Base-isolated Structures Research Articles

Enhancing Friction Pendulum Isolation Systems Using Passive and Semi-Active Dampers

Friction pendulum systems (FPSs) are a common solution for isolating civil engineering structures under ground movements. The result is a base-isolated structure in which the base exhibits low shear stiffness in such a way that the input energy of the earthquake is concentrated and dissipated into it, leaving the superstructure free of damage. As a consequence, large displacements of the FPS may be demanded depending on the earthquake intensity and the fundamental period of the FPS. To accommodate these displacements, large-size isolators with high friction coefficients are usually required. However, the FPS will then exhibit poor re-centering capacity and the risk of future shocks will increase due to previous residual displacements, especially for low-intensity earthquakes. An alternative solution is to include a semi-active damper to the FPS, keeping the friction coefficient low and achieving both, limited base displacement under high-intensity earthquakes and good re-centering capacity under low-intensity ones. Thus, this work presents a design methodology for base isolators formed by an FPS with a damper added. The design methodology is applied to an FPS with a passive damper and to an FPS with a semi-active damper. Two ON-OFF control strategies are studied: (i) a fairly simple phase control, and (ii), a mechanical energy-predictive based algorithm. The advantages of semi-active FPSs with low friction coefficients with respect to FPS with high friction coefficients are demonstrated. The results with the designed semi-active FPS are compared with the single FPS and the FPS with a passive damper. Finally, the use of semi-active FPS allows us to enhance the FPS performance as the isolator size can be reduced while keeping the capacity to cope with low and high-intensity earthquakes without residual displacements.

Reliability-based design optimization of adaptive sliding base isolation system for improving seismic performance of structures

The reliability-based design optimization (RBDO) of base-isolated structures by means of the state-of-the-art methods has been confronted with the high computational cost because the base isolation system often exhibits intrinsic nonlinearities and has to be modelled using a number of parameters in practice. In this regard, the present study aims at developing an efficient scheme for the RBDO of base isolation systems by integrating the probability density evolution method (for global reliability solution) and the variance-based sensitivity analysis (for design parameter reduction).To attain an adaptive seismic mitigation, the newly developed sliding implant-magnetic bearings are employed for constituting the base isolation system. It is shown that although the deployment of sliding implant-magnetic bearings allows a safer structure, the base isolation system still suffers from an unexpected failure probability of deflection under the rarely-occurring earthquake. As a result, the design optimization for strengthening the reliability of the base isolation system needs to be carried out. For identification of the critical design parameters to be optimized, the variance-based sensitivity analysis is performed, showing that the parameters of the exponential function that represents the Coulomb sliding friction associated with the sliding implant-magnetic bearing are mostly of concern. Furthermore, the RBDO of the base isolation system is carried out by employing the genetic algorithm. It is revealed that the global reliability of the base-isolated structure after optimization gains a significant improvement compared to that before optimization.

Research on Torsional Effect of Friction Pendulum Isolation Structure Based on Nonlinear Time History Analysis

A multi-layer irregular reinforced concrete frame structure in the 8-degree area is taken as the research object, and the working principle and hysteretic model of the friction pendulum bearing (FPB) are explained in detail. With the help of SAP2000 finite element software, a friction pendulum isolation model and its corresponding non-isolation model were established. Plastic hinge constitutive models were defined at both ends of the beams and columns of the RC frame structure, and 100 sets of two-way elastoplastic time history analysis of ground motion are carried out by using dynamic time history method. The results show that: under rare earthquakes, the seismic response of interlayer displacement, interlayer shear, interlayer torque and interlayer torsion angle of the structure after friction pendulum isolation is significantly reduced. Among them, the maximum torsional angle of the base-isolated structure is reduced by nearly 60% compared to the maximum inter-layer torsional angle of the non-isolated structure, and the torque of the first-layer base-isolated structure is reduced by nearly 74.1% compared to the torque of the non-isolated structure, which can prevent serious torsional damage under rare earthquakes and ensure the safety of the structure.

