• 한국기계가공학회지
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• 제21권1호
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• pp.41-48
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• 2022

Vehicle vibration was introduced in the time and frequency domains using fast Fourier transform (FFT) analysis. In particular, a vibration mode analysis and characteristics of the frequency response function (FRF) in a sport utility vehicle (SUV) passing over a bump barrier at different speeds was performed systematically. The response behavior of the theoretical acceleration was obtained using a numerical method applied to the forced vibration model. The amplitude and frequency of the external force on the vehicle cause various power spectra with individual intrinsic system frequencies. In this regard, several modes of power spectra were acquired from the spectra and are discussed in this paper. The proposed technique can be used for monitoring the acceleration in a vehicle passing over a bump barrier. To acquire acceleration signals, various experimental runs were performed using the SUV. These acceleration signals were then used to acquire the FRF and to conduct mode analysis. The vehicle characteristics according to the vehicle condition were analyzed using FRF. In addition, the vehicle structural system and bump passing frequencies were discriminated based on their power spectra and other FRF spectra.

• Earthquakes and Structures
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• 제4권5호
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• pp.557-585
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• 2013

The recent huge earthquake ground motion records in Japan result in the reconsideration of seismic design forces for nuclear power stations from the view point of seismological research. In addition, the seismic design force should be defined also from the view point of structural engineering. In this paper it is shown that one of the occurrence mechanisms of such large acceleration in recent seismic records (recorded in or near massive structures and not free-field ground motions) is due to the interaction between a massive building and its surrounding soil which induces amplification of local mode in the surface soil. Furthermore on-site investigation after earthquakes in the nuclear power stations reveals some damages of soil around the building (cracks, settlement and sand boiling). The influence of plastic behavior of soil is investigated in the context of interaction between the structure and the surrounding soil. Moreover the amplification property of the surface soil is investigated from the seismic records of the Suruga-gulf earthquake in 2009 and the 2011 off the Pacific coast of Tohoku earthquake in 2011. Two methods are introduced for the analysis of the non-stationary process of ground motions. It is shown that the non-stationary Fourier spectra can detect the temporal change of frequency contents of ground motions and the displacement profile integrated from its acceleration profile is useful to evaluate the seismic behavior of the building and the surrounding soil.

• Geomechanics and Engineering
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• 제21권1호
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• pp.35-51
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• 2020

In this study, the seismic response analysis of embankment dams was investigated through numerical modeling. The seismic behavior of dams under main earthquake records and wavelet-based records were studied. Earthquake records were decomposed using de-noising method (DNM) and down-sampling method (DSM) up to five levels. In decomposition process, low and high frequencies of the main earthquake record were separated into two signals. Acceleration response, spectral acceleration, and Fourier amplitude spectrum at the crest of embankment dams under different decomposition levels were evaluated. The seismic behavior under main and decomposed earthquake records was compared. The results indicate an acceptable agreement between the seismic responses of embankment dams under wavelet-based decomposed records and main earthquake motions. Dynamic analyses show that the DNM-based decomposed earthquake records have a better performance compared to DSM-based records. DNM-based records up to level 4 and DSM-based records up to level 2 have a high accuracy in assessment of seismic behavior of embankment dams. The periods corresponding to the maximum values of acceleration spectra and the frequencies corresponding to the maximum values of Fourier amplitude spectra of embankment dam crest under main and decomposed records are in good agreement. The results demonstrate that the main earthquake records can be replaced by wavelet-based decomposed records in seismic analysis of embankment dams.

