• Journal of the Computational Structural Engineering Institute of Korea
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• v.28 no.4
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• pp.401-408
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• 2015

Conventional model updating methods for the structures have used global structural responses which are modal parameters obtained through vibration measurements. Although models updated by modal parameters estimate global structural responses accurately, they have difficulties to predict local responses for safety assesment of structural members. The safety of structural members in the structures has been evaluated through the stress estimation based on strain measurements. Thus, this study additionally uses measured strain responses of structural members to perform model updating besides modal parameters. In the proposed method, the objective functions are set to the differences of the global and local responses obtained from updated model and measurement and those functions are minimized by NSGA-II, one of the multi-objective optimization techniques. The strain responses predicted from updated model are used for safety assessment of the steel frame structures. The proposed method are verified by numerical and experimental studies through the impact hammer tests for a steel frame specimen.

• Plant Journal
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• v.15 no.4
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• pp.36-42
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• 2019

In recent, according to the tightening of environment regulation policy, the height of the site of the power plant is increased and the length of the cooling water pipe is increased. This has a serious impact on the stability of the plant. This study analyzes the transient phenomenon using LIQT 7.2, an unsteady state one-dimensional analysis software, to secure the stability of 1,000 MW high-capacity coal-fired power plant cooling water system with high head. To prevent water hammer, The effects on performance characteristics were predicted by individual and combination application. The surge pressure of the cooling water which occurs when the pump was stopped without installing the anti-surge devices was the largest at the pump outlet side. The most effective and simple way to reduce surge pressure in these cooling water systems is to combine a vacuum breaker with a hydraulic non-return valve, which is an essential device for pump protection.

• Smart Structures and Systems
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• v.18 no.6
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• pp.1189-1202
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• 2016

This paper describes an application of wavelet analysis for damage detection of a steel frame structure with bolted connections. The wavelet coefficients of the acceleration response for the healthy and loosened connection structure were calculated at each measurement point. The difference of the wavelet coefficients of the response of the healthy and loosened connection structure is selected as an indicator of the damage. At each node of structure the norm of the difference of the wavelet coefficients matrix is then calculated. The point for which the norm has the higher value is a candidate for location of the damage. The above procedure was experimentally verified on a laboratory-scale 2-meter-long steel frame. The structure consists of 11 steel beams forming a four-bay frame, which is subjected to impact loads using a modal hammer. The accelerations are measured at 20 different locations on the frame, including joints and beam elements. Two states of the structure are considered: healthy and damaged one. The damage is introduced by means of loosening two out of three bolts at one of the frame connections. Calculating the norm of the difference of the wavelet coefficients matrix at each node the higher value was found to be at the same location where the bolts were loosened. The presented experiment showed the effectiveness of the wavelet approach to damage detection of frame structures assembled using bolted connections.

• Journal of Korean Society of Steel Construction
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• v.24 no.4
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• pp.399-410
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• 2012

In this study, vibration tests are performed on cantilevered and clamped-clamped laminated composite rectangular plates using experimental modal analysis technique. The damages are simulated by applying progressive line cracks to the laminated composite plates for damage evaluations due to crack growth. The changes of frequency response functions(FRFs), MAC values, and modal parameters (frequency, mode shape and damping ratio) of the damaged composite plates, which are obtained by the modal testing of impact hammer, are investigated. Each experimental modal parameter of the progressively damaged composite plates is compared with natural frequencies and mode shapes obtained by finite element analysis. It is seen that the damage can be evaluated from the changes in the geometric properties and structural behaviors of the laminated composite plates resulting from the model updating process of the finite element model as a benchmark.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.4
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• pp.343-350
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• 2011

Recently, as the catastrophic disasters due to earthquake happen frequently all over the world, it draws lots of attention to seismic capacity evaluation and/or structural integrity assessment of deteriorated civil infra-structures. However, there have been few studies on the existing dam flood gates, expecially in Korea. In this study, a proper vibration testing method applicable to a dam flood gate has been suggested, since the dynamic characteristics of a darn flood gate can be fundamental data for seismic capacity evaluation or structural integrity assessment. The frequency domain decomposition technique has been incorporated for modal parameter identification. Two kinds of vibration tests using an impact hammer and ambient vibration sources were carried out on two types of dam floodgates with different shapes. Through the field tests, the effectiveness of the ambient vibration tests were verified.

