Frequency Content Of Ground Motion Research Articles

Evaluation of pulse effect on frequency content of ground motions and definition of a new characteristic period

Evaluation of pulse effect on frequency content of ground motions and definition of a new characteristic period

INFLUENCE OF NUMERICAL SOIL MODEL TYPE ON DYNAMIC RESPONSE OF REINFORCED CONCRETE FRAME STRUCTURES

In engineering practice, numerical models of structures are typically supported using nondeformable media. However, the compliance and stratification of the underlying soil influence the seismic response of the structure. Its influence on structures subjected to dynamic loads has been investigated using various approaches to soil foundation modeling. This study was conducted for 70 different cases. Three actual soil profiles and two structures were analyzed, and the soil was modeled in three ways. Dynamic analysis was performed using five records of previous earthquakes. Finally, the relationship between the magnitude of the top displacement of the structure and the frequency content of ground motions was established.

An improvement over Fourier transform to enhance its performance for frequency content evaluation of seismic signals

The Fourier analysis is conventional technique for approximating the frequency content of earthquake ground motions. These approaches assume that earthquake excitation is a stationary process. Time frequency analysis techniques such as wavelet transform consider an earthquake to be a non-stationary process that shows a variation in frequency content over time, but these techniques are not easy to apply and most engineers are not familiar with their sophisticated mathematics. This paper introduces a new method for investigating of frequency content of earthquake records which is more accurate than Fourier spectrum but yet easy to apply for structural design and seismological studies. The efficiency of this new approach is investigated by analyzing a variety of single and multiple degree of freedom systems against near, mid and far field earthquake components. The results demonstrate the efficiency of the proposed method for a range of systems having different natural fundamental periods.

A generic soil velocity model that accounts for near-surface conditions and deeper geologic structure

Near-surface soil conditions can significantly alter the amplitude and frequency content of incoming ground motions – often with profound consequences for the built environment – and are thus important inputs to any ground-motion prediction. Previous soil-velocity models (SVM) have predicted shear-wave velocity profiles based on the time-averaged shear-wave velocity in the upper 30 m (VS30). This article presents a generic soil-velocity model that accounts both for near-surface conditions (VS30) and deeper geologic structure, as represented to the depth at which the profile reaches a velocity of 1.0 km/s (Z1.0). To demonstrate the advantages of our new SVM, we apply it to the Cascadia Region of North America, where numerous geologic basins and glaciated landscapes give rise to a wide range of VS30 and Z1.0 combinations. This soil velocity model yields good estimates of site response across all site conditions, and significantly improves upon a model calibrated using only VS30 data. In conjunction with existing models that describe the deep velocity structure of the region (e.g., (Stephenson et al., 2017) [27]; the proposed model is particularly suited for use in regional-scale predictions of site response, liquefaction, landslides, infrastructure damage, and loss. The proposed methodology is broadly applicable to the development of SVMs elsewhere, and with improved understanding of near-surface and deep velocity structures, can facilitate more accurate ground-motion predictions globally.

A coupled numerical approach to simulate the effect of earthquake frequency content on seismic behavior of submarine tunnel

Roughly 90% of all natural vibrations have epicenters in offshore zones and may cause destruction of submarine and floating structures. Such excitations can influence the safe performance of facilities set up on the seabed, like tunnels, jacket legs and subsea oil pipelines. Some researches on this theme have been carried out to demonstrate the importance of seaquake analyses and their effects have been underlined. The present study intends to numerically simulate a two-dimensional fluid-structure interaction (FSI) problem in order to examine the dynamic response of submarine tunnel under real horizontal earthquakes. Pressure is considered as independent nodal variables to represent the fluid flow effects and the induced time-dependent acceleration in porous medium equation is incorporated in the analysis and the tunnel shell is considered as flexible. This work highlights the importance of the input ground motion frequency content that governs the development of the induced seismic stress/strain around the lining of the tunnel. The results demonstrate that for deep sea the increment rate of the circumferential stress caused by surface gravity waves is below 7% when compared to the no-wave interface condition. Moreover, it is confirmed that long-period record may amplify the overall response of the system (up to 60%) specially the lateral and vertical displacements, as well as the principal stress to a lesser extent. The developed numerical model can attend to further analysis of tunnels embedded in a half-space in conjunction with fluid undergoing the severe long-period earthquakes.

