• Steel and Composite Structures
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• v.24 no.6
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• pp.689-695
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• 2017

The constitutive relation is an important factor in analysis of confined concrete in composite structures. In order to propose a constitutive model for nonlinear analysis of confined concrete, lateral restraint mechanism of confined concrete is firstly analyze to study the generalities. As the foundation of the constitutive model, peak stress and peak strain is the first step in research. According to the generalities and the Twin Shear Unified Strength Theory, a novel unified equation for peak stress and peak strain are established. It is well coincident with experimental results. Based on the general constitutive relations and the unified equation for peak stress and peak strain, we propose a unified and convenient constitutive model for confined concrete with fewer material parameters. Two examples involved with steel tube confined concrete and hoop-confined concrete are considered. The proposed constitutive model coincides well with the experimental results. This constitutive model can also be extended for nonlinear analysis to other types of confined concrete.

• Computers and Concrete
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• v.8 no.5
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• pp.597-616
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• 2011

With respect to rehabilitation, strengthening and retrofitting of existing and deteriorated columns in buildings and bridges, CFRP sheets have been found effective in enhancing the performance of existing RC columns by wrapping and bonding CFRP sheets externally around the concrete. Concrete columns and piers that are confined by both lateral steel reinforcement and CFRP are sometimes referred to as "hybrid" concrete columns. With the availability of experimental data on concrete columns confined by steel reinforcement and/or CFRP, the study presents modeling using artificial neural networks (ANNs) to predict the compressive strength of hybrid circular RC columns. The prediction of the ultimate confined compressive strength of RC columns is very important especially when this value is used in estimating the capacity of structures. The present ANN model used as parameters for the confining materials the lateral steel ratio (${\rho}_s$) and the FRP volumetric ratio (${\rho}_{FRP}$). The model gave good predictions for three types of confined columns: (a) columns confined with steel reinforcement only, (b) CFRP confined columns, and (c) hybrid columns confined by both steel and CFRP. The model may be used for predicting the compressive strength of existing circular RC columns confined with steel only that will be strengthened or retrofitted using CFRP.

• Journal of the Korea Safety Management & Science
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• v.18 no.3
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• pp.47-54
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• 2016

A confined space means places where the risk of damage to health caused by lack of oxygen or hazardous gases in inadequate ventilation conditions and the risk of fire and explosion caused by flammable substances. Asphyxiation accidents in a confined space occured every year and also occurred more than two people at the same time. In this study, we surveyed the domestic statistical data occurred the lack of oxygen in confined space for the last 10 years(2006-2015) and, analyzed the accident by industries sector, workplace size etc. 17 fatal work accidents that occurred in confined spaces in Korea between 2013 and 2015 were investigated and analyzed using the database of the KOSHA and suggested interventions to minimize asphyxiation accidents in confined spaces. This paper is expected to be used to establish interventions planning and training as a preventive measures in workplace having confined spaces.

• Structural Engineering and Mechanics
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• v.63 no.3
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• pp.371-384
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• 2017

Ultra high performance concrete (UHPC) has recently been applied as an alternative to conventional concrete in construction due to its extremely high compressive and tensile strength, and enhanced durability. However, up to date, there has been insufficient information regarding the confinement behavior of UHPC columns. Therefore, this study aims to perform an assessment of axial stress-strain model for UHPC confined by circular steel tube stub columns. The equations for calculating the confined peak stress and its corresponding strain of confined concrete in existing models suggested by Johansson (2002), Sakino et al. (2004), Han et al. (2005), Hatzigeorgiou (2008) were modified based on the regression analysis of test results in Schneider (2006) in order to increase the prediction accuracy for the case of confined UHPC. Furthermore, a new axial stress-strain model for confined UHPC was developed. To examine the suitability of the modified models and the proposed model for confined UHPC, axial stress-strain curves derived from the proposed models were compared with those obtained from previous test results. After validating the proposed model, an extensive parametric study was undertaken to investigate the effects of diameter-to-thickness ratio, steel yield strength and concrete compressive strength on the complete axial stress-strain curves, the strength and strain enhancement of UHPC confined by circular steel tube stub columns.

• Structural Engineering and Mechanics
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• v.84 no.1
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• pp.101-111
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• 2022

It is widely known that axially loaded fiber-reinforced polymer (FRP) confined concrete presents significant and enhanced mechanical properties with reference to the unconfined concrete. Therefore, to predict the mechanical behavior of FRP-confined concrete two quantities-peak strength and ultimate strain are required. Despite the significant advances, the determination of the ultimate strain of FRP-confined concrete is one of the most challenging problems to be resolved. This is often attributed to our persistence in desiring the conventional methods as the sole technique to examine this phenomenon and the complex nature of the ultimate strain of FRP-confined concrete. To bridge the research gap, this study adopted two machine learning (ML) techniques-artificial neural network (ANN) and Gaussian process regression (GPR)-to analyze observations obtained from 627 datasets of FRP-confined concrete circular and non-circular sections under axial loading test. Besides, the techniques are also used to predict the ultimate strain of FRP-confined concrete. Seven parameters namely width/diameter of the specimens, corner radius ratio, the strength of concrete, FRP elastic modulus, FRP thickness, FRP tensile rupture strain, and the axial strain of unconfined concrete-are the input parameters used to predict the ultimate strain of FRP-confined concrete. The results of the current study highlight the merit of using AI techniques in structural engineering applications given their extraordinary ability to comprehend multidimensional phenomena of FRP-confined concrete structures with ease, low computational cost, and high performance over the existing empirical models.

