Near-field Pulse-like Ground Motions Research Articles

Comprehensive evaluation of seismic fragility in base isolated buildings enhanced with supplemental dampers considering pounding phenomenon

Base isolation is a proven technology effective in minimizing earthquake-induced damage to both the structural and non-structural elements (especially freestanding contents) of buildings. Nevertheless, it has been shown that allowing the flexible isolation layer to endure substantial demand displacements might subject the isolated structure to failure. This vulnerability arises from potential superstructure yielding due to increased deformations and reduced stiffness, such as those occurring through pounding against a moat wall during intense earthquakes. One alternative to controlling these large deformations is to use supplementary dampers at the isolation plane of the building, creating a hybrid base isolation system (HIS), which adds damping to the isolation system. In this research, the seismic performance of base isolated structures with lead-rubber bearing (LRB) are studied in conjunction with linear viscous dampers (LVD). Due to this aim, a base-isolated steel frame with 2 story X-type braces and a surrounding moat wall is modeled with two various configurations: LRB, and LRB with LVD. Fragility curves are developed for an ensemble of near-field (NF) pulse-like ground motions and for the maximum inter-story drift ratio (MIDR) as an engineering demand parameter (EDP). The incremental dynamic analysis (IDA) is conducted to calculate the damage probability of the structure by assuming different threshold values for damage states. It was found that adding viscous dampers to the isolation system balanced the behavior of the structure under near field pulse-type records; resulting in reduced isolation displacement while its adverse effects are insignificant and can be ignored. Moreover, the median spectral acceleration, namely the spectral acceleration associated to a probability of exceedance (POE) equal to 50 %, at the specific limit state, in the hybrid base isolation system is higher compared to the conventional one.

Seismic mitigation analysis of base-isolated structure with sliding magnetic bearings considering local-site conditions

Base-isolation systems applied in building structures and infrastructures often suffer from damage even failure during strong earthquakes. The adaptability of isolators to local sites is a main concern for the robust design of base-isolation systems. In this study, seismic mitigation analysis of base-isolated structure with newly-developed sliding hydro-magnetic bearing (HMB) and sliding implant-magnetic bearing (IMB) considering local-site conditions is carried out. Extension of modeling of sliding magnetic bearings is first conducted based on the data of shaking-table tests of a reduced-scale base-isolated structure. To assess the instantaneous frequency for quantifying sliding magnetic bearings’ resisting force, the Hilbert–Huang transform (HHT) is applied. The influences of local-site conditions upon mitigation performance of the sliding magnetic bearings are then addressed, including far-field and near-field, ground motions with and without velocity pulses, and hard-soil and soft-soil. For comparison purposes, the seismic mitigation analysis of the base-isolated structure attached with lead rubber bearing (LRB) and curved surface slider (CSS) is carried out as well. Numerical results show that the sliding magnetic bearings outperform the state-of-the-art seismic isolators in both seismic mitigation and deformation constraint, and the IMB has excellent applicability for engineering since its perfect adaptability to local sites. However, the LRB cannot achieve the desired seismic mitigation under near-field pulse-like ground motions.

Numerical Study on Seismic Performance of Rocking Self-Centering (RSC) Bridge Piers Under Bidirectional Excitation

To study the seismic performance of rocking self-centering (RSC) bridge piers, a numerical model of RSC pier is established based on the OpenSees platform. The model is verified by experimental results. The seismic performance of RSC piers is analyzed under quasi-static and seismic loads, respectively, in terms of peak lateral drift, residual deformation, compressive zone height of the cross-section and tendon stress. Far-field, near-field without pulse and near-field pulse-like ground motion records are selected for the time-history analysis. The effects of prestressing tendon reinforcement ratio, energy-dissipating bar ratio and axial load are examined, respectively. The results show that the strength of the RSC pier, the depth of the compressive zone of the cross-section and the self-centering capacity increase with increasing prestressing tendon reinforcement ratio. When the energy-dissipating reinforcement ratio exceeds 1%, the boost of seismic performance of the pier becomes insignificant. The increasing axial load can increase the lateral strength of the pier. However, it could lead to premature yielding of the prestressing tendons.