Research on the Influence of Near-Fault Round Motion Pulse Characteristics on Seismic Response of Base-Isolated Structures

The time-frequency distribution of near-fault pulsed ground motion is different from that of ordinary ground motion because of its long period, short duration and high energy acceleration pulse effect, which has a significant influence on the structure with long natural vibration period, such as isolated structure. In this paper, the pulse characteristics of near-fault seismic records and their influence on the seismic response of isolated layer and superstructure are studied by selecting near-fault pulsed seismic records and far-field common seismic records as external excitation. The results show that compared with the far-field ordinary seismic waves, the 3D Hilbert amplitude spectrum of near-fault pulsed seismic waves is distributed in the low frequency region and the middle and late time domain, and the amplitude is also larger. The first natural vibration frequency of the isolated structure is located at the peak of the power spectrum of the near fault pulse wave, which makes the seismic response of the isolated structure in the near fault pulse type seismic record obviously larger than that in the far field common seismic record. The intensity indexes Sa(T1) and Sd(T1) of near-fault pulsed ground motion are greater than those of ordinary ground motion. The relationship between them and structural seismic response should be studied in depth.

Real-Time Hybrid Simulation Analysis of Moat Impacts in a Base-Isolated Structure

Base isolation is a well-known technique used to reduce accelerations and inertial forces in structures during earthquakes. However, excessive displacements of the structure due to flexibility of the isolation layer bearings may contribute to moat wall impact events. These impact events have the potential to cause substantial structural damage. This impact behavior and the effects on structural dynamics have been shown to be highly complex and difficult to model by means of pure numerical simulation. In this paper, the cyber-physical technique called Real-Time Hybrid Simulation (RTHS) is employed to capture the uncertainties in force profiles of moat wall impacts and to analyze the complex interactions between impacts and the dynamics of base-isolated structures during earthquake excitations. It is shown that RTHS is capable of accurately capturing the interactions between the isolation layer and moat wall during impact events induced by ground motions. In addition, the RTHS technique is used to analyze the role played by moat wall material nonlinearities in reducing the inertial demand on the structure during impact events. Finally, possible extensions of the research to larger scales as well as consideration of additional moat wall variants are proposed.

Horizontal-Vertical-Rocking Coupled Response Analysis of Vertical Seismic Isolated Structure under Near-Fault Earthquakes

For vertical isolated structures with excessive vertical eccentricity for mass and vertical stiffness, horizontal-vertical-rocking response needs to be better understood for vertical isolated structures located in near-fault areas, where long-period velocity pulse can be produced. In this study, a seismic isolation system including quasizero stiffness (QZS) and vertical damper (VD) is used to control near-fault (NF) vertical earthquakes. The responses of horizontal-vertical-rocking coupling base-isolated structure including quasizero stiffness (QZS) and vertical damper (VD) subjected to NF horizontal and vertical ground motions are investigated. Nonlinear dynamic analyses are conducted to study the effects of essential parameters such as isolation system eccentricity, static equilibrium position, vertical isolation period, and vertical damping ratio on seismic responses of vertical isolated structure. It is found that increasing vertical period and damping ratio causes the vertical isolated structures to behave well in reducing rocking responses of structure. The effect of horizontal-vertical-rocking coupling on vertical seismic isolation efficiency is insignificant. The vertical seismic isolation remains effective as compared to the system supported on rubber bearings. The vertical damping can significantly control the vertical displacement and rocking moment.

Seismic resilience evaluation of base-isolated RC buildings using a loss-recovery approach

