In this study, the effect of near-field and far-field ground motions on the seismic response of the soil pile system is investigated. The forward directivity effect, which includes a large velocity pulse at the beginning of the velocity time history of the ground motion is the most damaging phenomenon observed in near-field ground motions. To investigate the effect of near-field and far-field ground motions on the seismic response of a soil-pile system, a three-dimensional model consisting of the two-layer soil, liquefiable sand layer over dense sand, and the pile is utilized. Modeling is conducted in FLAC 3D software. The P2P Sand constitutive model is selected for sandy soil. Three fault-normal near-field and three far-field ground motion records were applied to the model. The numerical results show that near field velocity pulses have a considerable effect on the system behavior and sudden huge displacement demands were observed. Also, during the near-field ground motions, the exceeded pore water pressure coefficient (Ru) increases so that liquefaction occurs in the upper loose sand layer. Due to the pulse-like ground motions, a pulse-like relative displacement is created in response to the pile. Meanwhile the relative displacement response of the pile is entirely different due to the energy distribution during the far-field ground motions.

The foundation conditions of piers for multi-span long-distance heavy-haul railway bridges inevitably vary at different locations, which may lead to non-uniform ground motions at each pier position, potentially causing adverse effects on the bridge's seismic response. To investigate the seismic response of bridges and the running safety of heavy-haul trains as they cross the bridge during an earthquake, a three-dimensional heavy-haul railway train-track-bridge (HRTTB) coupled system model was developed using ANSYS/LS-DYNA. This model incorporates the nonlinear behavior of critical components such as bearings, lateral restrainers, piers, and wheel-rail contact interactions, and it has been validated against field-measured data to ensure reliable dynamics parameters for seismic analysis. A multi-span simply supported girder bridge from a heavy-haul railway (HHR) was employed as a case study, in which a spatially correlated non-stationary ground motion field was generated based on spectral representation harmonic theory. Comparative analyses of the seismic responses under spatially varying ground motions (SVGM) and uniform seismic excitation conditions were performed for the coupled system. The results indicate that the presence of heavy-haul trains prolongs the natural period of the HRTTB system, thereby appreciably altering its seismic response. At lower apparent wave velocities, more piers exhibit a low-response state, and some pier bases enter the elastic-plastic stage under local site effects. Compared with the piers, the bearings show higher sensitivity to seismic inputs; fixed bearings experience damage when subjected to traveling wave effects and local site effects, which is subsequently followed by the failure of lateral restrainers. Train running safety is markedly reduced when crossing local soft soil site conditions. The conclusions drawn from this study can be applied in the seismic design and running safety assessment of HHR bridge systems under SVGM.

This study examines the fragility response of an earthen embankment supported on a liquefiable deposit subjected to pulse and nonpulse ground motions. Fragility curves are developed based on two key parameters, namely, median seismic intensity and overall variability in the analysis. Such curves represent the vulnerability of an earthen embankment under two distinct types of ground motions. Numerical simulations are performed using two-dimensional finite-element analysis under plane strain conditions. The saturated sandy deposits in the foundation are modeled with the UBC3D-PLM constitutive model and calibrated with appropriate parameters. Two damage indexes are introduced: normalized embankment settlement and lateral embankment deformation. Nonlinear incremental dynamic analysis is performed for various ground motions, and fragility parameters are developed for different damage levels. The results show that pulse-type earthquakes cause more serious damage to earthen structures than nonpulse-type earthquakes, increasing the vulnerability. Further, the liquefiable layer thickness in the foundation soil plays a significant role in the vulnerability assessment of the embankment. The foundation liquefiable layer with less thickness may lead to an early onset of damage and lower the seismic demand on the embankment structure at lower damage levels. With an increase in the layer thickness, seismic demand reduces, with the drainage path playing a critical role.

Cloud and incremental dynamic analysis (IDA) are the two most commonly used methods for seismic fragility analysis. The two methods differ significantly in the number of ground motions and whether these motions are scaled. This paper designed a random selection procedure to thoroughly discuss the influence of ground motion combinations encompassing different numbers of motions on the Cloud-based and IDA-based seismic fragility analysis for underground subway station structures. Focusing on a shallow-buried single-story station structure, a nonlinear dynamic time-history finite element analysis model of soil-structure interaction was developed. 400 ground motions were selected for random combination to perform Cloud-based seismic fragility analysis, and 20 ground motions were selected for random combination to perform IDA-based analysis. The results showed that the number of ground motions has a significant influence on the seismic fragility analysis in both Cloud and IDA, especially on the prediction of damage probability for higher seismic performance levels and when the PGA exceeded 0.3 g. In regions with a low probability of strong earthquakes, this paper recommended using no fewer than 10 and 220 ground motions in the IDA-based and Cloud-based seismic fragility analyses, respectively. In regions with a high probability of strong earthquakes, the optimal number of ground motions should be increased to 300 for Cloud-based analysis and 15 for IDA-based analysis.