An ameliorative duration of pulse-like ground motion records for nonlinear structural analysis

The near-field pulse-like ground motion contains obvious long-period and large pulse characteristic, which makes the nonlinear response of the structure exceed the general prediction of seismic response results. In order to reserve the obvious pulse component while selecting the strong ground motion duration part of the near-field pulse-like ground motion record without causing a large loss of spectral characteristics, the paper proposes an ameliorative truncated strong ground motion duration—useful duration. It is proposed based on the elastic acceleration response spectrum. Its principle is to make the acceleration response spectrum corresponding to useful duration and that corresponding to the original ground motion record as close and consistent as possible. The research results show that from the perspective of elastic response spectrum matching, the response spectrum matching error corresponding to useful duration is smaller than that of significant duration. In addition, the five types of periods corresponding to useful duration are closer to the period of original ground motion. Ulteriorly, from the displacement ratio spectrum of the elasto-plastic single-degree-of-freedom system, it is found that the average value of the elasto-plastic displacement ratio spectrum of useful duration and the original ground motion record is 0.98, which is very close to 1.00. It shows that the truncated ground motion can be used to replace the original ground motion to fully predict the nonlinear response of typical structures. Additionally, the effectiveness is further verified in the nonlinear response of three reinforced concrete frame structures, and the nonlinear response results of the structures are calculated using the time-history of useful duration, with less discreteness.

Influence of concrete's poor-quality management on the seismic behavior of RC frames subject to near-field pulse-like ground motions

Influence of concrete's poor-quality management on the seismic behavior of RC frames subject to near-field pulse-like ground motions

Simulation of near-field pulse-like ground motions using a correlated bimodal fractional stochastic model

This paper introduces a two-part stochastic method to simulate completely nonstationary pulse-like records and capture several peaks in the power spectrum. Based on the evolutionary power spectral density compatibility of the acceleration ground motion in different stages, the model is fitted to the target records. The proposed model is flexible enough to consider the decay rate variability of the high-and low-frequency content of the ground motions. Also, using the circular shift and IFFT techniques, the discretization of the model is performed such that the simulation cost is significantly reduced. The horizontal components of 178 near-filed pulse-like ground motions with a rupture distance of less than 10 km, with strike-slip and dip-slip mechanisms, are simulated using the proposed method. The model's efficiency is investigated using different criteria such as the elastic response spectra, Arias intensity, zero-crossing, Fourier spectrum, and the 2DOF system response. Moreover, the simulated records' pulse-like characteristics are investigated, and the model's capability to generate an ensemble of desirable ground motions is studied. Comparing the results with the previous models reveals that the model can generate more compatible records with the seed ones according to various aspects.

Fragility analysis of offshore wind turbine expose to near-field pulse-like ground motion

Fragility analysis of offshore wind turbine expose to near-field pulse-like ground motion

Seismic Performance Assessment of Velocity Pulse-Like Ground Motions Under Near-Field Earthquakes

As a part of the earthquake ground motion, large velocity pulses are probably one of the important factors leading to serious rock structural damage and geological disasters. It is vitally important to identify the response characteristics of large-scale structures under near-field pulse-like ground motions for earthquake prevention and disaster reduction. In this study, the pulse parameters from the 1999 Chi-Chi and 2018 Hualien earthquakes are extracted using the wavelet transform method, and the potential consequences of velocity pulses on the seismic response of the buildings are assessed. Our results show that the near-field ground motions affected by the directivity effect contain some rich large pulse contents in the velocity time histories. The Chi-Chi earthquake presents a more obvious directivity effect than the Hualien earthquake, and the near-field velocity pulses are closely related to the severely damaged structures. The potential damage of both earthquakes to high-rise buildings can be evaluated by the relationship between the characteristic period of the velocity pulse and the fundamental natural period of the engineering structure. By comparing the periods, it can be revealed that some long-period spectral values in the range of 0.8–3.0 s exceed the seismic design values in the vicinity of the causative faults, and the vulnerability of high-rise buildings against pulse-like ground motions in the Chi-Chi earthquake is stronger than that in the Hualien earthquake. Interestingly, the ratio of peak ground velocity to acceleration (PGV/PGA) and maximum pseudo response velocity (PSV) are suitable as seismic intensity indicators of pulse-like ground motions, which can reflect the rupture direction of source characteristic and site amplification of near-surface soft deposition area around the causative fault. This study provides some references for seismic hazard assessment and post-earthquake structural design.