• 한국방재학회:학술대회논문집
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• 2011.02a
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• pp.166-166
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• 2011

한국건설기술연구원내에 세계 최초로 UHPC(초고성능 콘크리트)를 활용한 보도사장교를 설계 및 시공하여 공용중이다. 초고성능 콘크리트(UHPC, Ultra High Performance Concrete)는 기존의 콘크리트의 단점을 극복하기 위하여 강섬유 및 혼화재료를 사용하여 고강도화와 더불어 인장강도, 휨강도, 균열에 대한 저항성, 전단강도 및 내충격성을 대폭 개선시켜서 구조부재의 연성 및 강도를 확보하기 위해 개발된 것으로서 한국건설기술연구원이 독자적 연구를 바탕으로 UHPC를 개발하였고, KICT-UHPC의 특성을 충분히 반영한 보도사장교를 한국건설기술원내에 실구조물로 시공하였다. 한국건설기술연구원내 본관 1, 2동 연결 보도사장교(SUPER BRIDGE 1)의 진동실험을 통해 UHPC 보도사장교의 동특성(고유진동수 및 모드형상)을 평가하였다. 대상교량에는 보도사장교의 캔틸레버 구조 특성에 의해 발생하는 진동을 제어하기 위한 질량 264kg의 난간형 수직진동제어장치(TMD) 4대가 설치되어 있으며, 부가질량의 운동을 기계적으로 구속하거나 구속을 해제하여 TMD의 작동을 켜거나 끌 수 있도록 되어있다. TMD의 가동 및 정지시 동특성을 비교하는 정밀한 검증 실험을 통하여 TMD 설치에 의한 동특성의 변화도 평가하였다. 가진망치에 의한 충격실험으로부터 획득한 가속도 시간이력 데이터와 가진망치의 가진력 시간이력 데이터를 사용하여 주파수응답함수(FRF, frequency response function)를 계산하고, 이로부터 Super Bridge I의 동적모드형상과 고유진동수를 추출하였다. 1차 모드의 고유진동수는 TMD 정지시 2.2949 Hz, TMD 작동시 2.0996 Hz로서 8%정도 감소하였다. 충격해머에 의한 가진 실험을 통해 세계 최초로 시공한 UHPC 보도사장교의 진동특성과 관련한 신뢰성 있는 자료를 확보할 수 있었다.

• Computers and Concrete
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• v.12 no.6
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• pp.803-818
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• 2013

In this study, analytical and experimental modal analyses of a scaled bridge model are carried out to extract the dynamic characteristics such as natural frequency, mode shapes and damping ratios. For this purpose, a scaled bridge model is constructed in laboratory conditions. Three dimensional finite element model of the bridge is constituted and dynamic characteristics are determined, analytically. To identify the dynamic characteristics experimentally; Experimental Modal Analyses (ambient and forced vibration tests) are conducted to the bridge model. In the ambient vibration tests, natural excitations are provided and the response of the bridge model is measured. Sensitivity accelerometers are placed to collect signals from the measurements. The signals collected from the tests are processed by Operational Modal Analysis; and the dynamic characteristics of the bridge model are estimated using Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods. In the forced vibration tests, excitation of the bridge model is induced by an impact hammer and the frequency response functions are obtained. From the finite element analyses, a total of 8 natural frequencies are attained between 28.33 and 313.5 Hz. Considering the first eight mode shapes, these modes can be classified into longitudinal, transverse and vertical modes. It is seen that the dynamic characteristics obtained from the ambient and forced vibration tests are close to each other. It can be stated that the both of Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods are very useful to identify the dynamic characteristics of the bridge model. The first eight natural frequencies are obtained from experimental measurements between 25.00-299.5 Hz. In addition, the dynamic characteristics obtained from the finite element analyses have a good correlation with experimental frequencies and mode shapes. The MAC values obtained between 90-100% and 80-100% using experimental results and experimental-analytical results, respectively.

