Near-fault Motions 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.

Unified framework for stochastic dynamic responses and system reliability analysis of long-span cable-stayed bridges under near-fault ground motions

Long-span cable-stayed bridges are complex systems with numerous components, and prone to failures of multiple components under near-fault ground motions with long-period velocity pulses. The evaluation of dynamic system reliability of cable-stayed bridges is an important challenging issue. In this work, a unified framework for simultaneously determining both the stochastic dynamic responses and system reliabilities of long-span cable-stayed bridges under near-fault stochastic ground motions is proposed based on direct probability integral method (DPIM) with adaptive strategy. Firstly, the 3-dimensional finite element model of typical cable-stayed bridge and the stochastic synthesis model of near-fault ground motions are established. Based on probability density integral equation, new formulas in input probability space of time-variant moments of response are derived. Then, DPIM with adaptive strategy is suggested to calculate the mean value and standard deviation of response via new formulas. Dynamic system reliability of cable-stayed bridge under near-fault stochastic ground motions is evaluated by establishing failure criteria of extreme value mappings of multiple performance functions. Finally, numerical results indicate that the long-period velocity pulse of near-fault motions imposes an important effect, resulting in large mean responses and failure of cables. Failure probability of cables is significantly increased with the occurrence of velocity pulses, which causes the decrease of system reliability of cable-stayed bridge. Thus, the randomness and velocity pulse effect of near-fault ground motions should be seriously considered for reliability-based design and safety assessment of cable-stayed bridges.

Quantifying near-fault motion effects on soil liquefaction through effective stress site response analysis

Near-fault (NF) pulse-like ground motions have a significant impact on the performance of structures, but their effects on soil liquefaction potential have been relatively understudied. This paper uses 1D effective stress site response analysis capable of modeling porewater pressure (PWP) generation to investigate the seismic site response under NF and general ground motions. The aim is to assess the effects of NF motions on soil liquefaction by considering different soil models and PWP generation models. The results show that NF ground motions generally induce larger ground responses in terms of PWP generation, especially when the durations of the motions are short. This is due to the higher cumulative absolute velocity associated with NF ground motions compared with general ground motions due to a high-velocity pulse under the same peak ground acceleration. However, this effect diminishes as the duration of motion increases. To avoid underestimating seismic demand, especially for small to moderate earthquakes, a preliminary magnitude scale factor modified for the NF effect is suggested for use in conventional soil liquefaction triggering analysis.

Near-Fault Response and Loading Protocols of Box Columns in Steel Frames with Self-Centering Braces to Provide Recentering Capability

The main objective of this study was to establish loading protocols for self-centering braced frames (SCBFs) while taking into account the impact of near-fault ground motions. SCBFs are self-centering frames that have lower residual drift than buckling-restrained braced frames (BRBFs) under near-fault earthquakes; thus, SCBFs can better control damage to buildings. Typically, first-story columns are employed to represent the seismic behavior of a structure; therefore, loading protocols were developed for first-story columns in SCBFs under near-fault earthquakes. Nonlinear static and dynamic analyses of SCBFs designed with various periods were conducted for better understanding the seismic performance of SCBFs. Finally, the loading protocols developed for SCBFs were compared with those for BRBFs under the same near-fault motions.

On the Use of Instrumental and Macroseismic Data to Evaluate Ground-Motion Models: The 2019 Mw 6.4 Durres, Albania, Earthquake Sequence

Abstract In this study, we analyze the existing ground-motion models (GMMs) applicable in Albania for horizontal peak ground acceleration (PGA) and spectral acceleration (SA) using instrumental ground motions, and also incorporate online citizen responses from “Did you feel it?” (DYFI) to compensate for the sparse distribution of strong-motion stations and provide better constraints for near-fault motions. Our evaluation focuses primarily on the damaging 26 November 2019 Mw 6.4 Durres earthquake, incorporating 1360 DYFI online citizen responses collected after the Durres mainshock event, along with two significant September foreshocks and two large November aftershocks with a moment magnitude Mw>5.0. In general, the DYFI intensities exhibit higher values than instrumentation data, and we find that SA at 0.3 s better represents the observed macroseismic intensities for all events. In the meantime, the reversible relationships between macroseismic intensities and PGA/SA, as established by Oliveti et al. (2022) based on a dataset from the European region (Italy), show a better fit for the converted DYFI observations when compared to instrumental data, in contrast to the fit of the converted DYFI observations by Worden et al. (2012). This underscores the importance of regional characterization when considering the datasets from online citizen responses. The extensive DYFI intensities set, particularly in near-fault regions, significantly improves the evaluation of GMMs due to the sparse distribution of instrumentation data. Moreover, we account for data variance, and applied the log-likelihood approaches to select and rank a candidate set of GMMs. In addition to recommending a set of GMMs suitable for the Albania region, our study highlights the valuable applications of using online citizen responses like DYFI for ground-motion estimations, which are crucial in regions with limited instrumental station coverage. These online citizen response datasets contribute to better constraining the selection of GMMs, although careful consideration is necessary when relating intensity to ground motion for regional characterization. Our study makes a significant contribution to GMM selection and provides a valuable reference for the logic tree structure in subsequent seismic hazard assessments on both national and regional scales.

