MECHANICAL ENERGY EVALUATION METHOD FOR SEISMIC ISOLATION SYSTEMS WITH RUBBER BEARINGS UNDER LARGE DEFLECTION

Abstract

 Since seismic isolation systems have high seismic performance, the number of the general commercial construction of base-isolated buildings has been increasing recently. For Nuclear Power Plants (NPPs), on the other hand, even though there have been a lot of researches to apply seismic isolation systems, the base-isolated plant has not been built in Japan. Recent seismic regulation requires to evaluate seismic safety including ultimate behavior for NPPs generally, therefore, ultimate behavior of NPPs adopted the seismic isolation system must be also considered in the seismic residual risks: e.g., the hardening or the breaking of seismic isolators in the horizontal deformation.
 In this research, the energy balance-based seismic response prediction methods and its design method will be treated as the seismic safety evaluation in the large horizontal deflection region. In the paper, especially, a scheme of a mechanical energy balance evaluation is proposed for investigating ultimate behavior in the large deflection region, where one can estimate mechanical energy transition from experimental records of relations restoring force and shear deflection on a isolation-layer of a base-isolated system or on a sole isolator. In the scheme, the restoring force is modeled by dividing a conservative force and a non-conservative force, and the conservative force is modeled as the Duffing-typed model, in order to completely identify the response characteristics of hardening of a seismic isolator.
 To show the effectiveness of the proposed scheme, numerical examples of the energy transition are demonstrated for applying to a series of the E-defense shaking table tests on a large-scaled base-isolated specimen, when some of lead rubber bearings (LRBs) behave in the response of hardening or breaking under large horizontal deflection. To clarify the feature of the Duffing model-based scheme proposed here, the conventional Equivalent linear model-based scheme is also examined, and the both results are compared in the energy transitions of strain and absorption components. Further, changes in the mechanical energy balance before and after breaking of LRBs are discussed in the energy balance.
 The main results in the paper are summarized as follows:
 1) The Duffing model-based scheme proposed here gives the continuous energy transition of the seismic isolation system toward time-axis even when hardening response is appeared in some of LRBs, whereas in such non-linear regions the Equivalent linear model-based scheme gives accurate estimates only at the final of every half loop of the response.
 2) As compared the both energy transitions of the broken and the unbroken LRBs, it is confirmed that the elastic strain energy of the breaking LRBs related with the conservative forces becomes smaller, and the energy absorption due to hysteresis damping of the breaking LRBs related with the non-conservative forces becomes larger, when some of the LRBs are breaking. These results are considered that the elastic strain energy, that can be transmitted to the next loop if the breaking phenomena are not occurred, is changed into the energy absorption, and as the results the deflection in the next loop becomes smaller than the one in the assumption that the LRBs were not broken. Further, we observed the elastic strain energy of the damaged LRBs is decreased clearly after the breaking. The fact indicates that the storage capacity of the elastic strain energy is reduced by the breaking of LRBs.
 3) According to the results mentioned above, it will be expected the scope of energy balance-based seismic response prediction methods and its design method are extended to non-linear region when hardening or breaking is occurred in some of LRBs.