In the dynamic response analysis of slopes, the displacement of sliding surfaces is an important indicator for assessing stability. However, due to the uniform dynamic parameters of the Newmark slide block method, it is difficult to accurately analyze the displacements of large-scale slopes. To address this issue, the spatial distribution characteristics of dynamic parameters need to be considered to accurately analyze the stability of slopes. Under the combined action of rainfall and reservoir water level change, the Shiliushubao old landslide in the Three Gorges Reservoir area remains stable. To investigate the seismic stability of slopes, simulated seismic waves were employed. Firstly, the dynamic triaxial test, designed with cyclic loading, was employed to investigate the variation rules of the dynamic parameters of slope soil, and to establish a functional relationship. Then, the stochastic seismic motion model was used to generate artificially seismic waves in the Three Gorges Reservoir Area. Finally, to assess the stability of the old landslide, finite element software, GeoStudio 2018 was used to obtain the spatial distribution characteristics of the dynamic parameters and to calculate the permanent displacements of the reservoir bank slope by inputting random ground motion loads and dynamic characteristic functions. It is demonstrated that under the most unfavorable working conditions of heavy rainfall and the earthquake in the specific region, the permanent displacement of the Shiliushubao old landslide will be less than the critical permanent displacement, the old landslide is not to experience instability again.

Despite the complexity of real earthquake motions, the incident wavefield excitation for soil-structure interaction (SSI) analysis is conventionally derived from one-dimensional site response analysis (1D SRA), resulting in idealized, decoupled vertically incident shear and compressional waves for the horizontal and vertical components of the wavefield, respectively. Recent studies have revealed potentially significant deviation of the 1D free-field predictions from the actual three-dimensional (3D) site response and obtained physical insights into the mechanistic deficiencies of this simplified approach. Particularly, when applied to vertical motion estimation, 1D SRA can lead to consistent overprediction due to the refraction of inclined S waves in the actual wavefield that is not correctly accounted for in the idealized vertical P wave propagation model. However, in addition to the free-field site response, seismic demands on structures and non-structural components are also influenced by the dynamic characteristics of the structure and SSI effects. The extent to which the utilization of vertically propagating waves influences the structural system response is currently not well understood. With the recent realization of high-performance broadband physics-based 3D ground motion simulations, this study evaluates the impact of incident wavefield modeling on SSI analysis of representative building structures based on two essential ingredients: (1) realistic spatially dense simulated ground motions in shallow sedimentary basins as the reference incident motions for the local SSI model and (2) high-fidelity direct modeling of the soil-structure system that fully honors the complexity of the incident seismic waves. Numerical models for a suite of archetypal two-dimensional (2D) multi-story building frames were developed to study their seismic response under the following incident wavefield modeling conditions: (1) SSI models with reference incident waves from the 3D earthquake simulation, (2) SSI models with idealized vertically incident waves based on 1D SRA, and (3) conventional fixed-base models with base translational motions from 1D SRA. The impact of these modeling choices on various structural and non-structural demands is investigated and contrasted. The results show that, for the horizontal direction, the free-field linear and nonlinear site amplification and subsequent dynamic filtering of the base motions within the structure can be reasonably captured by the assumed vertically propagating shear waves. This leads to generally fair agreements for structural demands controlled by horizontal motions, including peak inter-story drifts and yielding of structural components. In contrast, vertical seismic demands on structures are overpredicted in most cases when using the 1D wavefields and can result in exacerbated structural damage. Special attention should be given to the potentially severe vertical floor accelerations predicted by the 1D approach due to the combined effects of fictitious free-field site amplification and significant vertical dynamic amplification along the building height. This can pose unrealistic challenges to seismic certification of acceleration-sensitive secondary equipment necessary for structural and operational functionality and containment barrier design of critical infrastructures. It is also demonstrated that vertical SSI effects can be more significant than those in the horizontal direction due to the large vertical structural stiffness and should be considered in vertical floor acceleration assessments, especially for massive high-rise buildings.

