• Journal of the Korea Concrete Institute
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• v.24 no.5
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• pp.585-596
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• 2012

In this study, the flexural characteristics of reinforced polymer concrete T-beams strengthened with GFRP, typically used for bridges and parking structures, are investigated. A method to determine the flexural failure mode of reinforced polymer concrete T-beams comprised of compression failure (CF), tension failure (TF), and fiber sheet failure (FF) for different levels of GFRP strengthening is proposed. Moreover, the present study provides a formula to calculate the design flexural strength for each failure mode. In reinforced polymer concrete T-beams strengthened with GFRP, an ideal failure mode can be achieved when the failure occurs in the following order: 1) yield of steel reinforcement, 2) failure of GFRP, and 3) compression failure of concrete. In the case of FF mode, due to GFRP failure before the polymer concrete crushing in compression region, a concept of equivalent rectangular block based on the ultimate limit state of concrete should not be used. Thus, this study suggests an idealized stress-strain curve for polymer concrete and finds parameters for stress block, ${\alpha}$ and ${\beta}$ based on the strain distribution in polymer concrete. Furthermore, the present study suggests an aspect ratio of 2.5 by examining the compressive stress distribution and design flexural strength characteristics for different aspect ratio of T-beams. This study also provides a design flexural strength formula, and validates its acceptability based on experiment and theoretical analysis.

• Computers and Concrete
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• v.15 no.4
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• pp.607-642
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• 2015

The stress-strain relationship of concrete in flexure is one of the essential parameters in assessing the flexural strength and ductility of reinforced concrete (RC) columns. An overview of previous research studies revealed that the presence of strain gradient would affect the maximum concrete stress developed in flexure. However, no quantitative model was available to evaluate the strain gradient effect on concrete under flexure. Previously, the authors have conducted experimental studies to investigate the strain gradient effect on maximum concrete stress and respective strain and developed two strain-gradient-dependent factors k3 and ko for modifying the flexural concrete stress-strain curve. As a continued study, the authors herein will extend the investigation of strain gradient effects on flexural strength and ductility of RC columns to concrete strength up to 100 MPa by employing the strain-gradient-dependent concrete stress-strain curve using nonlinear moment-curvature analysis. It was evident from the results that both the flexural strength and ductility of RC columns are improved under strain gradient effect. Lastly, for practical engineering design purpose, a new equivalent rectangular concrete stress block incorporating the combined effects of strain gradient and concrete strength was proposed and validated. Design formulas and charts have also been presented for flexural strength and ductility of RC columns.

• Computers and Concrete
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• v.20 no.2
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• pp.129-143
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• 2017

Seismic design of reinforced concrete (RC) structures requires a certain minimum level of flexural ductility. For example, Eurocode EN1998-1 directly specifies a minimum flexural ductility for RC beams, while Chinese code GB50011 limits the equivalent rectangular stress block depth ratio at peak resisting moment to achieve a certain nominal minimum flexural ductility indirectly. Although confinement is effective in improving the ductility of RC beams, most design codes do not provide any guidelines due to the lack of a suitable theory. In this study, the confinement for desirable flexural ductility performance of both normal- and high-strength concrete beams is evaluated based on a rigorous full-range moment-curvature analysis. An effective strategy is proposed for flexural ductility design of RC beams taking into account confinement. The key parameters considered include the maximum difference of tension and compression reinforcement ratios, and maximum neutral axis depth ratio at peak resisting moment. Empirical formulae and tables are then developed to provide guidelines accordingly.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.10a
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• pp.529-532
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• 1999

With increasing use of high-strength concrete tied columns in structural engineering, it becomes necessary to examine the applicability of related sections of the current design codes. This experimental study was conducted to investigate the behavior of eccentrically loaded high-strength concrete columns. Column specimens with concrete strength 234, 437, 703kgf/㎠ were tested under monotonically increasing eccentric compression. The test parameters included the longitudinal reinforcement ratio, eccentric distance and concrete compressive strength. The analytical results obtained from the stress-strain relationship and the ACI's equivalent rectangular stress block are compared with experimental test results.

