Base-isolated Building Structures Research Articles

Constant-strength inelastic displacement ratio spectra for assessment of superstructures in seismically isolated buildings

A reliable estimation of maximum displacement demands of seismically isolated inelastic superstructures is crucial for assessment of base-isolated buildings under beyond-design earthquakes. Constant-strength inelastic displacement ratio, CR, spectra, as applied in the performance-based seismic assessment procedure, can be used to estimate the inelastic displacement demands of structures. Currently, a suitable analytical expression of CR is lacking for base-isolated structures considering the influencing parameters comprehensively. This paper seeks to fill this gap. The constant-strength spectra are computed from nonlinear dynamic analyses of simplified two-degree-of-freedom models considering the inelastic behavior of the superstructure and isolator system, using a set of earthquake ground motions recorded on stiff soil sites. The effects of strength reduction factor of superstructure, superstructure elastic period, secondary stiffness ratio of superstructure, mass ratio, isolation period, normalized strength of isolation system and superstructure viscous damping on the mean and dispersion of CR are investigated. In particular, the effects of variations in earthquake intensity and isolator strength on CR are taken into account by the normalized strength of isolation system. Besides, limiting values of CR as the superstructure elastic period tends to zero or infinity are discussed and verified using the statistical values of CR. Finally, based on the trends of CR with respect to the design parameters of the superstructure and isolation system, and the boundary conditions to the CR formula, an analytical estimating equation with reasonable accuracy is developed to approximate the mean constant-strength inelastic displacement ratios during seismic assessment of base-isolated building structures built on stiff soil sites.

Full-scale laboratory investigation on laminated rubber bearings for metro-induced vibration mitigation

The vibration response generated by the operation of a metro system has a profound impact on the physical and mental well-being of urban residents. Vibration control is essential to effectively isolate the multi-band vibration response and structure-borne noise of buildings. Laminated rubber bearings between the foundation and the superstructures serve as a common, cost-effective solution for the seismic isolation of structures and have great potential to be used for metro-induced vibration mitigation. In this study, two types of full-scale laminated rubber bearings (i.e., linear natural rubber bearing and lead rubber bearing) were tested under various compressive stresses, ranging from 5 MPa to 15 MPa, to examine the static and dynamic characteristics of the bearings, including vertical hysteresis curves, vertical static stiffness, time history response, and frequency spectrum. Then, the numerical simulation of a base-isolated building structure equipped with laminated rubber bearings was conducted to evaluate the vertical vibration isolation performance and structure-borne noise isolation capability. It was demonstrated that the laminated rubber bearings have stable vertical bearing capacities and performance in isolating vertical vibrations.

Seismic behavior of base-isolated building structures with lead rubber bearings (LRBs)

This study is designed to evaluate the efficacy of multi-storey buildings in mitigating seismic responses, particularly through the implementation of a base isolation system. Our focus centers on two reinforced concrete structures, one eight stories high and the other ten stories high, both outfitted with a base isolation system featuring lead rubber isolators to proactively minimize seismic risks. The primary objective of introducing a base isolation system is to curtail the potentially destructive impact of earthquakes on a building by attenuating the transmission of seismic forces from the superstructure to the foundations. This seismic protection technique involves strategically installing isolators between the building's superstructure and its foundations. The integration of lead rubber bearing (LRB) isolators into the building's base constitutes a pivotal approach to significantly augmenting the seismic performance of a structure. This method boasts several advantages, chief among them being a notable reduction in floor drift, a critical factor for ensuring the safety and stability of a structure amidst seismic activity. The outcomes of our study underscore the remarkable effectiveness of base isolation in substantially decreasing the overall seismic responses of buildings in the studied region. The observed reductions range from 50% to 70%, affirming the robustness and resilience that this innovative approach imparts to buildings in earthquake-prone areas.

Design and analysis of LRB base-isolated building structure for multilevel performance targets