• Geotechnical Engineering
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• v.3 no.1
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• pp.25-38
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• 1987

The prediction of driving stresses in piles is necessary for optimum selection of driving hammers, better design of precast piles, enact assessment of drivabilities and complete description of piling specifications. However, the existing pile-driving formulas based on the theory of Newtonian impact have some defects and shortcomings; the numerical method by the wave equation analysis using electronic computer usually Involves various uncertainties and limitations which can yield erroneous outcomes because it employs soil constants of which the nature is unknown as essential parameters and ignores the effect of residual stresses set up in the pile .after each hammer blow; and the electronic measuring technique needs extra time and expense. The method developed herein is presented for the purpose of giving field engineers a reliable and convenient analytical procedure for the prediction of driving stresses along the full length of pile using the most effetive parameters without resort to electronic computer. This method is based on the fundamental mechanics of stress waves in elastic rods and takes into account the effect of residual stresses induced by reversed friction in piles.

• Structural Engineering and Mechanics
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• v.54 no.2
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• pp.257-289
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• 2015

Shear connectors are generally used to link the slab and girders together in slab-on-girder bridge structures. Damage of shear connectors in such structures will result in shear slippage between the slab and girders, which significantly reduces the load-carrying capacity of the bridge. Because shear connectors are buried inside the structure, routine visual inspection is not able to detect conditions of shear connectors. A few methods have been proposed in the literature to detect the condition of shear connectors based on vibration measurements. This paper proposes a different dynamic condition assessment approach to identify the damage of shear connectors in slab-on-girder bridge structures based on power spectral density transmissibility (PSDT). PSDT formulates the relationship between the auto-spectral densities of two responses in the frequency domain. It can be used to identify shear connector conditions with or without reference data of the undamaged structure (or the baseline). Measured impact force and acceleration responses from hammer tests are analyzed to obtain the frequency response functions at sensor locations by experimental modal analysis. PSDT from the slab response to the girder response is derived with the obtained frequency response functions. PSDT vectors in the undamaged and damaged states can be compared to identify the damage of shear connectors. When the baseline is not available, as in most practical cases, PSDT vectors from the measured response at a reference sensor to those of the slab and girder in the damaged state can be used to detect the damage of shear connectors. Numerical and experimental studies on a concrete slab supported by two steel girders are conducted to investigate the accuracy and efficiency of the proposed approach. Identification results demonstrate that damages of shear connectors are identified accurately and efficiently with and without the baseline. The proposed method is also used to evaluate the conditions of shear connectors in a real composite bridge with in-field testing data.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.6
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• pp.512-519
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• 2004

This paper described the free vibration characteristics of Optimized H Type (OHT) spacer grids (SG) supporting the PWR fuel rod. The vibration test and the finite element (FE) analysis are performed under the free boundary condition and the clamped at two points (or three points) in the bottom which is the same one as the experimental condition for the dummy rod continuously supported by spacer grids. A modal test is conducted by the impulse excitation method using an impulse hammer and an accelerometer, and the TDAS module of the I-DEAS software is used to acquire and analyze the sensor signals. The softwares related to the FE analysis are the I-DEAS for the geometrical shape modeling and meshing, and the ABAQUS for solving. The fundamental frequency of the OHT SG by experiment under a clamped condition at two points is 175.18 Hz, and shows a bending mode. We think there is no resonance between the fuel rod and the SG because the SG's frequency is higher than that of the fuel rod existing in the range from 30 to 120 Hz. The fundamental frequency of the SG under the free boundary condition is 349.2 Hz showing a bending mode, and the results between the test and the analysis have a good agreement with maximum 7 ％ in error It is also found that the FE analysis model of the OHT SGs to analyze an impact, a buckling and vibration et al. has been generated with reliability.