Seismic isolation systems, especially friction pendulum bearings (FPBs), are used as ideal seismic protection strategies in performance-based design. Furthermore, FPBs can increase the functionality of buildings by minimizing interruption of use of the facility (e.g., operational performance level) and reducing seismic demands on the structural and non-structural elements. The primary objective of this paper is to investigate the seismic resilience of reinforced concrete (RC) buildings, especially essential buildings in compliance with a loss-recovery approach. To this end, an analytical examination is conducted concerning a six-story RC building, which was mainly designed under the Uniform Building Code (UBC in International Conference of Building Officials, Whittier, CA, 1997) for a high-risk seismic zone. The incremental dynamic analyses of the fixed-base and base-isolated structures subjected to a set of near and far-fault ground motions are carried out for getting seismic fragility and functionality curves as well as the resilience of structures using a loss-recovery approach. The proposed methodology is employed to special buildings such as hospital buildings, with and without using the FPBs considering different seismic hazard scenarios that could occur within the investigated region. At the end of the paper, a parametric study is conducted by changing the building and pendulum parameters. The benefit of the suggested approach is found in terms of loss-recovery and seismic resilience that will be well recognized by the decision-maker for better planning to decrease seismic losses, rather than waiting for an earthquake and paying for it afterward.

Effective live load mass for storage buildings on friction-pendulum isolators

This paper presents the results from finite element models of based-isolated storage buildings subjected to ground motion excitation and supporting rigid blocks with the possibility to slide. Main components of the models were first compared and calibrated with the results from numerical solutions, finite element software, and shake table tests. The successfully verified finite element models were then used to assess ASCE 7–16 design provisions for the treatment of storage loads as seismic weight in base-isolated structures. The analyses included multi-story shear buildings of different heights and three-dimensional buildings of three-stories with both regular and irregular plans under combined horizontal and vertical ground excitation. Alternative low and high friction between the rigid blocks (representing the storage live load) and the floor deck were evaluated in the analyses as well as the effect of light versus heavy storage live loads. Ground excitation consisted of 100 records, covering scenarios that range from low to high seismic risk. The combination of cases included in the parametric study led to thousands of nonlinear time history analyses. Selected engineering design parameters (EDP) to conduct the evaluation consisted of isolator deformation and maximum force demands in the lateral load resting system. Calculated EDPs from the detailed models (accounting for inelastic response of the lateral load resisting system coupled with potential sliding of rigid blocks) were compared with calculated EDPs from simplified models with no blocks but having additional floor mass equal to 25% of the design live load. The latter models represented analysis conditions that ASCE 7–16 minimum provisions would allow in consulting practice. This study demonstrates that using the Standard’s minimum provision can lead to: i) significant underestimations of the deformation demand and thus unconservative designs of base isolators; ii) significant underestimations of design forces, and consequently, improper design of the lateral load resisting elements. In order to address these issues, a simple expression recently developed by the authors to estimate the portion of the design live load as seismic weight was also evaluated. It is shown that using the portion of the live load given by this equation in the simplified models with no blocks but having additional floor mass produce very similar EDPs as those obtained from the detailed models with sliding blocks and thus representing a significant improvement over the existing ASCE 7–16 minimum provisions when applied to base-isolated storage structures.

Experimental Study of Seismic Isolated Structures with Sliding Implant-Magnetic Bearings

ABSTRACT Sliding implant-magnetic bearing (IMB) is a new type of seismic isolators that benefits from its adaptive energy-dissipation behavior. In order to assess the performance of the sliding IMB for seismic protection, a complete shaking table test on the base-isolated structure is reported in the present paper. Tri-directional-recorded earthquake ground motions and mono-directional artificial earthquake ground motions pertaining to both site classes II and III are employed to drive structural vibration in the laboratory experiment, respectively. Seismic mitigation of the base-isolated structure with respect to dynamic characteristics, story accelerations, inter-story drifts, horizontal torsions, and local strains is addressed. Experimental study shows expected results that the base-isolated structure with sliding IMBs attains more profitable mitigations on story accelerations and inter-story drifts under earthquake ground motions pertaining to the stiff soil site than those pertaining to the soft soil site. Significant mitigations on horizontal torsion of the structure and on local strain at the column bottom are attained as well. In addition, seismic performance of sliding IMBs during the shaking table test is assessed. It is revealed that the isolators exhibit satisfactory energy-dissipation capacity and excellent frictional behavior, and maintain good working state during the entire experimental test.

