Near-fault Ground Motions Research Articles

Seismic response of isolated structures with an improved model of the UFREI

An improved formulation of the horizontal shear behavior of the unbonded fiber-reinforced elastomeric isolators (UFREI), which predicts both the pre-rollover and the post-rollover behavior considering the effect of the vertical load, is proposed. A simplified and computationally inexpensive formulation of the horizontal shear behavior of the UFREI has also been proposed afterward. Further, these formulations are verified using experimental results and compared with the results obtained using the other earlier methods in the recent literature. The horizontal shear behavior predicted by both of the proposed formulations is observed to fit the experimental results very well. The shear behavior predicted by these formulations is observed to be more accurate than the corresponding predicted by the other formulations from the recent literature. A parametric study characterizing the effect of the vertical pressure and the initial elastomeric shear modulus on the horizontal shear behavior of isolators is also carried out. The proposed formulations are further used to model the behavior of the UFREI employed to isolate the benchmark structure considered in the study.Supplementary damping devices are further installed alongside the nonlinear UFREI isolation system to mitigate the excessive displacement demand on isolators under near-fault ground motions. The isolated benchmark structure is thus coupled with a Magneto-rheological Damper (MRD) with an improved control algorithm for nonlinear isolation systems in three configurations, i.e., semi-active, passive-on, and passive-off. A comparative study of these configurations shows that MRD in passive-on mode provides the minimum isolators displacement response, but an overall improved structural response is achieved with MRD in the semi-active mode of operation.

Effects of near-fault pulse-type ground motions on high-speed railway simply supported bridge and pulse parameter analysis

Based on an innovative high-speed railway simply supported bridge (HSRSSB) model, the potential influence of near-fault pulse-type ground motions on HSRSSB is analyzed. Eighty-five original records of such ground motions are imposed on the simulation model. The dynamic responses of the bridge are calculated, including girder displacement, rail displacement, pier bending moment, bearing deformation, and residual displacement. Linear regression analysis reveals the relationship between the dynamic response and intensity measures (IMs). Moreover, six typical ground motions are chosen from these eighty-five records as prototypes in order to perform a pulse parameter analysis. Through an artificial ground motion synthesis, the effects of parameters, such as pulse amplitude, pulse period, and pulse number, on the behavior of HSRSSB are investigated and discussed. It is concluded that the response of HSRSSB depends on the acceleration-type IMs and presents a strong linear relationship with spectral IMs at the fundamental period of the structure. Regarding the pulse parameters, the HSRSSB dynamic response shows a positive correlation relationship with pulse amplitude, but a negative correlation with pulse period.

P-Delta effects on nonlinear dynamic response of steel moment-resisting frame structures subjected to near-fault pulse-like ground motions

The objective of this study is to explore the P-Delta effects on the steel moment-resisting frame structures (MRFs) subjected to the near-fault ground motions with large, medium, and small pulse periods. The 3-, 9-, and 20-story MRFs designed for the American SAC Phase II Steel Project are used as benchmark models. Three near-fault and one far-field ground motion groups, each with 11 different records, are selected for the nonlinear time-history analysis. The P-Delta effects are quantified based on peak inter-story drift ratio (PIDR) demands. When the near-fault ground motions act on the structures, even if the structures have the low height and the earthquake is weak, the P-Delta effects could not be ignored. The nonlinear response of the group with Tp near to T1 is the largest, but the largest P-Delta effects appear in the group with TP slightly larger than T1. P-Delta effects do not influence the prediction of the weakest floor and the PIDR demands due to the ground motions which can trigger higher-mode effects. However, for long-period buildings, P-Delta effects can sharply increase the components’ distortion at the low half of the structure and even change the structural collapse direction, although P-Delta effects would decrease the demands at the middle stories. The panel zones are significantly important components for the seismic performance of MRFs and not only the weakest floor but also all the floors at the low half of the building are worthy of attention in the seismic design of MRFs.