Seismic response of fault-crossing tunnel under near-fault motion

Seismic response of fault-crossing tunnel under near-fault motion

System-level seismic fragility of high-speed railway track-bridge system with component-replaceable U-shaped combined steel damper

A component-replaceable U-shaped combined steel damper (UCSD) was presented as an energy dissipation device to mitigate the seismic damage of high-speed railway track-bridge system (HSRTBS). The energy-dissipation modules inside the UCSD can be replaced by removing the high-tensile bolts. A low cyclic test of the energy dissipation module was carried out. Parametric analysis of the UCSD with varying U-shaped plate thicknesses was conducted using finite element analysis, including 8, 10, 12, 14, and 16 mm. To study the effect of the UCSD on HSRTBS under near-fault ground motions, twenty-eight near-fault records recommended by FEMA-P695 were selected, and the UCSD was installed adjacent to bearings in HSRTBS. Component-level fragility curves of HSRTBS with UCSDs with the different thicknesses of U-shaped plates were compared. To comprehensively and efficiently evaluate the contribution of the UCSDs on HSRTBS, system-level fragility curves were obtained by the product of the conditional marginal (PCM) method. Results indicate that the UCSD exhibits excellent hysteresis performance and energy dissipation capacity, improving with the U-shaped plate. At the maximum considered earthquakes (MCE) level, the exceeding probability of complete damage of the system is reduced from 100 % to 95 %, 79 %, 57 %, 35 %, and 20 % respectively after adding UCSDs in five different U-shaped plate thicknesses. This reveals that the proposed UCSD can effectively mitigate HSRTBS seismic damage under near-fault motions, progressively improving with increased U-shaped plate thickness.

Bond-slip at anchorage under unidirectional and bidirectional seismic excitation: Search for an efficient mathematical model

Reinforcement anchorage slip can significantly contribute to the seismic response of bridge columns, especially to residual deformation. For a reliable numerical model of slip action, the present study evaluates three advanced models, each founded on distinct principles: (a) a macroscopic model (M−I), (b) a microscopic model (M−II), and (c) a mixed model that employed micro-model material properties derived from a macro-model (M−III). The performance of each model under unidirectional shaking has been first validated. The M−I cannot account for bidirectional loading, and as such, the performance of the M−II and M−III has, for the first time, been rigorously examined against the two-component shake table test results. Beginning with the simplified pulses, the responses of representative bridge columns are computed to unidirectional and bidirectional shaking for several near-fault motions. The M−I, M−II, and M−III are fairly similar in mimicking the slip action for unidirectional loading. Also, the M−II and M−III can closely represent the responses to bidirectional shaking. A simple extension of the M−I is proposed to capture the effects of bidirectional loading. The performance of the M−I (extended), M−II, and M−III are shown similar from the viewpoint of representing (i) the seismic fragilities, (ii) the uncertainties of the influential parameters, and (iii) the consequences of a sequence of motions. The detailed scrutiny of a range of models helps select the most appropriate scheme in a specific design context. On the other hand, the less-rigourous model, viz., M−I with the proposed extension, can be used within the limitations of commercial software and can prove useful in routine design for bidirectional seismic action.

