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A New Approach to Seismic Base Isolation Using Air-Bearing Solutions

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A New Approach to Seismic Base Isolation Using Air-Bearing Solutions A New Approach to Seismic Base Isolation Using Air-bearing Solutions MohammadHossein HarbiMonfared Department of Engineering and Built Environment Anglia Ruskin University This dissertation is submitted for the degree of Doctor of Philosophy December 2015 This work is dedicated to those who have lost their loved ones in an earthquake. ii Acknowledgement I would like to express my sincere appreciation to Prof. Hassan Shirvani, my supervisor for his insightful guidance, inspiring encouragement and the continuous support of my Ph.D. study and related research. My special thanks also goes to Dr. Ayoub Shirvani, my former supervisor whom without his precious support it would have not been possible to conduct this research. Besides, I would like to thank the rest of my supervisory team: Dr. Sunny Nwaubani and Dr. Ahad Ramezanpour for their valuable comments and encouragement. My warm thanks goes to all my colleagues and friends at Anglia Ruskin University who helped me in this research project; Dr. Ehsan Eslamian for sharing his knowledge in fluid mechanics and Mr. Dan Jackson, for workshop preparation and laboratory tests. I am also grateful to Mr. Reza Ghadimzadegan of GeoSIG for providing the measurement instruments used in experimental tests. Last but not the least, I would like to express my heart-felt gratitude to my family for their love, patience and supports throughout my life. iii Declaration I certify that except where due acknowledgment is made in the text, the contents of this dissertation are genuine results of my own work and have not been submitted in any form for another degree or diploma. iv Abstract Earthquake, the natural phenomena, is conceived by the movement of the tectonic plates that induce shocks and impulse of devastating magnitude at ground level. Reducing losses during an earthquake has always been one of the most critical concerns of humans in earthquake prone areas. The main goal has been always to attenuate the shocks induced by ground motions on man-made structures. Two approaches have been conducted; increasing the earthquake resistant capacity of a structure, and reducing the seismic demands on a structure. With regard to the concept of reducing seismic demands on a structures, seismic base isolation is considered as an efficient method in mitigation of earthquake damages. A proper base isolation framework offers a structure great dynamic performance and in this way, the structure will be able to remain in elastic mode during an earthquake. On the other hand, not all isolation systems can provide the target structure with efficient seismic performance. The majority of currently available isolation systems still have some practical limitations. These limitations affect the functionality of a structural system and impose some restrictions to its proper use and protection level, causing it not to achieve anticipated level of performances. In this dissertation, an innovative seismic isolation system is proposed and investigated via laboratory tests and computer simulation to introduce a practical and effective seismic isolation system. The proposed system has aimed to modify some drawbacks of current seismic isolation system whilst at the same time keeping their advantages. The innovative isolation system in this study incorporates air-bearing benefits together with roller bearings and bungee cords in a complex system for horizontal base isolation. An experimental study was carried out to test a scale structure model (1/10th in length) as a case study for this research, to observe the behaviour of the structure with and without isolation system and to extract the dynamic characteristics of the structure by measuring fundamental frequencies and damping through a free vibration test. Computer simulation was conducted to simulate the dynamic behaviour of the structure when it is subjected to three different types of earthquakes; and with different base v configurations (fixed base and base isolated). The simulation was performed to gain an insight into the performance of the proposed isolation system under the given structure. Results from computer simulation were compared and validated with findings from experimental tests. It was confirmed that the present isolation system offers a significant reduction in acceleration demand in the structure leading to the reduction of base shear and consequently the level of damage to the structure. Results revealed that the proposed isolation system is able to mitigate the seismic responses under different ground motion excitations while exhibiting robust performance for the given structure. Furthermore, the system can also be used to isolate sensitive equipment