• Transactions of the Korean Society of Mechanical Engineers A
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• 제27권10호
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• pp.1712-1719
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• 2003

This paper concerns on application of active magnetic bearing(AMB) system to levitate the elevation axis of an electro-optical sight mounted on moving vehicles. In such a system, it is desirable to retain the elevation axis within the predetermined air-gap while the vehicle is moving. An optimal base acceleration feedforward control is proposed to reduce the base motion response. In the consideration of the uncertainty of the system model, a filtered-x least-mean-square(FXLMS) algorithm is used to estimate the frequency response function of the feedforward control which cancels base motions. The frequency response function is fitted to an optimal feedforward control. Experimental results demonstrate that the proposed control reduces the air-gap deviation to 27.7% that by feedback control alone.

• Journal of the Earthquake Engineering Society of Korea
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• 제20권1호
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• pp.45-54
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• 2016

This study suggests a prediction model of ground motion spectral shape considering characteristics of earthquake records in Korea. Based on the Graizer and Kalkan's prediction procedure, a spectral shape model is defined as a continuous function of period in order to improve the complex problems of the conventional models. The approximate spectral shape function is then developed with parameters such as moment magnitude, fault distance, and average shear velocity of independent variables. This paper finally determines estimator coefficients of subfunctions which explain the corelation among the independent variables using the nonlinear optimization. As a result of generating the prediction model of ground motion spectral shape, the ground motion spectral shape well estimates the response spectrum of earthquake recordings in Korea.

• Journal of Domestic Journal Test
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• 제11권2호
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• pp.221-227
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• 2023

− A rotordynamic system consists of components that undergo rotational motion. These components include shafts, impellers, thrust collars, and components that support rotation, such as bearings and seals. The motion of this type of rotating system can be modeled as two-dimensional motion and, accordingly, the equa- tion of motion for the rotordynamic system can be represented using complex coordinates. The directional fre- quency response function (dFRF) can be derived from this complex coordinate system and used as an effective analytical tool for rotating machinery. However, the dFRF is not widely used in the field because most pre- vious studies and commercial software are based on real coordinate systems. The objective of the current study is to introduce the dFRF and show that it can be an effective tool in rotordynamic analysis. In this study, the normal frequency response function (nFRF) and dFRF are compared under rotordynamic analysis for isotropic and unisotropic rotors. Results show that in the nFRF, the magnitude of the response is the same for both pos- itive and negative frequencies, and the response is similar under all modes. Consequently, the severity of the mode cannot be identified. However, in the dFRF, the forward and backward modes are clearly distinguishable in the frequency domain of the isotropic rotor, and the severity of the mode can be identified for the uniso- tropic rotor.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 한국지진공학회 2006년도 학술발표회 논문집
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• pp.637-644
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• 2006

Acceleration time history used in the seismic analysis of nuclear porter plant structure should envelop a target power spectral density (PSD) function in addition to design response spectrum. Current regulation guide defines the target PSD function only for the U.S. URC RG 1.60 Design Response Spectrum. This paper proposes a technical scheme to obtain the target PSD function compatible with generally defined design response spectrum. The scheme includes the methodology for design-spectrum compatible motion history in order to minimize the variation of the derived target PSD function. The PSD calculation procedure follows simple and practical methods allowed within regulation. Effectiveness of the proposed scheme is identified through an example problem. The design response spectrum In the example is based on U.S. NRC RG 1.60 but amplifies the spectral acceleration amplitudes above 9Hz. The target PSD function with little variation can be constructed with the reduced time history ensemble.

• Journal of the Earthquake Engineering Society of Korea
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• 제24권2호
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• pp.59-65
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• 2020

The empirical Green's function method is applied to the foreshock and the mainshock of the 2016 Gyeongju earthquake to simulate strong ground motions of the mainshock and scenario earthquake at seismic stations of seven metropolises in South Korea, respectively. To identify the applicability of the method in advance, the mainshock is simulated, assuming the foreshock as the empirical Green's function. As a result of the simulation, the overall shape, the amplitude of PGA, and the duration and response spectra of the simulated seismic waveforms are similar with those of the observed seismic waveforms. Based on this result, a scenario earthquake on the causative fault of Gyeongju earthquake with a moment magnitude 6.5 is simulated, assuming that the mainshock serves as the empirical Green's function. As a result, the amplitude of PGA and the duration of simulated seismic waveforms are significantly increased and extended, and the spectral amplitude of the low frequency band is relatively increased compared with that of the high frequency band. If the empirical Green's function method is applied to several recent well-recorded moderate earthquakes, the simulated seismic waveforms can be used as not only input data for developing ground motion prediction equations, but also input data for creating the design response spectra of major facilities in South Korea.

