Vibration Isolation on the Foundation of Shaking Tables and its Effects on Operational Performance of the Table and its Surrounding Environment

Using shaking tables in experimental studies on buildings or other installations subjected to earthquake actionsare considered inevitable in many circumstances. Due to their unique abilities in simulating ground motion effectson prototype systems, such facilities can provide the researcher with the most accurate and sophisticated results onthe role of earthquake actions on the specimen under investigation. However, providing accuracy in ground motionsimulation for shaking tables needs careful attention to its subsystems and dynamic interaction between them(specimen, table, actuators, control system and the foundation). In fact there is the possibility of a strong interactionbetween the shaking table itself and the specimen under investigation depending on its weight and frequencycontents. Interaction between the table and its foundation assembly can also be considered important in certaincases. In addition, during shaking tables operation some vibrational force and energy can be transferred to itssurrounding environment and causes undesirable effects in the nearby buildings. In this case, vibration isolation forthe foundation of shaking table can partially solve this problem. However, it may also intensify the interactionbetween the table and its foundation that needs to be addressed properly to maintain the accuracy of the system inground motion simulation. In this work a simplified model for investigation on the role of vibration isolation onfoundation of shaking table assembly is proposed that takes into account the interaction problems among allsubsystems of shaking tables. A large number of time integration analysis subjected to actual earthquake records(applied in the table’s degree of freedom) have been carried out on this model. According to the results of this study,vibration isolation on a 4000 tons foundation of a typical middle size shaking table (6 by 6 meters in size with theweight of 40 tons) reduces, on average, the level of maximum acceleration transferring to the ground to about 1/3of that for the non-isolated system. This has happened in the expenses of about ±3 mm maximum lateraldisplacement for the foundation with respect to the ground. However, the results in this case also signifies sensiblechanges in the actuator’s force, stroke and frequency demands that have to be provided to meet the requiredaccuracy in the ground motion simulation. In addition, according to the results of this study the dominant frequencyof vibration that transfers to the ground and the surrounding environment in case of non-isolated foundation is in therange of 8 to 15 Hertz while in the isolated system a low amplitude dominant frequency exist within the range of 5Hz. Such results vary with the change in the mass and natural frequency of the specimen subsystem. According tothese results vibration isolation for the foundation of shaking table is quite effective in reducing vibrational effectstransferring to the surrounding environment and nearby buildings. On the other hand, using vibration isolationtechnique in design process for foundation of shaking tables may require a major upgrade in the technical specifications for actuators of the table and a need for more sophisticated control system hardware and its add-onalgorithm and software.