Comparison of meta-heuristic approaches for the optimization of non-linear base-isolation systems considering the influence of superstructure flexibility

The main aim of this study is to investigate the meta-heuristic approaches called Harmony Search (HS), Grey Wolf Optimizer (GWO), Teaching-Learning Based Optimization (TLBO), and Jaya Algorithm (JA) in the optimization process of non-linear base isolation systems under near-fault earthquakes considering the effect of the superstructure flexibility. The optimization processes were performed by achieving the objective function set as minimizing the peak top floor acceleration to peak ground acceleration ratio with and without base displacement limits. The context of the optimization process, the analytical model of the superstructure, and the number of isolators are considered as the design constants. The mechanical parameters of the non-linear isolation systems, such as the isolation system period, the total characteristic strength ratio, and the yield displacement, are determined as the independent design variables. According to the results obtained in this study, GWO, JA, and TLBO algorithms can be more suitable for solving such design problems among the considered algorithms. Although the objective function values are amplified with the increase in the superstructure flexibility, these values remain around 1.0, even for the most stringent base displacement limit. It can be expressed as a successful seismic performance, especially considering such strong near-fault ground motions, which are the most challenging types for seismically isolated buildings.

Numerical studies on the seismic responses of precast segmental columns-supported bridge structures subjected to near-fault ground motions

Precast segmental columns have attracted wide attentions in bridge construction industry recently due to its ability to accelerate construction speed, improve construction quality and reduce environmental impact. It is of great practical importance to study the seismic performances of segmental columns, and many experimental and numerical studies have been performed. However, most previous studies focused on segmental column itself, the research on the seismic performance of a whole bridge structure supported by precast segmental columns is very limited. In particular, no systematic studies have been performed to investigate the seismic performances of segmental columns-supported bridges subjected to near-fault (N-F) ground motions (GMs). To bridge this research gap, the seismic responses of three bridge structures (i.e. a segmental columns-supported bridge, a bridge supported by segmental columns with internal energy dissipation bars, and a conventional bridge supported by monolithic columns) subjected to four types of earthquake ground motions (i.e. a N-F GM with fling-step (F-S) effect, a N-F GM with forward-directivity (F-D) effect, a non-pulse-like N-F GM, and a typical far-field (F-F) GM are investigated and compared in the present study based on the validated numerical models developed in ABAQUS. The influences of permanent ground displacement in the F-S ground motion and pulse period are also systematically investigated. The numerical results demonstrate that the permanent ground displacement in the N-F ground motions has very different effects on the segmental-columns supported bridge and monolithic columns supported bridge.

Temperature-dependent performance of LRBs and its effect on seismic responses of isolated bridges under near-fault earthquake excitations

As the widely used seismic isolators in engineering structures, the mechanical property of lead rubber bearings (LRBs) is vital to the seismic performance of isolated structures. Due to cyclic loadings, the temperature of lead core raises, causing the performance degradation of LRBs. This paper investigates the seismic performance of isolated bridges with consideration of the temperature-dependent behavior of LRBs, and the seismic responses of isolated bridges are quantified based on both the energy and deformation. The analytical results indicate that the absorbed energy and shearing strain of bearings increase when considering the effect of lead core heating under seismic loadings, and these variations cannot be ignored. Besides, the relative variations of energies and deformations of isolated bridges are unrelated to the period of the velocity pulse of near-fault ground motion. The temperature-dependent behavior of LRBs under seismic loadings is suggested to be taken into account in the seismic design of isolated bridges. Otherwise, the seismic damage risk of isolated bridges will be underestimated significantly.

Damping enhanced novel re-centering seismic isolator incorporating superelastic SMA for response control of bridges under near-fault earthquakes

To enhance the bridge resilient performance, a novel re-centering seismic isolator (RSI) is developed by incorporating damping enhanced (DE)-sliding-lead rubber bearing (LRB) with superelastic shape memory alloy (SMA), which functions with the yielding-sliding hysteretic mode. The numerical model of the novel RSI is developed in OpenSees platform and validated by comparing the numerical and experimental hysteretic loops of the superelastic SMA and DE-sliding-LRB. A parametric design procedure is proposed to determine the optimum parameters of the novel RSI system. A three-span continuous girder bridge is selected to investigate the response mitigation efficiency of the novel RSI system compared with LRB system under near-fault ground motions. A systematic parametric study is conducted to design the optimal parameters of the novel RSI system for bridges. Case study is conducted to investigate the effectiveness of the proposed design procedure and the novel RSI system for response mitigation of bridges. Results show that the damping capability is effectively enhanced by designing the friction coefficient of the sliding element. The novel RSI system can achieve dual mitigation of the displacement responses and base force in piers for bridges. Case study demonstrates the effectiveness of the novel RSI and parametric design procedure.

