Base-isolated Structures Research Articles

Theoretical and experimental analysis of an electromagnetic seismic isolation system

A near-fault (NF) ground motion with a strong velocity pulse component can induce considerable base displacement and structural acceleration in a base-isolated structure. Conventional viscous dampers are typically used to reduce the excessive responses of a seismic base-isolated system. It is generally recognized that the manufacturing tolerance requirements and costs of a long-stroke damper are higher than those of a typical viscous damper due to the buckling criterion. Thus, to satisfy the large stroke and high damping requirements of isolation systems in NF regions, an electromagnetic seismic isolation system (EMSIS) were developed in this study. The rotational electromagnetic (EM) damper addresses the stroke limitation. The mathematical model and theoretical formulas of the EMSIS were first established. Next, a prototype EMSIS was manufactured and its performance was validated using a shaking table apparatus. The equivalent EM damping ratio of the EMSIS was calibrated using a series of sine sweep tests. The test results indicated that each EM damper could provide a damping force proportional to the relative velocity of its two ends. Thus, the EM damper behaved as a linear viscous damper, which is suitable for seismic isolation systems. Finally, the EMSIS was subjected to both NF and far-field (FF) ground motions. The theoretical results were in agreement with the experimental findings. The shaking table test results revealed that the EM damper can effectively mitigate the displacement and acceleration responses of the developed isolation system under NF ground motions.

An Experimental Study on the Mechanical Properties of a High Damping Rubber Bearing with Low Shape Factor

A high damping rubber bearing (HDRB) is widely utilized in base-isolation structures due to its good energy dissipation capacity and environmentally friendly properties; however, it is incapable of isolating the vertical vibration caused by earthquakes and subways effectively. Thick rubber bearings with a low shape factor have become one of the important vertical isolation forms. This paper provides an experimental comparative study on high damping rubber bearings with low shape factor (HDRB-LSF), thick lead–rubber bearings (TLRB), and lead–rubber bearings (LRB). The abilities of the bearing and energy dissipation of the above bearings are analyzed contrastively considering the influence of vertical pressure, loading frequency, shear strain, and pre-pressure. Firstly, the HDRB-LSF, TLRB, and LRB are designed according to the Chinese Code for seismic design of buildings. Secondly, cyclic vertical compression tests and horizontal shear tests, as well as their correlation tests, are conducted, respectively. The vibrational characteristics and hysteresis feature of these three bearings are critically compared. Thirdly, a corrected calculation of vertical stiffness for the thick rubber bearings is proposed based on the experimental data to provide a more accurate and realistic tool measuring the vertical mechanical properties of rubber bearings. The test results proved that the HDRB-LSF has the most advanced performance of the three bearings. For the fatigue property, the hysteresis curves of the HDRB-LSF along with TLRB are plump both horizontally and vertically, thus providing a good energy dissipation effect. Regarding vertical stiffness, results from different loading cases show that the designed HDRB-LSF possesses a better vertical isolation effect and preferable environmental protection than LRB, a larger bearing capacity, and, similarly, a more environmentally friendly property than TLRB. Hence, it can avoid the unfavorable resonance effect caused by vertical periodic coupling within the structure. All the experimental data find that the proposed corrected equation can calculate the vertical stiffness of bearings with a higher accuracy. This paper presents the results of an analytical, parametric study that aimed to further explore the low shape factor concepts of rubber bearings applied in three-dimensional isolation for building structures.

Stochastic seismic analysis of base-isolated structures with electromagnetic inertial mass dampers considering different soil conditions

Stochastic seismic analysis of base-isolated structures with electromagnetic inertial mass dampers considering different soil conditions

Consideration of Three Seismic Isolation Performances as Design Objectives for Equivalent Linear Analysis of Bilinear Hysteretic Isolation Systems

