論文の公開元へBase isolation system with negative stiffness for displacement mitigation under long-period and near-fault ground motions
概要
Base isolation technique is one of the most widely accepted design philosophy for earthquake resistant design of both the large-scale structures and small-scale structures such as sensitive instruments, cultural assets, furniture, and art monuments. Base isolation detaches the structures/objects to impede the seismic wave propagation from the ground or floor on which they are supported. However, the long-period ground motion anticipated from the near-fault ground motion greatly influence the conventional base isolation systems. Such long-period ground motion has the intensity to resonant the base-isolated object with a long fundamental natural period, generating the large lateral displacement which might cause catastrophic damage to the isolated object due to damage in the components of the base isolation system. Conventional base isolation systems are effective mainly for earthquakes with the high-frequency components. Only a few studies of the earthquakes with the long-period components can be found but even in these earthquakes, the frequency range of long-period component is limited to 2-3 s as compared to earthquakes anticipated in Japan with fundamental period 5-10 s. Mitigating the displacement due to both the near-fault and the long-period ground motions is important from a seismic resistant design point of view. Very few studies have done for the displacement mitigation of the isolated objects subjected to both types of earthquake excitations. Those studies show the presence of the residual displacement due to use of friction device to control displacement. There have been no reported studies on the development of an elastic linear system for displacement mitigation, which is necessary for seismic-resistant design due to both types of ground motions. In these recent years, use of negative stiffness and variable stiffness for base isolation have been increased rapidly. Despite, till date, the performance of negative stiffness with variable stiffness is unknown. This study mainly focuses on displacement mitigation of base-isolated objects or the target objects such as sensitive instruments, critical computer servers, and furniture. Thus, the objectives of this study have been: (1) to study the performance of isolation systems with variable negative stiffness, (2) to develop new device comprising a unit of negative and positive springs (NP unit) arranged in series for displacement mitigation of conventional base isolation system for both near-fault and long-period ground motions simultaneously, and (3) to verify the performance of the proposed model when subjected to observed and simulated earthquake.
In this study first, a comparative study is performed to determine the performance of base isolation systems with variable negative stiffness. For this negative stiffness is varied parametrically using different functions in terms of the displacement response of the isolated objects. To study the performance of base-isolated objects subjected to near-fault and long-period ground motions, equations of motions are constructed. The numerical studies show that varying the stiffness with high-order power, elliptical and exponential functions, effectively mitigate the displacement response. However, with an increase in the order of the power function acceleration response subjected to near-fault ground motion increase appreciably. An optimal order of power function satisfying the allowable limits of both the displacement and acceleration responses in practical use is proposed.
From a practical viewpoint, varying negative stiffness is complicated in comparison with varying positive stiffness because negative stiffness itself is unstable so, stability should also be taken into account. Therefore, a system with varying positive stiffness satisfying the performance of varying negative stiffness with the optimal order of power function is proposed which is also useful for retrofitting of the existing conventional base isolation system.
An external device using NP unit is proposed to fulfill the second objective. This device consists of NP unit arranged in parallel with positive spring and a damper. The positive stiffness of the NP unit is varied linearly in term of displacement response of the base isolated object. The performance of the proposed model is investigated by comparing it with the conventional base isolation model.
Time history analysis is performed by numerical integration using Newmark’s method to verify the performance of the proposed systems. Kobe NS 1995, Ojiya EW 2004, Shin-Tokai EW, and Tomakomai EW 2003 earthquakes are used for applying ground acceleration üg to the systems. Through the numerical analysis, this study verifies the improvement in the performance of base isolation system by using negative-positive springs unit with variable stiffness. The results show that the proposed device markedly reduces the lateral displacement response of the system against both near-fault and longperiod ground motions. The proposed model, exhibits a significant decrease in relative displacement of the object with respect to the base for both types of earthquake excitations. An optimal range of damping values and slope, satisfying the allowable limits of both the displacement and acceleration responses when subjected to near-fault and long-period for practical use is proposed.
For realization of the system, first the variable positive spring of NP unit is to be realized practically. Hence, in this study, approximation of negative-positive stiffness in practical has been discussed using discontinuous stiffness-displacement (K-D) relationship. From the numerical analysis, this study verifies that both the acceleration and displacement responses can be limit to allowable ranges using discontinuous K-D relationship.
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