According to China's current code for the seismic design of buildings (GB50011-2010), in the course of designing base-isolated building structures, the isolation system and superstructure are designed separately. Hence, the horizontal seismic isolation coefficient in terms of the design-based earthquake (DBE) and horizontal deformation of the isolation system under the maximum considerable earthquake (MCE) cannot be considered simultaneously. Consequently, the optimum mechanical parameters for isolation bearing and the corresponding horizontal seismic isolation coefficient used in the seismic design of the superstructure cannot be obtained. To address these shortcomings, we propose an optimum seismic design procedure for LRB base-isolated building structures to meet multilevel performance fortification criteria, where explicit mathematical formulations for horizontal seismic isolation coefficients and the horizontal deformation of isolation systems in terms of the mechanical parameters of the LRB isolation system are established by combining equivalent linearization and modal decomposition response spectrum methods. Through a parametric study, we find the critical yield strength ratio, i.e., the ratio of the yielding shear of lead core to the total weight of the LRB base-isolated structure, which leads to the minimal value of the horizontal seismic isolation coefficient. Beyond this critical yield strength ratio, an optimum solution is unavailable, as the horizontal seismic isolation coefficient and horizontal displacement of the isolation system are of a conflicting nature. By converting the multi-criterion formulation into an auxiliary problem with a scalar objective, the optimum design method of an LRB base-isolated building structure oriented to multilevel performance fortification criteria is proposed, so as to obtain the optimum yield strength ratio and the optimum second shape coefficient for the seismic design of the superstructure. A numerical example demonstrates that the seismic horizontal seismic isolation coefficient and maximum horizontal displacement of the isolation system calculated from the optimum yield strength ratio and the optimum second shape coefficient agree well with the results from the time-history method. In a comparative study, LRB base-isolated building structures designed by the critical yield strength ratio, the optimum yield strength ratio and the optimum second shape coefficient, the optimum yield strength ratio and the corresponding fixed-base superstructure are subjected to MCE and mega earthquakes, where the LRB base-isolated structure shows good seismic performance in terms of the optimum yield strength ratio, even when subjected to mega earthquakes.

SEISMIC VULNERABILITY ASSESSMENT OF BASE-ISOLATED RC FRAME BUILDING USING DDBD METHOD

The direct displacement-based design (DDBD) method is widely used worldwide because it allows structures to be designed based on their expected performance criteria. The DDBD approach is utilized so that base-isolated building structures can meet the seismic performance parameters established by the displacement threshold. In the present study, an 8-story, 10-story and 12-storey base-isolated RC frame building is designed using the latest Indian seismic standards, with lead rubber bearing (LRB) as the base-isolation system. Force-based and DDBD methodology is implemented and compared for the selected base-isolated RC frame buildings. The nonlinear time-history analysis is conducted to evaluate the nonlinear lateral response of the RC frame buildings. The results indicated that the traditional force-based method shows almost 12% less top-storey displacement than the DDBD approach. The DDBD approach provides enhanced reliability, simplicity, and convenience in seismic design for base-isolated building frames with LRBs, making the observations more dependable and easier to employ.

Optimal design of negative-stiffness dampers for improved efficiency of structural seismic isolation

Over the past two decades, negative-stiffness devices have attracted increasing interest in the community of structural vibration isolation and control. They are typically used in combination with supplemental damping devices, and the combined systems are often referred to as negative-stiffness dampers (NSDs). Different types of NSDs have been developed and studied using both theoretical and experimental methods. However, comprehensive comparative studies between optimally designed NSDs for improving the efficiency of structural seismic isolation are inadequate. In this paper, three types of NSDs, i.e., the negative-stiffness viscous damper (NSVD), negative-stiffness viscoelastic damper (NSVeD), and negative-stiffness inertoviscous damper (NSiVD) are considered and compared in terms of their efficiencies for structural seismic isolation. Analytical formulas are derived to calculate the optimal damping coefficients of the NSVeD and NSiVD to maximize the modal damping ratio, thus minimizing the peak transmissibility. Frequency- and time-domain responses of low-frequency structures separately equipped with the three types of NSDs are compared when they are designed to achieve equal modal damping ratios. Moreover, based on the derived analytical optimal formulas, a multi-objective minimization design method is presented to determine the optimal characteristic parameters of NSDs, and simultaneous reduction of isolator displacement and floor response acceleration of a benchmark base-isolated building structures under a suit of design-level ground motions is achieved. The results indicate that the three types of NSDs can be more effective than conventional viscous dampers in reducing floor response accelerations of base-isolated structures without compromising the isolator displacement, and the NSVeD has superior performance over the NSVD and NSiVD in terms of improved efficiency of structural seismic isolation.

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.

