Forward-directivity Ground Motions Research Articles

Nonlinear seismic response of mid-rise modular buildings subjected to near-field ground motions

In recent earthquakes, structures near seismic sources have suffered severe damage due to near-fault ground motions. When compared with ordinary ground motions, these ground motions create high demands on structures. The presence of a strong energy pulse has been observed in near-fault ground motions. This impulsive motion, propagating towards the site, will result in forward-directivity effects. This study is intended to assess the structural effects of mid-rise steel Modular Building Systems (MBSs) under the near-fault pulse-like and non-pulse-like records and to investigate design procedures that explicitly comprise near-fault effects. The main objective of this paper is the nonlinear seismic responses of MBSs to near-fault forward-directivity (FD) and non-forward-directivity (NFD) ground motions, as well as whether the equivalent pulse present in forward-directivity earthquake ground motions dominate the structural response. This paper discusses the key properties that can be used to characterize near-fault ground motions with forward-directivity effects and non-forward-directivity effects on the engineering demand parameters. For this purpose, using 72 pulse-like ground motions and 120 non-pulse-like ground motions, a Cloud analysis is conducted to obtain statistically significant results. Nonlinear time history analysis is performed using OpenSees based on distributed plasticity approach to develop a finite-element model of a 6-story mid-rise MBS. The results depict that near-field pulse-type ground motions generate larger demands to the MBS compared with the non-pulse ground motions. In addition, when correlated with simple intensity measures such as PGA, PGV, and spectral acceleration at the first mode period, the structural response to forward-directivity ground motions exhibits a higher dispersion than the structural response to non-forward-directivity ground motions. The representative equivalent pulses of the considered forward-directivity ground motions were decomposed using the continuous wavelet transform method to explore their effects on the MBS response. Finally, the influence of the pulse parameters, including period and amplitude of the forward-directivity, are ascertained.

Seismic Response Analysis of Isolated and Nonisolated Continuous Girder Bridges under Multidimensional Near-Field Ground Motions

In this study, the responses of isolated bridges and nonisolated bridges are studied under multidimensional seismic motions. First, damage constitutive models of steel bars and concrete materials were combined with the fiber beam-column element model, and the isolated bearing model considering bearing failure was selected. The bridge numerical analysis model was then established. The seismic responses of isolated and nonisolated bridges were analyzed under near-field ground motions (fling-step and forward-directivity ground motions) and far-field ground motions. It was found that the seismic responses of nonisolated bridges, such as deck acceleration, pier displacement, pier damage, and bearing displacement under near-field ground motions, were higher than those under far-field ground motions. Under far-field ground motions, isolation bearings effectively reduced various seismic responses of structures and had isolation effects. Under forward-directivity TCU102 ground motions, the minimum isolation ratios of isolation bearings for peak acceleration of the girder in the Z direction, pier displacement, and pier shear force were −0.14, −2.65, and −0.05, respectively. The low-isolation ratio and significant damage to the isolation bearings indicate that the isolation bearings cannot be directly used under near-field conditions.

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

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.

The cross-over frequency between pulse and high-frequency components of near-fault forward-directivity ground motions

Due to the scarcity of near-fault records with forward-directivity effects, synthetic near-fault pulse-like ground motions through superimposing pulse models and high-frequency components are commonly used in the earthquake engineering. However, in existing studies, the cross-over frequency in generating high-frequency components is usually empirical and unclear. In this article, a hybrid decomposition and resynthesis method is developed to quantify the cross-over frequency, in which the wavelet decomposition and high-pass filter are, respectively, used to get the pulse and high-frequency components for near-fault records. Using the 30 near-fault pulse-like records, the distribution of cross-over frequencies is obtained. It is interestingly found that the cross-over frequency is inversely proportional to the pulse period.

