• Title/Summary/Keyword: dynamic tests

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Dynamic Deformation Characteristics of Fiber Reinforced Soils (섬유혼합토의 동적물성변형특성)

  • Jung, Sung-Yong;Kim, Dae-Il;Park, Chul-Soo;Mok, Young-Jin
    • Proceedings of the Korean Geotechical Society Conference
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    • 2004.03b
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    • pp.968-976
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    • 2004
  • In the paper, deformation characteristics of fiber-mixed-soils, mixed polypropylene staple fibers of 0.3% fiber content with sands of various gradation, and their effectiveness of reinforcement were evaluated. A series of Resonant Column tests were performed with specimens prepared with varying Uniformity Coefficient and constant Curvature Coefficient. Maximum shear moduli 01 fiber-mixed-soils were increased by up to 30% and modulus reduction was also restrained in nonlinear range. Normalized shear modulus reduction curves of fiber-mixed-soils shift close to the upper limit of Seed curd Idriss's curves and are located within narrower band than those of unmixed soils, which proves the effectiveness on stiffness increment by reinforcing soils with fibers.

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Random Walk Test on Hedge Ratios for Stock and Futures (헤지비율의 시계열 안정성 연구)

  • Seol, Byungmoon
    • Asia-Pacific Journal of Business Venturing and Entrepreneurship
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    • v.9 no.2
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    • pp.15-21
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    • 2014
  • The long memory properties of the hedge ratio for stock and futures have not been systematically investigated by the extant literature. To investigate hedge ratio' long memory, this paper employs a data set including KOSPI200 and S&P500. Coakley, Dollery, and Kellard(2008) employ a data set including a stock index and commodities foreign exchange, and suggested the S&P500 to be a fractionally integrated process. This paper firstly estimates hedge ratios with two dynamic models, BEKK(Bollerslev, Engle, Kroner, and Kraft) and diagonal-BEKK, and tests the long memory of hedge ratios with Geweke and Porter-Hudak(1983)(henceforth GPH) and Lo's modified rescaled adjusted range test by Lo(1991). In empirical results, two hedge ratios based on KOSPI200 and S&P500 show considerably significant long memory behaviours. Thus, such results show the hedge ratios to be stationary and strongly reject the random walk hypothesis on hedge ratios, which violates the efficient market hypothesis.

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Design of Carbon Composite Prosthetic Feet using Finite Element Methods (유한요소 해석기법을 이용한 탄소복합소재 인공발의 설계)

  • Cho, Hyeon Seok;Cha, Gook Chan;Park, Jin Kook;Kim, Shin Ki;Lee, Suk Min;Mun, Mu Sung;Kim, Chang Bu
    • Journal of the Korean Society for Precision Engineering
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    • v.30 no.7
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    • pp.769-776
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    • 2013
  • The dynamic compliance characteristics of a prosthetic foot midgait are very important for natural performance in an amputee's gait and should be in a range that provides natural, stable walking. In this study, finite element analysis (FEA) and classical laminate theory were used to examine the mechanical characteristics of a carbon-epoxy composite laminate prosthetic foot as a function of variation in the lamination composition. From this analysis, an FEM model of a prosthetic keel, made from the composite material, was developed. The lamination composition of the keel was designed for improved stiffness. The prototype product was fabricated using an autoclave. Vertical loading response tests were performed to verify the simulation model. The results of the experiments were similar to those from simulations below the loading level of the gait, suggesting use of the proposed simulation model for prosthetic keel design.

Finite Element Model Updating and System Identification of Reinforced Concrete Specimen (철근콘크리트 실험체의 시스템 식별과 유한요소모델수정)

  • Kim, Hack-Jin;Yu, Eun-Jong;Kim, Ho-Geun;Lee, Sang-Hyun;Cho, Seung-Ho;Chung, Lan
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2008.04a
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    • pp.647-652
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    • 2008
  • This paper focused on the application of finite element model updating technique to evaluate the structural properties of the reinforced concrete specimen using the data collected from shaking table tests. The specimen was subjected to six El Centro(NS, 1942) ground motion histories with different Peak Ground Acceleration(PGA) ranging from 0.06g to 0.50g. For model updating, flexural stiffness values of structural members(walls and slabs) were chosen as the updating parameters so that the converged results have direct physical interpretations. Initial values for finite element model were determined from the member dimensions and material properties. Frequency response functions(i.e. transfer functions), natural frequencies and mode shapes were obtained using the acceleration measurement at each floor and given ground acceleration history. The weighting factors were used to account for the relative confidence in different types of inputs for updating(i.e. transfer function and natural frequencies). The constraints based on upper/lower bound of parameters and sensitivity-based constraints were implemented to the updating procedure in this study using standard bounded variable least-squares(BVLS) method. The veracity of the updated finite element model was investigated by comparing the predicted and measured responses. The results indicated that the updated model replicates the dynamic behavior of the specimens reasonably well. At each stage of shaking, severity of damage that results from cracking of the reinforced concrete member was quantified from the updated parameters(i.e. flexural stiffness values).

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Analysis of Pressure Fluctuations in a Thrust Chamber with Chamber Pressure Variation (연소실 압력 변화에 따른 연소기 압력 섭동 분석)

  • Ahn, Kyu-Bok;Lim, Byoung-Jik;Kim, Jong-Gyu;Han, Yeoung-Min;Choi, Hwan-Seok
    • Journal of the Korean Society of Propulsion Engineers
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    • v.14 no.5
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    • pp.8-14
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    • 2010
  • For the development of a liquid rocket engine, hot-firing tests of a regeneratively cooled thrust chamber were performed at chamber pressures of approximately 30 and 60 bars. In the paper, pressure fluctuation data, which were obtained from the dynamic pressure transducers installed in propellant manifolds and combustion chamber, were analyzed. Compared to the data at chamber pressure of 60 bar, the results at chamber pressure of 30 bar showed low-frequency oscillations around 150 Hz in the combustion chamber. The low-frequency waves in the combustion chamber were coupled with those in the manifolds. However, the RMS values of the chamber pressure fluctuations at chamber pressure of 30 bar were only 0.8% of the chamber pressures. Thus, it can be inferred that the thrust chamber operates in the stability boundary even at low chamber pressure.

