VFPI: an isolation device for aseismic design

Sliding isolators with curved surface are effective base isolation systems incorporating isolation, energy dissipation and restoring mechanism in one unit. However, practical utility of these systems, such as friction pendulum system (FPS) has limitations due to constant isolator period and restoring force characteristics. A new isolator called the variable frequency pendulum isolator (VFPI) that overcomes these limitations while retaining all the advantages has been described in this paper. VFPI has oscillation frequency decreasing with sliding displacement, and the restoring force has an upper bound so that the force transmitted to the structure is limited. The mathematical formulations for the response of a SDOF structure and energy balance are also described. Parametric studies have been carried out to critically examine the behaviour of structures isolated with VFPI, FPS and PF system. From these investigations, it is concluded that the VFPI combines the advantages of both FPS and PF system, without their undesirable properties. The VFPI performance is also found to be stable during low‐intensity excitations, and fail‐safe during high‐intensity excitations. VFPI is found to exhibit robust performance for a wide range of structure, isolator and ground motion characteristics clearly demonstrating its advantages. Copyright © 2000 John Wiley & Sons, Ltd.     

Component and shaking table tests for full‐scale multiple friction pendulum system

The use of base isolation in developed countries including the U.S. and Japan has already been recognized as a very effective method for upgrading the seismic resistance of structures. In this study, an advanced base‐isolation system called the multiple friction pendulum system (MFPS) is investigated to understand its performance on seismic mitigation through full‐scale component and shaking table tests. The component tests of the advanced Teflon composite coated on the sliding surface show that the friction coefficient of the lubricant material is a function of the sliding velocity in the range of 0.03–0.12. The experimental results also indicate that there were no signs of degradation of the sliding interface observed after 2000 cycles of sliding displacements. A full‐scale MFPS isolator under a vertically compressive load of 8830 KN (900 tf) and horizontally cyclic displacements was tested in order to assess the feasibility of the MFPS isolator for its practical use. After 248 cycles of horizontal displacement reversals, the behaviour of the base isolator was almost identical to its behaviour during the first few cycles. The experimental results of the shaking table tests of a full‐scale steel structure isolated with MFPS isolators show that the MFPS device can isolate seismic transmitted energy effectively under soft‐soil‐deposit site earthquakes with long predominant periods as well as strong ground motions with short predominant periods. These test results demonstrate that the MFPS isolator possesses excellent durability and outstanding earthquake‐proof capability. Furthermore, the numerical results show that the mathematical model proposed in this study can well predict the seismic responses of a structure isolated with MFPS isolators. Copyright © 2006 John Wiley & Sons, Ltd.     

Polynomial friction pendulum isolators (PFPIs) for seismic performance control of benchmark highway bridge

The seismic response of a benchmark highway bridge isolated with passive polynomial friction pendulum isolators (PFPIs) is investigated and subjected to six bidirectional ground motion records. The benchmark study is based on a lumped mass finite-element model of the 91/5 highway overcrossing located in Southern California. The PFPI system possesses two important parameters; one is horizontal flexibility and the other is energy absorbing capacity through friction. The evaluation criteria of the benchmark bridge are analyzed considering two parameters, time period of the isolator and coefficient of friction of the isolation surface. The results of the numerical study are compared with those obtained from the traditional friction pendulum system (FPS). Dual design performance of the PFPI system suppressed the displacement and acceleration response of the benchmark highway bridge. The dual design hysteresis loop of the PFPI system is the main advantage over the linear hysteresis loop of the FPS. The numerical result indicates that the seismic performance of the PFPI system is better than that of the traditional FPS isolated system. Further, it is observed that variations of the isolation time period and coefficient of friction of the FPS and PFPI systems have a significant effect on the peak responses of the benchmark highway bridge.     