Stochastic Modeling and Synthesis of Near-Fault Forward-Directivity Ground Motions

The stochastic modeling and synthesis of near-fault forward-directivity ground motions for the prescribed seismic scenario is presented in this study. This stochastic model, which is established in the orientation of the strongest pulse, combines the directivity pulse component represented by the Gabor wavelet pulse model and the high-frequency component characterized by the modulated filtered white noise model. In this regard, the pulse feature, evolutionary intensity and frequency contents are reasonably considered. Then, the empirical predictive equations that relate the parameters of the proposed stochastic model to the variables that describe earthquake source, path, site conditions, and near-fault rupture-to-source geometry are developed based on both regression and correlation analysis. The basic procedure for the synthesis of possible forward-directivity ground motions under the prescribed seismic scenario is provided as well. The efficacy of the stochastic model and the simulation procedure is confirmed by comparing the simulated motions with the recorded ones in terms of time-series and response spectra values, which indicate that the synthetic ground motions are capable of reproducing the salient characteristics, as well as the natural variability, of the recorded ones. Therefore, the proposed stochastic modeling and simulation method is expected to be a desirable alternative and complement for recorded ground motions in the near-fault seismic risk assessment.

Effects of pulse-like nature of forward directivity ground motions on the seismic behavior of steel moment frames

In the structures with high level of ductility, the earthquake energy dissipation in structural components is an important factor that describes their seismic behavior. Since the connection details play a major role in the ductile behavior of structure, in this paper, the seismic response of 3-, 5- and 8-story steel special moment frames (SMFs) is investigated by considering the effects of panel zone modeling and the influence of forward-directivity near-field ground motions. To provide a reasonable comparison, selected records of both near and far-field are used in the nonlinear time-history analysis of models. The results of the comparison of the median maximum inter-story drift under excitation by near-field (NF) records and the far-field (FF) ground motions show that the inter-story drift demands can be obtained 3.47, 4.86 and 5.92 times in 3-, 5- and 8-story structures, respectively, undergoing near-field earthquakes.

Copula-based seismic fragility assessment of base-isolated structures under near-fault forward-directivity ground motions

The seismic fragility of base-isolated structures subjected to near-fault forward-directivity ground motions is investigated. A general framework for deriving the system-level fragility curve is proposed, where the vulnerability contributions of multiple interrelated components to the overall system are incorporated. In this framework, the joint probabilistic seismic demand model (JPSDM) is established, where the joint probability distribution of multiple engineering demand parameters (EDPs), conditioned on the intensity measure (IM) level, is characterized via copula approach, and the sampling-based JPSDM, along with the capacity models of these components, is employed to generate the overall system fragility curves. This proposed framework is applied in the case study of a typical seismically-isolated RC frame structure, where the peak lateral displacement in the base-isolation layer and the maximum inter-story drift in the superstructure are considered as the two major component EDPs. The analysis results indicate that the combination of t copula, which is quantitatively identified as the best-fit copula function, and the conditional lognormal marginal distribution adequately captures the joint probability distribution of these two EDPs conditioned on the IM level. Moreover, the impact of different copulas selection on the system-level fragility varies depending on the relative fragility contributions of different components in the overall system.

Evaluation of seismic performance of reinforced concrete (RC) buildings under near-field earthquakes

Near-field ground motions are significantly severely affected on seismic response of structure compared with far-field ground motions, and the reason is that the near-source forward directivity ground motions contain pulse-long periods. Therefore, the cumulative effects of far-fault records are minor. The damage and collapse of engineering structures observed in the last decades’ earthquakes show the potential of damage in existing structures under near-field ground motions. One important subject studied by earthquake engineers as part of a performance-based approach is the determination of demand and collapse capacity under near-field earthquake. Different methods for evaluating seismic structural performance have been suggested along with and as part of the development of performance-based earthquake engineering. This study investigated the results of illustrious characteristics of near-fault ground motions on the seismic response of reinforced concrete (RC) structures, by the use of Incremental Nonlinear Dynamic Analysis (IDA) method. Due to the fact that various ground motions result in different intensity-versus-response plots, this analysis is done again under various ground motions in order to achieve significant statistical averages. The OpenSees software was used to conduct nonlinear structural evaluations. Numerical modelling showed that near-source outcomes cause most of the seismic energy from the rupture to arrive in a single coherent long-period pulse of motion and permanent ground displacements. Finally, a vulnerability of RC building can be evaluated against pulse-like near-fault ground motions effects.