Assessing residual deformation of bridge piers under bidirectional near-fault motions with forward directivity and fling-step

Assessing residual deformation of bridge piers under bidirectional near-fault motions with forward directivity and fling-step

Seismic resilient design of rocking tall bridge piers using inerter-based systems

Seismic resilient design of tall bridge piers with rocking foundation by using inerter-based systems is investigated in this work. A simplified model is developed in OpenSees to simulate the rocking bridge pier equipped with a tuned viscous mass damper (TVMD) system, wherein nonlinearity of the pier response is incorporated through a distributed-plasticity fiber-based section approach; the rocking behavior is simulated through elastic-no-tension spring elements located at the foundation interface of the pier as per FEMA 356 recommendations; an equivalent model comprising a rotational mass linked with the two terminals of the TVMD through appropriate constraint conditions is utilized for modeling the inerter-based system. Optimal design parameters of the TVMD are determined through an extensive parametric analysis, aiming to minimize the tilt angle of the bridge pier. The effectiveness of the selected design parameters of the TVMD on the seismic mitigation performance of rocking tall piers is then investigated numerically. Besides, guidelines are proposed for the design of TVMD parameters in engineering practice based on these analytical results. Nonlinear time-history analysis is conducted under both far-field and near-fault motions, followed by seismic fragility analysis for the overturning damage state, assuming the peak ground acceleration as intensity measure and the dimensionless tilt angle as the engineering demand parameter. The parametric analysis results show that the TVMD is less efficient for near-fault motions, due to reduced energy dissipation capacity caused by pulse-type feature. Additionally, the optimal parameters obtained from parametric analysis could be generalized to a higher number of seismic excitations, achieving a similar seismic performance, which indicates robustness of the TVMD. The TVMD is found able to reduce the tilt angle at the rocking interface especially during strong earthquake events, consequently increasing the overturning stability of rocking foundations adopted for tall bridge piers.

Soil-structure interaction and frequency components of near-fault records on the performance-based confidence levels of steel setback MRFs

This paper investigates the role of soil-structure interaction (SSI) in the confidence levels of setback steel moment-resisting frames (MRFs) for meeting the structural performance objectives, under the action of the components of pulse-like ground motions. To this end, a group of nine flexible-base 10-story steel MRFs with irregular configurations of the setback type, as well as a 10-story regular frame (for comparison purposes) were subjected to the components of 55 near-fault records containing forward directivity pulses. For investigating the role of near- and far-fault shakings, a group of 32 far-fault ground motions was also applied to the structures. The role of soil-structure interaction was considered by using the beam on nonlinear Winkler foundation theory (BNWF) and results were compared with those from the rigid-base frames in the previous study. The pulse components were extracted from the original records through wavelet analysis so that their capability for representing the original near-field excitation would be assessed to impose severe damage on structures. The pulse components were also categorized into four groups according to the pulse-to-structure fundamental period ratio (TPULSE/TSTRUCTURE).The results reveal that SSI has beneficial and destructive effects on the performance-based confidence levels (PBCL) of all the studied structures under near- and far-fault records, respectively. The detrimental effects of SSI on PBCLs are more obvious for setback frames in comparison with the regular ones. Furthermore, using directivity pulses instead of original near-fault motions does not seem reliable while evaluating the PBCL of flexible-base setback structures, unlike the regular MRFs. Finally, it is highly recommended to consider the TPULSE/TSTRUCTURE ratio effects on the PBCL of flexible-base structures, especially for setback frames.

Response of structures to motions with fling step: Does fling matter?

An essential signature of the near-fault motions is the fling-step, which contains a one-sided velocity pulse and ultimately results in permanent ground displacement. It is well-known that the standard processing of ground motion removes such fling. However, some researchers believe that dynamic action is hardly affected by such removal of permanent static displacement. So, such processed motions can be used to examine the effect of fling-step. In the current investigation, two mathematical analogues, rigid sliding and rigid rocking blocks are extensively studied under near-fault motions with fling-step. The response is computed using the processed motions (without explicit fling) and the processed motions with fling mathematically reintroduced. A comparison in response confirms that using motions without explicit fling is essentially deficient from a structural engineering perspective. The study has also noted the peculiarities in the dynamics of rocking oscillators, which may be of significance for assessing the consequences of the sequence of motions.

