Seismic Response Of Base-isolated Structures Research Articles

Seismic analysis and performance of semi-active spring for base-isolated structures

The present study investigates the performance of a semi-active spring (SAS) in the mitigation of the seismic response of base-isolated structures. Initially, under stationary filtered white-noise earthquake excitation, the response of the multi-floor flexible base-isolated structure with SAS is examined to observe the response control effects. The equivalent linearization technique is used to obtain the stochastic response, as the force-deformation behaviour of SAS is non-linear. The performance of SAS in terms of added stiffness, damping, and response mitigation to the isolated structure is also investigated. It is found that the SAS controls the isolator displacement effectively. Furthermore, it was noted that there is an optimum stiffness value for the SAS devices for a particular system and excitation, at which point the RMS top floor acceleration reaches a minimum value. Next, approximate formulas are proposed for the RMS isolator displacement, the top floor absolute acceleration, and the optimum stiffness of the SAS. It is observed that these formulas accurately predict the expected response and can be applied to the initial design of base-isolated structures using SAS. Finally, using the non-linear model of the SAS, the seismic response of flexible base-isolated structures is determined for actual near-fault earthquakes, considering different values of the isolation periods and stiffness of the SAS device. The SAS was effective in controlling the isolator displacement under near-fault motions. The trends of the results of isolated structures with SAS devices under near-fault earthquake motions were also in good agreement with those under stochastic excitation.

On the displacement control performance of base-isolated structures incorporating nonlinear negative stiffness device

Low-frequency dominant far-field harmonic-like long period ground motions (FGMs) excitations have the potential to amplify the seismic response of base-isolated structures (BIS) and may result in isolator displacements exceeding their maximum limits. The application of negative stiffness devices (NSD) in isolated structures primarily focuses on SDOF systems, which fails to capture the displacement response of the superstructure on the isolation story. To address this issue, this study proposes the application of NSD to a 2DOF system in a BIS, with root mean square value as the metric for displacement transmissibility. Firstly, the displacement transmissibility of base-isolated structures with negative stiffness devices (BIS-NSD) and BIS is derived using the triple-harmonic balance (T-HB) method and the single-harmonic balance (S-HB) method, respectively. Subsequently, the applicability of the T-HB method and the S-HB method in obtaining analytical solution for the displacement transmissibility of BIS-NSD and BIS is revealed and validated through numerical solution. Parameters are selected based on fixed-point theorem for comparative analysis of the displacement and acceleration transmissibility between BIS-NSD and BIS. Finally, a comparative analysis between BIS-NSD and BIS under FGMs confirms the application advantages of NSD. The results indicate that compared to BIS, BIS-NSD significantly reduces the displacement transmissibility of the superstructure and the isolation story in the low-frequency range. As the primary resonance frequency decreases, NSD widens the vibration bandwidth of BIS. BIS-NSD effectively mitigates the increase in displacement caused by low-frequency ground motions. The acceleration transmissibility demonstrates that, without sacrificing the acceleration of the superstructure, it effectively controls the displacement of the isolation story.

λ-Factors for the Upper and Lower Bound Analyses of Base-Isolated Structures: Historical Review of Code Provisions for Elastomeric Bearings

The seismic response of base-isolated structures is notably influenced by mechanical properties of isolation devices due to their essential role in structural behavior. Consequently, the variability of such properties should be accounted for in the design process. The current seismic codes prescribe a simplified approach based on structural analyses in two extreme situations resulting from the upper and lower bound design properties of bearings (upper and lower bound analyses). In the case that experimental data are not provided by manufacturers, seismic codes provide the so-called “property modification factors” or “λ-factors”, i.e., modification coefficients to be applied to the nominal dynamic properties of bearings to obtain their upper or lower design properties. The aim of this paper is to provide a historical review of values provided for such factors by the main seismic codes by highlighting the limits, as well as some clerical errors, present in some codes. In particular, the European seismic codes are illustrated in detail, i.e., the Eurocode for bridges (EN 1998-2) and product standard on anti-seismic devices (EN 15129). Both these codes account for different sources of variability, such as the bearings production and the environmental and behavioral effects. For all these effects, the same λ-factor values are provided by the two codes, deriving from the second version of the AASHTO guide specifications for seismic isolation of bridges (AASHTO 1999), which are based on limited and/or old data, especially for high damping rubber bearings (HDRBs), and were never updated in the successive versions. More recent standards are also illustrated, providing different perspectives that deserve attention, even though they require further investigations to be applied in the design practice.

