• Geomechanics and Engineering
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• v.25 no.4
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• pp.341-345
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• 2021

1D sand compression response to ko-loading experiences volume contraction from low to high effective stress regimes. Previous study suggested compressibility model with physically correct asymptotic void ratios at low and high stress levels and examined only for both remolded clays and natural clays. This study extends the validity of Enhanced Terzaghi model for different sand types complied from 1D compression data. The model involved with four parameters can adequately fit 1D sand compression data for a wide stress range. The low stress obtained from fitting parameters helps to identify the initial fabric conditions. In addition, strong correlation between compressibility and the void ratio at low stress facilitates determination of self-consistent fitting parameters. The computed tangent constrained modulus can capture monotonic stiffening effect induced by an increase in effective stress. The magnitude of tangent stiffness during large strain test should not be associated with small strain stiffness values. The use of a single continuous function to capture 1D stress-strain sand response to ko-loading can improve numerical efficiency and systematically quantify the yield stress instead of ad hoc methods.

• Journal of Korean Biological Nursing Science
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• v.25 no.2
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• pp.123-130
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• 2023

Purpose: Sitting time, smoking, and perceived stress strongly influence physical health independent of physical activity. However, the associations among perceived stress, sedentary behavior, and smoking are poorly understood. Therefore, we examined the relationships between sitting time, smoking, and perceived stress in Korean adults aged < 65 years. Methods: We analyzed data from the seventh National Health and Nutrition Survey. In this cross-sectional study, data from 6,890 Korean adults aged < 65 years were analyzed. Complex-sample logistic regression was used to examine the relationships between sitting time, smoking, and perceived stress. Results: The group with a high sitting time (≥ 8 h/day) and those who smoked had significantly higher odds of experiencing stress than the low sitting time (< 8 h/ day) and non-smoking groups (odds ratio: 1.88, 95% confidence interval: 1.42-2.50). Conclusion: Perceived stress was positively correlated with current smoking status and increased sitting time. High sitting time (≥ 8 h/day) and current smoking were associated with a higher risk of perceived stress in Korean adults aged < 65 years.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.10
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• pp.1432-1441
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• 2012

In this study, an improved LCCT(Inductor-Capacitor-Capacitor-Trans) Z-source inverter(Improved LCCT ZSI) with characteristics of Quasi Z-source inverter(QZSI) and LCCT Z-source inverter(LCCT ZSI) is proposed. The proposed inverter can also reduce the voltage stress and input current/capacitor voltage ripples compared with conventional LCCT ZSI and Quasi ZSI. A two winding trans in Z-impedance network of the conventional LCCT ZSI is replaced by a three winding trans in the proposed inverter. To verify the validity of the proposed inverter, a DSP controlled hardware was made and PSIM simulation was executed for each method. Comparing the current and voltage ripples of each method under the condition of input DC voltage 70[V] and output AC voltage 76[Vrms], the input current and capacitor voltage ripple factors of the proposed inverter were low as 11[%] and 1.4[%] respectively. And, for generation of the same output AC voltage of each method, voltage stress of the proposed inverter was low as 175[V] under the condition of duty ratio D=0.15. As mentioned above, we could know that the proposed inverter have the characteristics of low voltage stress, low ripple factor and low operation duty ratio compared with the conventional methods. Finally, the efficiency according to load change/duty ratio and the transient state characteristics were discussed.

• Journal of the Korea Institute of Building Construction
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• v.2 no.4
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• pp.145-152
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• 2002

High fluid concrete unlike OPC concrete is made with various material, and the phase of fresh concrete is considerably different. In order to understand fluidity phase and mix properties of high fluid concrete, concrete is required to access as suspension structure which consists of aggregate and paste. The focus of this paper is to analyze the test results and quantify the effect of mix proportions of mortar and fineness modulus of sand on the properties of fresh mortar. The effect of water-binder ratio. sand-binder ration. contents of ggbs (by mass of total cementitious materials). and various contents of water reducing agent on the yield stress and plastic viscosity of the mix is studied. Based on the experimental results, the fellowing conclusions can be drawn: (1) The mixing time needed for high fluid mortar was approximately two times more than that of ordinary portland mortar. (2) The fluidity phase of mortar could be explained by yield stress of mix and the fluidity of mortar. (3) As the content of ggbs increased, yield stress of mortar was decreased and plastic viscosity of it was increased. (4) For the high fluid mortar, it was appeared that sand-binder ratio should be below 1.5.

• Steel and Composite Structures
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• v.22 no.6
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• pp.1417-1443
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• 2016

Cellular and castellated steel beams are used to obtain higher stiffness and bending capacity using the same weight of steel. In addition, the beam openings may be used as a pass for different mechanical fixtures such as ducts and pipes. The aim of this study is to investigate the effect of different parameters on both elastic and inelastic critical buckling stresses of steel web plates with openings. These parameters are plate aspect ratio; opening shape (circular or rectangular); end distance to the first opening; opening spacing; opening size; plate slenderness ratio; steel grade; and initial web imperfection. The web/flange interaction has been simplified by web edge restraints representing simply supported boundary conditions. A numerical parametric study has been performed through linear and nonlinear finite element (FE) models, where the FE results have been verified against both experimental and numerical results in the literature. The web plates are subject to in-plane linearly varying compression with different loading patterns, ranging from uniform compression to pure bending. A buckling stress modification factor (${\beta}$-factor) has been introduced as a ratio of buckling stress of web plate with openings to buckling stress of the corresponding solid web plate. The variation of ${\beta}$-factor against the aforementioned parameters has been reported. Furthermore, the critical plate slenderness ratio separating elastic buckling and yielding has been identified and discussed for two steel grades of DIN-17100, namely: ST-37/2 and ST-52/3. The FE results revealed that the minimum ${\beta}$-factor is 0.9 for web plates under uniform compression and 0.7 for those under both compression and tension.