Seismic Response and Damage Analysis of an Isolated Bridge Subjected to Near-Fault Ground Motion

In this study, the seismic response and damage behavior of an isolated bridge under near-fault ground motion are studied and compared with the non-isolated bridge. To consider the local damage of the bridge pier and its evolution process, the nonlinear behavior of the bridge pier in the analysis is simulated by the fiber beam-column element model combined with the damage constitutive model of steel and concrete materials. The mechanical behavior of the isolation bearing is simulated by the three-dimensional isolation bearing model that can consider the instability and failure of the bearing. It is found that the isolation bearings do not work, and even amplify the seismic response of the bridge (pier displacement and pier shear) under the near-fault ground motion. This amplification also leads to more severe damage to the piers of the isolated bridge. In the case of the analyzed concrete bridge type and near-fault earthquake data, it can be concluded that the used isolating lead rubber bearings should not be recommended. In general, it can be stated that the selection of the types of bridge bearings in near-fault areas should always be analyzed in detail by designers in order to avoid unintentional amplifying of the bridge’s seismic response.

Modeling response spectrum compatible pulse-like ground motion

The seismic response analysis of near-fault pulse-like ground motions is severely restricted due to the scarcity of pulse-like records. The requirement in regulations that the response spectra of artificial ground motions should be compatible with the target response spectrum makes the relevant studies more difficult. As a result, this study proposes a trigonometric series-based stochastic method to simulate pulse-like ground motions, with the advantage that the corresponding pseudo-spectral acceleration is compatible with the given target response spectrum. This goal is achieved by two parts. (1) The envelope function of pulse-like records obtained by the Hilbert transform is utilized as the amplitude modulation function to ensure that the simulated ground motion contains a pulse. (2) A novel iteration scheme based on random frequency parameters is proposed to guarantee the response spectrum compatibility. The velocity ground motion is first simulated since the pulse usually exists in velocity. The ground-motion acceleration subsequently obtained by differentiating the velocity is adopted to calculate the response spectrum. Two cases are implemented and verified the effectiveness of the proposed method in enriching existing pulse-like databases and generating pulse-like ground motion in areas that lack records. Moreover, the amplitude modulation function and target spectrum, as two key factors in the proposed method, determines the presence of a pulse and the pulse periods, respectively. This property makes the proposed method potentially universal applicability for stochastic pulse-like ground motion simulation in engineering.

Collapse of the Baihua Bridge under Near-Fault Ground Motion: Finite-Element Analysis with Multiscale Model

Collapse of the Baihua Bridge under Near-Fault Ground Motion: Finite-Element Analysis with Multiscale Model

A unified approach for time-invariant and time-variant reliability-based design optimization with multiple most probable points

Reliability-based design optimization (RBDO) can fulfill performance-based design of static and dynamic structures considering various uncertainties. For time-invariant and time-variant RBDO problems with multiple most probable points (MPPs, also termed as design points), the existing approaches encounter some difficulties in searching for all MPPs and performing time-variant RBDO which requires complicated time-variant reliability analysis. In this paper, the direct probability integral method (DPIM) combining with the change of probability measure (COM), i.e., DPIM-COM approach, is proposed to attack time-invariant and time-variant RBDO problems with multiple MPPs in a unified framework. Firstly, the DPIM in conjunction with Heaviside function is developed to estimate time-invariant and time-variant probabilistic constraints directly and explicitly. Then, an efficient formula for calculating the sensitivity of probabilistic constraints with respect to random design variables is derived based on the DPIM and COM strategy, where the representative points for reliability computation keep unchanged and only the corresponding assigned probabilities need to be updated. Accordingly, the corresponding structural responses of representative points at the current design variables can be reused to evaluate the sensitivity of probabilistic constraints, which saves much computational cost, especially for time-variant RBDO problems. Finally, three numerical examples, including the ten-story building with tuned mass damper under deterministic earthquake ground motion and near-fault stochastic impulsive ground motions, demonstrate the effectiveness and versatility of the proposed DPIM-COM approach for addressing time-invariant and time-variant RBDO problems with multiple MPPs.