Seismic performance of supplemental inerter and spring with on-off effects for base-isolated structures

The present study investigates the performance of supplemental inerter and spring with on-off effects (ISWOE) in the mitigation of the seismic response of base-isolated structures. Firstly, the response of the rigid base-isolated structure with ISWOE is investigated to see the response control effects of ISWOE under stationary earthquake excitation. The performance of ISWOE in the response reduction of the isolated structure is compared with the corresponding passive inerter and spring. The equivalent damping that was added to the base-isolated structures by the ISWOE was used to measure its performance in mitigating the seismic response. The equivalent damping of the ISWOE is obtained for different values of the isolation period and damping ratio. Subsequently, equations for the equivalent damping of the ISWOE and displacement responses are proposed, and it is observed that they match well with the obtained numerical results. Secondly, using the non-linear model of the ISWOE, the seismic response of flexible base-isolated structures is determined for actual earthquakes, considering different values of the isolation period and ISWOE parameters. The trends of the results of isolated structures with ISWOE under the actual earthquake motions were in good agreement with those under stochastic excitation. Finally, the seismic response of isolated structures with ISWOE by non-linear analysis is compared with the corresponding linear analysis with equivalent parameters of the ISWOE. The isolator displacement of the structures with the ISWOE by the non-linear analysis was observed to match those achieved using the equivalent parameters and by the linear analysis.

Response of base–isolated structures-evaluation of different isolation systems

Base isolation is a seismic mitigation technique that separates the structure from the ground motion by placing isolators between the foundation and the superstructure. This creates a flexible base that allows the structure to move independently of the ground, reducing the seismic forces transmitted to the superstructure. Different isolation systems have been developed, each with advantages and disadvantages. This paper evaluates the seismic response of base-isolated structures with varying isolation systems under various earthquake ground motions. The results show that base isolation can significantly reduce the seismic response of structures and that the effectiveness of different isolation systems depends on ground motion characteristics.

Performance of Supplemental Inertial Devices for Base-Isolated Structures

The seismic response of base-isolated structures with supplemental inertial devices subjected to stationary and real earthquake excitation is presented. Three types of devices, namely inertial mass damper (IMD), tuned mass damper-inerter (TMDI), and clutching inerter damper (CID), are considered. The seismic response of the base-isolated structure with and without inertial devices is compared to assess their effectiveness. Under stationary white-noise excitation, the optimum parameters of these devices were obtained. The criterion selected for optimality is the minimization of the mean-square relative displacement and absolute acceleration of the isolated structure. An equivalent linearization method was used for base-isolated structures with CID under stochastic excitation. It was observed that the IMD is not very effective in controlling the response of base-isolated structures. Optimally designed TMDI, on the other hand, was found to be effective in controlling the displacement and acceleration response of the isolated structure, with the effectiveness increasing as the inertance mass ratio increased. The CID involved in the base-isolated structure reduces the structural natural frequency and increases structural damping, thereby reducing isolator displacement and structural acceleration. The high-frequency components were present in the absolute acceleration of base-isolated structure with inertial devices (more pre-dominating for IMD and CID), which may have detrimental effects on installed high-frequency sensitive equipment.

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.

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

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

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.

