Base-isolated Structures Research Articles

Development of a two-phased nonlinear mass damper for displacement mitigation in base-isolated structures

This paper developed a two-phased arc nonlinear energy sink (2PA NES) for displacement mitigation in base-isolated structures. The 2PA NES is a type of nonlinear mass damper that can produce nonlinear restoring force through the auxiliary mass moving on a track. The track is designed in two phases that provide softening restoring force for the 2PA NES. In the base-isolated structures, the 2PA NES is attached to the top story as the building roof without restriction of the location and space. The 2PA NES can be precisely tuned to match the hysteretic behavior of the isolated structures. Restoring force of the 2PA NES and equations of motion of the 2PA NES-base-isolated system were derived first. Subsequently, the design philosophy of the 2PA NESs was proposed and applied in an eight-story base-isolated structure. The displacement mitigation capacity of the 2PA NES was then computationally investigated under a suite of pulse-type seismic excitations. Analysis results showed that the 2PA NES was more effective than a comparable tuned mass damper (TMD) in reducing the base displacement. In addition, the stroke of the 2PA NES was smaller than that of the TMD and the supplement of the 2PA NES had a slight effect on the response of the superstructure. With little installation limitation and high control effectiveness, the 2PA NES provides a promising solution for displacement mitigation in base-isolated structures.

Seismic response mitigation of structures with a friction pendulum inerter system

Introducing a tuned mass damper (TMD) into the friction pendulum system (FPS) has been proven to be an effective approach for improving the seismic performances of base-isolated structures. However, its seismic response mitigation effect is related to the quality of the mass employed, which unavoidably requires a necessary large mass under the requirement of a high seismic performance level. To avoid introducing the extra mass of the tuned mass damper (TMD) in the friction pendulum system (FPS) of a base-isolated structure, we herein introduce a lightweight inerter subsystem that has a series-parallel layout; comprises an inerter, a spring, and a damping element; and adds almost no mass. A structure isolated by the proposed friction pendulum inerter system (FPIS) was studied by nonlinear stochastic response analysis within a probabilistic framework, and an optimal design method for a structure with the FPIS was developed to simultaneously reduce the base shear force and the base isolation floor displacement. Based on the stochastic analysis results, parametric studies and a robustness analysis were conducted, and the impact of the FPIS’s mechanical layout on the seismic response mitigation effect was investigated. The analysis results demonstrated that the FPIS significantly reduced structural responses under different types of seismic excitations. Using the proposed optimal design method, target base shear force can be achieved at a minimized cost to the base isolation floor displacement. Compared to the FPS system with a TMD, the proposed FPIS enhances the seismic response mitigation effect by avoiding the extra mass that increases the seismic energy input to the base isolation floor.

Identification of the nonlinear characteristics of rubber bearings in model-free base-isolated buildings using partial measurements of seismic responses

Rubber-bearing isolation is one of the most successfully and widely used isolation technologies to provide lateral flexibility and energy dissipation capacity for reducing structural vibration and protecting the superstructure from damage. The seismic performance of the base-isolated structures partly depends on the nonlinear characteristics of the base isolation system. However, it is hard to establish proper mathematical models for the nonlinear hysteretic behaviors of base isolation due to the complexities of nonlinearities. Consequently, it is strongly desired to develop model-free methodologies for the nonlinear hysteretic performance identification with no assumption on the nonlinear hysteretic models of base isolation. In this paper, a novel method is proposed for this purpose. Firstly, the base isolation is in the linear state when the structure is under the weak earthquake, the restoring force is only provided by linear stiffness and viscous damping of base isolation, and the structural physical parameters can be estimated based on the extended Kalman filter approach. Then, the base isolation is in the nonlinear state when the structure is under the strong earthquake. The nonlinear hysteretic restoring forces from base isolation are treated as “unknown fictitious inputs” to the corresponding structural systems without base isolation. The generalized Kalman filter with unknown input algorithm is adopted for the simultaneous identification of the corresponding structural systems and the hysteretic restoring force of base isolation using only partial structural responses. No information about the structure is needed, and the responses at the location of the base isolation are not required, the proposed method is capable of identifying nonlinear characteristics of base isolation by the direct use of partial structural dynamic response. To validate the performances of the proposed method, some numerical simulation examples of identifying nonlinear hysteretic restoring forces of base isolation in different models are used.