The design displacement, its corresponding acceleration performance, and the re-centering performance of bilinear hysteretic isolation systems are adopted as previously determined design objectives for equivalent linear analysis. To demonstrate the applicability and generalization of the analysis procedure, two sets of values for damping modification factors are employed in the analysis: those provided by ASCE/SEI 7-16, and those estimated for different ranges of the ratios of effective periods of seismic isolation systems to pulse periods of ground motions. To investigate a broad range of seismic responses of base-isolated structures, 15 pulse-like near-fault ground motions are used for numerical demonstration. The analysis procedure is numerically verified to be practically feasible. A numerical comparison also shows that the three design objectives previously determined in the analysis procedure are sufficiently conservative compared with analysis results from nonlinear dynamic response history, even when subjected to pulse-like near-fault ground motions. Regarding the approximation to maximum inelastic acceleration and displacement responses, it is particularly more conservative for the former when the design displacement is greater and when adopting values of the damping modification factors provided in ASCE/SEI 7-16. For the approximation to dynamic residual displacement responses, the influences of pulse-like near-fault ground motions and different design objectives on the re-centering performance of bilinear hysteretic isolation systems still need further study.

A lateral load distribution for the static analysis of base-isolated building frames under the effect of far-fault and near-fault ground motions

This paper attempts to examine the effect of the scaling of ground motion records on the seismic responses of base-isolated buildings by using the three scaling methods including the ASCE 7–10, acceleration spectrum intensity, SIa, and the spectral acceleration corresponding to the effective period of the base-isolated structure at the design displacement, Sa(TD). The buildings had steel moment-resisting and X-braced systems and were isolated with lead rubber bearings (LRBs) at the base. Near-fault ground motions with Forward Directivity (FD), Fling-Step (FS) and No Pulse (No P.) characteristics as well as far-fault ground motion records were studied in this paper. The discrepancy in the seismic responses as well as the effect of the pulse period in the case of pulse-like ground motions was scrutinized for the three scaling methods. Also, the response spectrum analysis (RSA) and static method with the equivalent lateral force distribution prescribed in ASCE 7–10 were carried out. The results indicate that the RSA as well as the static analysis, prescribed in ASCE 7–10, underestimates the story shears of the base-isolated buildings. The underestimation increases in the case of the FF and No P. ground motions having a large amount of spectral acceleration in the higher modes region. Also, the underestimation increases with the increase in the height of the base-isolated buildings as well as in the case of the moment-resisting frames compared to the X-braced ones. Finally, by means of the results derived from the time history analyses, a formula was proposed to determine the lateral force distribution along the height of the base-isolated buildings with LRBs considering the characteristics of the isolator and the structure for different types of the ground motions.

Investigation of one dimensional multi-layer periodic unit cell for structural base isolation

Conventional base isolators like lead rubber bearing, high damping rubber bearing and friction pendulum bearing are not suitable for structures containing crucial pipelines and other sensitive structural components, since these structures with conventional isolators are usually subjected to large relative displacements between the superstructure and the base. Concepts from solid state physics have led to the evolution of a new technique of base isolation for civil engineering structures through the use of a foundation system consisting of periodic materials, namely periodic foundation. Periodic structures have the ability to block the passage of elastic waves within a certain frequency range through them. This frequency range is called the band gap or the attenuation zone of the periodic structure. Periodic foundations can be one dimensional (1D) or two dimensional (2D) or three dimensional (3D) periodic foundations depending of the arrangement of the periodic materials in the foundation. The band gap characteristics of 1D periodic foundations can be computed through the Plane Wave Expansion (PWE) method by expressing material properties in the form of Fourier series expansion in the reciprocal lattice space and obtaining the band gap through an eigenvalue problem using Bloch’s theorem. In the present study the PWE method has been formulated to analyse a generic multi-layer periodic unit cell. The generalized formulation has then been applied to the case of a 1D periodic foundation with three material layers. The results obtained from the analysis have shown wider attenuation zones with lower starting frequencies in the 1D three-layer periodic foundation as compared to a 1D two-layer periodic foundation. The effect of material and geometric parameters on the first attenuation zone of 1D three-layer foundation has also been investigated in the present study. Additionally, experimental studies on a single storey frame and numerical studies on a ten storey frame conducted with chosen configurations of periodic foundations showed that both 1D two-layer and 1D three-layer periodic foundations are effective in attenuating the vibration responses of structural systems.

