An Innovative System for Protecting Precision Machinery against Earthquakes by Nonlinear Seismic Isolators

• Main Authors: Wen-ChunHuang, 黃文駿
• Other Authors: George C. Yao
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/58858502460532347928

博士 === 國立成功大學 === 建築學系碩博士班 === 98 === Precision machinery in hi-tech factories is generally very sensitive to vibration. Unfortunately, the natural frequencies of elements inside precision machines are often low and may be destructively resonant with the earthquake oscillation frequency of the factory floor. For instance, after the 921 Chi-Chi Earthquake in 1999, losses from damage to precision machines and product in Hsinchu’s Science Based Industrial Park were estimated at US$ 400 million, despite a horizontal PGA of only around 120 gal was measured in Hsinchu, which caused almost no structural damage to factory buildings. This event revealed the fragility of vibration-sensitive precision machinery and showed the need to provide suitable protection system to avoid loss caused by the damage to critical equipment, nonstructural components (NC), and operational and functional components (OFC) in buildings. In this study, firstly, a guideway sliding isolator (GSI) system with very low frictional coefficient (below 0.01) were developed to protect precision machinery against horizontally floor seismic motions. Nonlinear restoring forces, including gap springs and magnetic springs, were applied to improve the performance of the GSI system. The gap spring is initially separated from the system by a gap, causing the GSI to slide freely when the displacement is smaller than the gap distance, and to perform nonlinearly once the gap is closed, therefore reducing the likelihood of resonance. The magnetic spring uses a noncontact magnetic repulsion force, also causing the GSI to achieve a nonlinear property. A numerical simulation model of the GSI system with magnetic/gap springs using step-by-step integration in Matlab Simulink program was developed. Full scaled shake table tests on the GSI systems with a 22-ton specimen were performed to verify the performance of the nonlinear GSI system and the accuracy of the numerical model. The testing results showed that the nonlinear GSI systems were effective in reducing the response accelerations to below 100 gal in most experimental cases. The GSI with magnetic springs could control the response displacements to about 10 mm and 20 mm when the system subjected to 160 gal and 940 gal far-field seismic motions of the floors, respectively. A parametric analysis of the magnetic springs in the GSI system under far-field seismic motions showed that sufficient magnetic forces in the small stiffness region can reduce the system’s response displacements. For GSI with gap springs, the response displacements could be controlled less than 10 mm, 25 mm, and 45 mm when the GSI system was excited by the floor’s seismic responses of small (160 gal) far-field motion, large far-field motion (940 gal), and small near-fault motion (160 gal), respectively. It was found that the GSI system with appropriate gap springs was more effective in controlling response displacements than was a free sliding system. Furthermore, the optimal gap for a system subjected to far-field earthquakes was found to be 5 mm in this study. Besides reducing horizontal response accelerations during earthquakes by the nonlinear GSI systems, we should also consider that the vibration-sensitive machinery requires a very rigid foundation for maintaining daily operation. For example, a scanner requires a foundation with a high dynamic stiffness of 100000000 N/m in the frequency band between 20 and 30 Hz. To satisfy this requirement, the practical foundation is usually glued to the floor by epoxy; nevertheless, it eliminates the possibility of sliding. For the GSI system developed in this study, the required dynamic stiffness can be provided by semi-active restraint, which is designed to use attractively magnetic forces that are active during the machine’s daily operation, but inactive when earthquakes are detected by the accelerometers installed on the machine. Therefore, a rigid machine foundation fixed to the floor by semi-active magnetic forces rather than epoxy adhesion can satisfy both the needs for the high dynamic stiffness of the precision foundation and the possibility of sliding. Moreover, the attraction force could be provided by electrical magnets made of superconductor. Rather than a continuous power supply, it needs the power only at the instants when magnetic forces are activated or deactivated. Hence, there is no additional energy consumption. At last, a bidirectional nonlinear GSI system was also applied to the base of a 1.8 ×1.8 ×0.25 m raised access flooring (RAF) system. A tri-axial shake table testing series using excitations achieved to 960 gal were performed. The seismic inputs were compatible to the AC156 response spectrum, the acceptance criterion for seismic qualification by shake table tests on NC/OFC. The results showed that the GSI system was effective in reducing the response acceleration to below 250 gal, and controlling the response displacements to be around 75 mm. It revealed that the nonlinear GSI system developed in this study has been qualified to protect most of the NC/OFC on building floors against horizontally seismic motions that are threatening to them.