Nonlinear seismic response of a gravity dam under near-fault ground motions and equivalent pulses

In this paper, the nonlinear seismic response of gravity dams to forward-directivity and ordinary (non forward-directivity) near-fault earthquake ground motions is investigated. Considering Pine Flat dam as case study, it is numerically modeled along with its full reservoir using the finite element method. Two sources of nonlinearity are considered in the analysis: (1) the material nonlinearity of dam concrete, and (2) the geometric nonlinearity by inserting a joint at the base of the dam. Seventy-five forward-directivity and sixty ordinary near-fault ground motions are used to obtain statistically significant results. The equivalent representative pulses of the selected forward-directivity ground motions are extracted using a well-known methodology. The dam-reservoir model is analyzed under the equivalent pulses as well to identify the cases for which the equivalent pulses can capture the structural response to the actual forward-directivity ground motions. Finally, the effects of pulse period, amplitude and energy on the seismic response of the dam-reservoir system are studied.

Effects of Near-Fault Motions and Artificial Pulse-Type Ground Motions on Super-Span Cable-Stayed Bridge Systems

AbstractThis paper evaluates the seismic responses of a super-span cable-stayed bridge subjected to near-fault ground motions. A new method, record-decomposition incorporation (RDI), is proposed to synthesize the artificial near-fault pulse-type ground motions by combining the high-frequency background record (BGR) with simple equivalent pulses. The effects of forward-directivity pulses and fling-step pulses on the responses are assessed for the Sutong cable-stayed bridge (with a main span of 1,088 m) in China. The results show that the near-fault pulse-type ground motions generate larger displacement and internal force to the bridge compared with the nonpulse ground motions, and the fling-step ground motions are more damaging to the bridge than those of the forward-directivity ground motions. The high-frequency components of near-fault records have a significant effect on the responses of the bridge. Subsequently, the longitudinal displacement and bending moment of the tower induced by artificial ground ...

Effects of tectonic regime and soil conditions on the pulse period of near-fault ground motions

In this article, the effects of site conditions (rock vs. soil) and tectonic regime (interplate vs. intraplate) on the pulse period of near-fault forward-directivity ground motions are investigated through linear regression analyses, and appropriate scaling laws that relate the pulse period to the earthquake magnitude are derived. The analyses of rock and soil sites are performed using interplate records only. The results show that for earthquakes of smaller magnitude, the pulse period is longer at soil sites than at rock sites. As the earthquake magnitude increases, the pulse period values at rock and soil sites converge. This observation is in agreement with findings reported in previous studies. However, as shown in this article, the effect of soil conditions on the pulse period is not statistically significant at the 10% level based on the available near-fault ground motion datasets. Regression analyses for interplate and intraplate records are also performed, including regressions with and without self-similarity. The results show that the pulse periods of interplate records are significantly longer (factor of 2.45, on average) than the pulse periods of intraplate records with similar magnitude. It is demonstrated that this difference should also be reflected in the rise times of the events originating in the two different tectonic regimes. Using the specific barrier model as a “tool” of analysis, it is shown that the significant difference in the pulse periods also implies a significant difference (factor of 4) in the local stress drops for the events of the two tectonic regimes. This latter conclusion is supported by recent reports in the literature of the reanalysis of eastern North American earthquakes, accounting more carefully for attenuation.

Rocking isolation of nonductile moderately tall buildings subjected to bidirectional near-fault ground motions

Recent studies show that slender structures with shallow foundations located on soil medium can benefit from rocking isolation effects during strong earthquakes. In such condition, foundation uplifting and soil yielding provide supplemental energy dissipation potential at substructure level. As a result, the structural demands would be significantly reduced. In this study, building structures with various geometrical properties mounted on surface raft foundations are examined. A set of 91 component pairs of near-fault forward-directivity ground motions recorded at soft as well as dense sites are selected. Three dimensional nonlinear soil–structure interaction (SSI) including foundation uplifting and soil yielding is considered. The results show that the protective effects of rocking isolation can play vital role in survival of medium-to-high-rise building structures subject to catastrophic earthquakes which are excessively greater than design limits. Evidently, rocking isolation has enhanced the elastic structural demands up to 50% for low-aspect-ratio as well as 75% for high-aspect-ratio structures. Such beneficial effects keep the superstructure in significantly larger safety margins. In addition, site effects on seismic demands of rocking structures, as well as liquefaction potential in case of buildings located on soft site are investigated.