Seismic performance of secondary systems housed in isolated and non-isolated building

The concept of base isolation for equipment is well known. Its application in buildings and structures is rather challenging. Introduction of horizontal flexibility at the base helps in proper energy dissipation at the base level thus reducing the seismic demand of the super structure to be considered during design. The present study shows the results of a series of numerical simulation studies on seismic responses of secondary system (SS) housed in non-isolated and base-isolated primary structures (PS) including equipment-structure interactions. For this study the primary structure consists of two similar single bay three-store reinforced cement concrete (RCC) Frame building, one non-isolated with conventional foundation and another base isolated with Lead plug bearings (LPB) constructed at IIT Guwahati, while the secondary system is modeled as a steel frame. Time period of the base isolated building is higher than the fixed building. Due to the presence of isolator, Acceleration response is significantly reduced in both (X and Y) direction of Building. It have been found that when compared to fixed base building, the base isolated building gives better performance in high seismic prone areas.

A smart mechatronic base isolation system using earthquake early warning

In earthquake-prone countries such as Japan, effects of earthquakes are one of the major design objectives in buildings and infrastructures. Recent advances in earthquake engineering have improved the safety and reliability of buildings and infrastructures. Better understanding of geomorphology of earthquakes also enables us to better predict and estimate propagation of such natural hazard. For example, the Earthquake Early Warning (EEW) system in Japan provides advanced warnings using the different arrival times of P and S waves. On the other hand, techniques and methods in passive, semi-active and active vibration control have flourished. Base-isolation is a proven technique which enhances earthquake resilience of buildings and bridges. In general, this technique involves decoupling a superstructure from its foundation and lengthens its natural period of vibration. This technique is mature and commercialization has taken place worldwide. However, wind-resistance of base-isolated structures is generally a concern as the lateral stiffness of a base-isolated structure is low and serviceability wind effects may induce unacceptable lateral movement and/or vibration of the structure. This paper presents a new concept which the base-isolation system is activated by EEW system through a mechatronic system. In other times the structure is not base-isolated and provides strong lateral resistance against wind loads. As a backup design, the proposed system also equips with its own network of accelerometers which can trigger base isolation system independent from EEW system. The smart mechatronic base isolation system is fully automated, and resets itself after each ground motion. The paper describes the conceptual framework of proposed system and presents laboratory-scaled proof-of-concept experiments. Four historical earthquake time histories were used as ground excitations. Structural responses are compared between the two triggering mechanisms and results are discussed. Finally, the concept of Internet of Things (IoT) which make use of EEW is discussed, allowing connectivity between a single control unit and a network of infrastructures to achieve economy of scale.

Response spectrum analysis of frame structures: reliability-based comparison between complete quadratic combination and damping-adjusted combination

In the framework of seismic design of structures, response spectrum analysis (RSA) is the most commonly used approach in practice. The most popular combination rule is the complete quadratic combination (CQC) which is also prescribed by the most of seismic design codes and is based on the assumptions that the seismic acceleration is a white noise process and the peak factor ratios associated to the total and modal responses are unitary. Recently, the damping adjusted combination (DAC) rule has been developed for base-isolated structures to overcome the aforementioned simplified assumptions. Although it has been proved that the simplifications about peak factors lead to noticeable errors in the case of base-isolated structures, the accuracy gain of DAC with respect to CQC in the case of fixed-base structures is still unknown. Therefore, the paper presents an in-depth study on the RSA of three-dimensional frame structures, aimed to evaluate the accuracy of the above methods. Two reference classes of frame structures having different degree of complexity are considered. Average interstorey drift and floor torsion responses, obtained from a set of Time History Analyses are compared with those of the modal combination rules. Lognormal joint probability density functions of the predictive errors from CQC and DAC are finally evaluated for a reliability assessment of the two combination rules under bidirectional seismic excitations.

