............................................................................. 343.4.1. Viscosity behavior obtained from simple relaxation tests .................................. 343.4.2. Scheme to identify optimal overstress parameters .............................................. 373.5. Summary ............................................................................................................... 42Chapter 4: NUMERICAL VERIFICATION .......................................................................... 434.1. General.................................................................................................................. 434.2. Simulation results ................................................................................................. 434.4.1. Relaxation tests ................................................................................................... 434.4.2. Cyclic shear tests ................................................................................................. 584.3. Summary ............................................................................................................... 69Chapter 5: SELF-HEATING OF HIGH DAMPING RUBBER BEARINGS ........................ 705.1. General.................................................................................................................. 705.2. Specimens and test conditions .............................................................................. 705.3. Effect of inside temperature of HDRBs on their mechanical characteristics under sinusoidal loading ............................................................. 715.4. Effect of self-heating on the design practice ........................................................ 785.4.1. Seismic analysis of single degree of freedom (SDOF) system ........................... 785.4.1.1. Motion equation and time-stepping procedure ............................................ 795.4.1.2. Bilinear model ............................................................................................. 825.4.1.3. Input data of seismic analysis ...................................................................... 845.4.1.4. Numerical results ......................................................................................... 855.4.2. Temperature rise insde HDRBs under earthquakes ............................................ 875.5. Summary ............................................................................................................... 88Chapter 6: SUMMARY AND CONCLUSIONS ..................................................................... 906.1. Experimental observations at room and low temperatures ................................... 906.2. Proposed model and parameter identification ...................................................... 906.3. Numerical verification .......................................................................................... 916.4. Self-heating of high damping rubber bearings ..................................................... 916.5. Future studies ........................................................................................................ 92REFERENCE ........................................................................................................................... 93
High damping rubber bearings (HDRBs) are seismic isolating devices used widely in Japan, especially after Kobe earthquake in 1995. They provide a cost effective and reliable technology for earthquake protection of the bridge structure. In some guide specifications for the seismic design of bridges with HDRBs, the nonlinear characteristics of HDRBs are expressed in terms of a bilinear model. However, the mechanical behavior of HDRBs is characterized by strong strain-rate dependency. The current bilinear model cannot represent this behavior.Rate-dependent behavior of high damping rubber bearing (HDRBs) is investigated at subzero temperatures (-30℃ and -10℃) and also at room temperature (23℃) under horizontal cyclic shear deformations with a constant vertical compression load. On the basis of experimental observations, an improved elasto-viscoplastic rheology model has been proposed where the total stress has been decomposed into rate independent elasto-plastic stress, nonlinear elastic stress and the nonlinear visco-elastoplastic overstress. To represent the nonlinear viscosity behavior at both subzero and room temperatures, the overstress branch of an existing elasto-viscoplastic rheology model has been modified by replacing the linear elastic spring of the branch with two parallel branches that include a linear elastic spring and a nonlinear elasto-plastic model (spring-slider). This part is connected in series with the dashpot. To identify constitutive relations of each element in the resultant rheology model, an experimental scheme comprised of three types of tests, namely cyclic shear (CS) tests, multi-step relaxation (MSR) tests, and simple relaxation (SR) tests, are carried out at constant strain rates for the three reference temperatures. An optimum calculation approach is developed to determine a unique set of overstress parameters capable of representing CS, MSR, SR tests and also the sinusoidal loading tests with variable input strain rates. Finally, the ability of the proposed model and parameter identification procedure are verified by comparing numerical simulation results with experimental data.The final objective is to enhance the capability of the proposed model by incorporating self-heating effect into the model. As the first step, an experimental program is conducted to investigate the self-heating effect on the mechanical characteristics of HDRBs under sinusoidal loading at room and low temperatures. Experimental results show that the inside temperatures of HDRBs increases by cyclic loading, especially at low temperatures. Moreover, it is shown that the stress-strain relationships of HDRBs are governed by the inside temperatures. Therefore, a seismic model for HDRBs at low temperatures should be based on the inside temperatures.
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