Dynamic behavior and seismic performance of base-isolated structures with electromagnetic inertial mass dampers: Analytical solutions and simulations

The newly-developed electromagnetic inertial mass damper (EIMD) consisting of an inerter element and a damping element can significantly enhance the seismic performance of frame structures. However, whether an EIMD can enhance the seismic performance of a base-isolated structure (BIS) remains largely unknown. This paper presents a comprehensive theoretical study of the BIS with an EIMD, stressing on the dynamic behavior, parameter optimization, and seismic performance. Based on a simplified two-degree-of-freedom model, we derive the closed-form solutions of the dynamic characteristic parameters, modal participation factors, and dynamic amplification factors. A parametric study is conducted to investigate the effects of the EIMD parameters on the dynamic behavior of the system, through which the seismic response control mechanism of the EIMD in BIS is comprehensively analyzed. When the BIS subjected to a total of 20 earthquake records, the parameters of the EIMD are optimized for minimizing the interstory drift of the superstructure, the absolute acceleration response of the superstructure, and the relative displacement response of the base floor, individually. The effectiveness of the EIMD in enhancing the seismic performance of base-isolated structures is validated via numerical simulation of a five-story isolated building structure. Under a typical earthquake ground motion, the peak relative displacement of the base floor is reduced by up to 56.57% in comparison to traditional BIS; whereas the peak relative displacement and the absolute acceleration of the superstructure are reduced by up to 60.00% and 52.11%, respectively. Both analytical and numerical results demonstrate that the EIMD can significantly reduce the dynamic responses of both the base floor and the superstructure.

Base-isolated steel structure with spring limitersunder near-fault earthquakes: Experiment

The seismic performance of a steel frame base-isolated structure with steel spiral spring limiters is experimentally investigated under near-fault ground motions. A series of shake table simulator tests are carried out to analyze the dynamic response of isolation layer and superstructure. The test setup consists of a one-eighth scale five-story steel frame with steel spiral spring limiters that are designed and modeled in different parameters and stiffness, as well as various reserved gap sizes between the testing structure and limiters. The main output parameters are the maximum deformation at the isolation level, the maximum vertical force of isolation bearings, and inter-story drift. The results further reveal the seismic impact on the isolation layer and superstructure dynamic response caused by the limiter stiffness and reserved gap size.

Risk Assessment of Reinforced Concrete Buildings with Rubber Isolation Systems Designed by the Italian Seismic Code

ABSTRACT This paper presents an insight on the seismic response and seismic risk of base-isolated structures performed within the RINTC project, considering different configurations of rubber-based isolation systems. This represents an in-depth revision of past works due to the adoption of more robust and accurate isolator model. Four archetype 6-storeys RC frame buildings are examined, representing both new designed isolated buildings and existing buildings retrofitted with seismic isolation, differing in age of construction and location (the cities of L’Aquila (central Italy) and Naples (southern Italy)). All the isolation systems are designed in accordance with the current Italian Seismic Code. Seismic performance assessment is performed by multi-stripe nonlinear time history analysis. The annual failure rates for two different performance levels, namely Global Collapse and Usability Preventing Damage, are computed. The results point out that all isolation systems work effectively in limiting damage while they show a quite low margin with respect to collapse, regardless of the dominant failure mode, which is governed by the more fragile component among isolation system and superstructure. The annual failure rates derived for base-isolated buildings are critically reviewed and then compared with those obtained for similar new designed fixed-base RC buildings.

Optimal Earthquake Intensity Measures for Probabilistic Seismic Demand Models of Base-Isolated Nuclear Power Plant Structures