Online tuning of fractional order fuzzy PID controller in smart seismic isolated structures

This paper aims at designing an adaptive fractional order fuzzy proportional–integral-derivative controller for seismic control of smart base-isolated structure by means of variable friction dampers (VFD). One main challenge occurs when large displacement of the isolator happens due to near-field motions. To overcome this challenge, a solution is to use VFDs. However, the floor accelerations of the superstructure can increase because of sudden changes in the damper friction force of VFDs. Therefore, a suitable control strategy is desired to handle the displacement of isolator without any increase in superstructure acceleration responses during both far-field and near-field earthquakes. First, a sub-level fractional order fuzzy PID (FOFPID) controller is designed to reduce the isolator displacement without significant increase in roof acceleration based on a multi-objective optimization algorithm. Using an adaptive strategy, the fuzzy rule weights of the FOFPID controller are then tuned on-line based on information sensed from both the earthquake and the building responses. Considering nine important performance criteria and several real-data earthquakes, numerical studies are carried out for a benchmark base-isolated structure equipped with VFDs. Simulation results show the superior performance of the proposed controller in mitigation of seismic responses of base-isolated structure against various types of earthquakes in comparison with other controllers presented in previous researches.

Effects of vertical component of near-field ground motions on seismic responses of asymmetric structures supported on TCFP bearings

The effects of vertical component of earthquakes on torsional amplification due to mass eccentricity in seismic responses of base-isolated structures subjected to near-field ground motions are studied in this paper. 3-, 6- and 9-story superstructures and aspect ratios of 1, 2 and 3 have been modeled as steel special moment frames mounted on Triple Concave Friction Pendulum (TCFP) bearings considering different period and damping ratios. Three-dimensional linear superstructures resting on nonlinear isolators are subjected to both 2 and 3 component near-field ground motions. Effects of mass eccentricity and vertical component of 25 near-field earthquakes on the seismic responses including maximum isolator displacement and base shear as well as peak superstructure acceleration are studied. The results indicate that the effect of vertical component on the responses of asymmetric structures, especially on the base shear is significant. Therefore, it can be claimed that in the absence of the vertical component, mass eccentricity has a little effect on the base shear increase. Additionally, the impact of this component on acceleration is remarkable so the roof acceleration of a nine-story structure has been increased 1.67 times, compared to the case that the structure is subjected to only horizontal components of earthquakes.

Optimal design of tuned liquid column dampers for seismic response control of base-isolated structures

In this paper, the use of a tuned liquid column damper (TLCD) as a cost-effective means to control the seismic response of a base-isolated structure is studied. A straightforward direct approach for the optimal design of such a device is proposed, considering a white noise model of the base excitation. On this base, a direct optimization procedure of the TLCD design parameters is performed and optimal design charts are presented as a ready-to-use practical design tool. Comparison with the optimal parameters obtained considering a classical iterative statistical linearization technique proves the reliability of the proposed approach. The performance of the base-isolated TLCD-controlled structure is examined and compared with that of the simple base-isolated one (without TLCD), using a set of 44 recorded ground motions as base excitation. Theoretical and numerical results show that the TLCD is rather effective in reducing the response of base-isolated structures under strong earthquakes. Therefore, considering its advantageous characteristics and its overall beneficial effects, TLCDs can be considered as practical and appealing means to control the seismic response of base-isolated structures.

Seismic responses of asymmetric steel structures isolated with the TCFP subjected to mathematical near-fault pulse models

In this paper, the effects of mass eccentricity of superstructure as well as stiffness eccentricity of isolators on the amplification of seismic responses of base-isolated structures are investigated by using mathematical near-fault pulse models. Superstructures with 3, 6 and 9 stories and aspect ratios equal to 1, 2 and 3 are mounted on a reasonable variety of Triple Concave Friction Pendulum (TCFP) bearings considering different period and damping ratio. Three-dimensional linear superstructure mounted on nonlinear isolators are subjected to simplified pulses including fling step and forward directivity while various pulse period (Tp) and Peak Ground Velocity (PGV) amounts as two crucial parameters of these pulses are scrutinized. Maximum isolator displacement and base shear as well as peak superstructure acceleration and drift are selected as the main engineering demand parameters. The results indicate that the torsional intensification of different demand parameters caused by superstructure mass eccentricity is more significant than isolator stiffness eccentricity. The torsion due to mass eccentricity has intensified the base shear of asymmetric 6-story model 2.55 times comparing to symmetric one. In similar circumstances, the isolator displacement and roof acceleration are increased 49 and 116 percent respectively in the presence of mass eccentricity. Furthermore, it is demonstrated that torsional effects of mass eccentricity can force the drift to reach the allowable limit of ASCE 7 standard in the presence of forward directivity pulses.

