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The seismic isolation mechanism of the core-suspended isolation (CSI) system comprises a double layer of inclined rubber bearings installed on top of a reinforced core structure. A multilevel structure is then suspended from a hat truss or umbrella girder constructed on the seismic isolation mechanism. The first building to use the CSI system was the Safety and Security Center in Tokyo, Japan, whose structural health monitoring system has detected and recorded 231 earthquake motions since 2006, including the 2011 Tohoku Pacific Earthquake (2011-TPE). The present study estimates the dynamic characteristics of this CSI-equipped building in earthquakes, and the effects of the CSI system are revealed via the observed earthquake records. The temporal changes of the fundamental period and damping factor are estimated from the 2011-TPE; the fundamental period increases with the deformation of the isolation level, whereas the fundamental damping factor is only related weakly to that deformation. The 231 observed earthquake records reveal that the CSI system performs seismic isolation by reducing the response acceleration of the suspended structure by roughly a half for peak ground accelerations exceeding 30 cm/s2. Daily microtremor observations are used to diagnose earthquake damage; the fundamental frequencies in each direction remain almost constant and were not changed by the 2011-TPE.

Summary After an earthquake, non-negligible residual displacements may affect the serviceability of a base isolated structure, if the isolation system does not possess a good restoring capability. The permanent offset does not affect the performance unless the design is problematic for utilities, also considering possible concerns related to the maintenance of the devices. Starting from experimental and analytical results of previous studies, the restoring capability of Double Concave Friction Pendulum bearings is investigated in this paper. A simplified design suggestion for the estimation of maximum expected residual displacements for currently used friction pendulum systems is then validated. The study is based on controlled-displacement and seismic input experiments, both performed under unidirectional motion. Several shaking table tests have been carried out on a three-dimensional isolated specimen structure. The same sequence of seismic inputs was applied considering three different conditions of sliding surfaces corresponding to low, medium and high friction. The accumulation of residual displacements is also investigated by means of nonlinear dynamic analysis. Copyright © 2017 John Wiley & Sons, Ltd.

SUMMARY In a midstory isolated building, the isolation system is incorporated into the midstory rather than the base of the building. Because of the flexibility of the substructure below the isolation system in a midstory isolated building, the contribution of higher modes to the seismic responses of the midstory isolated structure may not be negligible, especially when the coupling of higher modes exists. To investigate this modal coupling effect, a simplified three-lumped-mass structural model of the midstory isolated building is assumed in this study. Through the equivalent linear analysis and shaking table tests, it is found that the coupling of higher modes may lead to enlarged acceleration responses at the super-floor and superstructure above the isolation layer. Accordingly, a simple method to prevent the midstory isolation design from the coupling of higher modes attributed to the improper design of the substructure and superstructure is proposed in this paper. Copyright © 2012 John Wiley & Sons, Ltd.

SUMMARY This paper examines the rocking response and stability of rigid blocks standing free on an isolated base supported: (a) on linear viscoelastic bearings, (b) on single concave and (c) on double concave spherical sliding bearings. The investigation concludes that seismic isolation is beneficial to improve the stability only of small blocks. This happens because while seismic isolation increase the ‘static’ value of the minimum overturning acceleration, this value remains nearly constant as we move to larger blocks or higher frequency pulses; therefore, seismic isolation removes appreciably from the dynamics of rocking blocks the beneficial property of increasing stability as their size increases or as the excitation pulse period decreases. This remarkable result suggests that free- standing ancient classical columns exhibit superior stability as they are built (standing free on a rigid foundation) rather than if they were seismically isolated even with isolation system with long isolation periods. The study further confirms this finding by examining the seismic response of the columns from the peristyle of two ancient Greek temples when subjected to historic records. Copyright © 2011 John Wiley & Sons, Ltd.

Sliding base-isolation systems used in bridges reduce pier drifts, but at the expense of increased bearing displacements under near-source pulse-type earthquakes. It is common practice to incorporate supplemental passive non-linear dampers into the isolation system to counter increased bearing displacements. Non-linear passive dampers can certainly reduce bearing displacements, but only with increased isolation level forces and pier drifts. The semi-active controllable non-linear dampers, which can vary damping in real time, can reduce bearing displacements without further increase in forces and pier drifts; and hence deserve investigation. In this study performance of such a 'smart' sliding isolation System, used in a 1:20 scaled bridge model, employing semi-active controllable magneto-rheological (MR) dampers is investigated, analytically and experimentally, under several near-fault earthquakes. A non-linear analytical model, which incorporates the non-linearities of sliding bearings and the MR damper, is developed. A Lyapunov control algorithm for control of the MR damper is developed and implemented in shake table tests. Analytical and shake table test results are compared. It is shown that the smart MR damper reduces bearing displacements further than the passive low- and high-damping cases, while maintaining isolation level forces less than the passive high-damping case.

This article summarizes the work done by the authors in seismic isolation over the past six years in Chile. First, a general evaluation of the optimal values of the yield level of the isolation system is performed, focusing on the idea of, but not restricted to, the use of lead–rubber bearings. These optimal values are obtained for two performance objectives: to minimize the base shear in the superstructure and to control the isolator deformation. They were used in the design and construction of two important isolated buildings that are described herein; a short description of the more relevant aspects of the design and implementation of the isolation system in these two buildings is also presented. Furthermore, results from a long testing program conducted on more than 260 full-size elastomeric isolators are summarized and discussed. It is shown that these experimental results enable the elastomeric compounds to be characterized quite accurately by testing reduced-scale specimens with elastomer thickness identical to that used in the full-size isolators. Also, results from isolator constitutive modeling, scragging, and creep in the short term are briefly discussed. Inelastic analyses were performed in the structures in order to evaluate realistic interstory drift and floor acceleration response reduction factors due to the isolation design used. It is shown that in spite of the isolation system, minor inelastic excursions of the primary structure are expected, leading to smaller drift and acceleration reduction factors than those obtained from assuming an elastic response of the superstructure. In any case, seismic isolation is shown to be a very competitive alternative, technically and economically, for building design in Chile. Although it may be difficult to extrapolate this experience to other environments, the results presented herein demonstrate that seismic isolation is a technique that can be effectively used to mitigate seismic hazards in developing countries. Copyright © 2004 John Wiley & Sons, Ltd.