The Structural Performance Analysis of Base-Isolated Hospital Buildings with Analysis Modal (Case Study: General Hospital in Labuhan Batu Utara Regency Area)

The development of earthquake analysis towards structures is required to prevent damages and loss in buildings due to earthquakes. The base isolation system is a simple design approach for earthquake-resistant buildings to protect the structures and components from the risk of earthquake damages by using the concept of reducing earthquake forces. This research aims to analyze the performance of a general hospital building in Labura Regency area in order to know the safety of the building in terms of period, frequency, base shear force, displacement and earthquake force, used the base isolators and without the base isolators. The method used is response-spectrum dynamic analysis by ETABS v2016 program. From the calculation of structural analysis, the application of base isolation is able to build up the period of the structure, therefore, the maximum acceleration of earthquakes can be reduced at certain period. There is an average increase by 48.21% of the structural period compared to non-isolated base structure, and the frequency that occurs in structures using base isolators is smaller than without base isolators. The friction force obtained is smaller compared to the structures without dampers. Base-isolated building structures observed have bigger displacement than non-base isolated structures. The average rise of the building displacement is 27.14% at x and 2.74% at y directions. In base-isolated structures, earthquake forces are reduced averagely by 57.51% at x and 82.73% at y directions. The analysis of structural performance, General Hospital in Labura Regency is categorized to Immediate Occupancy (IO) in which the building structures are safe with no significant risk of fatalities due to structural failures, there are no any significant damages and the building can be used and functioned/operated again immediately.

A direct displacement-based design procedure for base-isolated building structures with lead rubber bearings (LRBs)

This paper proposes a direct-displacement based design (DDBD) procedure, to make base-isolated building structure with lead-rubber bearing (LRBs) satisfy the performance objectives prescribed by displacement thresholds. To this end, the DDBD for regular building structures is incorporated with the two-degree-of-freedom (2DOF) model of base-isolated building structures. Fundamentals of the standard DDBD procedure is briefed at first. The equivalent linearized 2DOF model of a base-isolated building structure is then detailed. Modal analysis and response spectrum analysis of this 2DOF model has been carried out, which suggests a simplistic and readily-invertible closed-form relationship from structural characteristics to seismic displacements. Given the displacement requirements, the relationship facilitates the solution of the structural parameters of the base-isolated building structure. Following the basic steps of DDBD but utilizing this 2DOF model as the substitute for the original nonlinear structural model, an alternative DDBD procedure is then developed and detailed. This DDBD procedure is verified by numerical examples, in which the nonlinear time-history analysis of the designed structures yields seismic displacement results matching the preset target thresholds well. The design solutions of the proposed procedure are also compared with those given by an existing one. Numerical results indicate that the proposed DDBD procedure is reliable, straightforward, and convenient for the seismic design of base-isolated building structures with LRBs.

Experimental investigation on the effectiveness of laminated rubber bearings to isolate metro generated vibration

Vibration control is important in maintaining the functionality of the buildings surrounding a metro. Among the many methods of vibration control, track isolation is by far the most efficient approach. However, for buildings constructed near a metro line which has no track isolation, base isolation of the building structures is useful to alleviate metro generated vibration. Laminated rubber bearings are commonly used for seismic isolation of building structures; however, their effectiveness in isolating metro generated vibration has not yet been well explored. In this study, scaled model tests were carried out to investigate the isolation effectiveness of laminated rubber bearings in base isolated building structures. Specifically, four types of laminated rubber bearings, i.e. thick natural rubber bearings, thin natural rubber bearings, thick high damping rubber bearings, and thin high damping rubber bearings, were tested. Tests were carried out under the designed compressive stress, 70% of the design compressive stress and at various excitation intensities. Our results show that the isolation effectiveness of both thick natural rubber bearings and thick high damping rubber bearings were significant and stable, while thin rubber bearings offered insignificant and unstable isolation.

Life-Cycle Cost Model and Design Optimization of Base-Isolated Building Structures

Design of economic structures adequately resistant to withstand during their service life, without catastrophic failures, all possible loading conditions and to absorb the induced seismic energy in a controlled fashion, has been the subject of intensive research so far. Modern buildings usually contain extremely sensitive and costly equipment that are vital in business, commerce, education and/or health care. The building contents frequently are more valuable than the buildings them-selves. Furthermore, hospitals, communication and emergency centres, police and fire stations must be operational when needed most: immediately after an earthquake. Conventional con-struction can cause very high floor accelerations in stiff buildings and large interstorey drifts in flexible structures. These two factors cause difficulties in insuring the safety of both building and its contents. For this reason base-isolated structures are considered as an efficient alternative design practice to the conventional fixed-base one. In this study a systematic assessment of op-timized fixed and base-isolated reinforced concrete buildings is presented in terms of their initial and total cost taking into account the life-cycle cost of the structures.