Influence of frequency content of ground motions on seismic fragility of equipment in nuclear power plant

The objective of this study is to investigate the influence of frequency content of ground motions on the seismic fragility of nuclear power plant (NPP) equipment. Transfer functions of a steam generator, a reactor vessel and an auxiliary building of a Korean APR1400 type NPP are extracted from finite element models. The transfer functions are used to investigate the influence of frequency contents of input ground motion on the seismic fragilities of the equipment. A methodology is proposed to transform fragility functions that are defined in terms of peak ground acceleration (PGA) of the design response spectrum to those that are defined in terms of realistic response parameters. The method is applied to a three-dimensional finite element model of the auxiliary building as a case study. The results show that all equipment has a higher probability of failure when the auxiliary building is subjected to ground motions with high-frequency content.

2D nonlinear site response analysis of shallow bedrock sites using integrated subsurface profiles

In this study, an attempt has been made to generate the subsurface profiles using integrated geophysical approach and a site-specific 2-D nonlinear site response analysis at shallow bedrock sites in Peninsular India. The site selected for the study is Tarapur, India in which three survey lines were selected for the sub-surface mapping. The subsurface profiling of these three locations was done by an integrated geophysical approach by carrying out Multichannel Analysis of Surface Waves - 2D survey, Ground Penetrating Radar survey, and conventional drilling borehole and measurement of Standard Penetration Test N values. Thirteen intraplate recordings from all around the world are selected for the site response analysis. The subsurface profiles generated by the integrated geophysical approaches are modeled in FLAC - 2D nonlinear module and site response parameters are estimated. Further 1D site response analyses are carried out at selected points in 2- D profiles using the program DEEPSOIL and the 2D site response results are compared. Results of site response analysis are expressed in terms of short-period amplification factor (Fc) and long period amplification factor (Fs) and the results show that the 2D geometry of sub-surface is more sensitive to the higher frequency content of ground motion especially for subsurface with more heterogeneities. Also, a parametric study has been done on a synthetic 2D profile to quantitatively evaluate the effect of different ground motion characteristics like amplitude and frequency content on 2D site response analysis. The parametric study shows that underground heterogeneity is very sensitive to the ground motions with high-frequency content and low amplitude.

Evaluation of seismic site response of submarine clay canyons using centrifuge modelling

The morphological features of submarine canyons significantly modify the amplitude, duration and frequency content of free-field seismic ground motions. This paper examines the influence of these morphological features through a series of seismic centrifuge tests on kaolin clay models with canyon sloping angles of 15, 30 and 45°. These tests were performed using the IFSTTAR (the French Institute of Science and Technology for Transport, Development and Networks) geotechnical centrifuge at an acceleration of 40g by applying a set of dynamic excitations, varying in amplitude and frequency content. The response of the clay canyon models was monitored by means of accelerometers and pore-pressure transducers. In-flight characterisation was performed on each clay model with bender element and T-bar tests prior to the application of seismic excitations. The study shows that the topographic shape of the canyon can substantially affect peak values and the frequency content of the free-field motions along the slope surface. Low-intensity seismic excitations and frequency contents along with the dominant frequency of the soil deposit were found to cause substantial amplification. In addition, the reduction of the shear modulus and the corresponding increase in damping tend to increase the fundamental site period.

Strength‐reduction factors for the design of light nonstructural elements in buildings

SummaryStrength‐reduction factors that reduce ordinates of floor spectra acceleration due to nonlinearity in the secondary system are investigated. In exchange for permitting some inelastic deformation to occur in the secondary system or its supports, these strength reduction factors allow to design the nonstructural elements or their supports for lateral forces that are smaller than those that would be required to maintain them elastically during earthquakes. This paper presents the results of a statistical analysis on component strength‐reduction factors that were computed considering floor motions recorded on instrumented buildings in California during various earthquakes. The effect of yielding in the component or its anchorage/bracing in offering protection against excessive component acceleration demands is investigated. It is shown that strength‐reduction factors computed from floor motions are significantly different from those computed from ground motions recorded on rock or on firm soils. In particular, they exhibit much larger reductions for periods tuned or nearly tuned to the dominant modal periods of the building response. This is due to the large differences in frequency content of ground motions and floor motions, with the former typically characterized by wide‐band spectra whereas the latter are characterized by narrow‐band spectra near periods of dominant modes in the response of the building. Finally, the study provides approximate equations to estimate component strength‐reduction factors computed through nonlinear regression analyses.