Adjustable vertical vibration isolator with a variable ellipse curve mechanism

This paper presents a passive vertical quasi‐zero‐stiffness vibration isolator intended for relatively small objects. The present isolator has features of compactness, long stroke, and adjustability to various load capabilities. To realize these features, we use constant‐force springs, which sustain constant load regardless of their elongation, and propose a variable ellipse curve mechanism that is inspired by the principle of ellipsographs. The variable ellipse curve mechanism can convert the restoring force of the horizontally placed constant‐force springs to the vertical restoring force of the vibration isolator. At the same time as converting the direction, the vertical restoring force can be adjusted by changing the ratio of the semi‐minor axis to the semi‐major one of the ellipse. In this study, a prototype of a class of quasi‐zero‐stiffness vibration isolator with the proposed variable ellipse curve mechanism is created. Shaking table tests are performed to demonstrate the efficacy of the present mechanism, where the prototype is subjected to various sinusoidal and earthquake ground motions. It is demonstrated through the shaking table tests that the prototype can reduce the response acceleration within the same specified tolerance even when the mass of the vibration isolated object is changed. Copyright © 2017 John Wiley & Sons, Ltd.     

Development of a tunable friction pendulum system for semi‐active control of building structures under earthquake ground motions

The Friction Pendulum System (FPS) isolator is commonly used as a base isolation system in buildings. In this paper, a new tunable FPS (TFPS) isolator is proposed and developed to act as a semi‐active control system by combining the traditional FPS and semi‐active control concept. Theoretical analysis and physical tests were carried out to investigate the behavior of the proposed TFPS isolator. The experimental and theoretical results were in good agreement, both suggesting that the friction force of the TFPS isolator can be tuned to achieve seismic isolation of the structure. A series of numerical simulations of a base‐isolated structure equipped with the proposed TFPS isolator and subjected to earthquake ground motions were also conducted. In the analyses, the linear quadratic regulator (LQR) method was adopted to control the friction force of the proposed TFPS, and the applicability and effectiveness of the TFPS in controlling the structure's seismic responses were investigated. The simulation results showed that the TFPS can reduce the displacement of the isolation layer without significantly increasing the floor acceleration and inter‐story displacement of the superstructure, confirming that the TFPS can effectively control a base‐isolated structure under earthquake ground motions.     

An isolation system for limited seismic gaps in near‐fault zones

The ability of a recently proposed seismic isolation system, with inherent self‐stopping mechanism, to mitigate or even eliminate seismic pounding of adjacent structures is investigated under severe near‐fault earthquakes. The isolation system is referred to as roll‐in‐cage (RNC) isolator. It is a rolling‐based isolator that provides in one unit the necessary functions of vertical rigid support, horizontal flexibility with enhanced stability, hysteretic energy dissipation, and resistance to minor vibration loads. In addition, the RNC isolator is distinguished by a self‐stopping (buffer) mechanism to limit the bearing displacement under excitations stronger than a design earthquake or at limited seismic gaps, and a linear gravity‐based self‐recentering mechanism to prevent permanent bearing displacement without causing vertical fluctuation of the isolated structure. A previously developed multifeature SAP2000 model of the RNC isolator is improved in this paper to account for the inherent buffer mechanism's damping. Then, the effectiveness of the isolator's buffer mechanism in limiting peak bearing displacements is studied together with its possibly arising negative influence on the isolation efficiency. After that, the study investigates how to alleviate or even eliminate those possibly arising drawbacks, due to the developed RNC isolator's inner pounding as a result of its buffer activation, to achieve efficient seismic isolation with no direct structure‐to‐structure pounding, considering limited seismic gaps with adjacent structures and near‐fault earthquakes. The results show that the RNC isolator could be an efficient solution for aseismic design in near‐fault zones considering limited seismic gaps. Earthquake Engineering and Structural Dynamics. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of horizontal stiffness of fibre‐reinforced elastomeric isolators

Unbonded fibre‐reinforced elastomeric isolator (U‐FREI) is relatively new seismic base isolator in which fibre layers are used as reinforcement to replace steel shims as are normally used in conventional isolators. Further, the top and bottom end steel connector plates of conventional isolators are also removed. In general, the horizontal response of U‐FREI is nonlinear because of reduction in contact area due to rollover deformation and reduction in shear modulus of isolator under large deformation. Thus, evaluation of horizontal stiffness of U‐FREI is a challenging problem. Most previous studies were focused on the investigation of horizontal response of scaled models of U‐FREIs with low shape factors. A few analytical approaches were suggested for predicting the horizontal response of U‐FREI; but their results were not in good agreement with experimental observations. In the present study, the horizontal responses of prototype U‐FREIs are evaluated under a constant vertical pressure and cyclic loading using both experiments and finite element analysis. Prototype U‐FREIs with different shear moduli and with different shape factors are considered. Finite element simulations of corresponding bonded FREIs are also performed under the same loadings as in U‐FREIs. A rational analytical approach including the influence of rollover deformation and simultaneous reduction in shear modulus is proposed as a basic analytical tool for predicting the horizontal stiffness of FREIs (both bonded and unbonded). It is in reasonably good agreement with the results obtained from experiments and numerical analysis. Copyright © 2017 John Wiley & Sons, Ltd.     