The purpose of this study is to evaluate the optimal earthquake intensity measures (IMs) for probabilistic seismic demand models (PSDMs) of the base-isolated nuclear power plant (NPP) structures. The numerical model of NPP structures is developed using a lumped-mass stick model, in which a bilinear model is employed to simulate the force-displacement relations of base isolators. In this study, 20 different IMs are considered and 90 ground motion records are used to perform time-history analyses. The seismic engineering demand parameters (EDPs) are monitored in terms of maximum floor displacement (MFD), the maximum floor acceleration (MFA) of the structures, and maximum isolator displacement (MID). As a result, a set of PSDMs of the base-isolated structure is developed based on three EDPs (i.e., MFD, MFA, and MID) associated with 20 IMs. Four statistical parameters including the coefficient of determination, efficiency (i.e., standard deviation), practicality, and proficiency are then calculated to evaluate optimal IMs for seismic performances of the isolated NPP structures. The results reveal that the optimal IMs for PSDMs with respect to MFD and MID are velocity spectrum intensity, Housner intensity, peak ground velocity, and spectral velocity at the fundamental period. Meanwhile, peak ground acceleration, acceleration spectrum intensity, A95, effective peak acceleration, and sustained maximum acceleration are efficient IMs for PSDMs with respect to MFA of the base-isolated structures. On the other hand, cumulative absolute velocity is not recommended for determining the exceedance of the operating basis earthquake of base-isolated NPP structures.

Comparative Study, Design and Analysis of a G+12 Structure in Earthquake Zone in India

Seismic isolation is a technology that decouples a building structure from the damaging earthquake motion. It is a simple structural design approach to mitigate or reduce potential earthquake damage. In base-isolated structures, the seismic protection is obtained by shifting the natural period of the structure away from the range of the frequencies for which the maximum amplification effects of the ground motion are expected; thus, the seismic input energy is significantly reduced. At the same time, the reduction of the high deformations attained at the base of the structure is possible, thanks to the energy dissipation caused by the damping and the hysteretic properties of these devices, further improving the reduction of responses of the structures. Base isolation is also an attractive retrofitting strategy to improve the seismic performance of existing bridges and monumental historic building.  The method of base isolation was developed in an attempt to mitigate the effects of earthquakes on buildings during earthquakes and has been practically proven to be the one of the very effective methods in the past several decades.  Base isolation consists of the installation of support mechanism which decouples the structure from earthquake induced ground motions.  Base isolation allows to filter the input forcing functions and to avoid acceleration seismic forces on the structure.  If the structure is separated from the ground during an earthquake, the ground is moving but the structure experienced little movement. To minimize the transmission of potentially damaging earthquake ground motions into a structure is achieved by the introduction of flexibility at the base of the structure in the horizontal direction while at the same time introducing damping elements to restrict the amplitude or extent of the motion caused by the earthquake somewhat akin to shock absorbers. In recent years this relatively new technology has emerged as a practical and economic alternative to conventional seismic strengthening. This concept has received increasing academic and professional attention and is being applied to a wide range of civil engineering structures. To date there are several hundred buildings in Japan, New Zealand, United States, India which use seismic isolation principles and technology for their seismic design.

Output feedback-based adaptive fuzzy sliding mode control for seismic response reduction of base-isolated buildings

In this research, a model-free control strategy for vibration suppression of base-isolated structures, which are excited by an earthquake, is investigated. For this purpose, the semi-active base isolated system including a magnetorheological (MR) damper is utilized. In this way, the displacement of the base isolator is measured by a linear variable differential transformer (LVDT). Accordingly, an adaptive fuzzy sliding mode observer is implemented to estimate the vector of state tracking error. This estimated vector is then employed by an adaptive fuzzy sliding mode control scheme to calculate the input voltage of the MR damper. The efficiency of the implemented model-free control strategy is studied by some numerical simulations. The advantages of the proposed controller can be summarized as its flexibility and robustness against disturbance rejection and model uncertainty.

Nonlinear dynamic response of an isolation system with superelastic hysteresis and negative stiffness

The negative stiffness exhibited by bi-stable mechanisms together with the tunable superelasticity offered by shape memory alloy (SMA) wires can enhance the dynamic resilience of a structure in the context of vibration isolation. The effects of negative stiffness and superelastic damping in base-isolated structures are here explored by carrying out an extensive study of the nonlinear dynamic response via pathfollowing, bifurcation analysis, and time integration. The frequency-response curves of the isolated structure, with and without the negative stiffness contribution, are numerically obtained for different excitation amplitudes to construct the acceleration and displacement transmissibility curves. The advantages of negative stiffness, such as damping augmentation and reduced acceleration/displacement transmissibility, as well as the existence of rich bifurcation scenarios toward quasi-periodicity and chaos, are discussed.