Estimating seismic response to bidirectional excitation per unidirectional analysis: A reevaluation for motions with fling-step using SDOF systems

Within the context of near-fault motions with fling-step signature, the present paper reevaluates the applicability of a straightforward alternative to estimate the response to bidirectional shaking per unidirectional analysis. The method originally evolved for motions with forward-directivity, uses unidirectional analysis in a single incidence angle. The performance of the method is examined using reinforced concrete (RC) bridge piers as the representative single-degree-of-freedom (SDOF) oscillators. Synthetic fling has been introduced to two horizontal components of processed (base-line corrected and filtered) strong motions. It has been shown that the response to bidirectional excitation can be closely estimated by means of a unidirectional analysis using the ground motion components rotated to the most preferred orientation. Existence of the least preferred orientation in which the unidirectional analysis results in the maximum difference from the bidirectional analysis appears to reinforce the development. For a ground motion pair, these orientations can be identified in advance through characteristic ground motion parameters, viz., energetic length and characteristic intensity for peak and cumulative demand, respectively. The usefulness of the said orientations to estimate the Park-Ang damage index is also established. The efficacy of the simple methodology has also been demonstrated through fragility analysis. The straightforward method can hence be used in the near-fault regime regardless of the ground motion characteristics.

Performance of clutched inerter damper for base-isolated structures under near-fault motions

The performance of the supplemental clutched inerter damper (CID) for the base-isolated multi-story structures subjected to near-fault earthquakes is investigated. The isolation system is considered as lead-rubber bearings with bi-linear characteristics and viscous damping. The resisting force of the CID is proportional to the relative acceleration between two terminals under the attached condition and zero when detached. The governing equations of motion of base-isolated structure and the CID are derived and solved using numerical techniques under seven near-fault ground motions data. The variation of peak bearing displacement, top floor absolute acceleration, total base shear, and the CID force is plotted against the inertance mass ratio of the CID. The above peak responses were also analyzed for different values of damping, period of isolation, yield strength of LRB, and superstructure stories. Application of the CID is observed to effectively facilitate the reduction in bearing displacement while the combined effect of isolation and the CID prevents the top floor acceleration to shoot up. The optimum value of inertance mass ratio is also determined by minimizing the total base shear which is the measure of equivalent lateral force on the structure. The optimum inertance lies in the range of 35%–45% of the total mass of the isolated building under near-fault motions. In addition, the performance of the CID base-isolated structure subjected to cycloidal pulses is also investigated. It is observed that the CID is quite effective in controlling the displacement of the isolation system under cycloidal pulses.

Pounding performance between a seismic-isolated long-span girder bridge and its approaches

For girder bridges located in regions with a high seismic hazard level, seismic isolation bearings are usually implemented to suppress the seismic inertial force transmitted from the superstructure to substructures, protecting the pier columns and foundations. However, whether potential pounding might happen for these seismic-isolated girder bridges between adjacent units are scarcely considered, especially for those with long spans where the wave passage effect is of great significance. This paper develops a finite element model for a typical hybrid truss-girder bridge, based on which the influence of seismic isolation bearings and wave passage effect on pounding is carefully investigated considering both near-fault and far-field motions. The results show that for this type of long-span girder bridges, pounding may happen even the adjacent bridge units are designed with similar vibration periods, especially when near-fault motions and wave passage effect are considered. Under non-uniform excitations with different apparent wave velocities, the employment of isolation bearings can reduce the relative displacement at expansion joints and thus improve the performance against pounding; however, the occurrence of pounding and damage of expansion joints cannot be fully avoided. Further, motions with lower apparent velocities generally correspond to more devastating responses, which should be particularly considered during engineering practice.

Closed-form critical response of undamped bilinear hysteretic MDOF system under pseudo-double impulse for estimating resonant response under one-cycle sine wave

Approximate closed-form maximum interstory drifts under the critical pseudo-double impulse (PDI) are derived for undamped multi-story shear building models with bilinear hysteresis by using the proposed updated mode-controlled energy-based approach (UMEA). PDI was proposed in the previous paper to simulate the critical responses of elastic-plastic MDOF models under near-fault fling-step motions. Although a double impulse (DI) is treated as a ground motion, PDI is treated as a set of impulsive lateral forces. PDI excites only the fundamental-mode responses of elastic proportionally-damped MDOF models because the fundamental participation vector is applied to the influence coefficient vector of PDI. UMEA is a kind of displacement control analyses, and the displacement increment, which is proportional to the fundamental mode evaluated by the tangent story stiffnesses, is given to the model. It is demonstrated that UMEA effectively captures the main characteristics of the maximum interstory drifts under the critical PDI and the critical one-cycle sine wave because most of the kinetic energy, which the fundamental mode has just before yielding, is given to the plastic fundamental mode just after yielding except for the case that the post-yielding stiffness is almost zero. The use of UMEA leads to much more efficient estimation of the maximum interstory drifts than the time-history response analysis because UMEA requires only 2N-time evaluations of the elastic and plastic fundamental modes at most for N-story shear mass models with bilinear hysteresis. It is shown through numerical examples that the maximum interstory drifts under the critical PDI and the corresponding one-cycle sine wave correspond well to those evaluated by the proposed method. It is also shown that the correspondence of the responses of elastic-plastic MDOF models under recorded near-fault motions and the estimated responses by the proposed method is fairly good.

