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Evaluation of bonding state of tunnel shotcrete using impact-echo method - numerical analysis (충격 반향 기법을 이용한 숏크리트 배면 접착 상태 평가에 관한 수치해석적 연구)

  • Song, Ki-Il;Cho, Gye-Chun;Chang, Seok-Bue
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.10 no.2
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    • pp.105-118
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    • 2008
  • Shotcrete is one of the main support materials in tunnelling. Its bonding state on excavated rock surfaces controls the safety of the tunnel: De-bonding of shotcrete from an excavated surface decreases the safety of the tunnel. Meanwhile, the bonding state of shotcrete is affected by blasting during excavation at tunnel face as well as bench cut. Generally, the bonding state of shotcrete can be classified as void, de-bonded, or fully bonded. In this study, the state of the back-surface of shotcrete is investigated using impact-echo (IE) techniques. Numerical simulation of IE technique is performed with ABAQUS. Signals obtained from the IE simulations were analyzed at time, frequency, and time-frequency domains, respectively. Using an integrated active signal processing technique coupled with a Short-Time Fourier Transform (STFT) analysis, the bonding state of the shotcrete can be evaluated accurately. As the bonding state worsens, the amplitude of the first peak past the maximum amplitude in the time domain waveform and the maximum energy of the autospectral density are increasing. The resonance frequency becomes detectable and calculable and the contour in time-frequency domain has a long tail parallel to the time axis. Signal characteristics with respect to ground condition were obtained in case of fully bonded condition. As the ground condition worsens, the length of a long tail parallel to the time axis is lengthened and the contour is located in low frequency range under 10 kHz.

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Investigation of Stiffness Characteristics of Subgrade Soils under Tracks Based on Stress and Strain Levels (응력 및 변형률 수준을 고려한 궤도 흙노반의 변형계수 특성 분석)

  • Lim, Yujin;Kim, DaeSung;Cho, Hojin;Sagong, Myoung
    • Journal of the Korean Society for Railway
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    • v.16 no.5
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    • pp.386-393
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    • 2013
  • In this study, the so-called repeated plate load bearing test (RPBT) used to get $E_{v2}$ values in order to check the degree of compaction of subgrade, and to get design parameters for determining the thickness of the trackbed foundation, is investigated. The test procedure of the RPBT method is scrutinized in detail. $E_{v2}$ values obtained from the field were verified in order to check the reliability of the test data. The $E_{v2}$ values obtained from high-speed rail construction sites were compared to converted modulus values obtained from resonant column (RC) test results. For these tests, medium-size samples composed of the same soils from the field were used after analyzing stress and strain levels existing in the soil below the repeated loading plates. Finite element analyses, using the PLAXIS and ABAQUS programs, were performed in order to investigate the impact of the strain influence coefficient. This was done by getting newly computed $I_z$ to get the precise strain level predicted on the subgrade surface in the full track structure; under wheel loading. It was verified that it is necessary to use precise loading steps to construct nonlinear load-settlement curves from RPBT in order to get correct $E_{v2}$ values at the proper strain levels.

Seismic Characteristic Evaluation on Strip-type Damping Devices with Optimized Shape (최적 형상 스트립형 감쇠장치의 내진 특성 평가)

  • Hwang, Jung-Hyun;Ock, Jong-Ho
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.23 no.6
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    • pp.26-37
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    • 2019
  • This paper aims to investigate the seismic characteristics of strip-type damping devices possessing optimized shapes for the moment-resisting mechanism throughout analytical and experimental studies. Predicting equations for initial stiffness and yielding strength were introduced and compared with analytical results obtained from finite element analyses (FEAs) using commercial FEA program ABAQUS. In order for establishing predicting equations, two idealized processes were considered and both predicting equations showed that they could provide enough approximations for seismic applications in building structures. Throughout experimental studies, it was noted that structural uncertainties on mild steels, connection details and structural types linking damping devices with building structures could interrupt predicting structural behavior of the devices. Also, it was observed that shear stress concentrations should be considered if shear yielding type devices are applied into building structures. Nevertheless, it was shown that structural conservatism can be established using the predicting equations and seismic applications of the damping devices can enhance the seismic performance of building structures efficiently in the viewpoint that they have high resistance to low-cycle fatigue failures.