Damage assessment of shear connectors with vibration measurements and power spectral density transmissibility

  • Li, Jun;Hao, Hong;Xia, Yong;Zhu, Hong-Ping
    • 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.

Demands and distribution of hysteretic energy in moment resistant self-centering steel frames

  • Lopez-Barraza, Arturo;Ruiz, Sonia E.;Reyes-Salazar, Alfredo;Bojorquez, Eden
    • Steel and Composite Structures
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    • v.20 no.5
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    • pp.1155-1171
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    • 2016
  • Post-tensioned (PT) steel moment resisting frames (MRFs) with semi-rigid connections (SRC) can be used to control the hysteretic energy demands and to reduce the maximum inter-story drift (${\gamma}$). In this study the seismic behavior of steel MRFs with PT connections is estimated by incremental nonlinear dynamic analysis in terms of dissipated hysteretic energy ($E_H$) demands. For this aim, five PT steel MRFs are subjected to 30 long duration earthquake ground motions recorded on soft soil sites. To assess the energy dissipated in the frames with PT connections, a new expression is proposed for the hysteretic behavior of semi-rigid connections validated by experimental tests. The performance was estimated not only for the global $E_H$ demands in the steel frames; but also for, the distribution and demands of hysteretic energy in beams, columns and connections considering several levels of deformation. The results show that $E_H$ varies with ${\gamma}$, and that most of $E_H$ is dissipated by the connections. It is observed in all the cases a log-normal distribution of $E_H$ through the building height. The largest demand of $E_H$ occurs between 0.25 and 0.5 of the height. Finally, an equation is proposed to calculate the distribution of $E_H$ in terms of the normalized height of the stories (h/H) and the inter-story drift.

Study on the local damage of SFRC with different fraction under contact blast loading

  • Zhang, Yongliang;Zhao, Kai;Li, Yongchi;Gu, Jincai;Ye, Zhongbao;Ma, Jian
    • Computers and Concrete
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    • v.22 no.1
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    • pp.63-70
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    • 2018
  • The steel fiber reinforced concrete (SFRC) shows better performance under dynamic loading than conventional concrete in virtue of its good ductility. In this paper, a series of quasi-static experiments were carried out on the SFRC with volume fractions from 0 to 6%. The compressive strength increases by 38% while the tension strength increases by 106% when the fraction is 6.0%. The contact explosion tests were also performed on the ${\Phi}40{\times}6cm$ circular SFRC slabs of different volume fractions with 20 g RDX charges placed on their surfaces. The volume of spalling pit decreases rapidly with the increase of steel fiber fraction with a decline of 80% when the fraction is 6%, which is same as the crack density. Based on the experimental results, the fitting formulae are given, which can be used to predict individually the change tendencies of the blast crater volume, the spalling pit volume and the crack density in slabs with the increase of the steel fiber fraction. The new formulae of the thickness of damage region are established, whose predictions agree well with our test results and others. This is of great practical significance for experimental investigations and engineering applications.

Model Updating of a RC Frame Building using Response Surface Method and Multiobjective Optimization (반응표면법 및 다목적 최적화를 이용한 철근콘크리트 건물모델의 모델 개선)

  • Lee, Sang-Hyun;Yu, Eunjong
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.30 no.1
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    • pp.39-46
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    • 2017
  • In this paper, a model updating procedure based on the response surface method combined with the multi-objective optimization was proposed and applied for updating of the FE models representing a low-rise reinforced concrete building before and after the seismic retrofit. The dynamic properties to be matched were obtained from vibration tests using a small shaker system. By varying the structural parameters according to the central composite design, analysis results from the initial FE model using a commercial software were collected and used to produce two regression functions each of which representing the errors in the natural frequencies and mode shapes. The two functions were used as the objective functions for multi-objective optimization. Final solution was determined by examining the Pareto solutions with one iteration. The parameters representing the stiffnesses of existing concrete, masonry, connection stiffness in expansion joint, new concrete, retrofitted members with steel section jacketing were selected and identified.

Application of Equivalent Walking Loads for Vibration Analysis of Building Floor Subjected to Footstep Loadings (보행하중을 받는 건축물 바닥판의 진동해석을 위한 등가 보행하중의 적용)

  • 김기철;이동근
    • Journal of the Earthquake Engineering Society of Korea
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    • v.5 no.5
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    • pp.35-45
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    • 2001
  • Recently, the floor systems those require large open space may have low inherent damping due to the decline of the use of curtain walls. Furthermore, the use of the high strength materials has resulted in more flexible and longer spanning in floor systems. The long span structures such as shopping malls, offices and large assembly rooms may lead to significant dynamic response due to human activities. Excessive vibrations make the occupants uncomfortable and deteriorate the serviceability of buildings. It is now proved that footfall loading is the major source of floor vibrations. The common method of application of walking loads for the vibration analysis of structures subjected to walking loads is to inflict measured walking loads and periodic function at a node. But this method could not account for the moving effect of walking. In this study, natural frequency and damping ratio of example structure are evaluated by heel drop tests. And the application of equivalent walking loads is used for on efficient vibration analysis of the plate structures subjected to walking loads.

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