Simulation of the response of base-isolated buildings under earthquake excitations considering soil flexibility

The accurate analysis of the seismic response of isolated structures requires incorporation of the flexibility of supporting soil. However, it is often customary to idealize the soil as rigid during the analysis of such structures. In this paper, seismic response time history analyses of base-isolated buildings modelled as linear single degree-of-freedom （SDOF） and multi degree-of-freedom （MDOF） systems with linear and nonlinear base models considering and ignoring the flexibility of supporting soil are conducted. The flexibility of supporting soil is modelled through a lumped parameter model consisting of swaying and rocking spring-dashpots. In the analysis, a large number of parametric studies for different earthquake excitations with three different peak ground acceleration （PGA） levels, different natural periods of the building models, and different shear wave velocities in the soil are considered. For the isolation system, laminated rubber bearings （LRBs） as well as high damping rubber bearings （HDRBs） are used. Responses of the isolated buildings with and without SSI are compared under different ground motions leading to the following conclusions： （1） soil flexibility may considerably influence the stiff superstructure response and may only slightly influence the response of the flexible structures; （2） the use of HDRBs for the isolation system induces higher structural peak responses with SSI compared to the system with LRBs; （3） although the peak response is affected by the incorporation of soil flexibility, it appears insensitive to the variation of shear wave velocity in the soil; （4） the response amplifications of the SDOF system become closer to unit with the increase in the natural period of the building, indicating an inverse relationship between SSI effects and natural periods for all the considered ground motions, base isolations and shear wave velocities; （5） the incorporation of SSI increases the number of significant cycles of large amplitude accelerations for all the stories, especially for earthquakes with low and moderate PGA levels; and （6） buildings with a linear LRB base-isolation system exhibit larger differences in displacement and acceleration amplifications, especially at the level of the lower stories.

Seismic Responses of Asymmetric Base-Isolated Structures under Near-Fault Ground Motion

An inter-story shear model of asymmetric base-isolated structures incorporating deformation of each isolation bearing was built, and a method to simultaneously simulate bi-directional near-fault and far-field ground motions was proposed. A comparative study on the dynamic responses of asymmetric base-isolated structures under near-fault and far-field ground motions were conducted to investigate the effects of eccentricity in the isolation system and in the superstructures, the ratio of the uncoupled torsional to lateral frequency of the superstructure and the pulse period of near-fault ground motions on the nonlinear seismic response of asymmetric base-isolated structures. Numerical results show that eccentricity in the isolation system makes asymmetric base-isolated structure more sensitive to near-fault ground motions, and the pulse period of near-fault ground motions plays an import role in governing the seismic responses of asymmetric base-isolated structures.

Numerical Analysis of Seismic Elastomeric Isolation Bearing in the Base-Isolated Buildings

Base isolation concept is currently accepted as a new strategy for earthquake resistance structures. According to different types of base isolation devices, laminated rubber bearing which is made by thin layers of steel shims bonded by rubber is one of the most popular devices to reduce the effects of earthquake in the buildings. Laminated rubber bearings should be protected against failure or instability because failure of isolation devices may cause serious damage on the structures. Hence, the prediction of the behaviour of the laminated rubber bearing with different properties is essential in the design of a seismic bearing. In this paper, a finite element modeling of the laminated rubber bearing is presented. The procedures of modeling the rubber bearing with finite element are described. By the comparison of the numerical and the experimental, the validities of modelling and results have been determined. The results of this study perform that there is a good agreement between finite element analysis and experimental results.

A modified Kelvin impact model for pounding simulation of base-isolated building with adjacent structures