Dynamic response analysis of story-adding structure with isolation technique subjected to near-fault pulse-like ground motions

The isolation technique is widely studied and applied in structural engineering due to its excellent seismic performance. However, the existing research mostly focuses on the conventional structure under the action of regular ground motions. In this paper, a story-adding seismic structure, a base-isolated structure, and a story-adding isolated structure were simulated by using numerical simulation methods. The dynamic response characteristics of the three structures under near-fault pulse-like ground motions are analysed and compared with the far-fault ground motions. The results showed that using the base isolation can significantly extend the period of the main structure and reduce the seismic response on the upper structure. The inter-story drift ratio, inter-story shear force, and the story acceleration of all three structures under near-field pulse-like ground motions were all larger than that of far-field earthquakes. Both base-isolated structure and story-adding isolated structure showed excellent damping performance. Besides, the damping properties of the base-isolated structure are better than the story-adding isolated structure.

Simulating the near-field pulse-like ground motions of the Imperial Valley, California, earthquake

Long-period pulse-like ground motions in the near-field are likely to induce severe damage to large structures due to resonance at the natural frequencies of engineered construction. To investigate the origin and characteristics of large velocity pulses from the seismic source, this study identifies 14 stations with velocity pulses from 32 strong-motion seismographs located in the near-field of the Imperial Valley MW 6.5 earthquake. The pulse-like ground motions are simulated by a kinematic approach using a fourth-order accurate finite-difference method. The results show that the slip asperity in the northern part of the fault plane contributes more to large velocity pulses than the asperity that originates in the hypocentral zone. The two-sided pulses on the fault-normal component and the one-sided pulses on the fault-parallel component are mainly regulated by the fault rupture and slip directions, respectively. Compared with the pseudo-velocity response spectrum, the displacement response spectrum better captures the resonance effect of long-period pulse-like ground motion on large structures. The location of the maximum ground motion coincides with the surface projection area of the large asperity, and the amplitude and the attenuation rate of each horizontal component vary with increasing distance from the fault. The difference in the ground motion peaks on both sides of the fault implies that the fault-normal component is more sensitive to the fault dip angle than the fault-parallel component. We reproduce the pulse-like ground motions up to 1 Hz by fitting the simulated velocity time histories, response spectrum and ground motions to observed data. Using the pulse records of the Imperial Valley earthquake, the 1994 Northridge MW 6.7 earthquake, and the 1999 Chi-Chi MW 7.6 earthquake, we compare the effects of earthquake magnitude on peak value and distribution of large velocity pulses, in which a reasonable consistency can be observed. Our simulated results of near-field pulse-like ground motions are beneficial to clarify the pulse mechanism.

Intensity measures for the seismic response evaluation of buried steel pipelines under near-field pulse-like ground motions

Ground-motion Intensity Measures (IMs) are used to quantify the strength of ground motions and evaluate the response of structures. IMs act as a link between seismic demand and seismic hazard analysis and therefore, have a key role in performance-based earthquake engineering. Many studies have been carried out on the determination of suitable IMs in terms of efficiency, sufficiency and scaling robustness. The majority of these investigations focused on ordinary structures such as buildings and bridges, and only a few were about buried pipelines. In the current study, the optimal IMs for predicting the seismic demand of continuous buried steel pipelines under near-field pulse-like ground motion records is investigated. Incremental dynamic analysis is performed using twenty ground motion records. Using the results of the regression analysis, the optimality of 23 potential IMs are studied. It is concluded that specific energy density (SED) followed by $$\sqrt {VSI[{\omega _1}(PGD + RM{S_d})]} $$ are the optimal IMs based on efficiency, sufficiency and scaling robustness for seismic response evaluation of buried pipelines under near-field ground motions.

Influence of Near-field Pulse-like Ground Motion on Seismic Response of Steel-concrete Plate Composite Beam Bridges with Seismic Isolation

For steel-concrete composite bridges, different seismic excitations are selected, and nonlinear time-history analysis method is used to study the influence of near-field seismic impulse effect on seismic response of steel-concrete plate composite beam bridges and damping effect of steel-concrete plate composite beam bridges. Under Near-Field Seismic excitation, cable-stayed seismic isolation bearings are used. The seismic responses of main girder displacement, bearing deformation, shear force and pier seismic force are compared and analysed through two directions of input. The influence of impulse earthquake on seismic response of steel-concrete composite beams with isolation bearings is studied. The seismic response of steel plate composite beam bridges is basically smaller than that of non-isolation girder bridge after the isolation bearings are laid out, and the response decreases by more than 40%. The isolation bearings have good damping effect. Therefore, under the action of impulsive ground motion, it is suggested to adopt isolation bearings, such as Cable-sliding Friction Aseismic Bearing.