Semiactive conceptual fuzzy control of magnetorheological dampers in an irregular base-isolated benchmark building optimized by multi-objective genetic algorithm

Seismic base isolations are well-established passive control systems, used for reducing structural responses and preventing interior sensitive equipment and nonstructural elements from damaging. However, in a base-isolated structure under near-fault earthquakes, isolated layers sustain large displacements that might not be allowable. Further, any passive limitation in these displacements amplifies vibration transmissions to the superstructure. Smart hybrid base isolations using magnetorheological (MR) dampers, as well-known semiactive devices, can overcome this problem by smart dampening. Nonetheless, highly nonlinear hysteretic behavior of MR dampers is one of the challenges for model-based control algorithms. In this study, a smart multi-objective fuzzy-genetic control for dampening the vibrations of structures in an irregular base-isolated benchmark building subjected to different earthquake scenarios is presented. The control aims, on one hand, to conserve (or even improve) top isolation characteristics in unique restraining of floor accelerations and drifts and, on the other hand, to mitigate large base displacements under near-fault earthquakes using MR dampers simultaneously. Unlike many smart controls with a black-box performance, the proposed fuzzy core is constructed conceptually employing control expertise with respect to the plant. To achieve control objectives, a conceptual innovative feedback is proposed for fuzzy decision making. To optimize this human-designed controller, a multi-objective genetic algorithm is applied. For evaluation, a three-dimensional nonlinear irregular base-isolated benchmark building planned by the American Society of Civil Engineers is employed. In comparison with classical and smart controls, the results demonstrate that despite having the smallest structure with fast performance, the proposed control effectively diminishes conflicting responses and has better performance than other controls.

Modal property of base-isolated high-rise structure considering soil–structure interaction effect

A simplified methodology is rigorously studied in this article to analyze the modal properties of base-isolated high-rise structures with dynamic soil–structure interaction being considered. The proposed methodology is developed based on a more reasonable 2-degree-of-freedom model and the existing simplified methodology which is only applicable for non-isolated structures. The base-isolated structure model with 2 degrees of freedom is supported by swaying and rocking springs and by the corresponding dashpots. Rigorous mathematical derivation is performed, and closed-form formulas of natural periods, modes, and modal damping ratios are derived. Furthermore, the overall accuracy of the proposed methodology was checked against the results of the rigorously derived complex eigenvalue approach proposed by Constantinou and Kneifati. A parametric study is also conducted on the soil–structure interaction effects of base-isolated structures, which indicates that tall and slender structures with stiff isolation systems are more affected by soil–structure interaction effects in comparison to flexible superstructures. The proposed method provides a feasible way to evaluate the soil–structure interaction effects of base-isolated structures efficiently during the schematic design phase.

Wavelet analysis of soil-structure interaction effects on seismic responses of base-isolated nuclear power plants

Seismic base isolation has been accepted as one of the most popular design procedures to protect important structures against earthquakes. However, due to lack of information and experimental data the application of base isolation is quite limited to nuclear power plant (NPP) industry. Moreover, the effects of inelastic behavior of soil beneath base-isolated NPP have raised questions to the effectiveness of isolation device. This study applies the wavelet analysis to investigate the effects of soil-structure interaction (SSI) on the seismic response of a base-isolated NPP structure. To evaluate the SSI effects, the NPP structure is modelled as a lumped mass stick model and combined with a soil model using the concept of cone models. The lead rubber bearing (LRB) base isolator is used to adopt the base isolation system. The shear wave velocity of soil is varied to reflect the real rock site conditions of structure. The comparison between seismic performance of isolated structure and non-isolated structure has drawn. The results show that the wavelet analysis proves to be an efficient tool to evaluate the SSI effects on the seismic response of base-isolated structure and the seismic performance of base-isolated NPP is not sensitive to the effects in this case.