Seismic response control of smart sliding isolated buildings using variable stiffness systems: an experimental and numerical study

Effectiveness of a new semiactive independently variable stiffness (SAIVS) device in reducing seismic response of sliding base isolated buildings is evaluated analytically and experimentally. Through analytical and experimental study of force—displacement behaviour of the SAIVS device, it is shown that the device can vary stiffness continuously and smoothly between minimum and maximum stiffness. Passive sliding base isolation systems reduce interstorey drifts and superstructure accelerations, but with increased base displacements, which is undesirable, under large velocity near fault pulse type earthquakes. It is a common practice to incorporate non‐linear passive dampers into the isolation system to reduce bearing displacements. Incorporation of passive dampers, however, may result in increased superstructure accelerations and drifts; while, properly designed passive dampers can be beneficial. A viable alternative is to use semiactive variable stiffness systems, which can vary the period of the sliding base isolated buildings in real time, to simultaneously reduce bearing displacements and superstructure responses further than the passive systems, which deserves investigation. This study investigates the performance of a 1:5 scaled smart sliding base isolated building model equipped with the SAIVS device analytically and experimentally, under near fault earthquakes, by developing a new moving average non‐linear tangential stiffness control algorithm for control of the SAIVS device. The SAIVS device reduces bearing displacements further than the passive cases, while maintaining isolation level forces and superstructure responses at the same level as the passive minimum stiffness case, indicating the significant potential of the SAIVS system. Copyright © 2005 John Wiley & Sons, Ltd.     

Structural and nonstructural performance of a seismically isolated building using stable unbonded fiber‐reinforced elastomeric isolators

Stable unbonded fiber‐reinforced elastomeric isolators (SU‐FREIs) exhibit a characteristic horizontal softening and stiffening response, similar to other adaptive devices such as the triple friction pendulum and sliding systems with variable curvature. The transition between the softening and stiffening occurs at a displacement corresponding to a unique deformation known as full rollover. In this paper, the full rollover displacement of SU‐FREIs is altered by using modified support geometry (MSG), a geometric modification of the upper and lower supports applied to tailor the hysteresis loops of the isolator. Experimental results are used to calibrate a numerical model of a base‐isolated structure. The model demonstrates that the stiffening regime provides minimal restraint against displacements during events that meet or exceed the maximum considered earthquake. A parametric study revealed that the level of stiffening required to restrain displacements during large events is significant. This increase in stiffness is reflected in an increase in the response of the structure and light nonstructural components. Full rollover and MSG is considered advantageous to maintain horizontal stability and provide control over the stiffening of SU‐FREIs. Copyright © 2015 John Wiley & Sons, Ltd.     

Uplift‐restraining Friction Pendulum seismic isolation system

This paper extends the scope of seismic isolation by introducing an innovative uplift‐restraining Friction Pendulum system. Termed the XY‐FP isolator, the new isolation device consists of two orthogonal opposing concave beams interconnected through a sliding mechanism that permits tension to develop in the bearing, thereby preventing uplift. Owing to its distinct configuration, the XY‐FP isolator possesses unique properties for a seismic isolator, including uplift restraint, decoupling of the bi‐directional motion along two orthogonal directions, and capability of providing independent stiffness and energy dissipation along the principal horizontal directions of the bearing. The study concentrates on introducing the concept and establishing the underlying principles of operation of the new XY‐FP isolator, formulating the mathematical model for the XY‐FP isolator, and presenting its mechanical behaviour through a displacement‐control testing program on a single XY‐FP isolator. Copyright © 2005 John Wiley & Sons, Ltd.     