Despite the emergence of recent advancements, machine learning (ML) based methods for estimating the fragility curves of structures through probabilistic ground motion selection techniques pose a challenge due to the computational cost associated with data preparation. The primary aim of this research is to reduce the data preparation time involved in estimating the fragility curves of structures using a ground motion selection approach that considers earthquake magnitude, distance from the seismic source, and shear wave velocity of soil as essential parameters. To achieve this objective, ML algorithms are employed to calculate the fragility curves of various reinforced concrete moment resisting (RC/MR) frames with different periods, utilizing codebased and generalized conditional intensity measure (GCIM) ground motion selection methods. The SMOTE-ENN technique, a data resampling method, is used to balance the training data for the ML algorithms to address potential bias resulting from imbalanced training data. To validate the fragility curves obtained through ML, analytical fragility curves are derived for a specific structure at three damage levels and compared with the ML curves. The results demonstrate that the percentage of the enclosed area between the analytical and ML curves, relative to the area under the analytical curve, is below 10 % and 5 % for the GCIM and code-based methods, respectively. Fragility curves were generated for various structures, including regular and irregular buildings, to investigate the generalizability. Results indicate that, for the specific structures analyzed in this study, excluding torsional ones, the structure's period is a sufficient structural feature for generating fragility curves.

We propose a test procedure to quantify the response of dry sand to cyclic compressional loading under constrained conditions. The test procedure is designed to represent the response of sand layers to upward propagating P-waves during an earthquake event. Such P-waves are prominent within the vertical component of earthquake ground-motions, which is often ignored or simplified in common practice of seismic hazard analysis, despite its potential damaging effects. In the proposed method, the lateral deformation is restrained within a triaxial device, through variations of the cell pressure, thus maintaining pure compression while allowing moderate to large axial strains. Both dry and saturated samples are tested, and the compressive stiffness is computed from the full stress-strain loops. We show that as long as drained conditions are maintained and volume changes are allowed - the response of a saturated sample to slow cyclic loading is representative of the response of dry sand to seismic loading, despite the differences in saturation and in strain rates. Finally, we compare the proposed method to cyclic loading within a rigid cell and discuss the differences and limitations that the new proposed method overcomes.

This study focuses on the challenge of identifying the most destructive earthquakes to minimize earthquakeinduced damage, with particular attention to the seismic behavior of special reinforced concrete moment frames (RCMFs) and the influence of soil-structure interaction (SSI). To achieve this objective, a numerical model was developed in OpenSEES platform to analyze RCMFs with heights of 2, 6 and 10 stories on four different soil types (Site Classes B to E). Also, to consider the effect of SSI, the study utilized a Beam on Nonlinear Winkler Foundation approach (BNWF), incorporating springs and dashpots. An extensive set of earthquake records, including 274 horizontal ground motion records, categorized based on shear wave velocity for each site class, was employed. Incremental dynamic analysis (IDA) was used to identify the most destructive earthquake scenarios, with maximum inter-story drift serving as the damage measure (DM) for the four seismic performance levels proposed by HAZUS and peak ground acceleration (PGA) as the intensity measure (IM). After performing correlation analysis between the 57 ground motion parameters (GMPs) and the maximum inter-story drift, followed by an inter-correlation analysis among the candidate GMPs, it was ultimately determined that the GMPs: Vmax/Amax, Tm and F5PSD, accurately represent the potential for seismic damage. IDA results highlighted the significant influence of SSI on the seismic performance of structure, especially in taller buildings constructed on softer soil types. Finally, two equations were developed based on the identified GMPs to determine and rank destructive earthquakes for both SSI and no-SSI (NSSI) conditions.

Although the influence of duration of ground motion on the seismic response of aboveground structures is clearly recognized, its influence on underground structures remains unclear. To this end, this study performs incremental dynamic analyses under both short and long duration ground motions, to quantify the significance of the duration effect on the seismic fragility of subway stations. A two-dimensional soil-structure system is established on the basis of the Daikai subway station, consisting of an elastoplastic soil model and a concrete damage plasticity model. A set of thirty spectrally matched ground motions with varying significant durations (D5-95) are employed. In particular, using the center column total compressive damage index (DTCD) and peak inter-story drift ratio (IDR) as structural demand measures (DM), the percentage difference in fragility curves between long and short duration is evaluated by accounting for six suits of damage state thresholds. Correlations between the two DMs and D5-95 show that ground motion duration affects significantly the seismic fragility of subway stations. Overall, the duration effect is not detected in the minor damage state and becomes more pronounced in the collapse state, suggesting that the duration effect increases as the damage state threshold increases. The median collapse capacity for long duration ground motions is up to approximately 60% or 37% lower than that for short duration ground motions, when a peak IDR or DTCD are adopted, respectively. The results of this study highlight the great importance of properly considering duration when selecting earthquake records for seismic fragility assessment of subway stations.