جداسازی لرزه ای نیمه فعال در زلزله های نزدیک گسل
Abstract
1. Introduction
2. Structural model
3. Synthetic earthquake pulse model
4. Application of Monte-Carlo simulation
5. Random variables and probability distribution functions
6. Results
7. Conclusions
Acknowledgements
References
Abstract
Structural control can be used for protecting buildings and its vibration-sensitive contents from earthquakes. Seismic isolation is a passive control system that lowers effective earthquake forces by utilizing flexible bearings. However, supplemental damping in the isolation system may become necessary to reduce large isolator displacements under near-fault earthquakes. Semi-active dampers are preferred over passive dampers because of their capacity in minimizing possible amplifications in floor accelerations due to increased damping. Semi-active dampers are also preferred over the active ones because of their higher stability and lower power consumption. On the other hand, seismic performance of semi-active isolation may vary due to variations in the mechanical properties of semi-active devices and/or seismic isolators. Such uncertainties alongside the uncertainties associated with ground motion parameters should be taken into consideration to develop a realistic picture of the behavior of seismically isolated buildings equipped with semi-active control devices. The objective of this study is to examine the effectiveness of semi-active isolation in protecting vibration-sensitive equipment and integrity of a structure by considering the aforementioned uncertainties and present the reliability of semi-active seismic isolation under near-fault earthquakes. For this purpose, this paper introduces a method that uses synthetically generated near-fault earthquakes and Monte-Carlo Simulations. This method is used to determine the reliability of a 3-story and a 9-story benchmark buildings with semi-active isolation systems under near-fault earthquakes of various magnitudes and varying fault distances. The results are presented in the forms of comparative plots of probability of failure and reliability.
Introduction
Structural control systems can be used in modern engineering practice to protect structures from destructive effects of earthquakes. Structural control systems are mainly categorized as passive, active, semi-active, and hybrid systems. The basic concept of passive control systems is to lower the effective earthquake forces by elongating the fundamental period of the structure, thereby, lowering both the floor accelerations and the inter-story drift ratios to keep them within desired limits. The main challenge that such systems face is the large displacement requirements of the isolation system in case of nearfault earthquakes [1], which may exceed practical and economical limits [2]. Such large base displacements may even exceed the seismic gap, thus, posing serious risks [3]. Providing high passive isolation damping may be of help but such a solution may cause increase in floor accelerations and inter-story drifts depending on the earthquake characteristics [4]. Various other studies [1,5–7] also revealed that additional damping that is necessary for controlling base displacement may not guarantee good performance in structural response under the near-fault earthquakes, which would be a problem for missioncritical buildings such as hospitals that house vibration-sensitive contents [2,8]. Thus, active or semi-active isolation systems that can adapt to earthquake excitations of different frequency contents are necessary. Although active control systems are effective, they require very high power to operate [9,10]. Moreover, adding mechanical energy actively to the building may cause stability problems to the structure [11–13]. These drawbacks increase the interest in semi-active control systems, which can provide the proper amount of damping without causing any stability problems. Also, they need much less power to operate than the active control systems [12,13]. Symans et al. [14] showed that both isolation displacement and superstructure response can be limited by such adaptive base isolation systems. Likewise, [15–17] confirmed that the safety performance of a seismic isolation system that is equipped with semi-active dampers are quite high and it is effective in simultaneously limiting both base displacement and superstructure responses.