Seismic Performance Assessment of RC Frame Structures Subjected to Far-Field and Near-Field Ground Motions Considering Strain Rate Effect

This paper investigates the influence of strain rate effect on the seismic performance of Reinforced concrete (RC) frame structures subjected to far-field and near-field ground motions. An approach for the nonlinear dynamic analysis of RC frame structures considering the strain-rate sensitivity of concrete and reinforcing steel materials is proposed and its effectiveness is validated by the experimental data of RC columns under dynamic loadings. A non-dimensional index is put forward to reveal the regularities of strain rate under different types of ground motions with various intensity levels. The influences of strain rate effect and input ground motion on the seismic performance of the exemplar RC frame are studied by comparing the seismic responses and fragilities of rate-dependent structural models with those of rate-independent ones. Numerical results indicate that the strain rates in structural members increase with the ground motion intensity and the strain rates induced by the near-field pulse-like earthquakes are higher than those by the far-field and near-field non-pulse-like earthquakes. The global response, critical member response, local damage and seismic fragility are all influenced by the strain rate effect, especially under the near-field pulse-like ground motions. Neglecting the influences of strain rate effect, variations in strain rates of different structural members and inputs of pulse-like ground motions may lead to erroneous seismic performance assessments of RC frame structures.

A spectral-velocity-based combination-type earthquake intensity measure for super high-rise buildings

A spectral-velocity-based combination-type ground motion intensity measure (IM), which is inspired by the superior sensitivity of spectral velocity to structural response compared with other two spectral quantities, is proposed for super high-rise building structures with the consideration of the characteristics of ground motions and structures themselves. Two super high-rise buildings with typical frame/core-tube/outrigger lateral resisting system and a wide range of structural height (H = 258 m~660 m) are deliberately selected to identify the correlation between the maximum inter-story drift ratio and nineteen IMs developed in recent decade together with the proposed IM, with sixty chosen far-field and near-field pulse-like ground motions. With the suggested optimal number of lower vibration modes and corresponding combination factors, the efficiency, sufficiency and the scaling robustness of the proposed IM is further demonstrated. In addition, the relative sufficiency measures of other IMs with respect to the proposed IM are investigated. The results from the study indicate that, the proposed IM is believed, from the standpoint of efficiency, sufficiency, relative sufficiency measure and scaling robustness, to be a more specialized and desirable tool for super high-rise buildings, either for far-field or near-field ground motions.

New insights on effects of directionality and duration of near-field ground motions on seismic response of tall buildings

Summary The effects of ground motion directionality on seismic response of buildings are at the center of ongoing debate among earthquake engineering professionals and researchers. This has prompted a renewed interest to have a better understanding of directionality effects of near-field pulse-like ground motions on seismic response of tall buildings to further improve seismic design in this respect. In particular the prediction of the maximum displacement response along the structural axis which is called the critical displacement response. This paper presents the results from parametric studies that investigate the directionality effects on nonlinear dynamic response of simple structures and a tall building. The outcome of these analyses was the development of a method, which relies on the maximum velocity to provide a good approximation to the critical displacement response. The method developed is computationally efficient and involves less calculation than other methods. In addition, it was determined that the building responses to records rotated to fault-normal can lead to significant underestimation of the maximum response along the structural axis, using the fault-parallel ground motion also may lead to large response differences and smaller yet significant differences when using the maximum direction ground motion.

Response of self-centering braced frame to near-field pulse-like ground motions

A low damage self-centering braced frame equipped with post-tensioning strands is capable of directing damage to replaceable butterfly-shaped fuses. This paper investigates the seismic performance of rocking braced frame under near-field pulse-like ground motions compared to far-field records. A non-linear time history analysis is performed for twelve self-centering archetypes. A sensitivity analysis is carried out to examine the influences of ground motion types and modeling parameters. Findings represent the proper efficiency of the self-centering system under both far-field and near-field pulse-like ground motions.