Seismic fragility of structures isolated by single concave sliding devices for different soil conditions

This study deals with the seismic fragility of elastic structural systems equipped with single concave sliding (friction pendulum system (FPS)) isolators considering different soil conditions. The behavior of these systems is analyzed by employing a two-degree-of-freedom model, whereas the FPS response is described by means of a velocity-dependent model. The uncertainty in the seismic inputs is taken into account by considering artificial seismic excitations modelled as timemodulated filtered Gaussian white noise random processes of different intensity within the power spectral density method. In particular, the filter parameters, which control the frequency content of the random excitations, are calibrated to describe stiff, medium and soft soil conditions. The sliding friction coefficient at large velocity is also considered as a random variable modelled through a uniform probability density function. Incremental dynamic analyses are developed in order to evaluate the probabilities of exceeding different limit states related to both the reinforced concrete (RC) superstructure and isolation level, defining the seismic fragility curves within an extensive parametric study carried out for different structural system properties and soil conditions. The abovementioned seismic fragility curves are useful to evaluate the seismic reliability of base-isolated elastic systems equipped with FPS and located in any site for any soil condition.     

Physical model for dynamic analysis of structures with FPS isolators

This article presents a physical model for frictional pendulum isolators (FPS) that is ready to be implemented in most commercial software. The model is capable of accounting for effects such as large deformations, sticking, and uplift and impact by sensing the normal loads in the isolators through a gap element. Sticking has been incorporated into the model by extending the Park–Wen hysteretic model to the case of large deformations. The proposed model has been tested against a theoretically ‘exact’ formulation leading to essentially identical results. To facilitate its use, the physical FPS model has been cast into a typical non‐linear structural element format, i.e. with deformation as input and restoring force as output. Examples of a building and a bridge have been chosen to show the potential of the element and to provide further insight into the earthquake response of structures with FPS isolators; in particular, in aspects such as the orientation in placement of the isolator, sticking, P? Δ, and other large deformation effects. Copyright © 2003 John Wiley & Sons, Ltd.     

Evaluation of simplified methods of analysis for structures with triple friction pendulum isolators

Triple friction pendulum isolators, that exhibit behavior with amplitude‐dependent strength and instantaneous stiffness, represent a new development in seismic isolation. The application of simplified methods of analysis for this type of seismically isolated structures requires development of tools of simplified analysis and demonstration of their accuracy. This paper describes these tools and presents validation studies based on a large number of nonlinear response history analysis results. It is shown that simplified methods of analysis systematically provide good and often conservative estimates of isolator displacement demands and good estimates of isolator peak velocities. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance of a guideway seismic isolator with magnetic springs for precision machinery

This paper proposes the use of the nonlinear restoring force in an isolation system to improve the performance of a seismic isolator. Nonlinear magnetic springs applied to guideway sliding isolators (GSI) that protect precision machinery against seismic motion were studied. The magnetic springs use a non‐contact magnetic repulsion force to achieve a nonlinear property. A numerical simulation model of the GSI system using step‐by‐step integration in the time domain was developed. A full‐scale shaking table test was performed to verify the accuracy of the numerical model. Simulation and experimental results show that the GSI system with magnetic springs has good performance when subjected to floor vibrations during earthquakes. A parametric analysis of the magnetic springs in the GSI system under seismic motion was theoretically investigated. It was found that sufficient magnetic forces can diminish the system relative displacements. Copyright © 2008 John Wiley & Sons, Ltd.     

Optimized friction pendulum and precast-prestressed pile to base-isolate a Chilean masonry house

In this study friction pendulum system (FPS) bearings and precast-prestressed pile (PPP) isolators are considered as base isolation devices for a Chilean confined masonry house. The house is numerically modeled using a multiple degree-of-freedom approach that is calibrated with experimental data. Dynamic behavior of the FPS and PPP isolators is simulated using analytical formulations based on laboratory testing. Optimization of the isolators is performed using an earthquake that is generated to match the design spectrum for the house based on Chilean seismic code. A non-dominated sorting genetic algorithm (NSGA-II) is applied to carry out the optimization. Seismic response of the base-isolated structure subjected to a suite of ground motions is compared to the performance of the traditionally-constructed structure by means of several performance indices (PIs). Numerical simulations indicate that the PPP isolation system is more effective in reducing the base and structural shear, interstory drift, and floor acceleration of the structure than the FPS isolation system, although both systems result in substantial reductions of the response.     