or hardware in buildings affected by seismic shocks. vi vii Table of Contents 1 Introduction 27 1.1 Seismic isolation 27 1.2 Motivations for this study 29 1.3 Gap in knowledge 29 1.4 Aim and objectives of this research 30 1.5 Research methodology 31 1.5.1 Air-bearing device 31 1.5.2 Scaled structure model 32 1.6 Dissertation scope 33 1.7 Dissertation outline 33 2 Literature review 35 2.1 Introduction 35 2.2 Seismic base isolation from historical perspective 35 2.3 Seismic base-isolation efforts in the modern time 36 2.4 Recent progress in seismic-base isolation 38 2.5 Seismic base-isolation systems 40 2.5.1 Elastomeric-bearing isolation systems 41 2.5.2 Sliding base-isolation systems 45 2.5.3 Dampers used for seismic isolation 46 2.5.4 Soft first-story building 47 2.5.5 Artificial soil layers 47 viii 2.5.6 Rolling base-isolation systems 48 2.5.7 Air-bearing for base isolation 48 2.5.8 Other isolation systems 49 2.5.9 Advantages and disadvantages of current seismic isolators 50 2.6 Air-bearing perspective 50 2.6.1 Applications 50 2.6.2 Technical research 52 2.7 Earthquake Early Warning systems 53 2.8 Summary 54 3 New base isolation system 55 3.1 Introduction 55 3.2 Seismic base-isolation principals 56 3.2.1 Horizontal isolation 57 3.2.2 Damping 62 3.3 Innovative isolation system using air bearing 67 3.4 Case study 69 3.4.1 Scaling procedure 70 3.5 Scaled model strucutre 72 3.5.1 Mechanical characteristics of the scaled model 74 3.5.2 Isolation system for scaled model 76 3.6 Analytical model of the scaled structure 79 3.6.1 Modal analysis 80 3.6.2 Modal analysis for scaled model 83 ix 3.7 Conclusion 90 4 Air-bearing device design and development 91 4.1 Introduction 91 4.2 Air bearing design 91 4.3 Simulation methods 94 4.3.1 Governing equations 94 4.3.2 Turbulence modelling 97 4.4 Numerical solutions 100 4.4.1 Geometry 100 4.4.2 Discretisation 101 4.4.3 Mesh generation 103 4.4.4 Numerical setting 103 4.4.5 Numerical results 105 4.5 Experimental study 109 4.5.1 Air-bearing device 110 4.5.2 Tests 112 4.5.3 Experimental results 113 4.6 Validation 117 4.6.1 Numerical study and experiments 117 4.6.2 Numerical validations 117 4.7 Conclusion 118 5 Experimental study on the model structure 120 5.1 Introduction 120 x 5.1.1 Experimental study (general argument) 120 5.1.2 Experimental study for this research 122 5.2 Test rig 124 5.3 Dynamic test 124 5.4 Tests’ procedure 126 5.4.1 Fixed base 127 5.4.2 Base isolated 127 5.5 Measurement devices 128 5.5.1 Data acquisition 128 5.5.2 Sensors 128 5.5.3 Data communication software 129 5.6 Data analysis and results 129 5.6.1 Absolute acceleration on top of the structure 130 5.6.2 Natural frequencies and periods 134 5.6.3 Story drifts 137 5.6.4 Damping 141 5.7 Conclusion 146 6 Computer simulation of the structure 148 6.1 Introduction 148 6.2 Physical model 150 6.3 Mathematical model 151 6.4 Simulation 152 6.4.1 Earthquakes 152 xi 6.5 Program settings 155 6.5.1 Mass 155 6.5.2 Stiffness 156 6.5.3 Time history input 157 6.6 Results 160 6.6.1 Frequencies and periods 160 6.6.2 Accelerations 160 6.6.3 Displacements 162 6.6.4 Base-shear 165 6.6.5 Earthquake input energy 166 6.7 Conclusion 170 7 Conclusion and future works 171 7.1 Conclusion 171 7.2 Contributions 173 7.3 Future works 173 8 References 175 Appendices 183 A.1 Research flowchart 183 A.2 BI projects around the world 184 A.3 Roller bearing specifications 189 A.4 Modal analysis (fixed-base model) 191 A.5 Modal analysis (base isolated) 194 xii A.6 Air pump specifications 198 A.7 GMSplus specifications and calibration 200 A.8 Accelerometer specifications and calibration 204 A.9 FFT algorithm and implementation 208 xiii List of figures Figure 2-1 Rubber bearing schematic view ..................................................................41 Figure 2-2 Laminated rubber bearing also known as Low damping rubber bearing (Buchanan, et al., 2011) ................................................................................................42 Figure 2-3 Lead rubber bearing section (Saiful-Islam, et al., 2011) .............................44 Figure 2-4 High damping rubber bearing (Saiful-Islam, et al., 2011) ..........................44 Figure 2-5 Spherical sliding system schematic view (Buchanan, et al., 2011) .............46 Figure 2-6 An air-bearing schematic view ....................................................................51 Figure 2-7 An Air float bearing schematic view (HOVAIR, 2013) .............................51 Figure 2-8 Comparison of coefficient of friction for three types of bearing (Newway, 2006) .............................................................................................................................52 Figure 3-1 Elastic design spectrum, (흃 denotes damping) (Chopra, 2007)...................56 Figure 3-2 Behavior of building structure with base-isolation system .........................57
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