The reliability of non‐linear dynamic analysis aimed to predict the seismic performance of structural and geotechnical systems, as well as their dynamic interaction, is significantly affected by the ...selection of suitable input motions. Several procedures for selecting actual earthquake records compatible with the seismic hazard at the site of interest and for evaluating synthetic accelerograms consistent with a seismological framework are available in the literature. However, the degree of uncertainty affecting these approaches might lead to unreliable performance predictions especially for strongly non‐linear problems, such as those involving the response of soils to cyclic and dynamic loading. In this vein, the paper presents a procedure for generating fully non‐stationary ground motions ensuring energy compatibility with a target motion. Two different approaches have been introduced: i) the time window method based on the central finite difference approach and ii) the simplified intensity‐compatible approach in which a closed form expression is provided to evaluate a modulating function that depends on the Arias intensity and on the strong motion duration of a target motion. To highlight the reliability and the accuracy of the proposed procedure, several sets of spectrum‐compatible artificial accelerograms, generated starting from a rock outcropping motion, were used as input motions in a series of one‐dimensional site response analyses. The analysis results are presented and discussed in the paper highlighting the influence of the main features of the proposed generation procedure on the variability of the computed site response.
A performance based approach for site response analysis requires a probabilistic approach accounting for the observed variability in soil stratification and engineering properties of the soil layers. ...The major variability in site-specific response analysis arises from the uncertainties induced by the (a) local seismic hazard assessment, (b) selection and scaling of the hazard compatible input earthquake time histories, (c) soil stratification and engineering properties of encountered soil and rock layers, and (d) method of site response analysis. Even though the uncertainties related to first item, local seismic hazard assessment, has primary importance on the outcome of the site-specific response analyses, the discussion in this article focuses on the possible uncertainties in selection and scaling of the hazard compatible input earthquake time histories, soil stratification, thickness, type and their engineering properties, depth of ground water table and bedrock and properties of the engineering bedrock. One alternative may be to conduct site response analyses for large number of soil profiles generated by Monte Carlo simulations using relatively large number of hazard compatible acceleration time histories to assess probabilistic performance based design acceleration spectra and acceleration time histories calculated on the ground surface with respect to different performance levels. A remaining issue may be considered as the variability induced by 1D, 2D, and 3D site response analysis.
•Definition of performance based site response analysis.•Comparison of 1D, 2D, and 3D site response analysis.
Frequency-dependent equivalent linear (FD-EQL) site response analysis methods, developed as potential substitutes for the EQL procedure to better simulate the nonlinear soil response, have been ...reported to overpredict the high-frequency wave propagation. Modified procedures proposed to overcome this limitation have demonstrated to improve the fits with the nonlinear analysis results. The method has not yet been applied to perform a deconvolution analysis, where a conventional EQL analysis often fails to converge or provide reliable estimate. The conventional EQL procedure and one of the modified FD-EQL method, which uses an empirical factor f to interpolate the strain spectrum between the EQL and FD-EQL outputs, are used to perform a series of deconvolution analyses using an idealized 1000 m profile and twelve Kik-net downhole arrays. The residuals of recorded and deconvoluted motions are shown to increase with strain for the EQL method even when using the recommended frequency cut-off. For the FD-EQL method, the range of f recommended for convolution analyses is shown to provide unrealistic responses in performing deconvolution analyses. A new range that produces unbiased residuals for all strain amplitudes is proposed. Comparisons highlight that the modified FD-EQL yields reliable predictions of the within motion for all profiles and motion intensities, automatically suppressing the amplification of high-frequency noise typically accompanied in a deconvolution 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.
•Studies near-fault pulse effects on liquefaction using effective site response analysis.•Analyzes two soil models and two PWP models for six sites experiencing near-fault and general motions with comparable seismic demands.•Near-fault motions, especially with short duration, induce more PWP due to higher cumulative absolute velocity.•Proposes magnitude scaling factors for near-fault motions to improve liquefaction assessment.