Base isolation can effectively reduce the seismic forces on a superstructure, particularly in lowto medium-rise buildings. However, under strong near-fault ground motions, pounding may occur at the isolation level between the baseisolated building （BIB） and its surrounding retaining walls. To effectively investigate the behavior of the BIB pounding with adjacent structures, after assessing some commonly used impact models, a modified Kelvin impact model is proposed in this paper. Relevant parameters in the modified Kelvin model are theoretically derived and numerically verified through a simple pounding case. At the same time, inelasticity of the isolated superstructure is introduced in order to accurately evaluate the potential damage to the superstructure caused by the pounding of the BIB with adjacent structures. The reliability of the modified Kelvin impact model is validated through numerical comparisons with other impact models. However, the difference between the numerical results from the various impact analytical models is not significant. Many numerical simulations of BIBs are conducted to investigate the influence of various design parameters and conditions on the peak inter-story drifts and floor accelerations during pounding. It is shown that pounding can substantially increase floor accelerations, especially at the ground floor where impacts occur. Higher modes of vibration are excited during poundings, increasing the inter-story drifts instead of keeping a nearly rigid-body motion of the superstructure. Furthermore, higher ductility demands can be imposed on lower floors of the superstructure. Moreover, impact stiffness seems to play a significant role in the acceleration response at the isolation level and the inter-story drifts of lower floors of the superstructure. Finally, the numerical results show that excessive flexibility of the isolation system used to minimize the floor accelerations may cause the BIB to be more susceptible to pounding under a limited seismic gap.

Identification of the nonlinear properties of rubber-bearings in base-isolated buildings with limited seismic response data

Seismic behaviors of base-isolated structures are highly affected by the nonlinear characteristics of the isolated systems. Most of the currently available methods for the identification of nonlinear properties of isolator require either the measurements of all structural responses or the assumptions of the proper mathematic models for the rubber-bearings. In this paper, two algorithms are proposed to identify the nonlinear properties of rubber-bearings in base-isolated buildings using only partial measurements of structural dynamic responses. The first algorithm is applicable to the case that proper mathematical models are available for the base isolators. It is based on the extended Kalman filter for the parametric identification of nonlinear models of rubber-bearing isolators and buildings. For the general case where it is difficult to establish a proper mathematical model to describe the nonlinear behavior of a rubber-bearing isolator, another algorithm is proposed to identify the model-tYee nonlinear property of rubber-bearing isolated system. Nonlinear effect of rubber-bearing is treated as ＇fictitious loading＇ on the linear building under severe earthquake. The algorithm is based on the sequential Kalman estimator for the structural responses and the least-squares estimation of the ＇fictitious loading＇ to identify the nonlinear force of rubber-bearing isolator. Simulation results demonstrate that the proposed two algorithms are capable of identifying the nonlinear properties of rubber-bearing isolated systems with good accuracy.

Adaptive base-isolation of civil structures using variable amplification

Semi-active dampers are used in base-isolation to reduce the seismic response of civil engineering structures. In the present study, a new semi-active damping system using variable amplification will be investigated for adaptive baseisolation. It uses a novel variable amplification device （VAD） connected in series with a passive damper. The VAD is capable of producing multiple amplification factors, each corresponding to a different amplification state. Forces from the damper are amplified to the structure according to the current amplification state, which is selected via a semi-active control algorithm specifically tailored to the system＇s tmique damping characteristics. To demonstrate the effectiveness of the VAD-damper system for adaptive base-isolation, numerical simulations are conducted for three and seven-story base-isolated buildings subject to both far and near-field ground motions. The results indicate that the system can achieve significant reductions in response compared to the base-isolated buildings with no damper. The proposed system is also found to perform well compared to a typical semi-active damper.

Experimental and theoretical study on vibration control of base-isolation with energy transducer

In order to evaluate the effects of structural control and energy transition for the base-isolation with energy transducer （BIET）, shaking table tests on a steel frame model （BIET system） with scale of 1：4 were conducted and the results were compared with the lead rubber beating （LRB） isolation system for the same model. Then numerical analysis of the system was carried out, in which the improved Wen analytic model was used to simulate the hysteretic law of transducers. The results show that the structural system can transform the partial earthquake energy to hydraulic energy ; furthermore, the effect of structural control can reach or be close to that of the LRB isolation system. The agreements between numerical analysis results and those of shaking table tests demonstrate the accuracy of the numerical model.