• 한국지진공학회논문집
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• 제13권6호
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• pp.27-37
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• 2009

지진파의 푸리에 가속도스펙트럼(Fourier Acceleration Spectrum)에 기반한 계측진도 평가방법(Sokolov and Wald, 2002)의 국내 적용성을 평가하기 위해 관련 논문(연관희 등, 2009)에서 평가된 국내 지진의 진도 MMI ${leq}$ IV 범위에 대한 진도별 FAS 평균(m)과 표준편차(${\sigma}$) 모델을 이용하여, FAS 진도평가방법의 타당성을 평가하여 보았다. FAS 통계특성 모델 평가시 사용된 지진관측자 료의 FAS를 이용하고 본 연구에서 프로그램으로 구현된 FAS 진도평가기법을 적용할 경우 관측된 진도를 ${\sigma}$ = 0.74 MMI의 오차로 추정할 수 있었으며, 오차의 지진규모-거리 의존성을 추가로 보정할 경우 오차를 ${\sigma}$ = 0.61 MMI 까지도 저감할 수 있었다. 또한 본 방법을 MMI ${\leq}$ IV에 대한 국내 FAS 통계특성 모델과 MMI ${\geq}$ V에 대한 전 세계 FAS 통계특성 모델을 함께 이용하여, 진도 VI 이상인 국내 피해지진의 진도를 미소지진관측자료의 지진원특성을 이론적으로 증가시켜 도출된 스펙트럼을 이용하여 추정한 결과 최대 진도추정 오차 0.63 이내로 예측할 수 있었다.

• 센서학회지
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• 제24권3호
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• pp.169-174
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• 2015

In this study, we developed a system to detect the various defects in the metallic objects using the phenomenon that the defects cause the changes of the natural resonant frequencies. Our system consists of a FFT Amp, an Auto Impact Hammer, a Hammer controller and a PC. Auto Impact Hammer creates vibrations in the metallic objects when tapped on the surface. These vibrational signals are converted to the voltage signals by an acceleration sensor attached to the metallic part surface. These analog voltage signals were fed into an ADC (analog-digital converter) and an FFT (fast fourier transform) conversion in the FFT Amp to obtain the digital data in the frequency domain. Labview graphical program was used to process the digital data from th FFT amp to display the spectrum. We compared those spectra with the standard spectrum to find the shifts in the resonant frequencies of the metal parts, and thus detecting the defects. We used PCB's acceleration sensor and TI's TMS320F28335 DSP (digital signal processor) to obtain the resolution of 2.93 Hz and to analyze the frequencies up to 44 kHz.

• Structural Engineering and Mechanics
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• 제48권1호
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• pp.1-16
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• 2013

The seismic response of RC space frame structures with isolated footing resting on a shallow soil stratum on rock is presented in this paper. Homogeneous soil stratum of different stiffness in the very soft to stiff range is considered. Soil, footing and super structure are considered to be the parts of an integral system. A finite element model of the integrated system is developed and subjected to scaled acceleration time histories recorded during two different real earthquakes. Dynamic analysis is performed using mode superposition method of transient analysis. A parametric study is conducted to investigate the effect of flexibility of soil in the dynamic behaviour of low-rise building frames. The time histories and Fourier spectra of roof displacement, base shear and structural response quantities of the space frame on compliant base are presented and compared with the fixed base condition. Results indicate that the incorporation of soil flexibility is required for the realistic estimate of structural seismic response especially for single storey structures resting on very soft soil.

• Smart Structures and Systems
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• 제23권5호
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• pp.507-520
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• 2019

In the structural health monitoring field, damage detection has been commonly carried out based on the structural model and the engineering features related to the model. However, the extracted features are often subjected to various errors, which makes the pattern recognition for damage detection still challenging. In this study, an automated damage identification method is presented for hanger cables in a tied-arch bridge using a convolutional neural network (CNN). Raw measurement data for Fourier amplitude spectra (FAS) of acceleration responses are used without a complex data pre-processing for modal identification. A CNN is a kind of deep neural network that typically consists of convolution, pooling, and fully-connected layers. A numerical simulation study was performed for multiple damage detection in the hangers using ambient wind vibration data on the bridge deck. The results show that the current CNN using FAS data performs better under various damage states than the CNN using time-history data and the traditional neural network using FAS. Robustness of the present CNN has been proven under various observational noise levels and wind speeds.