This study quantifies the effects of epistemic uncertainty in soil parameters on nonlinear (NL) site response analysis (SRA) results, validated against the data recorded at a well-instrumented ...geotechnical downhole array located in Japan. To this end, a one-dimensional soil column model of the Service Hall Array (SHA) near the Kashiwazaki-Kariwa Nuclear Power Plant (KKNPP) is developed using the finite element (FE) program LS-DYNA. The dynamic stress–strain relationship is characterized by a modified two-stage hyperbolic (MTH) NL backbone curve formulation capable of capturing soil behavior at both small- and large-shear strains. The model is then validated against the ground motion recordings to capture the model bias. The uncertainties associated with the shear-wave velocity profile (a small-strain soil property) and soil shear strength (a large-strain soil property) are incorporated in NL SRA to quantify their separate and joint randomization effects on the results. This study proposes using the Latin Hypercube Sampling (LHS) method as an efficient alternative to commonly used methods, such as Standard Monte Carlo (SMC), to account for uncertainty propagation in such reliability analysis. Both low-intensity and design-level records from the recordings at the SHA are applied to study the contribution of the small- and large-strain NL dynamic soil properties. Results from 46,200 NL FE analyses (23,100 per input motion) are presented. Measured and predicted site response, using recorded ground motions at this downhole array, is compared to assess the significance of soil parameter uncertainty on the observed ground motion dispersions. It is demonstrated that increasing the number of soft realizations and implementing higher level earthquake intensity lead to higher ground motion dispersion. Unlike past studies in randomization of Vs profiles with the SMC method, the LHS method is shown to have no significant effect on the predicted median surface response spectra and amplification factors (AFs) for this case study.
Ground classification refers to the process of categorizing the ground into different grades based on how soil properties impact ground motion. With the introduction of the National Standard of ...Seismic Design General (KDS 17 00 00) in South Korea, ground classification is now based on the average shear wave velocity of the soil layer (VSSO) and the depth to bedrock (BRDP). To consider the specific characteristics of the ground classification, this study develops a site amplification model based on a suite of nonlinear ground response analyses that utilize VSSO and BRDP as model variables. We collected borehole data from the National Geotechnical Information DB System and obtained VS profiles through geophysical surveys. In cases where no survey record was available, the N-VS relationship was used to estimate the profiles, or representative shear wave velocity profiles for each soil layer were obtained. The collected 93,486 profiles were then classified into 36 clusters based on VSSO and BRDP. 3,271 shear wave velocity profiles were randomly selected from each group and used for the site response analysis. This study utilized 34 observation records of horizontal components of ground motion from domestic and international earthquakes as input seismic waves. Then, each motion was scaled to 1, 1.5, and 2 times, resulting in 81 seismic waves with PGAs ranging from 0.1g to 1g. Nonlinear site response analyses were then performed using the NLHH(Nonlinear Hybrid Hyperbolic) module using a Python-based site response analysis module, PySeismoSoil. This module requires shear wave velocity profiles and input seismic waves as input parameters. The NLHH module uses the HH model, which employs an empirical dynamic curve (Darendeli, 2001)1 to estimate the damping ratio. A nonlinear site response analysis of 81 input seismic waves was performed at 3,271 boreholes using the NLHH module, and the response spectrum and site amplification ratio were determined. Based on these, this study develops a nonlinear site amplification model with the ground classification parameters, VSSO, BRDP, and input PGA level at the depth to bedrock as model variables.
The Philippines, being the archipelagic country that it is, has always understood the importance of inter-island transportation and, in a way, associated progress and development with it. Outside of ...the nation’s capital in Metro Manila, Davao Region is one of the strategic locations identified with the potential to thrive as a business center in Southern Philippines. In order to support and boost economic activity within the Davao Region, the Philippine government proposed the construction of a long-span extradosed bridge that connects two of the region’s major cities. This study presents the site characterization of the proposed bridge and the development of the site-specific ground motions. Situated on potentially liquefiable soil, this study implements an approach that couples porewater pressure generation with nonlinear site response analysis. The results show the expected period-dependent attenuation/amplification of the response spectra and reveal that, even at different hazard levels, the short- and intermediate-period spectral accelerations normalize to a certain intensity. The results are finally compared with the design spectra prescribed in the local design code as a way to demonstrate the effects of soil nonlinearity and liquefaction on the resulting ground motions. At the end of this paper, practical issues and key differences between this coupled approach and the conventional practice shall be highlighted.