Simple Method for Dynamic Responses of Soil-Pile-Isolated Structure Interaction System

To investigate the effect of soil-pile-structure interaction(SPSI effect)on the dynamic response of a baseisolated structure with buried footings on a pile foundation,certain shake table tests are previously conducted.Based on the test results and the existing related studies,an efficient simplified model and a corresponding calculation method are verified for estimating the dynamic characteristics of a base-isolated structure with buried footings on a pile foundation with the SSI effect.In this method,the solutions by Veletsos and co-workers for a non-isolated structure with the SSI effect are verified and advanced for a base-isolated structure,and the solutions by Maravas and co-workers for a non-isolated structure on a pile foundation are introduced to consider the effect of the piles.By comparison with the shake table test,this work proves that the simplified method can efficiently estimate the dynamic responses of a base-isolated structure with buried footings on a pile foundation.Using parameter analysis,this work also shows that the dynamic characteristics of a non-isolated structure are quite similar to those of the base-isolated structure when the soil foundation is sufficiently soft,which means that the isolation layer gradually loses its isolation function as the soil foundation softens.

Nonlinear analysis of r.c. framed buildings retrofitted with elastomeric and friction bearings under near-fault earthquakes

Reinforced concrete （r.c.） framed buildings designed in compliance with inadequate seismic classifi- cations and code provisions present in many cases a high vulnerability and need to be retrofitted. To this end, the insertion of a base isolation system allows a considerable reduction of the seismic loads transmitted to the super- structure. However, strong near-fault ground motions, which are characterised by long-duration horizontal pulses, may amplify the inelastic response of the superstructure and induce a failure of the isolation system. The above considerations point out the importance of checking the effectiveness of different isolation systems for retrofitting a r.c. framed structure. For this purpose, a numerical inves- tigation is carried out with reference to a six-storey r.c. framed building, which, primarily designed （as to be a fixed-base one） in compliance with the previous Italian code （DM96） for a medium-risk seismic zone, has to be retrofitted by insertion of an isolation system at the base for attaining performance levels imposed by the current Italian code （NTC08） in a high-risk seismic zone. Besides the （fixed-base） original structure, three cases of base isolation are studied： elastomeric bearings acting alone （e.g. HDLRBs）; in-parallel combination of elastomeric and friction bearings （e.g. high-damping-laminated-rubber beatings, HDLRBs and steel-PTFE sliding bearings, SBs）; friction bearings acting alone （e.g. friction pendulum bearings, FPBs）. The nonlinear analysis of the fixed-base and base-isolated structures subjected to horizontal com- ponents of near-fault ground motions is performed for checking plastic conditions at the potential critical （end） sections of the girders and columns as well as critical conditions of the isolation systems. Unexpected high val- ues of ductility demand are highlighted at the lower floors of all base-isolated structures, while re-centring problems of the base isolation systems under near-fault earthquakes are expected in case of friction beatings acting alone （i.e. FPBs） or that in combination （i.e. SBs） with HDLRBs.

Dynamic Responses Analysis of a Building Structure Subjected to Ground Shock from a Tunnel Explosion

Dynamic responses of a multi-storey building without or with a sliding base-isolation device for ground shock induced by an in-tunnel explosion are numerically analyzed. The effect of an adjacent tunnel in between the building and the explosion tunnel, which affects ground shock propagation , is considered in the analysis. Different modeling methods, such as the eight-node equal-parametric finite element and mass-lumped system, are used to establish the coupling model consisting of the two adjacent tunnels, the surrounding soil medium with the Lysmer viscous boundary condition, and the multi-storey building with or without the sliding base-isolation device. In numerical calculations , a continuous friction model, which is different from the traditional Coulomb friction model, is adopted to improve the computational efficiency and reduce the accumulated errors. Some example analyses are subsequently performed to study the response characteristics of the building and the sliding base-isolation device to ground shock. The effect of the adjacent tunnel in between the building and the explosion tunnel on the ground shock wave propagation is also investigated. The final conclusions based on the numerical results will provide some guidance in engineering practice.

Test and numerical simulation of a new type of hybrid control technique

In this paper, a new hybrid control technique, based on a combination of base-isolation and semi-active variable stiffness/damping in a superstructure, is presented. To illustrate the efficiency of the proposed control system, model tests on a mini-electromagnetic shaking table and a numerical simulation were performed. The test and numerical calculation results indicate that this new hybrid control mode with additional damping and smaller additional stiffness can achieve a better control efficiency.