Analysis for the 3-Dimensional Time History Response of a Base-Isolated Building Structure

In this paper, the models of traditional anti-seismic and base-isolated masonry and concrete frame composite structures were created by the three-dimensional finite element program and their time history responses were analyzed under the actions of different seismic waves. The results show that whether they were under the action of frequently occurred earthquake or rarely occurred earthquake, the story shears of base-isolated structure are far less than those of traditional anti-seismic structure; the lateral displacements of the former are nearly translational, while those of the latter are approximately parabolic; and the fundamental period of the former is much longer than the counterpart. In conclusion, the seismic fortification intensity of base-isolated structure is one or two degree less than that of traditional anti-seismic structure, and adopting the base-isolated structure will protect the life-safety and property of people and achieve good economic benefits as well.

An LMI approach to vibration control of base-isolated building structures with delayed measurements

In this article, we address a convex optimisation approach to the problem of state-feedback H ∞ control design for vibration reduction of base-isolated building structures with delayed measurements, where the delays are time-varying and bounded. An appropriate Lyapunov–Krasovskii functional and some free-weighting matrices are utilised to establish some delay-range-dependent sufficient conditions for the design of desired controllers in terms of linear matrix inequalities. The controller, which guarantees asymptotic stability and an H ∞ performance, simultaneously, for the closed-loop system of the structure, is then developed. The performance of the controller is evaluated by means of simulations in MATLAB/Simulink.

Vibration control of base-isolated structures using mixed H2/H∞ output-feedback control

A mixed H2/ H∞ output-feedback control design methodology for vibration reduction of base-isolated building structures modelled in the form of second-order linear systems is presented. Sufficient conditions for the design of a desired control are given in terms of linear matrix inequalities. A controller that guarantees asymptotic stability and a mixed H2/ H∞ performance for the closed-loop system of the structure is developed, based on a Lyapunov function. The performance of the controller is evaluated by means of simulations in MATLAB/Simulink.

Soil–structure interaction effects on base-isolated buildings founded on soil stratum

The present study investigates the effects of soil–structure interaction (SSI) on the response of base-isolated multistory buildings founded on an elastic soil layer overlying rigid bedrock and subjected to a harmonic ground motion. Initially a four-degree-of-freedom system (4-DOF) is developed and the equations of motion are formulated in the frequency domain. Frequency independent expressions are used to determine the stiffness and damping coefficients for the rigid surface foundation on the soil-stratum underlined by bedrock at shallow depth. Assuming the foundation mass to be negligible, an equivalent two-degree-of-freedom (2-DOF) system is derived. The first mode of motion of the equivalent 2-DOF system appears to be sufficient to describe the response of the overall system for all ranges of stiffness and inertia properties of the structure and its isolation. An extensive parametric study demonstrates that SSI effects are significant, primarily for squat, light structures, founded on soil-stratum of low stiffness. The methodology could serve as a means to perform a preliminary seismic design of base-isolated building structures founded on homogenous soil-stratum over bedrock.

Seismic protection of base-isolated building with nonlinear isolation system using smart passive control strategy

This paper numerically investigates the applicability of the magnetorheological (MR) damper-based smart passive control system with the electromagnetic induction (EMI) part to the seismic protection of base-isolated building structures with nonlinear isolation systems such as friction pendulum bearings and lead–rubber bearings. The EMI part consists of a permanent magnet and a solenoid coil, which produces the electric energy (i.e. induced current) according to Faraday's law of EMI. The produced electric energy is applied to the MR damper to change the damping characteristics by itself without any controller as well as sensors for reducing structural responses. To verify the efficacy of the smart passive control system to a nonlinear base-isolated building, a series of numerical simulations are carried out by considering the phase II benchmark nonlinear base-isolated structure control problem. It is demonstrated from the numerical simulation results that the smart passive control system has the comparable or superior control performance to the conventional MR damper-based semiactive control system. Therefore, the smart passive control system could be considered as one of the promising control devices for seismic protection of seismically excited nonlinear base-isolated structures. Copyright © 2008 John Wiley & Sons, Ltd.

Numerical Verification of the Effectiveness of the “Alpha” Method for the Estimation of the Maximum Rotational Elastic Response of Eccentric Systems

In previous research works, the authors have identified a key system parameter which controls the maximum rotational response under free and forced vibrations of one-story linear-elastic systems representative of asymmetric seismic base-isolated building structures. This parameter (called “ALPHA”) has also led to the identification of a simplified procedure (called “ALPHA method”) for the estimation of the maximum rotational response of such systems. The main goal of this article is to verify the properties of the ALPHA parameter and the predictive capabilities of the ALPHA method, when applied to one-story systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of 11,600 numerical simulations developed with reference to different representative structures subjected to historically recorded ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and the ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained: (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response; (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation; and (c) indicate that the ALPHA method is only weakly sensitive upon the period of vibration of the structure. The article also introduces a simple code-like provision for conservative estimations of the maximum rotation developed under seismic input by asymmetric structures based upon the confidence interval concepts.