Estimating the maximum response and maximum control force for high-rise base-isolated buildings with active structural control in along-wind direction

The combination of a passive base-isolated (PBI) structure and active structural control (ASC) has been employed in many buildings globally to improve control performance. Conventional approaches have utilized numerical simulations to calculate the maximum response and maximum control force, thereby complicating the design of control systems that satisfy design limitations. To solve this problem, this study extends the gust factor approach to buildings employing a combination of PBI structures and ASC. First, a method to theoretically estimate the mean control force and mean displacement response under along-wind force using only the static equilibrium, without requiring numerical simulations, is presented. Second, estimation methods of the gust factors for the displacement and control force are devised, and numerical verifications are used to validate the estimation methods. Using the estimation method for the mean control force, mean displacement response, and gust factors, the maximum control force and maximum displacement are estimated without using numerical simulations. Finally, a design method for determining the isolated period, isolated damping ratio, and feedback gain that satisfies the restrictions on the maximum control force and maximum displacement response is developed. Subsequently, the design method is validated using a numerical design example.

A Resilience and Robustness Oriented Design of Base-Isolated Structures: The New Camerino University Research Center

This paper analyses the new Research Centre designed for the University of Camerino and entirely financed by the national Civil Protection Department (DPC), following the seismic events in Central Italy in 2016. The building has been designed to guarantee speed of execution as well as a high level of safety, especially regarding seismic actions. The structural solution was to create an isolated system with a steel braced super-structure with pinned joints and r.c. sub-structures able to adapt to the complex morphology of the area. As described in the first part of the paper, design choices have been made to achieve a high level of resilience and robustness, i.e. to limit damage to structural and non-structural components and equipment under moderate and design seismic actions and to avoid disproportionate consequences in the event of extreme actions, larger than the design ones. In the second part of the paper, specific risk analyses have been carried out to evaluate the real performance of the building under increasing intensity levels, with reference to both serviceability and ultimate conditions. To this purpose a site-specific hazard study was first conducted, then nonlinear analyses were performed using a hazard-consistent set of records with return periods ranging from TR=60 years to TR=10000 years. The main demand parameters of both the isolation system and the super-structure were recorded and capacity values corresponding to different ultimate and damage limit conditions were defined. The results obtained in terms of demand hazard curves show that the building performances in terms of rustiness and resilience are very high, confirming the efficacy of the strategies adopted in the design.

Optimal seismic response using a Passive Tuned Mass Damper Inerter (TMDI)

The latest earthquakes history shows that resistant conception, safe and economical structures are a daily challenge for structural engineers. Among the newest vibration control devices figures the inerter which is a device which can develop a large fictive mass (it consists of a mass amplification effect) using rotational inertia. In this research work, the effectiveness of a traditional passive tuned mass damper (TMD) is compared with a tuned mass damper inerter (TMDI) which consists generally of tuned mass damper coupled with an Inerter. Both devices are used to control a base-isolated structure vibration submitted to several seismic records. In This study, a base-isolated structure of six stories (6 DOF) was equipped with (TMD) and (TMDI) and a time history analysis were performed under different earthquake records (El Centro, Kobe, Kocaeli). The mathematical model of the building is established in MATLAB Simulink. The simulation results illustrated he robustness, performance and effectiveness of the structure equipped with a (TMDI). The dynamical parameters of interest were base and top displacement as well as the base shear force and peak inerter force produced.

Experimental Assessment of the Seismic Response of a Base-Isolated Building Through a Hybrid Simulation Technique

Base-isolated structural systems have been more and more investigated through both numerical and experimental campaigns, in order to evaluate the effective advantages, in terms of vulnerability reduction. Thanks to the lateral response of proper isolation devices, large displacement demands can be accommodated, and the overall energy of the seismic event can be dissipated, by means of hysteretic behaviors. Among the common typologies of isolators, Curved Surface Slider devices represent a special technologic solution, with potentially high dissipative capacities, provided by innovative sliding materials. On the other hand, the overall behavior is highly non-linear, and a number of research works have been developed, aiming at the definition of the most comprehensive analytical model of such devices. The most realistic response of a base-isolated structure could be returned by a shake table test of a full-scale buildings. However, dimensions of the available shake tables do not allow to consider the common load conditions, which the isolation devices are subjected to, and consequently scaled specimens are needed, and unrealistic responses could be found. Hybrid simulations seem to solve such an issue, by accounting for an experimental sub-structuring, represented by a physical devices tested in a testing equipment, and a numerical sub-structuring, consisting of a numerical model of the superstructure. Thus, a much more realistic response of the full-scale structure can be computed. In this work the outcomes of a number of hybrid simulations have been deeply analyzed, and compared to similar numerical model, by accounting for proper non-linear constitutive laws for isolation devices, in order to evaluate the effectiveness of design and assessment procedures, commonly adopted in real practice applications.