The Behavior on Stress and Settlement of GCP Composite Ground with Different Shear Strength of Soil (GCP로 개량된 복합지반의 지반강도 별 응력 및 침하거동)

  • Na, Seung-Ju;Kim, Daehyeon;Kim, Gyeong-Eop
    • Journal of the Korean Geosynthetics Society
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    • v.16 no.3
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    • pp.63-74
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    • 2017
  • Although the Granular Compaction Pile (GCP) has been used for many decades, several failures still occur such as bulging, shear failure and other phenomena, indicating that more refined study is needed. The main objective of the study is to evaluate the stress concentration ratio for both area replacement ratio and shear strength of soil through literature review and numerical analysis. Numerical analysis using the finite element program ABAQUS has been performed for the composite ground with GCP. The behavior stress and settlement of composite ground have been analyzed for both the area replacement ratio (10~40%) and shear strength of soil (25~75 kPa). As a result of numerical analysis, as the soil strength and area replacement ratio increased, the average stree related coefficient and stress concentration ratio for depth tended to decrease, and stress related coefficient of upper layer tend to decrease equally, but the stress concentration ratio decreased. Therefore, tendency that the value in th upper layer differs from the value in other depths was displayed. Care should be taken because it is possible to make mistakes in designing the entire composite ground with the values measured in the upper layer. Also, the settlement reduction factor was compared with the existing equation and numerical analysis. And the value obatined from the existing equation and numerical analysis are similar.

Parametric Study on design Variables of Rectangular Concrete Filled Tubular Columns with High-Strength Steel (유한요소해석에 의한 고강도 강재를 사용한 각형 콘크리트 충전 강관 기둥의 설계인자 분석)

  • Choi, Hyun-Ki;Bae, Baek-Il;Choi, Yun-Cheol;Choi, Chang-Sik
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.19 no.2
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    • pp.10-21
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    • 2015
  • For the safe design of steel-concrete composite structure, usable yield strength of steels are limited in most of design standard. However, this limitation sometimes cause the uneconomical design for some kind of members such as slender columns which was affected by elastic buckling load. For the economical design for slender columns, parametric study of RCFT (Rectangular CFT) with high-strength steel is conducted, especially investigating the limitation of yield strength of high-strength steels. Using ABAQUS, finite element analysis program, the finite element model was constructed and calibrated with experimental study for RCFT with high strength steel which have yield strength up to 680MPa. Investigated design parameters are yield strength of steel, compressive strength of concrete, steel thickness and slenderness ratio. The effect of design parameters were compared with design standard, KBC-09. From the parametric study with 54 models and previous test specimens, RCFT can be safely design with higher yield strength of steels than currently limited by KBC for large range of slenderness ratio.

Analysis of Pull-out Behavior of Tunnel-type Anchorage for Suspended Bridge Using 2-D Model Tests and Numerical Analysis (2차원 모형실험 및 수치해석을 통한 현수교 터널식 앵커리지의 인발거동 특성 분석)

  • Seo, Seunghwan;Park, Jaehyun;Lee, Sungjune;Chung, Moonkyung
    • Journal of the Korean Geotechnical Society
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    • v.34 no.10
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    • pp.61-74
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    • 2018
  • In this study, the pull-out behavior of tunnel type anchorage of suspension bridges was analyzed based on results from laboratory size model tests and numerical analysis. Tunnel type anchorage has found its applications occasionally in both domestic and oversea projects, therefore design method including failure mode and safety factor is yet to be clearly established. In an attempt to improve the design method, scaled model tests were conducted by employing simplified shapes and structure of the Ulsan grand bridge's anchorage which was the first case history of its like in Korea. In the model tests, the anchorage body and the surrounding rocks were made by using gypsum mixture. The pull-out behavior was investigated under plane strain conditions. The results of the model tests showed that the tunnel type anchorage underwent wedge shape failure. For the verification of the model tests, numerical analysis was carried out using ABAQUS, a finite element analysis program. The failure behavior predicted by numerical analysis was consistent with that by the model tests. The result of numerical analysis also showed that the effect of Poisson's ratio was negligible, and that a plugging type failure mode could occur only when the strength of the surrounding rocks was 10 times larger than that of anchorage body.

Analysis of Behavior on GCP Composite Ground Considering Loading and Foundation Conditions (하중 및 기초조건에 따른 GCP 복합지반의 거동분석)

  • Kim, Gyeong-Eop;Park, Kyung-Ho;Kim, Dae-Hyeon
    • Journal of the Korean Geosynthetics Society
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    • v.17 no.1
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    • pp.127-137
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    • 2018
  • Gravel Compaction Pile (hereinafter referred to as GCP) is a ground improvement technique by packing crushed stones on fragile clay ground, pressing it, and forming stakes on the foundation. Although many researchers have analyzed stress behavior of GCP composite ground on domestic GCP technique using laboratory experiment and field experiment, analyses of stress behavior according to the difference of stiffness of mat foundation loaded on the upper foundation of GCP composite ground have not been done actively. Therefore, this study aimed to identify the stress concentration ratio in accordance with the difference of basis stiffness by interpreting figures. To perform this, replacement ratio was changed and modelled using ABAQUS, software for finite element analysis and analyzed the stress concentration ratio, amounts of settlement, and maximum amounts of horizontal displacement of composite ground in accordance with the difference of stiffness. An analysis showed that the stress concentration ratio of rigid foundation was highly assessed than unloading of flexible foundation in case of unloading, while amounts of settlement under flexible unloading condition were slightly higher than under rigid condition. This indicates that the characteristic of stress behavior on the different stiffness of upper foundation needs to be clarified. In addition, the maximum horizontal displacement was generated in a constant level regardless of the difference of stiffness.