Influence of the gap size on the response of a single-degree-of-freedom vibro-impact system with two-sided constraints: Experimental tests and numerical modeling

The large displacements caused by strong earthquakes in base-isolated structures (buildings, bridges, strategic facilities, equipment, ⋯) can excessively deform or damage the isolation system or lead to pounding with surrounding moat walls or adjacent structures, if the available seismic gap is not sufficient. The acceleration spikes caused by the impact can damage the structure itself as well as sensitive equipment housed in it. A possible mitigation measure consists in the interposition of deformable shock absorbers (bumpers). In this paper, the influence of the gap amplitude on the experimental response of a single-degree-of-freedom oscillator, excited by a harmonic base acceleration and symmetrically constrained by two unilateral deformable and dissipative bumpers, is investigated. The parametric investigation considered both positive, null, and negative gaps. Particular attention is paid to the study of the effect, on the system response, of the transition from positive to small negative gaps and of excessive negative gaps. Secondary resonances in the low frequency range, associated with the occurrence of multiple impacts, were experimentally observed for small positive gaps. Finally, the experimental results were reproduced, in a sufficiently accurate manner, using a suitable numerical model, whose parameters were identified based on the experimental data.

Optimal design of dual isolated structure with supplemental tuned inerter damper based on performance requirements

This paper investigated the use of tuned inerter damper (TID) to enhance the seismic performance of dual isolated structure (DIS). An optimization design strategy was developed based on the performance requirements and expressed as a constrained multi-objective optimization design problem, where by the displacement response ratios of isolation layers were adopted as the constraints, and the acceleration response ratio of the superstructure was selected as the objective function. Meanwhile, the time history analysis was conducted to verify the effectiveness of the optimization strategy. The results indicated that the proposed dual isolated structure could effectively decrease the displacement of the superstructure relative to the ground; furthermore, it reduces the isolation layers' deformations compared with the DIS and the base isolated structure (BIS).

A nonlinear soil-structure interaction analysis technique based on seismic isolation design response spectrum for seismically isolated nuclear structures with rigid basemat

Seismic Analysis of Safety-Related Nuclear Structures (ASCE 4-16) offers a two-step nonlinear soil-structure interaction analysis procedure for seismically base-isolated structures based on the seismic isolation design response spectrum. The two steps are (I) development of seismic isolation design response spectrum at the foundation level of an isolated structure using frequency domain soil–structure interaction (SSI) analysis for an equivalent linear SSI system and (II) nonlinear time domain analysis of the isolated structure using foundation input motions, which is consistent with the seismic isolation design response spectrum. In step II, the rocking component of the structure shall be considered when its effects are significant. However, the ASCE 4-16 does not present how to consider the rocking input. In this study, we propose a method of taking account the rocking component of a base-isolated structure with rigid basemat. Specifically, this paper presents a method of developing rocking input motion that is fully coupled with horizontal input motion at the base of the isolated structure. The method was verified by comparing numerical results using the rocking input for a base-isolated nuclear power plant with rigid basemat on a compliant soil medium with those of a refined nonlinear response history analysis.

Shear deformation detection in smart rubber bearing (SRB) using active sensing method

Rubber bearings are one of the key components of base-isolated structures. Therefore, it is important to monitor the state in rubber bearing effectively. Based on the Smart Rubber Bearing (SRB) proposed in the previous study, this paper studied the feasibility of SRB in shear deformation detection. Theoretical analysis revealed that the SRB has considerable potential in shear deformation detection while dose not affect its original horizontal stiffness. Detection tests were carried out on a full-scale SRB with different shear deformations, and a swept signal and white noise signal were used as the detection signals. A wavelet-packet-based shear deformation index is proposed based on the tests. The test result shows that the proposed shear deformation index can accurately represent the shear deformation in SRB when the transducers are installed on the same vertical axis, and the shear deformation index decreases approximately linearly with the increase of the shear deformation in SRB.