An improved response surface method for fragility analysis of base-isolated structures considering the correlation of seismic demands on structural components

An improved response surface method (RSM) for the seismic fragility analysis of base-isolated structures is proposed so that the correlation of seismic demands on the structural components can be effectively considered. In addition, the performance levels of base-isolated structure as well as the associated quantitative indices for various structural components are discussed and defined. Taking a typical base-isolated reinforced concrete plane frame with lead-rubber-bearings as an illustrative example, the seismic fragility analysis following the computational procedure based on the improved RSM are conducted. Through the illustrative example, it is demonstrated that the obtained response surface models for the seismic demands on the components as well as the correlation of components are accurate and reliable; moreover, the computational cost for the system seismic fragility analysis is significantly reduced due to the use of Monte Carlo simulation; finally, it is indicated that it is necessary to take into account the correlation of components in order to obtain more accurate system fragility curves of base-isolated structure.

Smart structures through nontraditional design of Tuned Mass Damper Inerter for higher control of base isolated systems

This paper introduces a smart structure design through the definition of an innovative passive control strategy, referred to as New Tuned Mass Damper Inerter (New TMDI), coupled with a base isolation system (BI), to control displacements in base isolated structures under seismic excitations. The herein proposed New TMDI comprises a recently developed nontraditional Tuned Mass Damper (known as New TMD), in which a secondary mass system is connected to the base plate of the BI system by a spring and to the ground by a dashpot, and of an inerter device placed in parallel with the damper.An optimization procedure which minimizes the base displacement variance of the BI system, considering the case of white noise base excitation, is proposed to determine optimal design parameters of the New TMDI, and pertinent approximate closed-form expression are derived. Note that, this device might be considered for applications in historic buildings, generally built in contiguity with the others. Adopting this nontraditional TMDI, it would be possible to get a smart historic building, offering higher control performance over an ordinary design. Further, the performance of a base-isolated New TMDI-controlled multi-degree-of-freedom structure is examined and compared with that of the simple base-isolated one, using a set of 44 recorded ground motions as base excitation. Results show that the New TMDI is rather effective in reducing the response of base-isolated structures under strong earthquakes.

Seismic structural responses reduction of double-layered containment nuclear power plant via BIS-TMD

Purpose In recent years, a new type of double-layered containment structure of nuclear power plant has been built in China, named ACP1000 NNP. This paper aims to propose a new method to mitigate the seismic responses of this type of the structure. Design/methodology/approach The new base isolation structure (BIS)- tuned mass damper (TMD) structure was proposed and implemented only by modifying the connection between various components of NPPs, and the application of this concept design in ACP1000 was skillfully realized. Findings The new structure adopts the combination of structural form and function, adopts appropriate amplification of the seismic response of the secondary component, possesses advantages in explicit damping mechanism, good damping effect, robustness of seismic structures, simple implementation process, etc., and meets the special seismic safety requirements of the NPPs. Originality/value This seismic and hazard mitigation and BIS-TMD structure can avoid the risk of ACP1000 accidents caused by horizontal earthquakes.