Seismic Responses of Isolated Structures with Mass Asymmetry Mounted on TCFP Subjected to Near-Fault Ground Motions

This study is devoted to investigate the effects of mass eccentricity on seismic responses of base-isolated structures subjected to near-field ground motions. Superstructures with 3, 6 and 9 stories with aspect ratios equal to 1, 2 and 3 have been idealized as steel special moment frames resting on a reasonable variety of triple concave friction pendulum bearings considering different period and damping ratios for the isolators. Three-dimensional linear superstructures mounted on nonlinear isolators have been subjected to three components of near-field ground motions. Under 25 near-field ground motions, effects of mass eccentricity on the main parameters of the systems have been studied. These parameters have been selected as the main engineering demands including maximum isolator displacement and base shear as well as peak superstructure acceleration. The results indicate that the mass eccentricities do not have a remarkable effect on isolator displacement. In contrary to displacement, torsional effect of mass eccentricity has raised the base shear up to 1.75 times in a three-story superstructure. Additionally, mass eccentricity can amplify the roof acceleration of a nine-story model approximately 3 times in comparison with a symmetric one. It is also concluded that eccentricity in the direction of the subjected earthquake has the most impact on the base shear, while the isolator displacement and roof acceleration are mostly influenced by the eccentricity perpendicular to the earthquake path.

Nonlinear incremental analysis of fire-damaged r.c. base-isolated structures subjected to near-fault ground motions

Amplification of structural response of r.c. base-isolated structures is expected under near-fault ground motions, yet there is a lack of knowledge of their behavior in the case of fire. To investigate the nonlinear seismic response following a fire, an incremental dynamic analysis is carried out on five-storey r.c. base-isolated framed buildings with fire-protected High-Damping-Laminated-Rubber Bearings (HDLRBs), designed in line with the Italian seismic code. Horizontal components of near-fault ground motions characterized by forward-directivity or fling-step pulse-type are considered. The nonlinear seismic response of base-isolated structures in a no fire situation is compared with that in the event of fire, at 45 (i.e. R45) and 60 (i.e. R60) minutes of fire resistance, assuming both damaged (i.e. DS) and repaired (i.e. RS) stiffness conditions. Five fire scenarios are considered assuming the fire compartment confined to the area of the first level (i.e. F1), the first two (i.e. F1/2) and the upper (i.e. Fi, i=3–5) levels, with the parametric temperature–time fire curve evaluated in accordance with Eurocode 1. The nonlinear seismic analysis is performed by using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the critical (end) sections of the girders and columns, where thermal mapping with reduced mechanical properties is evaluated with the 500°C isotherm method proposed by Eurocode 2. A viscoelastic model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, simulates the response of an HDLRB.

The effects of peak ground velocity of near-field ground motions on the seismic responses of base-isolated structures mounted on friction bearings

This research has been conducted in order to investigate the effects of peak ground velocity (PGV) of near-field earthquakes on base-isolated structures mounted on Single Friction Pendulum (SFP), Double Concave Friction Pendulum (DCFP) and Triple Concave Friction Pendulum (TCFP) bearings. Seismic responses of base-isolated structures subjected to simplified near field pulses including the forward directivity and the fling step pulses are considered in this study. Behaviour of a two dimensional single story structure mounting on SFP, DCFP and TCFP isolators investigated employing a variety range of isolators and the velocity (PGV) of the forward directivity and the fling step pulses as the main variables of the near field earthquakes. The maximum isolator displacement and base shear are selected as main seismic responses. Peak seismic responses of different isolator types are compared to emphasize the efficiency of each one under near field earthquakes. It is demonstrated that rising the PGVs increases the isolator displacement and base shear of structure. The effects of the forward directivity are greater than the fling step pulses. Furthermore, TCFP isolator is more effective to control the near field effects than the other friction pendulum isolators are. This efficiency is more significant in pulses with longer period and greater PGVs.