Effects of near-fault pulse-like ground motions on seismically isolated buildings

Forward-directivity effects create distinct velocity pulse in the fault-normal direction of near-fault ground motions. It has been shown that this pulse period governs the structural response of fixed base buildings. The present study examines the effects of the near-fault pulse-like ground motions on seismically isolated buildings, considering different effective isolation period to pulse period ratios. Three sets for near-fault ground motions with varying pulse periods are employed to determine the effects of pulse-like ground motions on isolator displacements, superstructure drift ratios, and superstructure load distributions. These effects are categorized as spectral shape and amplification. It is seen that the pulse period forms the spectral shape of records. Spectral acceleration relatively decreases in the short periods and increases in the long periods by enhancing the pulse period. This phenomenon reduces isolator displacement demand for near-fault ground motions with the large and medium pulse periods considering ASCE 7–16 scaling range. An intensity measure parameter is modified to indicate the spectral shape effect of pulse-like records on the isolated buildings. Also, the amplification ratio for the isolator displacement caused by the pulse period is obtained, excluding the spectral shape effect and considering the pulse period to effective isolation period ratio. Further, the R-μ-T relation of the first-floor acceleration, transferred acceleration to the superstructure, is acquired to determine the pulse period effect on superstructures. The results show that the superstructure ductility demand is very sensitive to the pulse period, and the effectiveness of the isolation system considerably decreases by enhancing the pulse period. Also, it is demonstrated that the pulse period governs the lateral load distribution for the superstructure.

Seismic behavior of precast segmental column bridges under near-fault forward-directivity ground motions

Seismic behavior of precast segmental column bridges under near-fault forward-directivity ground motions

Residual displacement demand for non-degrading bilinear SDOF oscillators with self-centering viscous-hysteretic devices

The self-centering viscous-hysteretic device (SC-VHD) is a high-performance self-centering system that was recently developed to reduce structural residual displacement and damage during severe earthquakes. This study focuses on developing a residual displacement ratio C r spectrum and a residual displacement estimation method for non‐degrading bilinear SDOF oscillators with SC-VHDs. Statistical analysis is conducted to investigate the effect of design parameters on the constant-ductility residual displacement ratio C r spectrum under near-fault pulse-like earthquake ground motions (NFPE) and far-fault earthquake ground motions (FFE). Based on statistical data, a formula for the mean C r spectrum is proposed. Subsequently, an iterative calculation method for residual displacement is presented according to the equivalent linearization method and the proposed mean C r spectrum formula. Finally, using the iterative calculation method, the residual displacement of a two-story steel moment-resisting frame (MRF) with SC-VHDs is predicted, demonstrating its application. The results indicate that the mean C r spectrum under the NFPE is significantly larger than that under the FFE. Besides, the mean constant-ductility C r spectrum decreases with increasing post-yield strength ratio, ratio of preload to yield strength, ratio of loading stiffness to elastic stiffness increases, or ratio of unloading stiffness to loading stiffness, but increases with increasing viscous damping ratio. When estimating the C r spectrum, increasing the ductility ratio and decreasing the ratio of preload to yield strength can both reduce record-to-record variability. Both the formula for the mean C r spectrum and the iterative calculation method for residual displacement are validated by comparing with statistical data. • The influence of design parameters on the residual displacement C r spectrum is investigated. • A formula for the normalized mean C r spectrum is developed. • An iterative calculation method for residual displacement is proposed. • An example application is used to validate the calculation method.

Modeling of non-ductile RC structure under near-fault ground motions: A nonlinear finite element analysis

The old existing reinforced concrete (RC) structures in Taiwan are susceptible to severe damage under earthquakes because of the soft-story mechanism. Moreover, the effects of near-fault ground motions often contain a long period of velocity pulse and permanent ground displacement. This study is based on the tri-axial shaking table test conducted at the National Center for Research on Earthquake Engineering in Taiwan and a series of seismic performance evaluations of a non-ductile RC structure in Taiwan. Finite element analysis (FEA) will be conducted to simulate the linear and nonlinear behavior of the seven-story building. This accommodates the damage plasticity model for concrete and the elastic-perfectly plastic model for reinforcement. Comparison of the results between the experimental test and numerical model showed that the similarity of the vibration period has a great influence on the simulation results. The findings showed that the data of acceleration and displacement behavior corresponded with the experimental results in a satisfactory margin. Also, the damage mode is very similar to the shaking table test results. The study found that using FEA can satisfactorily simulate the seismic performance of mid-rise buildings under a near-fault earthquake.