Seismic responses and energy dissipation of pure-friction and resilient‐friction base-isolated structures: A parametric study

A comprehensive parametric study is carried out on pure-friction base-isolated (P-FBI) and resilient‐friction base-isolated (R-FBI) structures in this paper. For this purpose, an 8-story building subjected to a band-limited white noise acceleration modeled based on a stationary process is considered. At first, the time histories of seismic response and superstructure energy for P-FBI and R-FBI structures are compared with those given for a fixed-base (F-B) structure. A vast range of values for the friction coefficient of the sliding surface, mass ratio of the base story, target period, and damping ratio of the restoring device are also considered for the parametric study. Then, the effect of changing these parameters on the maximum response of stories and superstructure energy is investigated. Considering a low friction coefficient, it is concluded that the restoring device of the R-FBI structure is able to significantly reduce displacement of the base story during earthquake excitations. It is also capable to return the base story to its original location at the end of an earthquake. Simulation results also show that the restoring device increases the maximum seismic input energy entered into the superstructure but reduces the maximum seismic damage energy of the superstructure. Results demonstrate that the amount of friction coefficient has the greatest effect on seismic responses of the studied structure. Base displacement is reduced with increasing friction coefficient, but top floor acceleration and superstructure drift are significantly increased, which results in an increment in maximum input and damage energies. Overall results indicate that an increment in damping ratio of the restoring device results in a reduction in maximum base displacement and top floor acceleration, and an increment in maximum top floor displacement and drift, while it has no effect on the residual base displacement. Moreover, base displacement and top floor drift are increased by increasing the period of the restoring device, while other seismic responses are reduced.

An innovative continuous Bayesian model updating method for base-isolated RC buildings using vibration monitoring data

Base isolation is the most widespread passive control strategy for seismic protection of low- and medium-rise Reinforced Concrete (RC) buildings. The excellent performance of base-isolated structures just after construction is well recognized but their seismic behavior after a few operation years can be significantly modified, due to isolators’ aging. In this perspective, the availability of long-term vibration data allows to eventually assess the evolution of degradation and plan maintenance or replacement actions. This paper presents an innovative continuous Bayesian model updating method specific for base isolated buildings. In the proposed method, the Bayesian functions of the isolation system are periodically updated to account for daily measured data. The results of a comprehensive study on a base isolated three-stories RC school building, located in Bojano (Italy), are presented. The investigation comprises Ambient Vibration Testing (AVT), modal identification, installation of a continuous monitoring system and calibration of a Finite Element Model (FEM) of the isolated structure. In order to reduce the computational cost of the model updating procedure, a surrogate model is constructed and calibrated using the complete FEM of the structure through the Response Surface Methodology (RSM). The advantage of having long-term monitoring data on the results of the Bayesian model updating for base isolated structures is highlighted.

A novel robust optimum control algorithm and its application to semi-active controlled base-isolated structures

A new Robust Optimum control algorithm that combines a Linear Quadratic Regulator and a nonlinear robust compensator is presented to improve the control of the seismic response of building structures with nonlinear isolation systems. The Linear Quadratic Regulator was used to achieve the optimal performance of a nominal linear model of the controlled structure, and a robust compensator was developed to restrain the effect of nonlinearities. The Robust Optimum control method was proven theoretically, and implemented to control the response of a base-isolated structure equipped with a Tunable Friction Pendulum System isolation under different ground motions. The simulation results validated the stability, robustness, and generalization ability of the proposed control algorithm, and suggested that it is effective in controlling base isolation displacement, inter-story drift, and floor acceleration. The designed robust compensator can successfully compensate for the effect of nonlinearities.

Numerical simulation of base-isolated buildings in collisions with surrounding moat walls during earthquakes: a nonsmooth mechanics approach

In this paper, we present a formulation and a numerical simulation method for earthquake-induced collisions between a seismically base-isolated building structure and its surrounding moat walls. The base isolator is assumed to undergo plastic deformation. We use the conventional shear building model for a superstructure. Based on nonsmooth mechanics, we formulate a measure differential inclusion that describes the time evolution of the entire structure. For numerical computation, we apply Moreau’s time-stepping scheme to the presented formulation. As a consequence, we solve a quadratic programming problem for each time-step. Our preliminary numerical experiments demonstrate that the presented method is more robust and numerically stable compared with the conventional Newmark method. We also present two numerical examples of mid-rise base-isolated building structures.