The effects of peak ground velocity of near-field ground motions on the seismic responses of base-isolated structures mounted on friction bearings

This research has been conducted in order to investigate the effects of peak ground velocity (PGV) of near-field earthquakes on base-isolated structures mounted on Single Friction Pendulum (SFP), Double Concave Friction Pendulum (DCFP) and Triple Concave Friction Pendulum (TCFP) bearings. Seismic responses of base-isolated structures subjected to simplified near field pulses including the forward directivity and the fling step pulses are considered in this study. Behaviour of a two dimensional single story structure mounting on SFP, DCFP and TCFP isolators investigated employing a variety range of isolators and the velocity (PGV) of the forward directivity and the fling step pulses as the main variables of the near field earthquakes. The maximum isolator displacement and base shear are selected as main seismic responses. Peak seismic responses of different isolator types are compared to emphasize the efficiency of each one under near field earthquakes. It is demonstrated that rising the PGVs increases the isolator displacement and base shear of structure. The effects of the forward directivity are greater than the fling step pulses. Furthermore, TCFP isolator is more effective to control the near field effects than the other friction pendulum isolators are. This efficiency is more significant in pulses with longer period and greater PGVs.

Shaking table tests for the seismic response of a base-isolated structure with the SSI effect

To estimate the effect of the soil–structure interaction (SSI) on the seismic response of base-isolated structures, four shaking table tests were carried out. The influence of the SSI effect on the seismic response of base-isolated structures is verified and some new findings are reported. According to the tests, the damping ratios of the interaction systems with the SSI effects are larger than those of isolated structures on rigid foundations. The acceleration magnification factor (AMF) at each floor of an isolated structure is larger with an SSI effect, and the change law is also affected. With a greater PGA of input motion, AMFs increase monotonically along the height of the isolated structure with the SSI effect. The more important finding is that the rotation response of a foundation can be amplified by the isolation layer, which further reduces the isolation efficiency of the isolation layer. For non-isolated structures on soil foundations, the softer soil layer can be considered as an efficient isolation layer that reduces the seismic response of the structure.

Shaking Table Test Study on Seismic Responses of Base-Isolated Buildings under Near-Fault Ground Motions

The shaking table tests of a 1:7 scale model of three-floor steel frame base-isolated building was completed to study the seismic responses of base-isolated buildings under near-fault ground motions. Under the action of the typical near-fault seismic wave, the seismic responses of base-isolated structures increase with the increase of PGA. The maximum story displacements of super-structure decrease with increase of story. The velocity pulse has an adverse effect to acceleration responses of base-isolated structures. The isolation effect of base-isolated super-structures is still favorable under near-fault ground motions, but it will be necessary to add damping in isolation system or limit the displacement of bearings to prevent the excessive deformation of isolation layer.

Impact of structures with double concave friction pendulum bearings on adjacent structures

The double concave friction pendulum bearing is a new kind of friction pendulum bearing that consists of two sliding surfaces. To study the seismic responses of three-dimensional base-isolated structures using double concave friction pendulum systems during impact with adjacent structures, after deriving the coupled differential equations of motion, a computer program was developed in order to obtain the time history response of base-isolated structures under earthquake excitations. In the first part of this paper, the seismic response of three-dimensional base-isolated structures using double concave friction pendulum bearings considering tri-linear and bi-linear behaviours without adjacent structures is investigated. The advantages and disadvantages of tri-linear over bi-linear behaviour, considering the principal parameters, are scrutinised. It is demonstrated that using tri-linear double concave friction pendulum bearing behaviour rather than the bi-linear one can decrease the base shear up to 48%. However, tri-linear double concave friction pendulum bearings cause larger displacements of sliding surfaces than bi-linear ones, by up to 57%. The second part of the paper studies the effects of essential parameters such as separation distance, stiffness of impact elements, isolation period and the behaviour of double concave friction pendulum bearings on the seismic response of base-isolated structures using double concave friction pendulum systems during impact with adjacent structures. Numerical simulations demonstrate that base shear force increases significantly and, vice versa, bearing displacement decreases considerably due to impact of the isolated structure with adjacent structures. In addition, the hysteretic curve of the bearing affects the responses of the isolated structure during impact with adjacent structures.