Sensitivity of 1-D ground motion predictions to analysis codes and material models using KiK-net vertical arrays

One-dimensional (1-D) ground response analysis (GRA) predictions are highly sensitive to the analysis methods and dynamic soil models employed, contributing to significant scatter in analysis outputs. In this paper, a parametric study of 1-D GRA is conducted to evaluate the performance of various models using 60 ground motion records of varying intensity from five Kiban-Kyoshin (KiK-net) downhole array stations in Japan. Four GRA codes are considered, including the equivalent-linear (EL) method, a modified EL method using frequency-dependent soil parameters (FDEL), and two nonlinear (NL) models computed in OpenSees and DEEPSOIL. The soil constitutive models are constrained using a set of five material curves representing a wide range of nonlinear soil behaviour. It was found that all models are sensitive to the material curves at high frequencies, and the NL models are highly sensitive to the parametrization of Rayleigh damping. As the levels of soil nonlinearity increase, all GRA methods tend to over-damp the higher frequency content of ground motions, with substantial discrepancies across the model predictions. Interestingly, the FDEL method displays a lower sensitivity to material curves and reduces the biases at high frequencies. The results of this study suggest that GRA codes and material curves contribute evenly to the overall dispersion when simulating strong ground motions. To quantify the variability of GRA models, along with the epistemic uncertainties, a linear regression relationship in bi-logarithmic space is established across all records between the total standard deviation and the shear strains developed within the soil profiles.

The correlation between the ground motion intensity measure parameters of earthquakes

In performance-based seismic engineering, the choice of ground motion goes hand in hand with dynamic analysis. This paper presents the relationship between the various intensity measures (IMs) parameter of ground motions that can be used in evaluating the behavior of structures. This is through the relationship between them and the ratio of the peak ground acceleration to the peak ground velocity (PGA/PGV). The PGA/PGV ratio has been used an empirical parameter to estimate ground motion frequency content and categorizes ground motion suites for performing nonlinear time history analyses of structures. Regression analyses are performed on data obtained from ground motion records with different PGA/PGV ratios. The regression analyses indicate that the best correlation IMs parameter with the PGA/PGV ratio in both near- and far-source earthquakes is the mean period (Tm). Also, the relationships between PGA/PGV ratio and some IMs parameters in near-source earthquakes are stronger than it in far-source earthquakes.

Influence of Earthquake Frequency Nonstationarity on Seismic Structural Response

Earthquake ground acceleration records exhibit nonstationarity in both amplitude and frequency content. In order to simplify the random vibration analysis, most earthquake models have often neglected the temporal variation of the frequency content. This is partly because it is difficult to incorporate this variation in ground motion models and also partly because it was believed that it had no substantial effect on structure response. The aim of this paper is to identify the temporal variation in frequency content effects, characterizing the earthquake strong ground motions, on structures. Two comparative stochastic earthquake ground motion models based on the concept of physical spectrum are used to characterize and simulate from a selected ground acceleration record two sets of artificial earthquakes exhibiting similar phases distribution and different frequency contents: the first set presents a time-variant frequency content, and the second has a time-invariant frequency content. Three hysteretic models have been considered, elasto-plastic model (bilinear), Clough’s stiffness degrading model and maximum point-oriented bilinear model. Comparative results are presented to quantify the effects of the time-variant frequency content of earthquake ground motion on the structural response of elastic and inelastic simple systems. It is found that the characteristics of elastic and inelastic structural response depend strongly on the time variation of the frequency content in the seismic input.