Effects of near-fault ground motions and equivalent pulses on Large Crossing Transmission Tower-line System

The focus of this paper is the seismic response of Large Crossing Transmission Tower-line System (LCTL) to near-fault ground motions, and whether simplified pulses are capable of representing the effects of the ground motion pulses present in near-fault ground motions on seismic response. The effects of forward-directivity pulses and fling-step pulses on the response of near-fault LCTL were assessed. Results showed that near-fault pulse-like ground motions impose a larger seismic response to LCTL compared to far-field ground motions. The response of LCTL to near-fault motions shows higher scatter than the response to far-field ground motions when correlated with simple intensity measures such as PGA. Moreover, the seismic responses increase with the pulse period of near-fault ground motions. The response of LCTL to the forward-directivity ground motions and fling-step ground motions were reproduced using two new equivalent pulse models. It is shown that the equivalent pulse models can capture the important response characteristics of the near-fault record. There are significant differences between the responses of LCTL subjected to forward-directivity ground motions and the responses subjected to fling-step ground motions. Finally, the tower-line coupling of LCTL subjected to near-fault pulse-like ground motion was investigated. Results show that the effect of tower-line coupling on seismic responses of LCTL to far-field ground motions is more obvious than the responses to near-fault ground motions.

The Seismic Response of High-Speed Railway Bridges Subjected to Near-Fault Forward Directivity Ground Motions Using a Vehicle-Track-Bridge Element

Based on the Next Generation Attenuation (NGA) project ground motion library, the finite element model of the high-speed railway vehicle-bridge system is established. The model was specifically developed for such system that is subjected to near-fault ground motions. In addition, it accounted for the influence of the rail irregularities. The vehicle-track-bridge (VTB) element is presented to simulate the interaction between train and bridge, in which a train can be modeled as a series of sprung masses concentrated at the axle positions. For the short period railway bridge, the results from the case study demonstrate that directivity pulse effect tends to increase the seismic responses of the bridge compared with far-fault ground motions or nonpulse-like motions and the directivity pulse effect and high values of the vertical acceleration component can notably influence the hysteretic behaviour of piers.

Effects of near-fault ground motions and equivalent pulses on multi-story structures

The focus of this paper is the structural response of multi-story structures to near-fault ground motions, and whether structural response is dominated by the ground motion pulses present in forward-directivity ground motions. Also considered is whether simplified pulses are capable of representing the effects of these pulses on structural response. Incremental Dynamic Analysis was employed to assess the effects of forward-directivity pulses on the response of near-fault multi-story structures. Three different generic multi-story shear buildings were subjected to fifty four near-fault ground motions including ordinary and forward-directivity records. The Maximum Story Displacement Ductility Demand was selected as the Engineering Demand Parameter. Results showed that pulse-like forward-directivity ground motions impose a larger ductility demand to the structure compared to ordinary ground motions. Moreover, the response of the structures to forward-directivity motions shows higher scatter than the response to ordinary ground motions when correlated with simple intensity measures such as PGA or spectral acceleration at the first mode period. The only intensity measure that appears to be valid for both ordinary and forward-directivity ground motions is the peak ground velocity. The structural response to the forward directivity ground motions was reproduced using an equivalent pulse model based on the modified Gabor Wavelet pulse. It is shown that when the ratio of pulse period to the fundamental structural period falls in a range of 0.5–2.5, the equivalent pulse model appropriately represents the structural response to forward-directivity ground motions. The simplified pulse parameters can be predicted using existing relationships and can be incorporated into probabilistic seismic hazard analysis to develop a seismic reliability analysis. Finally, the effects of damping ratio and P - Δ were investigated for forward-directivity ground motions. The effect of variations in the damping ratio on the ductility demand was insignificant while P - Δ -effects on the ductility demand are significant.