Earthquake responses of a base-isolated structure on a multi-layered soft soil foundation by using shaking table tests

Soil-structure interaction (SSI) has a significant effect on the earthquake response of a base-isolated structure, particularly on the rotation response of the SSI system and the isolation performance of the isolation layer, as demonstrated by previous shaking table tests (Zhuang et al., 2014). On a softer soil foundation, the SSI should have a greater influence on the seismic response of an isolated structure. To this end, a new shaking table test is conducted to estimate the effect of SSI on the dynamic characteristics of a base-isolated structure on a multi-layered soil foundation including a soft clay layer. As expected, the isolation efficiency of the isolation layer is reduced by the SSI effects, especially with increasing peak ground acceleration (PGA) of the input motion. Compared with the test results for an isolated structure on a harder soil foundation, the rotation responses of the pile cap and the isolation layer in this study are stronger. Additionally, the rotation responses of the pile cap are significantly amplified by the isolation layer. This type of amplification effect can become stronger with increasing PGA of the input motion, which differs from the results for previous tests with a base-isolated structure on a harder sand foundation. Meanwhile, when the natural isolation property of the softening soil layer is considered, the seismic responses of a base-isolated structure are reduced by the SSI effects because the natural isolation of the soft soil layer can compensate for the lost isolation ability of the isolation layer.

Performance evaluation of base-isolated structures with sliding hydromagnetic bearings

Being an efficient technique for mitigating the seismic response of structures, base isolation is widely used in earthquake engineering practice. The sliding hydromagnetic bearing is viewed as a promising isolation system for seismic protection, owing to its adaptive damping energy dissipation, mild deformation constraint, and easy assembly and maintenance. To verify the applicability and efficiency of sliding hydromagnetic bearings, a performance evaluation of base-isolated structures with a sliding hydromagnetic base-isolation system is carried out in this paper. Numerical simulations are performed to explore the dynamic behaviors of sliding hydromagnetic bearings fabricated recently, for which two models, the power function and polynomial models, are proposed. Parameter identification and model validation are carried out using the data from numerical simulations and quasi-static tests. Backward differential formulas are employed to solve the stiff differential equation invoked by the Coulomb friction associated with the models, which bypasses the computational challenges inherent in conventional integral schemes. For a real-time definition of the instantaneous frequency in the models, the Hilbert–Huang transform is utilized. Comparative studies reveal that the sliding hydromagnetic bearing has an advantage over the lead rubber bearing and the curved surface slider in the control of structural displacement, structural velocity, and structural acceleration. Furthermore, the residual displacement yielded in the sliding hydromagnetic base-isolation system merely shifts the starting position of the bearing on the next movement, and the sliding surface and deformation constraints remain approximately as the initial condition.

Seismic reliability-based design of inelastic base-isolated structures with lead-rubber bearing systems

In this paper, a seismic reliability-based approach is proposed to design inelastic steel moment frame structures isolated by lead-rubber bearing (LRB) systems. An equivalent two-degree-of-freedom system is assumed in which a bilinear behaviour is assigned to both the superstructure and the base. Furthermore, uncertainties associated with the equivalent superstructure mass, stiffness, and yield properties are taken into account by employing proper probability density functions. The proposed design approach is twofold: 1) Reliability curves that return the key design parameters of the inelastic base-isolated structure including: the period of the superstructure, the target base displacement, and the ductility-dependent strength reduction factor for a given target reliability. 2) Regression equations, which estimate the displacement ductility demand of the inelastic superstructure and the optimal design properties of the lead-rubber bearing system including: the total initial stiffness and the total yield force. These regression equations are calibrated against a large set of optimally-designed base-isolated buildings using Genetic Algorithm.