Accidental torsion due to overturning in nominally symmetric structures isolated with the FPS

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.     

Experimental and analytical studies of structures seismically isolated with an uplift‐restraining friction pendulum system

This paper presents findings from a comprehensive analytical and experimental study on the uplift‐restraining XY‐FP sliding isolation system. To investigate the effectiveness of the XY‐FP isolator and provide a rational basis for evaluating the efficacy of the developed mathematical model, an extensive experimental program was conducted on the earthquake simulator at the University at Buffalo. The experimental program involved a slender, five‐storey, scale‐model frame seismically isolated with four XY‐FP isolators subjected to simulations of historical horizontal and vertical ground motions. The experimental response demonstrates the validity of the concept and provides evidence for the effectiveness of the XY‐FP isolator in preventing uplift. A comprehensive analytical model capable of emulating the mechanical behaviour of the XY‐FP isolator is developed and implemented in program 3D‐BASIS‐ME. The newly enhanced program is used to predict the dynamic response of the seismically‐isolated model structure. Comparison of analytical predictions with experimental results attests to the efficacy of the analytical model for simulating the response of the XY‐FP isolator. With its appealing conceptual simplicity and its proven effectiveness, the new uplift‐restraining isolator has the potential to facilitate the application of seismic isolation even under the most extreme of conditions, including but not limited to near‐fault strong ground motions and uplift‐prone structural systems. Copyright © 2005 John Wiley & Sons, Ltd.     

Smart base isolated buildings with variable friction systems: H∞ controller and SAIVF device

A new control algorithm is developed for reducing the response of smart base isolated buildings with variable friction semiactive control systems in near‐fault earthquakes. The central idea of the control algorithm is to design a H_∞ controller for the structural system and use this controller to determine the optimum control force in the semiactive device. The H_∞ controller is designed using appropriate input and output weighting filters that have been developed for optimal performance in reducing near‐fault earthquake responses. A novel semiactive variable friction device is also developed and with the H_∞ controller shown to be effective in achieving response reductions in smart base isolated buildings in near‐fault earthquakes. The new variable friction device developed consists of four friction elements and four restoring spring elements arranged in a rhombus configuration with each arm consisting of a friction–stiffness pair. The level of friction force can be adjusted by varying the angle of the arms of the device leading to smooth variation of friction force in the device. Experimental results are presented to verify the proposed analytical model of the device. The H_∞ algorithm is implemented analytically on a five storey smart base isolated building with linear elastomeric isolation bearings and variable friction system located at the isolation level. The H_∞ controller along with the weighting filters leads to the smooth variation of friction force, thus eliminating the disadvantages associated with rapid switching. Several recent near‐fault earthquakes are considered in this study. The robustness of the H_∞ controller is shown by considering a stiffness uncertainty of ±10%. Copyright © 2006 John Wiley & Sons, Ltd.     

Seismic performance of cable-sliding friction bearing system for isolated bridges

During past strong earthquakes, highway bridges have sustained severe damage or even collapse due to excessive displacements and/or very large lateral forces. For commonly used isolation bearings with a pure friction sliding surface, seismic forces may be reduced but displacements are often unconstrained. In this paper, an alternative seismic bearing system, called the cable-sliding friction bearing system, is developed by integrating seismic isolation devices with displacement restrainers consisting of cables attached to the upper and lower plates of the bearing. Restoring forces are provided to limit the displacements of the sliding component. Design parameters including the length and stiffness of the cables, friction coefficient, strength of the shear bolt in a fixed-type bearing, and movements under earthquake excitations are discussed. Laboratory testing of a prototype bearing subjected to vertical loads and quasi-static cyclic lateral loads, and corresponding numerical finite element simulation analysis, were carried out. It is shown that the numerical simulation shows good agreement with the experimental force-displacement hysteretic response, indicating the viability of the new bearing system. In addition, practical application of this bearing system to a multi-span bridge in China and its design advantages are discussed.