Site-Specific Response Spectra Developed by Considering Near-Fault Motions in Taiwan

Site-Specific Response Spectra Developed by Considering Near-Fault Motions in Taiwan

Seismic Behavior of a Class of Mixed Reinforced Concrete-Steel Buildings Subjected to Near-Fault Motions

This paper investigates the seismic behavior of a class of mixed reinforced concrete-steel buildings. In particular, mixed buildings constructed by r/c (reinforced concrete) at their lower story(ies) and structural steel at their upper story(ies) are studied from the viewpoint of their wide application in engineering praxis. The need to investigate the seismic behavior for this type of mixed buildings arises from the fact that the existent literature is small and that modern seismic codes do not offer specific seismic design recommendations for them. To study the seismic behavior of mixed r/c-steel buildings, a 3-D numerical model is employed and five realistic r/c-steel mixed buildings are simulated. Two cases of the support condition, i.e., fixed or pinned, of the lowest steel story to the upper r/c one are examined. The r/c and steel parts of the mixed buildings are initially designed as separate structures by making use of the relevant seismic design guidelines of Eurocode 8, and then the seismic response of these buildings is computed through non-linear time-history analyses. The special category of near-fault seismic motions is selected in these time-history analyses to force the mixed r/c-steel buildings under study to exhibit a strong non-linear response. Seismic response indices in terms of inter-story drift ratio, residual inter-story drift ratio and peak floor absolute accelerations are computed. The maximum values of these indices are discussed by comparing the two aforementioned kinds of support conditions and checking the satisfaction of specific seismic performance limits. Conclusions regarding the expected seismic behavior of mixed r/c-steel buildings under near-fault seismic motions are drawn. Finally, the need to introduce specific design recommendations for mixed r/c-steel buildings in modern seismic codes is stressed.

Near-Fault Forward Directivity Effect on the Estimation of Ground Motion Amplification Factors

Near-fault forward directivity (NFFD) ground motions cause significant potential damage to civil infrastructure. Buildings at soil sites at which NFFD motions are expected might need probabilistic seismic hazard analysis (PSHA) for soil sites considering the NFFD effect. In the analysis, ground motion amplification factors (GMAFs) considering the NFFD effect are required. Thus, investigating the characteristics of GMAFs due to the NFFD effect is important. To achieve this objective, this paper performs probabilistic ground response analyses for two typical soil sites subjected to three ground motion suites, that is, NFFD motions, far-fault motions, and near-fault motions without pulses. The results indicate that pulse periods of NFFD motions significantly affect the characteristics of GMAFs, rendering mean GMAFs remarkably lower or higher than those of ordinary ground motions. This study also reveals that standard deviations of GMAFs are insensitive to types of input motions. By further investigating the influences of soil deposit on pulse periods, this study observes that the pulse periods tend to approach 1.5 times the fundamental period of the soil deposit after pulselike ground motions propagate through the soil deposit. This observation provides a useful clue to pulselike ground motion selection for soil sites.

Seismic performance of frame‐bent structures subjected to near‐fault ground motions

Reinforced concrete frame-bent structures are one of the structural forms commonly used in industrial factory buildings. In order to study the seismic behaviour of frame-bent structures located near fault areas, eight pulse-like, eight non-pulse near-fault motions and eight far-fault ground motions have been selected in this paper. The finite element model of a reinforced concrete frame-bent structure was established to study seismic responses to near-fault and far-field ground motions. Numerical analysis results indicate that the maximum base shear of the structure under the action of near-fault ground motion was 5.10 times that of far-fault and 1.47 times that of non-pulsed near-fault ground motion; the maximum story drift angle under pulse-like near-fault ground motion was 3.8 times that of non-pulse and 19.7 times that of far-fault ground motion. Therefore, engineering designers should pay more attention to the seismic performance of frame-bent structures under the action of near-fault ground vibration.