Strength reduction factors for near-fault forward-directivity ground motions

The seismic design of structures located close to active faults must account for the special characteristics of ground motions in the near-fault region, in particular when these motions contain forward directivity (FD) effects. FD effects result in short duration motions with intense velocity pulses. In this study, the response of nonlinear-single-degree-of-freedom (SDOF) systems to FD motions is characterized by ductility and period-dependent strength reduction factors. This study compares reduction factors from two near-fault data sets, one containing 82 pulse-type motions with FD effects and the other containing 63 motions without FD effects. A statistical analysis was performed to determine if differences between reduction factors for the two sets were statistically significant. Results show that reduction factors for FD and non-FD near-fault motions are different, particularly in the range of periods of FD pulses ( T = 0.5 to 3 s). Strength reduction factors for FD motions were shown to be dependent on earthquake magnitude and, to a lesser degree, site conditions. This magnitude dependence was not observed in non-FD motions. These conclusions indicate that common design specifications based on non-pulse ground motions cannot be directly applied to pulse-type, FD motions.

Near-fault ground motion bi-normalized pseudo-velocity spectra and its applications

Ground motions with forward-directivity effect in the near-fault region are obviously different from ordinary far-field ground motions. Design spectral models for this kind of motions have been proposed by correlating simple pulses with parameters attenuation relationships in a previous study of the authors. To further test the applicability of the established design spectral model, we analyze ground motion pseudo-velocity response spectra (PVS), normalized pseudo-velocity spectra (NPVS) and bi-normalized pseudo-velocity spectra (BNPVS) of 53 typical near-fault forward-directivity ground motions. It is found that BNPVS not only has more salient features to reflect the difference between soil and rock sites, but also has less scattering to reveal the nature of forward-directivity motions. And then, BNPVS is used for prediction of design spectra accounting for the influence of site conditions, and the constructed design spectra are compared with those spectra established previously. It is concluded that site condition can heavily affect ground motions, buildings on rock can be even more dangerous than those on soil sites, in particular for ordinary buildings with short to middle vibration periods. Finally, pulse models are also suggested for structural analyses in the near-fault region.

Seismic Site Response for Near-Fault Forward Directivity Ground Motions

Forward directivity effects in the near-fault region produce pulse-type motions that differ significantly from ordinary ground motions that occur at greater distances from the causative fault. Current code site factors are based on empirical observations and analyses involving less intense nonpulse ordinary ground motions. Nonlinear site response analyses with bidirectional shaking are performed using representative site profiles to quantify seismic site response effects for intense near-fault motions resulting from forward directivity. Input rock motions are represented with simplified velocity pulses that characterize the amplitude and period of forward directivity motions. Results indicate that site response affects both the amplitude and period of forward directivity pulses, and hence, local site conditions should be considered when evaluating seismic designs in the near-fault region. Stiff soil sites tend to amplify the peak ground velocity and increase the period of pulse-type motions, particularly, when the period of the rock motion coincides with the degraded period of the site. Amplification is limited at soft soil sites by the dynamic strength of the weak soil, so attenuation occurs for intense input motions. This nonlinearity is not reflected in the site factors in current building codes. Guidance is provided for estimating the amplitude and pulse period for velocity pulses at soil sites.

Design spectra including effect of rupture directivity in near-fault region

In order to propose a seismic design spectrum that includes the effect of rupture directivity in the near-fault region, this study investigates the application of equivalent pulses to the parameter attenuation relationships developed for near-fault, forward-directivity motions. Near-fault ground motions are represented by equivalent pulses with different waveforms defined by a small number of parameters (peak acceleration, A, and velocity V; and pulse period, Tv). Dimensionless ratios between these parameters (e.g., ATv/V, VTv/D) and response spectral shapes and amplitudes are examined for different pulses to gain insight on their dependence on basic pulse waveforms. Ratios of ATv/V, VTv/D, and the ratio of pulse period to the period for peak spectral velocity (Tv-p) are utilized to quantify the difference between rock and soil sites for near-fault forward-directivity ground motions. The ATv/V ratio of recorded near-fault motions is substantially larger for rock sites than that for soil sites, while Tv-p/Tv ratios are smaller at rock sites than at soil sites. Furthermore, using simple pulses and available predictive relationships for the pulse parameters, a preliminary model for the design acceleration response spectra for the near-fault region that includes the dependence on magnitude, rupture distance, and local site conditions are developed.