Aerial Seismology Using Balloon-Based Barometers

Seismology on Venus has long eluded planetary scientists due to extreme temperature and pressure conditions on its surface, which most electronics cannot withstand for mission durations required for ground-based seismic studies. We show that infrasonic (low-frequency) pressure fluctuations, generated as a result of ground motion, produced by an artificial seismic source known as a seismic hammer, and recorded using sensitive microbarometers deployed on a tethered balloon, are able to replicate the frequency content of ground motion. We also show that weak, artificial seismic activity thus produced may be geolocated by using multiple airborne barometers. The success of this technique paves the way for balloon-based aero-seismology, leading to a potentially revolutionary method to perform seismic studies from a remote airborne station on the earth and solar system objects with substantial atmospheres such as Venus and Titan.

An iterative linear procedure using frequency-dependent soil parameters for site response analyses

Local site amplification predictions that derive from the traditional equivalent linear (EL) procedure are deemed to be reliable when weak shaking is considered, while it is well recognized that significant discrepancies can occur for stronger earthquakes where soil nonlinearity dominates the site response. To better capture nonlinearity effects, frequency-dependent equivalent linear methods, commonly referred to as FDEL methods, were proposed by modifying the EL procedure with frequency-dependent soil parameters. However, these methods have never been widely used in earthquake engineering practice. Existing FDEL methods rely on generic spectral shape fitting parameters to approximate the variation of shear strain amplitudes in the frequency domain, which can lead to substantial inconsistencies in the model prediction when applied to a particular seismic event, with a specific signature. This study investigates a range of spectral seismic intensities to identify a proxy that could be used directly to update the shear strain spectra in accordance with the simulated ground motions. A new FDEL algorithm to compute effective shear strain as a function of frequency is proposed and tested on various scenarios. When simulating strong ground motions that exhibit higher levels of soil deformation, the frequency content of ground motions processed within the FDEL procedure is substantially improved as compared to the EL method, with results more consistent with those from NL analyses.

Mapping of ground motion amplifications for the Fraser River delta in Greater Vancouver, Canada

The Fraser River delta in Greater Vancouver, Canada consists of deep soft deposits of silts and clays, and it is well known that the deep soil deposits can amplify the low frequency contents of ground motions. This study aims to investigate the effects of deep soil deposits in the delta on ground motion amplifications by using thorough site response simulations that account for the full soil profiles and a suite of recorded ground motions that covers a wide range of intensity levels. Based on both equivalent-linear and nonlinear site response simulations, the effects of soil depth (represented by natural period of the soil, TS) on ground motion amplifications for various spectral periods are clearly demonstrated. The ground motion amplification maps for various spectral periods and rock ground motion intensity levels are also generated to be used in the regional seismic hazard assessment for infrastructure. It is found that ground motions for long periods are substantially amplified in the center of the delta, while those for short periods are de-amplified when input rock motions are large.

Effect of liquefaction on nonlinear seismic response in layered soils: a case study of Babol, North of Iran

Soft soil layer can significantly increase destruction during ground motions. Although, earthquake waves pass through mainly rocky media to reach ground surface, they often transit through a thin layer of surficial soil which can dramatically affect ground surface motions. This study thoroughly investigates the effect of liquefaction on the seismic response of soft soil layers using the finite difference method in specific areas of Babol City, north of Iran. Nonlinear effective stress analysis was carried out using FLAC 2 D, a finite difference based software. Liquefaction potential was evaluated based on both pore pressure ratio (R u) and shear strain amplitude criteria. The logical agreement of these two criteria was observed. Based on the results, near the river of this city, strong site effect observed due to the presence of soft soil layers and liquefaction, effectively altered the amplitude and frequency content of ground motions. Seismic analyses proved that spectral acceleration responses were attenuated for low periods due to the shear strain development and energy dissipation of motion but they increased for longer periods, which could strongly affect the seismic design of structures.