In reality, when seismic waves arrive, sites are shaken by multi-directional ground motions that are usually decomposed into three components: two perpendicular horizontal (E-W and N-S) and one ...vertical (UP). Numerous experimental and numerical studies have demonstrated that two horizontal (or bi-directional) motions can significantly aggravate the building and site responses compared to the scenario where only one horizontal motion is applied. However, 1D site response analysis (SRA) is still prevalent in quantifying site effects and responses due to its simplicity. Recently, we investigated the effect of bi-directional ground motions on site responses at potentially liquefiable sites. Results proved that bi-directional ground motions can intensify the site responses and increase the possibility of getting liquefied. In this study, we utilize a machine learning-based method to predict the soil liquefaction under four different excitation conditions, including 1D SRA using E-W, N-S and RotD100 motions, and 2D SRA subjected to bi-directional motions. Moreover, we evaluate the relative importance and correlations of a broad range of ground motion and soil parameters, i.e., peak ground acceleration (PGA), peak ground velocity (PGV), spectral acceleration, relative density, and Vs, 30, in evaluating the liquefaction hazards.
In recent years the applications of metamaterials for building protection have garnered considerable attention. In this study a new device for the seismic protection of new and existing structures ...based on resonant metamaterials has been presented. This device, called Periodic Foundation Piles, applies to deformable soil deposits in order to filter shear waves in a specific frequency range and consists of an array of vertical periodic and relatively small resonators, suitably tuned to one or more natural frequencies of the soil deposit. The performance of this system in reducing the amplitude of a seismic motion has been shown by using a non-linear one-dimensional shear beam model subjected to real acceleration records at the base. A soil deposit with increasing shear wave velocity profile has been considered; the nonlinear behavior of the soil has been modeled through the Iwan model which has been calibrated using a normalized secant shear modulus curve. Numerical results are shown in terms of peak ground acceleration, horizontal displacements and shear strain profiles, as well as in terms of Fourier amplitude and response spectra. The results of the analysis indicate that periodic foundation piles may provide efficient seismic protection for both new and existing structures.
Earthquake records collected at dense arrays of strong-motion stations are often utilized in microzonation studies to evaluate the changes in site response due to variability in site conditions ...across a region. These studies typically begin with calculating Fourier spectral amplification(s) and then transition to performing engineering site response analyses. It has proven difficult to utilize Fourier spectral amplification(s) to define the appropriate elastic response spectr(um)/(a) for a site or sites. This is because, first, the ground motions recorded at these strong-motion stations have lower intensity and hence do not show the nonlinear site effects observed during higher-intensity earthquakes and, second, Fourier and response spectral amplitudes measure different aspects of ground motions. The strong-motion stations in Anchorage, Alaska, have been recording earthquakes in the region for the last three decades. This study utilizes a database of 95 events from 2004 to 2019 to calculate Fourier spectral amplifications at 35 stations using the generalized inversion technique (GIT). Estimated response spectra have been evaluated at each site by applying those Fourier spectral amplifications to a response spectrum of a reference station through random vibration theory (RVT). Correction factors are also applied within the approach to account for nonlinear site effects. This RVT-based approach is tested using ground motions recorded during the MW 7.1 2018 Anchorage Earthquake, and close matches between measured and predicted response spectra are found. The method is then compared with site response analyses using a calibrated 1D equivalent linear (EQL) model of the Delaney Park Downhole Array site. Estimated spectra using the RVT-based approach are, finally, compared with those using Next Generation Attenuation Subduction (NGA-Sub) and NGA-West2 ground-motion models. The proposed method provides a coherent and straightforward way to use GIT-derived Fourier spectral amplifications to directly estimate site-specific response spectra, accounting for nonlinear site effects and without requiring engineering characterization of subsurface soil conditions.
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