Influence of non-stationary content of ground-motions on nonlinear dynamic response of RC bridge piers

This paper quantifies the impact of the non-stationary content (time-varying parameters that are not captured by power spectral content alone) of different ground-motion types (near/far field, with/without pulses time-series) on the nonlinear dynamic response of reinforced concrete bridge piers, taking into account the material cyclic degradation. Three groups of ground motions are selected to represent far-field, near-field without pulse and near-field pulse-like ground motions. Three analysis cases are considered corresponding to acceleration series matched to the mean response spectrum of: (1) far field, (2) near-field without pulse and (3) near-field pulse-like ground-motions, respectively. Using the selected ground motions, several nonlinear incremental dynamic analyses of prototype reinforced concrete bridge piers with a range of fundamental periods are conducted. Finally, a comparison between the response of the structures using the material model accounting for both buckling and low-cycle fatigue of reinforcing steel and the more conventional material model that does not account for these effects is made. The results show that the inelastic buckling and low-cycle fatigue have a significant influence on the nonlinear response of the RC bridge piers considered and that pulse effects can increase the mean acceleration response by about 50%.

Inelastic displacement ratios of fully self-centering controlled rocking systems subjected to near-source pulse-like ground motions

Modern self-centering controlled rocking (SC-CR) systems are capable of reducing residual drift after severe earthquakes by swaying on their bases and concentrating damage in energy dissipation devices. The present research determines constant-strength (CR) spectra for SC-CR systems subjected to far-field and near-field pulse-like (NF-pulse) ground motions. SC-CR systems are simulated as single degree of freedom models with a flag shape behavior. Nonlinear time-history analyses are conducted using 91 NF-pulse and 44 far-field records to derive the CR spectra. Each point of CR spectrum of a SC-CR system is the ratio of maximum inelastic to maximum elastic displacement. An extensive statistical study is carried out to examine the effects of ground motion and modeling parameters on the CR spectra. It is found that the influences of pulse period, predominant period of ground motion, and modeling parameters on inelastic displacement ratios (CR ratios) of SC-CR are significant, while other effects such as the source-to-site distance, earthquake magnitude, and site classes are not as much important. Finally, using two-stage regression analyses, formulas are developed for determining un-normalized (code-compliant) and normalized CR spectra of SC-CR systems subjected to far-field and NF-pulse ground motions.

Seismic Performance of Midstory Isolated Structures under Near-Field Pulse-Like Ground Motion and Limiting Deformation of Isolation Layers

Excessive deformation of the isolation layer in midstory isolated structures may occur under strong near-field pulse-like ground motion, which would result in the overturning collapse of the superstructure. The objective of the present research is to limit excessive deformation of the isolation layer and to reduce nonlinear response of midstory isolated structures. To this end, a protective system is presented to limit deformation of the isolation layer by soft pounding. Based on the Kelvin pounding model, a mechanical model is put forward for this protective system. In addition, a new method has been proposed that synthesizes artificial near-field pulse-like ground motion by combining the real near-field nonpulse ground motion with simple equivalent pulses. Also, the impact of artificial near-field pulse-like ground motion on the nonlinear response of midstory isolated structures and the deformation of the isolation layer has been investigated. The effectiveness of the midstory isolation with the protective system has been validated. The results show that the maximum deformation of the isolation layer significantly exceeds the allowable deformation of lead-rubber bearings when subjected to near-field pulse-like ground motion, and it causes the lead-rubber bearings destruction. The proposed protective system is effective in restricting the excessive deformation of the isolation layer and reducing nonlinear responses of the isolated structure, preventing collapse of the superstructure.

Identification of Near-Field Pulse-Like Ground Motions Recorded at Bajiao Station during the Mainshock of the Great Wenchuan Earthquake

When earthquake occurs, it is in near-fault that the most serious damage happens and velocity pulse appears. Velocity pulse could have huge potential to destroy the structure in near-fault. The set of records at Bajiao Station is one of the three famous near-field sets of strong ground motion records whose PGAs are the largest in all the sets of records obtained from the mainshock of the Great Wenchuan Earthquake. Our research is to identify the pulse-like characteristics from the set of records at Bajiao Station. It is found that velocity pulses in the records are “hidden pulses”.