• Computers and Concrete
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• v.27 no.4
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• pp.355-367
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• 2021

This paper investigates the mechanical response of simply supported one-way reinforced concrete slabs under fire through numerical analysis. The numerical model is constructed using the software ABAQUS, and verified by experimental results. Generally, mechanical response of the slab can be divided into four stages, accompanied with drastic stress redistribution. In the first stage, the bottom of the slab is under tension and the top is under compression. In the second stage, stress at bottom of the slab becomes compression due to thermal expansion, with the tension zone at the mid-span section moving up along the thickness of the slab. In the third stage, compression stress at bottom of the slab starts to decrease with the deflection of the slab increasing significantly. In the fourth stage, the bottom of the slab is under tension again, eventually leading to cracking of the slab. Parametric studies were further performed to investigate the effects of load ratio, thickness of protective layer, width-span ratio and slab thickness on the performance of the slab. Results show that increasing the thickness of the slab or reducing the load ratio can significantly postpone the time that deflection of the slab reaches span/20 under fire. It is also worth noting that slabs with the span ratio of 1:1 reached a deflection of span/20 22 min less than those of 1:3. The thickness of protective layer has little effect on performance of the slab until it reaches a deflection of span/20, but its effect becomes obvious in the late stages of fire.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.2C
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• pp.77-84
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• 2010

In planning underground roadway foundation on soft ground, deep cement mixing method (DCM) is employed. The proper mixing ratio using batch test and replacement rates that meet strength criteria are used for deep cement mixing column. Stiffness ratio and distance between deep cement mixing columns (C.T.C) are varied to find out influences on stress, displacement, and differential settlement. The replacement ratios that meet settlement criteria are 10~35%. As stiffness varies, stress reaches at 769.kPa that exceed criteria due to stress concentration when stiffness ratio difference is over 30. Also, when C.T.C is 5 m, stress spreads to soils, so C.T.C need to be considered carefully. The vertical displacement is 0.6~1.56 cm, and angular distortion is 1/909~1/510.

• Journal of the Korea institute for structural maintenance and inspection
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• v.10 no.1
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• pp.205-215
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• 2006

In this study 3-D shell FEM model was applied to analyze the behavior of curved steel box girders stiffened by diaphragms. The reliability of the analytical method has been proved by comparing with the existing results. It was also found from this analysis that main factors affecting a distortional stress are length of a girder, curvature of the girder, and spacing of diaphragms. A modelled bridge with 30m of span length and 40m of radius was analyzed to find an optimum spacing of diaphragm, and as a result of applying different spacings, 5m was found to be most appropriate to control the stress ratio regulated by specifications. In the effect of diaphragm shape, the rhamen-typed diaphragm is found to be more effective than the fully filled-up one in the range of opening ratio of 0.4 to 0.6. But, the fully filled-up diaphragm had more efficiency in terms of reducing the distortional stress than X-truss typed diaphragm.

• Journal of Mechanical Science and Technology
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• v.15 no.1
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• pp.117-124
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• 2001

This paper presents turbulent characteristics of an impinging F-O-O-F type injector in which fuel ad oxidizer impinge on each other to atomize under the different momentum ratio. Water was used as an inert simulant liquid instead of fuel and oxidizer. The droplet size and velocity in the impinging spray flow field were measured using a PDPA. The gradient of the spray half-width(b$_2$) along the long-axis direction declined throughout the entire spray flow field with increasing the momentum ratio from 1.19 to 6.48. However, the gradient of the half-width(b$_1$) along the short-axis direction decreased with increasing the momentum ratio. The turbulence intensity and turbulent kinetic energy were converged into the center of the center of the initial region with increasing the momentum ratio. As the momentum ratio increased from MR=1.19 to MR=6.48, the turbulent shear stress decreased. The results of this study can be used for the design of an impinging type injector for liquid rackets.

• Steel and Composite Structures
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• v.11 no.6
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• pp.469-488
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• 2011

This paper consists of two parts; the first part describes the laboratory work concerning the behavior of lightweight aggregate concrete filled steel tubes (LACFT). Based on eccentricity tests, fifty-four specimens with different slenderness ratios (L/D= 3, 7, and 14) were tested. The main parameters varied in the test are: load eccentricity; steel ratio; and slenderness ratio. The standard load-strain curves of LACFT columns under eccentric loading were summarized and significant parameters affecting LACFT column's bearing capacity, failure mechanism and failure mode such as confinement effect and bond strength were all studied and analyzed through the comparison with predicted strength of concrete filled steel tube columns (CFT) using the existing codes such as AISC-LRFD (1999), CHN DBJ 13-51-2003 (2003) and CHN CECS 28:90 (1990). The second part of this paper presents the results of parametric study and introduces a practical and accurate method for determination of the maximum compressive strength of confined concrete core ($f_{max}$), In addition to, the study of the effect of aspect-ratio and length-width ratio on the yield stress of steel tubes ( $f_{sy}$) under biaxial state of stress in CFT columns and the effect of these two factors on the ultimate load carrying capacity of axially loaded CFT/LACFT columns.