Probabilistic Risk-Based Performance Evaluation of Seismically Base-Isolated Steel Structures Subjected to Far-Field Earthquakes

The performance of base-isolated steel structures having special moment frames is assessed. The archetypes, which are designed per ASCE/SEI 7–2016, are simulated in the Finite Element (FE) computational platform, OpenSees. Adopting nonlinear dynamic analyses using far-field ground motions, the performance of Drift-Sensitive Structural Components (DS-SC), and Drift-/Acceleration-Sensitive Non-Structural Components (DS/AS NSC) at slight, moderate, extensive, and collapse damage states are investigated. The effects of structural height, effective transformed period (Teff), response modification coefficient (RI), and isolation type on the performance of 26 archetypes mounted on Lead Rubber Bearings (LRBs) and Triple Concave Friction Pendulums (TCFPs) are evaluated. Computing 50-year probability of exceedance using the fragility curves and seismic hazard curves of the site, increasing Teff reduces the role of RI in the structural performance; variations in the height, as well as RI, do not affect the risk of damages to the AS-NSC; the risk of collapse is not sensitive to the variations of Teff. The TCFP systems represent superior performance than LRB systems in lower intensities. For longer periods and taller structures, the isolation type has less effect on the performance of NSC. Finally, the archetypes have less than 1% risk of collapse in 50 years; nevertheless, high-rise structures with RI = 2.0 have more than 10% probability of collapse given the maximum earthquake.

Adaptive control of earthquake-excited nonlinear structures with real-time tracking on prescribed performance criteria

A model reference adaptive control method is proposed to suppress the nonlinear responses of earthquake-excited structures with the proposed implementation of performance tracking criteria. The model reference concept is adopted to propose a comprehensive framework for real-time tracking on prescribed performances, and a deterministic reference model is presented to explicitly represent the prior-designed structural performances. The tracking error and the time-varying adaptive control law are systematically presented and designed by using the backstepping theory and the Lyapunov stability analysis. Little prior or explicit knowledge of structural nonlinear behavior is considered for the proposed adaptive backstepping law. Two performance schemes, inspired by the classical linear quadratic regulator control theory and the unknown disturbance cancelation, are developed. Illustrative examples of a bilinear single-degree-of-freedom system and a seismically base-isolated benchmark building structure are employed with a detail illustration and discussion about the control performance of the proposed methodology. Finally, an additional parametric study on the issues of the actuator saturation, the optimum parameter selection, and the switching delay is presented and discussed.

The Use of Limestone Sand for the Seismic Base Isolation of Structures

The possibility of the use of a layer of natural material under foundations for seismic base isolation was investigated. The dissipation of seismic energy of a low-cost natural material with adequate thickness, bearing capacity, and lateral and vertical stiffness, which can serve as an optimal solution for seismic base isolation under the foundations of many structures, was tested. This paper presents the results of a brief experimental study to determine the effectiveness of ordinary limestone sand under the foundation of a cantilever concrete column to increase its seismic resistance. The behavior of small-scale columns with three substrates below the foundation (rigid base, the thin layer of limestone sand, and the thick layer of limestone sand) was investigated by the shake table. The column was exposed to a set of horizontal base accelerations until structure collapse. It was concluded that a layer of limestone sand of appropriate thickness and compressibility can serve as the means a seismic base isolation. The nonlinear numerical model for the dynamic analysis of planar concrete structures coupled with soil is briefly presented and verified by the performed experimental tests.