Impact of Near-Fault Ground Motions on Longitudinal Seismic Response of CHRF Bridges

Curved high-pier rigid frame bridges (CHRF bridges) are unavoidably affected by near-fault ground motions (NFGMs), and seismic pounding between adjacent components of CHRF bridge has a significant effect on the seismic performance of CHRF bridges. The seismic response and seismic pounding laws of CHRF bridges under NFGMs need further investigation. In this study, the influence of NFGMs with impulse and directional effects on the dynamic response of CHRF bridges was studied. Subsequently, the pounding responses between adjacent components of CHRF bridge were systematically analyzed. The results showed that the impulse and directional effects of NFGMs have a significant impact on the seismic response of CHRF bridges. The seismic response of CHRF bridges under near-fault impulse-like ground motions (IPGMs) is greater than that under near-fault non-pulse-like ground motions (NPGMs). CHRF bridges have the lowest seismic response under far-fault ground motions (FFGMs). The seismic response of CHRF bridges is significant under backward region ground motions (BRGMs) and the lowest under forward region ground motions (FRGMs). The IPGMs induce larger pounding force (PF) and a smaller number of poundings (PN) compared with FFGMs. The PF and the PN increase from the FRGMs to the BRGMs. Because of the pounding, the impulse and directional effects of NFGMs cause the shear force of the main pier and the auxiliary pier of CHRF bridges to increase significantly and the relative bending moment decreases. Moreover, the shear force and bending moment of the tie beam increases significantly owing to pounding.

Stochastic modelling of the pulse-like near-fault ground motions with time-frequency representation

Stochastic modelling of the pulse-like near-fault ground motions with time-frequency representation

Effectiveness of Instantaneous Power in predicting the collapse responses of structures under pulse-like ground motions by Esra ZENGİN

This study investigates the performance of the Instantaneous Power (IP(T1)) intensity measure (IM) for predicting the structural collapse of structures under near-fault pulse-like ground motions. The efficiency and sufficiency of vector IM consisting of elastic spectral acceleration at the fundamental period of the structure (Sa(T1)) and IP(T1) are compared with those obtained using Sa(T1), average spectral acceleration (Saavg), and spectral shape-based vector IMs. A proportional reduction in error, which quantifies to what extent the other ground-motion parameter eliminates the error in the collapse capacity prediction relative to IM of interest, is used to assess the sufficiency. Our findings reveal that [Sa(T1), IP(T1)] yields better collapse capacity prediction compared to all IMs considered and is found to be sufficient with respect to most of the seismological parameters, duration-related IMs, pulse period, and scale factor.

Parameter estimation for predicting near-fault strong ground motion and its application to Lushan earthquake in China

Stochastic finite-fault modelling based on the dynamic corner frequency is a widespread method for synthesizing near-fault ground motions at high frequencies and has been proven to be an effective tool for simulating high-frequency ground motions by researchers globally. Many input parameters, which have more uncertain effects on the synthetic result, are related to sources, sites, and paths in stochastic modelling. Thus, it is difficult to select reasonable parameters. When using the stochastic finite-fault method, the key link to be considered is how to determine the input parameters required for simulations. Based on this fact, this study analyzed and discussed the sensitivity of several important input parameters in the simulation parameters to the simulation results. The results showed that for the Class C site stations, the crustal and local site amplification factors must be considered simultaneously. Overall, the influence of the high-frequency attenuation parameter κ0 calculated using different methods on the simulation results is not evident. For near-fault stations, various upper-edge buried depth synthetic response spectra can be set, and finally, the average value is taken as the final synthetic recorded response spectra. When using the random source model, the rupture starting point must be set on the subfault in the middle along the fault length and width direction. The conclusions provide a basis for a highly reasonable and rapid determination of input parameters when using the stochastic finite-fault simulations for practical engineering applications and can improve the simulation accuracy to apply the finite source modelling to rebuild scenarios and evaluate and alleviate the seismic hazard in the target region.

Seismic assessment of a dual concentrically braced steel structure under near-fault ground motions

Seismic assessment of a dual concentrically braced steel structure under near-fault ground motions