Summarized IDA curves by the wavelet transform and bees optimization algorithm

Incremental dynamic analysis (IDA), as an accurate method to evaluate the parameters of structural performance levels, requires many non-linear time history analyses, using a set of ground motion records which are scaled to different intensity levels. Therefore, this method is very computationally demanding. In this study, a new method is presented to estimate the summarized (16%, 50%, and 84% fractiles) IDA curves of a first-mode dominated structure using discrete wavelet transform and bees optimization algorithm. This method reduces the number of required ground motion records for the prediction of the summarized IDA curves. At first, a subset of first list ground motion records is decomposed by means of discrete wavelet transform which have a low dispersion estimating the summarized IDA curves of equivalent SDOF system of the main structure. Then, the bees algorithm optimizes a series of factors for each level of detail coefficients in discrete wavelet transform. The applied factors change the frequency content of original ground motion records which the generated ground motions records can be utilized to reliably estimate the summarized IDA curves of the main structure. At the end, to evaluate the efficiency of the proposed method, the seismic behavior of a typical 3-story special steel moment frame, subjected to a set of twenty ground motion records is compared with this method.

A new method of deriving the acceleration and displacement design spectra of pulse-like ground motions based on the wavelet multi-resolution analysis

Pulse-like ground motions usually have higher spectral displacements and spectral accelerations than other motions in long periods and can cause severe damage on structures. However, there is still no a generally accepted method to incorporate the features of this type motion in the code-based design spectrum. The pulse period is one of the key parameters to characterize the features of pulse-like ground motions. The relations between the pulse period and the earthquake magnitude have been well studied, and many prediction equations have been proposed. However, these results cannot be considered in the process of deriving the design spectrum of pulse-like ground motions. This paper proposes a new method to derive the acceleration and displacement design spectra of pulse-like ground motions based on the wavelet multi-resolution analysis. To consider the influence of the frequency content of ground motions, the frequency band of ground motions is divided into several continuous sub-bands. One or more ground motion components obtained by the wavelet multi-resolution analysis are used to represent the frequency content in each sub-band. Differing from previous methods, the design spectral shape of the proposed method can be changed according to the frequency content of ground motions, and the prediction equations between the pulse period and the earthquake magnitude can be used to improve the current code-based design spectrum.

Seismic Response of Soft Soil Deposit Using Simplified Models

Near surface soils can greatly influence the amplitude, duration, and frequency content of ground motions. Surveys of the damage caused by earthquakes indicates that the lowest levels of damage occur in structures founded on rock or hard soil, while most of the damage occurs usually in structures founded in soft soil sites. With the aim to understand better the seismic response of soft soils deposits, not susceptible to liquefaction, this study made a comparison between the real seismic response registered in soft soil deposit in the 2011 Tohoku earthquake (Mw=9.1), with the response predicted by a propagation analysis with the equivalent linear method using the computer program SHAKE2000 [1]. An additional comparison is made applying the simplified method of Carlton (2014), developed specifically for soft soils. The site chosen for this analysis was a soft soil deposit, with NEHRP site classification type F, monitored by the seismic station TKCH07 of the KiK-net network located in Hokaiddo, Japan. The estimated response showed and acceptable approximation with the real response, although the response calculated with SHAKE2000 predicted high levels of amplification near the natural frequencies of the soft soil deposit.

A new practical equivalent linear model for estimating seismic hysteretic energy demand of bilinear systems

Hysteretic energy is defined as energy dissipated through inelastic deformations during a ground motion by the system. It includes frequency content and duration of ground motion as two remarkable parameters, while these characteristics are not seen in displacement spectrum. Since maximum displacement individually cannot be the appropriate criterion for damage assessment, hysteretic energy has been evaluated in this research as a more comprehensive seismic demand parameter. An innovative methodology has been proposed to establish a new equivalent linear model to estimate hysteretic energy spectrum for bilinear SDOF models under two different sets of earthquake excitations. Error minimization has been defined in the space of equivalent linearization concept, which resulted in equivalent damping and equivalent period as representative parameters of the linear model. Nonlinear regression analysis was carried out for predicting these equivalent parameter as a function of ductility. The results also indicate differences between seismic demand characteristics of far-field and near-field ground motions, which are not identified by most of previous equations presented for predicting seismic energy. The main advantage of the proposed model is its independency on parameters related to earthquake and response characteristics, which has led to more efficiency as well as simplicity. The capability of providing a practical energy based seismic performance evaluation is another outstanding feature of the proposed model.