Base-isolated structure equipped with tuned liquid column damper: An experimental study

Abstract In this study, a novel passive vibration control strategy is investigated experimentally, where a Tuned Liquid Column Damper protects a base-isolated structure. The Tuned Liquid Column Damper is attached to the base, in contrast to typical attachment points of passive energy dissipation devices in high-rise buildings at elevated levels. Experiments on a base-excited small-scale three-story shear frame are conducted in order to study effects of both control devices – base-isolation and Tuned Liquid Column Damper – on the structural model. The dynamic properties of the stand-alone shear frame and the base-isolation subsystem are derived using standard dynamic test methods based on displacement and acceleration response measurements. In the base-isolation subsystem both viscous damping and friction effects are identified. The water level of the Tuned Liquid Column Damper is tracked by means of computational image processing of video recordings, facilitating the identification of the fundamental liquid motion mode as well as the nonlinear damping properties. An experimental parametric study is conducted for three Tuned Liquid Column Damper devices with different frequency tuning ratios. The assessment of the hybrid control strategy is based on the determined transfer functions of the studied setups. Experimental outcomes confirm recent theoretical findings that a passive hybrid control strategy combining a base-isolation and a Tuned Liquid Column Damper reduces the displacement demand of the base-isolation subsystem as well as the total story acceleration demand if both control devices are properly tuned.

Improving the dynamic performance of base-isolated structures via tuned mass damper and inerter devices: A comparative study

Improving the dynamic performance of base-isolated structures via tuned mass damper and inerter devices: A comparative study

An optimum model reference adaptive control algorithm for smart base-isolated structures

In structural control, appropriate control parameters and a stable closed-loop mechanism are required for a controller to achieve optimal performance, particularly in the presence of uncertain structural parameters under external excitation. In this study, an optimum model reference adaptive control (OMRAC) algorithm combining the Linear quadratic regulator method and Model Reference Adaptive Control based on Lyapunov stability is proposed. The OMRAC algorithm is implemented and applied to the response control of a base-isolated structure equipped with magneto-rheological dampers under earthquake excitations. The results from a series of numerical simulations that consider the effects of uncertainty within structural parameters are reported. The performance of the OMRAC algorithm is compared with that of other control algorithms in terms of effectiveness and stability. The results suggest that the proposed OMRAC method can successfully compensate for uncertainties in structural parameters, leading the controlled structure to adaptively track the optimal response of the reference model.

Inelastic response of base-isolated structures subjected to impact

While base isolation provides enhanced structural and non-structural seismic performance, there are still concerns regarding large horizontal displacements in extreme earthquakes. Excessive horizontal displacements may induce detrimental pounding of base-isolated structures against the moat wall or the restraining rim of friction pendulum bearings, resulting in yielding or even collapse of the superstructure. This paper uses a simplified two degrees-of-freedom model to investigate the inelastic responses of various base-isolated superstructure designs subjected to impact. It is found that the stiffness of the superstructure largely dictates its response to the transient impact force. The stiff superstructure is very sensitive to the impact force and develops considerably large ductility demands compared to the case with the absence of impact. However, for the flexible superstructure, the ductility demand is more dependent on system yielding under the input motion and does not increase as dramatically when impact is introduced, indicating that it is less sensitive to impact.

Copula-based seismic fragility assessment of base-isolated structures under near-fault forward-directivity ground motions

The seismic fragility of base-isolated structures subjected to near-fault forward-directivity ground motions is investigated. A general framework for deriving the system-level fragility curve is proposed, where the vulnerability contributions of multiple interrelated components to the overall system are incorporated. In this framework, the joint probabilistic seismic demand model (JPSDM) is established, where the joint probability distribution of multiple engineering demand parameters (EDPs), conditioned on the intensity measure (IM) level, is characterized via copula approach, and the sampling-based JPSDM, along with the capacity models of these components, is employed to generate the overall system fragility curves. This proposed framework is applied in the case study of a typical seismically-isolated RC frame structure, where the peak lateral displacement in the base-isolation layer and the maximum inter-story drift in the superstructure are considered as the two major component EDPs. The analysis results indicate that the combination of t copula, which is quantitatively identified as the best-fit copula function, and the conditional lognormal marginal distribution adequately captures the joint probability distribution of these two EDPs conditioned on the IM level. Moreover, the impact of different copulas selection on the system-level fragility varies depending on the relative fragility contributions of different components in the overall system.