An innovative method for axial pressure evaluation in smart rubber bearing based on bidirectional long-short term memory neural network

Rubber bearings are a key component of base-isolated structures. Hence, it is important to effectively monitor the axial pressure in rubber bearings. A smart rubber bearing (SRB) and a wavelet-packet-based axial pressure index were introduced in the previous study. However, the wavelet-packet-based axial pressure index for evaluating the axial pressure in the SRB has notable shortcomings, because the system is susceptible to disturbances such as the change of sensitive subsets, generator-sensor couple and SRB specimen. This paper proposes the discrete wavelet packet transform enhanced bidirectional long-short term memory (DWPT-BiLSTM) method to evaluate the axial pressure state of the SRB. Three full-scale SRBs were tested, and two data augmentation methods were used to increase the dataset capacity and ensure the proper distribution of different axial pressure states in the dataset. By simultaneously considering the validation accuracy and training time, the optimal network structure is one hidden layer with 50 cells, and the optimal learning rate is 0.01. With these hyperparameters, the DWPT-BiLSTM method achieved an accuracy of 99.4% on the training data and an accuracy of 97.3% on the validation data in the evaluation of the axial pressure state of the SRB. The precision and recall of the axial pressure states were above 95% for both the training and validation set.

Experimental seismic performance assessment and numerical modelling of nonlinear inerter vibration absorber (IVA)‐equipped base isolated structures tested on shaking table

AbstractIn recent years, inerter vibration absorbers (IVAs), such as the tuned mass damper inerter (TMDI), attracted much attention in the literature for reducing seismic displacement demands of base isolated structures (BISs). Several theoretical studies reported reduced BIS seismic demands with increasing inertance endowed by a grounded inerter element, but adopted mostly idealized linear dynamical models. Herein, the potential of TMDI‐configured IVAs for seismic response reduction of BISs modelled as single‐mass structures is assessed under the combined effects of nonlinear inerter and structural behavior. To this aim, experimental data from a shaking table testing campaign are considered utilizing a custom‐built flywheel rack‐and‐pinion grounded inerter prototype with variable inertance, along with high damping rubber bearings in the isolation layer and in the BIS‐to‐absorber link. White noise excitation and an ensemble of six ground motions (GMs) with different frequency content are used in the tests for which bearings exhibit softening nonlinear behavior. Experimental results demonstrate improvement of BIS nonlinear seismic response in terms of displacement and base shear with increasing inertance for nonlinear and nonoptimally tuned IVAs. It is found though that the considered IVAs may be detrimental to BIS acceleration response depending on the GMs time‐varying frequency content signatures as captured by the continuous wavelet transform spectrogram. Finally, it is concluded that representing the inerter device by a simplified linear dissipative model as opposed to a nonlinear model with friction and gear backlash suffices to trace the BIS response with acceptable accuracy and, thus, can be used for optimal seismic TMDI design.

Enhanced seismic base isolation using inertial amplifiers

Harnessing the concept of inertial amplification, an inertial amplifier coupled base isolator (IABI) is proposed in this paper. The seismic performance, in terms of story drift, base shear reduction, of this proposed inertial amplifier coupled base isolator has been compared with that of the classical base isolator (CBI) and inerter-based isolation system (Inerter-BI). A thorough analytical study, implementing rationals of random vibration for harmonic and Gaussian white noise is conducted to identify the performance of the inertial amplifier coupled base isolator (IABI) in the frequency domain. Further, a numerical study is conducted to determine the story drift, base shear response in time-domain for the inertial amplifier based base-isolated structure and classical base-isolated structure for twenty-two real earthquake ground motions. The results elucidate the histogram of base shear and story drift reduction percentage shifts towards higher values for inertial amplifier coupled isolator compared to CBI and Inerter-BI. From both analytical and numerical study, it is observed that the seismic performance and response reduction capacity of proposed IABI is significantly 89.38% and 72% superior to the classical base isolator and inerter-based isolation system, respectively. This finding evidenced towards the supremacy of the inertial amplifier base isolator over the classical base isolator and inerter coupled base isolator in terms of seismic vibration control.