스마트 수동 제어 시스템을 이용한 면진 건물의 내진 성능 개선

본 연구에서는 면진 건물의 내진 성능을 개선하기 위하여 최근 개발된 스마트 수동 제어 시스템을 적용하고 이의 효용성을 수치 모의실험을 통해 검증하였다. 스마트 수동 제어 시스템은 효과적인 반능동 제어 장치로 알려진 MR 감쇠기에 전자기 유도부를 도입하여 응답 변화에 따라 MR 감쇠기로 입력되는 전류를 변화시킴으로써 MR 감쇠기의 감쇠 특성을 조절하는 새로운 개념의 스마트 제진 시스템이다. 스마트 수동 제어 시스템에서 전자기 유도부는 영구자석과 솔레노이드 코일로 구성되며, 기존 스마트 제진 시스템의 계측기, 제어기, 외부 전원장치를 한꺼번에 대체할 수 있다. 면진 건물에 대한 스마트 수동 제어 시스템의 내진 성능을 수치적으로 검증하기 위하여 미국토목학회에서 제시한 면진 건물에 대한 벤치마크 제어 문제를 활용하였다. 스마트 수동 제어 시스템의 제진 성능을 MR 감쇠기를 이용한 기존 스마트 제어 시스템의 성능과 비교하였다. 수치 모의실험 결과를 통해 스마트 수동 제어 시스템이 면진 건물의 내진 성능을 개선하는 데 매우 유용함을 확인하였다. In this study, the efficacy of the newly developed smart passive control system to improve seismic performance of base isolated building structures is numerically verified. The smart passive control system consists of a magnetorheological (MR) damper and an electromagnetic induction (EMI) part. The damping characteristics of an MR damper can be controlled by the current generated in an EMI part according to the Faraday's law of electromagnetic induction. An EMI part consisting of a permanent magnet and a solenoid coil could substitute a control system including sensors, a controller and an external power supply in a conventional smart control system. The benchmark control problem for a base isolated building presented by the american society of civil engineers is considered for numerical simulation. The control performance of the smart passive control system is compared to that of the conventional smart control system using MR dampers. It is demonstrated from the numerical simulation results that the smart passive control system is useful to improve the seismic performance of base isolated buildings.

Application of some semi-active control algorithms to a smart base-isolated building employing MR dampers

This paper investigates the effectiveness of the MR damper-based control systems for seismic protection of base-isolated building structures employing some semi-active control algorithms, such as the modified clipped-optimal control, the maximum energy dissipation, the modulated homogeneous friction, and fuzzy logic-based control algorithms, by examining the Phase I smart base-isolated benchmark building problem. The results of the numerical simulations showed that most of the control systems considered herein could be beneficial in reducing seismic responses, especially the base displacement or the isolator deformation, of the base-isolated building. If the base displacement is to be primarily reduced without regard to variation of the floor acceleration, the original clipped-optimal control algorithm could be recommended. On the other hand, if it may be required to reduce the base displacement without increasing the floor acceleration, the modified clipped-optimal control algorithm proposed in this study could be considered as one promising candidate for the linear benchmark base-isolated system. Copyright © 2005 John Wiley & Sons, Ltd.

BASE-ISOLATION DESIGN PRACTICE IN JAPAN: INTRODUCTION TO THE POST-KOBE APPROACH

Japan has twenty years of experience in designing and constructing base-isolated building structures. Construction has increased significantly since the 1995 Hyogoken-Nanbu (Kobe) earthquake, having reached over 150 annual construction projects. Many new developments and refinements have been made in the material, device, design, and construction of these structures. This paper summarises recent design and construction of base-isolated building structures in Japan, including statistical data with respect to the common usage as well as the number of new projects. It is notable that the size, height, and fundamental natural period of new base-isolated buildings increase steadily with time, indicating that base-isolation in Japan is reaching maturity. Base-isolators and dampers commonly adopted in Japan are also introduced, with emphasis on recent design efforts to enlarge the natural period of base-isolated structures and reduce the lateral forces induced in the superstructure. Basic design procedures are presented, including determination of design earthquake forces, modelling of base-isolation layers, modelling of the superstructure, selection of ground motions, time-history analyses, and performance criteria. A mandated peer-review system, unique for design of base-isolated structures, is also noted. Several characteristic issues in the design of base-isolated structures are discussed: Variation of base-isolation material properties, applications to high-rise buildings, effects of vertical ground motions, and response when subjected to near-fault ground motions.