Finite Element Analysis for Investigating the Behavior of Gravel Compaction Pile Composite Ground (GCP 복합지반의 거동분석을 위한 유한요소해석)

  • Kim, Gyeong-eop;Park, Kyung-Ho;Kim, Ho-Yeon;Kim, Daehyeon
    • Journal of the Korean Geosynthetics Society
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    • v.17 no.3
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    • pp.19-32
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    • 2018
  • Gravel Compaction Pile (GCP) method is currently being designed and constructed by empirical method because quantitative design method has not been developed, leading to various types of and frequent destruction such as expansion failure and shear failure and difficulties in establishing clear cause and developing measure to prevent destruction. In addition, despite the difference with domestic construction equipment and material characteristics, the methods applied to the overseas ground is applied to the domestic as it is, leading to remarkable difference between applied values and measured values in variables such as bearing capacity and the settlement amount. The purpose of this study was, therefore, to propose a reasonable and safe design method of GCP method by analyzing the settlement and stress behavior characteristics according to ground strength change under GCP method applied to domestic clay ground. For the purpose, settlement amount of composite ground, stress concentration ratio, and maximum horizontal displacement and expected location of GCP were analyzed using ABAQUS. The results of analysis showed that the settlement and Settlement reduction rate of composite ground decreased by more than 60% under replacement ratio of 30% or more, that the maximum horizontal displacement of GCP occurred at the depth 2.6 times pile diameter, and that the difference in horizontal displacement is slight under replacement ratio of 30%.

A Numerical Analysis on Application of Laser Peening to Dissimilar Metal Welds in a Safety Injection Nozzle of Integral Reactor (일체형 원자로 안전주입 노즐 이종금속 용접부에 대한 레이저 피닝 적용의 수치 해석적 연구)

  • Seo, Joong-Hyun;Kim, Jong-Sung;Jhung, Myung-Jo;Ryu, Yong-Ho
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.36 no.6
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    • pp.599-608
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    • 2012
  • A numerical analysis has been performed through implicit dynamic finite element analysis using the commercial package, ABAQUS in order to investigate effect of laser peening on welding residual stress mitigation of dissimilar metal welds in a safety injection nozzle of integral reactor. The implicit dynamic finite element analysis are compared with the previous experimental results. By comparison, it is identified that the implicit dynamic finite element analysis is valid for residual stress mitigation via laser peening. Implicit static finite element residual stress analysis has been performed for the dissimilar metal welds subject to inner repair welding. The analysis results represent that both axial and hoop residual stresses are tensile on inner surface of safety injection nozzle due to inner repair welding. Also Parametric study has performed to investigate effect of laser peening variables such as maximum impact pressure, duration time of pressure, spot diameter and peening direction on the welding residual stress mitigation. As a result, it is found that laser peening has the preventive maintenance effect to mitigate mainly residual stresses of region near inner surface.

Centralized Controller High-altitude Work Car Elevations Lift Structure Safety Assessment (중앙집중식 컨트롤러 고소작업차의 고소리프트의 구조안정성 평가)

  • Kim, Jun-tae;Lee, Gi-yeong;Lee, Sang-sik;Park, Won-yeop
    • The Journal of Korea Institute of Information, Electronics, and Communication Technology
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    • v.10 no.4
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    • pp.350-357
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    • 2017
  • This study was conducted as a post - study on the development of a centralized controller and a hydraulic lift system including structural analysis and remote control for the development of a vertically elevated car. The safety review was carried out through the structural modification of the elevator lift which was developed during the previous research. 3D modeling was performed with Solidworks, and a model of finite element was created through Hypermesh S / W. In addition, the loading environment of the work vehicle for the evaluation is a condition in which the loading amount is 250 kg per position (total, upper, upper, lower, and lower) on the work table, ), The structural analysis was carried out under the condition that the load was 600 kg, and safety was examined in various aspects. As a result, when the allowable load of 250 kg and the excess load of 600 kg are excluded (except Case-11), the stress level is below the yield strength. In the case of Case-11, there is a region exceeding the yield strength at the center support portion of the safety bar at the upper end even after excluding the component which generates the maximum stress, but it does not affect the safety aspect of the whole structure Respectively. Looking at the deflection results, it can be seen that in all cases the maximum deflection occurs in the same table, and the tendency of sagging in both 250 kg and 600 kg is the same.