• Structural Engineering and Mechanics
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• v.18 no.5
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• pp.541-563
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• 2004

For reaching large inelastic deformations without a substantial loss in strength, the potential plastic hinge regions of the reinforced concrete structural members should be confined by adequate transverse reinforcement. Therefore, simple and realistic representation of confined concrete behaviour is needed for inelastic analysis of reinforced concrete structures. In this study, a trilinear stress-strain model is proposed for the axial behaviour of confined concrete. The model is based on experimental work that was carried out on nearly full size specimens. During the interpretation of experimental data, the buckling and strain hardening of the longitudinal reinforcement are also taken into account. The proposed model is used for predicting the stress-strain relationships of confined concrete specimens tested by other researchers. Although the proposed model is simpler than most of the available models, the comparisons between the predicted results and experimental data indicate that it can represent the stress-strain relationship of confined concrete quite realistically.

• Journal of the Korea Safety Management & Science
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• v.5 no.4
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• pp.13-20
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• 2003

The purpose of this study is to know the status of prevention on health obstacle in industries with confined space. Total 190 respondents, 95 safety or health managers and 95 confined space workers from 95 industries with confined space are surveyed by hygienists from April 2003, to July 2003. The contents of survey include health-work program in confined space, instrument of oxygen sampling, equipment of ventilation, safety and health education, watching manager, head count, awakening of risk, air condition and emergency training. The results are as follows: 1. It is found 38% of respondents established health-work program in confined space. The percentage of respondents with instrument of oxygen sampling and equipment of ventilation, operation of safety and health education, posting of watching man and operation of head count are 42%, 35%, 75%, 46% and 56%, respectively. 2. The percentage of awakening of risk, confirm of air condition and operation of emergency training are 36%, 25% and 15%, respectively.

• Computers and Concrete
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• v.30 no.6
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• pp.377-391
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• 2022

The use of carbon fiber-reinforced polymers (CFRP) has widely increased due to its enhancement in the ultimate strength and ductility of the reinforced concrete (RC) structures. This study presents a prediction model for the axial compressive strength and strain of normal-strength concrete cylinders confined with CFRP. Besides, soft computing approaches have been extensively used to model in many areas of civil engineering applications. Therefore, the genetic expression programming (GEP) models to predict axial compressive strength and strain of CFRP-confined concrete specimens were used in this study. For this purpose, the parameters of 283 CFRP-confined concrete specimens collected from 38 experimental studies in the literature were taken into account as input variables to predict GEP based models. Then, the results of GEP models were statistically compared with those of models proposed by various researchers. The values of R² for strength and strain of CFRP-confined concrete were obtained as 0.897 and 0.713, respectively. The results of the comparison reveal that the proposed GEP-based models for CFRP-confined concrete have the best efficiency among the existing models and provide the best performance.

• Biomedical Science Letters
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• v.6 no.4
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• pp.269-279
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• 2000

This present study was investigated to elucidate degenerative changes according to the change of habitual environment on the myocytes of doves by restricting them from flight that is instinct behavior of this animal and strong exercise. To restrict doves from flight, they were confined in the cage (1 $m^3$) for 2 months. After this period, the myocardium of the experimental group was compared to that of wild doves in the ultrastructural and cytochemical ways. In addition, stereological changes were also examined. The results were as followings: 1. The body weight of the confined experimental groups was higher than that of the wild doves, but the ratios of the pectoral muscle/body weight (p<0.05) and the heart/body weight were lower. 2. At the ultrastructural level, the myocardium of confined doves appeared as wavy fibers in the smaller area than in the myocardium of wild doves. Also, the length of sarcomeres was longer in the confined doves. The number of sarcoplasmic reticulum and capillary was smaller in the myocardium of confined doves. 3. Cytochemical examinations showed that the activities of cytochrome oxidase were lowered in the confined doves. 4. Stereological analysis revealed that the density of myofibrils was greater in the confined doves. In contrast the volume density of sarcoplasmic reticulum (p<0.05) and the surface density of mitochondrial inner membrane (p<0.05) was lower in the confined doves, while the numerical density of mitochondrial inner membrane was higher (p<0.05). These results suggest that even the short period of restricted exercise can induce negative effects on the functions of myocytes of doves that are adapted for the strong exercise such as flight. Therefore, the maintenance of prolonged exercise seems to be one of the important factors that are critical to retain the functions of myocardium.

• Earthquakes and Structures
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• v.7 no.2
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• pp.201-216
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• 2014

In this paper the vulnerability of the confined masonry buildings is evaluated analytically. The proposed approach includes the nonlinear dynamic analysis of the two-story confined masonry buildings with common plan as a reference structure. In this approach the damage level is calculated based on the probability of exceedance of loss vs a specified ground motion in the form of fragility curves. The fragility curves of confined masonry wall buildings are presented in two levels of limit states corresponding to elastic and maximum strength versus PGA based on analytical method. In this regard the randomness of parameters indicating the characteristics of the building structure as well as ground motion is considered as likely uncertainties. In order to develop the analytical fragility curves the proposed analytical models of confined masonry walls in a previous investigation of the authors, are used to specify the damage indices and responses of the structure. In order to obtain damage indices a series of pushover analyses are performed, and to identify the seismic demand a series of nonlinear dynamic analysis are conducted. Finally by considering various mechanical and geometric parameters of masonry walls and numerous accelerograms, the fragility curves with assuming a log normal distribution of data are derived based on capacity and demand of building structures in a probabilistic approach.