Overturning of a structure causes variations in the normal loads of the isolators supporting that structure. For frictional isolators, such variation leads to changes in the frictional forces developed and, hence, in the strength distribution in plan. For frictional pendulum system (FPS) isolators, it also causes changes in the pendular action, i.e. in the stiffness distribution of the isolation interface. Therefore, although the structure is nominally symmetric it develops lateral–torsional coupling when it is subjected to two horizontal components of ground motion. This coupling is denoted herein as accidental torsion due to overturning, and its effect in the earthquake response of nominally symmetric structures is evaluated. Several parameters are identified to control this coupling, but the most important are the slenderness of the structure and the aspect ratio of the building plan. Results are presented in terms of the torsional amplification of the deformations of the isolation base and the interstorey deformations of the superstructure. The FPS system is modelled accurately by including true large deformations and the potential uplift and impact of the isolators. Impulsive as well as subduction-type ground motions are considered in the analysis, but results show small differences between them. An upper bound for the mean-plus-one standard deviation values of the torsional amplifications for the base due to this accidental torsion is 5%. This implies that for design purposes of the isolation system such increase in deformations could probably be neglected. However, the same amplification for the interstorey deformations may be as large as 50%, depending on the torsional stiffness and slenderness of the superstructure, and should be considered in design. In general, such amplification of deformations decreases for torsionally stiffer structures and smaller height-to-base aspect ratios. Copyright © 2003 John Wiley & Sons, Ltd.

This experimental investigation deals with the earthquake behaviour of a nominally symmetric and a mass-asymmetric three-storey structural model isolated with the frictional pendulum system (FPS). Both accidental and natural torsion are evaluated in the structure by using recorded accelerations in all building floors and measured deformations at the isolation level. A 3D-shaking table was used to subject the model to five different ground motions, including impulsive as well as far-field subduction-zone type earthquakes. Results show that the analytical predictions of the earthquake behaviour of the isolated structure, as obtained from a physical model of the FPS, are in close agreement with the true complex inelastic measured behaviour of the FPS. Besides, experimental results also validate previous observations about the importance of accounting for the variability of the normal loads in modelling the earthquake behaviour of FPS isolators. Measured torsional deformation amplifications at the base of the building vary, in the mean, from 2.5% to 6% for the symmetric and asymmetric structural configurations, respectively. In relation to the fixed base structure, the reduction factors for the base shear of the isolated structure are, in the mean, about 3.9 for both configurations. Finally, it is concluded that the FPS is capable of controlling the lateral-torsional motions of mass-asymmetric structures quite effectively by aligning the centre of mass of the superstructure with the centre of pendular and frictional resistance of the isolation system.

The seismic response of a critical rotating machine either rigidly attached to a floor or independently isolated housed within an initially aseismically designed or uncontrolled structure are investigated. A particular isolation system, the Resilient-Friction Base Isolator (RFBI), is employed. Finite element formulations of a rotor-disk-bearing model on a rigid base are developed. The equations of motion for the combined rotating machine–structure–RFBI systems are presented. Parametric studies are performed to investigate the effects of variations in system physical properties including friction coefficient, mass ratio, shaft flexibility, bearing rigidity, bearing damping and speed of rotation on the response of rotating machines for the combined rotating machine–structure–isolator systems. Comparative studies in the peak response of the rotating machine supported on various isolation systems and the corresponding fixed base system are carried out. The study indicates that the Resilient-Friction Base Isolator can significantly reduce the seismic response of rotating components to potentially damaging ground excitations. Copyright © 2000 John Wiley & Sons, Ltd.

An experimental study of a Coulomb friction damped aseismic base isolation system with fail-safe characteristics is described in this paper. The base isolation system utilized commercially made natural rubber bearings and a skid system which comes into operation at preset-levels of relative horizontal displacement between the structure and the foundation. The fail-safe skid provides hysteretic damping and prevents failure of the isolation system in the event of displacements larger than those assumed in the original design. The isolation system can be designed for an earthquake which can be reasonably expected within the lifetime of the structure; in the event of an earthquake of unanticipated intensity the failsafe system will prevent collapse of the structure. The testing of the system involved an 80,000lb model, approximately 1/3 scale to a real structure mounted on the 20′ × 20′ shaking table at EERC and subject to a variety of earthquake inputs. The results show that the hysteretic effect of the fail-safe system does not greatly increase the accelerations experienced by the structure but considerably reduces the relative displacements at the isolation bearings. The action of the fail-safe system was tested by using an earthquake input that produced a resonant response in the isolated mode of the model. The stability limit of the isolation system was exceeded and the bearings failed but complete failure of the isolation system and thus collapse of the model was prevented by the fail-safe system. The implementation of the system in full scale structures poses no technical or construction problems. An equivalent linearization technique was developed for this system for design purposes using response spectra. The accuracy of this approach was verified by comparison with the experimental results.

A method is presented whereby the response spectrum may be used to predict the response of equipment mounted on an isolation system with non-linear motion limiting constraints. The results of the approximate method are compared with results obtained by direct numerical integration for a representative piece of equipment.