• Geomechanics and Engineering
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• 제30권1호
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• pp.1-10
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• 2022

Seismic ground response evaluation is one of the main issues in geotechnical earthquake engineering. These analyses are subsequently divided into one-, two- and three-dimensional methods, and each of which can perform in time or frequency domain. In this study, a novel approach is proposed to assess the seismic site response using two-dimensional transfer functions in frequency domain analysis. Using the proposed formulation, a program is written in MATLAB environment and then promoted utilizing the equivalent linear approach. The accuracy of the written program is evaluated by comparing the obtained results with those of actual recorded data in the Gilroy region during Loma Prieta (1989) and Coyote Lake (1979) earthquakes. In order to precise comparison, acceleration time histories, Fourier amplitude spectra and acceleration response spectra diagrams of calculated and recorded data are presented. The proposed 2D transfer function diagrams are also obtained using mentioned earthquakes which show the amount of amplification or attenuation of the input motion at different frequencies while passing through the soil layer. The results of the proposed method confirm its accuracy and efficiency to evaluate ground motion during earthquakes using two-dimensional model. Then, studies on irregular topographies are carried out, and diagrams of amplification factors are shown.

• 한국지진공학회논문집
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• 제2권2호
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• pp.35-44
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• 1998

최근의 중.소규모의 연이은 지진활동은 한반도도 지진에 대하여 안전지대는 아닌 것으로 생각되고 있으며, 1995년 일본의 Kobe 지진 대 참사는 국내에도 지진에 관한 많은 관심을 고조시키고 있다. 그러나, 국내의 구조물의 내진설계를 위한 사용되는 지진파에 대한 연구는 매우 미흡하며, 최근까지도 국내에서는 외국의 설계용 지진파를 그대로 사용하거나 설계응답스펙트럼을 이용하고 있는 실정이다. 본 논문에서는 국내지진파의 물리적 특성, 즉, 주기-빈도 분포, 확률밀도분포, Fourier Spectrum 및 응답스펙트럼을 구하여 비교.분석하였다. 또한 이상화된 인공지진파를 산출하여 이를 현재 교통량의 폭발적인 증가와 도로의 선형성을 이유로 사각을 가진 교량구조물이 많이 건설되고 있는바, 70$^{\circ}$사각슬래브교에 대한 지진해석을 수행하여 인공지진의 합리적 해석 횟수를 규명하였다.

• 한국지진공학회논문집
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• 제13권6호
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• pp.11-25
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• 2009

본 연구에서는 지진파의 푸리에 가속도스펙트럼(Fourier Acceleration Spectrum)에 기반한 계측진도 평가방법(Sokolov and Wald, 2002)을 국내에 적용하기 위해 국내 지진 MMI ${\leq}$ IV 범위에 대한 진도별 FAS 통계특성(평균과 표준편차) 모델을 기상청 발표 진도자료로 부터 도출된 65개 지진에 대한 등진도구역도(Isoseismal Maps)와 등진도구역도 상의 일관된 부지특성을 갖는 580개 수평성분 FAS 자료를 기반으로 경험적으로 도출하였다. 일관된 부지특성을 갖는 FAS는 특정 지진관측소에서 관측된 FAS를 국내 지반물성을 대표할 수 있는 D 등급관측소(연관희와 서정희, 2007)의 부지증폭특성을 갖도록 변환하여 도출하였다. 국내 지진의 MMI ${\leq}$ IV의 FAS 평균값은 로그영역에서 진도에 비례하는 선형성을 나타내었으며, 진도 IV에 대한 FAS 평균은 전 세계 모델과 유사한 특징을 보였다. 또한 진도 MMI ${\leq}$ IV의 FAS 표준편차는 진도 V 이상의 전 세계적인 모델의 표준편차 보다는 크게 평가되었으나, 진도 IV에 대한 전 세계적인 모델의 표준편차보다는 현저히 낮은 값을 나타내었다.