• International Journal of Control, Automation, and Systems
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• 제4권3호
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• pp.325-332
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• 2006

The dynamic responses of human standing postural control were investigated when subjects were exposed to long-term horizontal vibration. It was hypothesized that the motion of standing posture complexity mainly occurs in the mid-sagittal plane. The motor-driven support platform was designed as a source of vibration. The AC Servo-controlled motors produced anterior/posterior (AP) motion. The platform acceleration and the trunk angular velocity were used as the input and the output of the system, respectively. A method was proposed to identify the complexity of the standing posture dynamics. That is, during AP platform motion, the subject's knee, hip and neck were tightly constrained by fixing assembly, so the lower extremity, trunk and head of the subject's body were individually immovable. Through this method, it was assumed that the ankle joint rotation mainly contributed to maintaining their body balance. Four subjects took part in this study. During the experiment, the random vibration was generated at a magnitude of $0.44m/s^2$, and the duration of each trial was 40 seconds. Measured data were estimated by the coherence function and the frequency response function for analyzing the dynamic behavior of standing control over a frequency range from 0.2 to 3 Hz. Significant coherence values were found above 0.5 Hz. The estimation of frequency response function revealed the dominant resonance frequencies between 0.60 Hz and 0.68 Hz. On the basis of our results illustrated here, the linear model of standing postural control was further concluded.

• 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.

• Earthquakes and Structures
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• 제14권1호
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• pp.33-44
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• 2018

Base-isolated nuclear power plant (BI-NPP) structures are founded on expanded basemat as a flexible floating nuclear island, are still lacking the recommendation of the consideration of incoherent motion effect. The effect of incoherent earthquake motion on the seismic response of BI-NPP structure has been investigated herein. The incoherency of the ground motions is applied by using an isotropic frequency-dependent spatial correlation function to perform the conditional simulation of the reference design spectrum compatible ground motion in time domain. Time history analysis of two structural models with 486 and 5 equivalent lead plug rubber bearing (LRB) base-isolators have been done under uniform excitation and multiple point excitation. two different cases have been considered: 1) Incoherent motion generated for soft soil and 2) Incoherent motion generated for hard rock soil. The results show that the incoherent motions reduce acceleration and the lateral displacement responses and the reduction is noticeable at soft soil site and higher frequencies.

• Journal of the Earthquake Engineering Society of Korea
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• 제15권4호
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• pp.33-43
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• 2011

This paper addresses a fundamental study that is necessary to complement and improve the current domestic design specifications for the strong motion duration criterion and the envelope function of artificial accelerograms that can be applied to the earthquake-proof design of nuclear structures. The criteria for the design response spectra and strong motion duration suggested by USNRC and ASCE Standard 4-98 are commonly being used in the profession, and they are first compared with each other and reviewed. By applying 209 real strong earthquake records that are greater than 5 in magnitude at rock sites to the strong motion duration criterion in ASCE 4-98, an empirical regression model that predicts the strong motion duration as a function of the earthquake magnitude was then developed. Using synthetically generated earthquake time histories for the 10 cases whose strong motion durations varied from 6 to 20 seconds, extensive seismic analyses were finally conducted to identify the effects of the strong motion durations on the seismic responses of the nuclear power plant containment structures.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 한국지진공학회 2006년도 학술발표회 논문집
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• pp.133-140
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• 2006

This paper addresses a fundamental research subject to complement and improve current domestic design specifications for the strong motion duration criterion and the envelop function of artificial accelerograms that can be applied to the earthquake-proof design of nuclear structures. The criteria for design response spectra and strong motion duration suggested by WRC RG 1.60 and ASCE Standard 4-98 are commonly being used in the profession, and they are first compared with each other and reviewed. By applying 152 real strong earthquake records that are over magnitude of 5 in the rock sites to the strong motion duration criterion in ASCE 4-98, an empirical regression model that predicts the strong motion duration as a function of earthquake magnitude is then developed. Using synthetically generated earthquake time histories for the five cases whose strong motion durations vary from 6 to 15 seconds, a seismic analysis is conducted to identify effects of the strong motion durations on the seismic responses of nuclear structures.