• Proceedings of the Korea Concrete Institute Conference
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• 1997.10a
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• pp.436-441
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• 1997

This paper presents the compressive stress distribution model appropriate to predict the ultimate strength of structural elements using ultra high-strength concrete. From the results of this investigation, the following conclusions are drawn: 1. The constant value of strain at extreme concrete compression fiber of 0.0027 is seen to represent satisfactorily the experimental result for ultra high-strength concrete. 2. The current ACI-318 rectangular stress block parameters were found to overestimate the moment capacity of ultra high-strength concrete columns with eccentrically loaded. 3. The equivalent trapezoidal stress distribution model with new parameter $\lambda_1$ and $\lambda_2$ was developed.

• Journal of the Korea institute for structural maintenance and inspection
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• v.7 no.2
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• pp.239-246
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• 2003

In the PCA Load Contour Method, the biaxial bending design coefficient of columns(${\beta}$) is based on the equivalent rectangular stress block (RSB). And coefficient of ${\beta}$ estimates the reinforcement index to be a influencing parameter on biaxial moment strength of RC columns without considering the arbitrary condition of bar arrangement. The experimental results of high strength concrete (HSC) columns subjected to combined axial load and biaxial bending moment were compared to the analysis results of RSB method. As result, the accuracy of RSB method is still acceptable for HSC columns and, as the reinforcement is placed densely in each corner of column section, the ${\beta}$ is decreased.

• Journal of the Korea Concrete Institute
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• v.11 no.4
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• pp.63-71
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• 1999

In evaluating the ultimate strength of a section for a reinforced concrete flexural member, the effect of member length is not usually considered, even though the strength tends to decrease with increase of member length. In this paper the influence of specimen length on flexural compressive strength of concrete was evaluated. For this purpose, a series of C-shaped specimens subjected to axial compression and bending moment were tested using four different length-to-depth ratios (from 1,2,3 and 4) of specimens with compressive strength of 590 kgf/$\textrm{cm}^2$. Results indicate that for the region of h/c <3.0 the reduction in flexural compressive strength with increase of length-to-depth ratios was apparent. A model equation was depth of an equivalent rectangular stress block was larger than that by ACI. It was also founded that the effect of specimen length on ultimate strain was negligible. Finally more general model equation is also suggested.

• Computers and Concrete
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• v.18 no.5
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• pp.983-998
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• 2016

Structural optimization is one of the most important topics in structural engineering and has a wide range of applicability. Therefore, the main objective of the present study is to apply the Lagrange Multiplier Method (LMM) for minimum cost design of singly and doubly reinforced rectangular concrete beams. Concrete and steel material costs are used as objective cost function to be minimized in this study, and ultimate flexural strength of the beam is considered to be as the main constraint. The ultimate limit state method with partial material strength factors and equivalent concrete stress block is used to derive general relations for flexural strength of RC beam and empirical coefficients are taken from topic 9 of the Iranian National Building Regulation (INBR9). Optimum designs are obtained by using the LMM and are presented in closed form solutions. Graphical representation of solutions are presented and it is shown that proposed design curves can be used for minimum cost design of the beams without prior knowledge of optimization and without the need for iterative trials. The applicability of the proposed relations and curves are demonstrated through two real life examples of SRB and DRB design situations and it is shown that the minimum cost design is actually reached using proposed method.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.3A
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• pp.227-234
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• 2011

Recently, a high strength concrete of more than 40 MPa has been increasingly used in practice. However, use of the high strength concrete may influence on design parameters, particularly stress distribution. This is very true since the current everyday practice employs equivalent rectangular stress distribution that is derived from normal strength concrete. Subsequently, the stress distribution seems to be reevaluated and then a new distribution with new parameters needs to be suggested for the high strength concrete. For this purpose, linear and multiple regression analyses have been carried out in term of using experimental data for the high strength concrete of 40 to 80 MPa available in literatures. Accordingly, new parameters associated with the stress distribution have been proposed and employed for the design of flexural and compressive members. Comparative design examples indicate that designs with new parameters reduce section dimensions compared to those with the current code parameters for concrete strengths of 40 to 70 MPa. In particular, for compressive members, design with new parameters exhibit conservative compressive force compared to those with the current code parameters.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.24 no.6
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• pp.586-594
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• 2013

This paper proposes a new coupling element of microwave slot coupler for designing waveguide slot array which can reduce the effect of undesired higher order mode coupling between coupling and radiating slots in the branch waveguide. The proposed device is composed of a centered longitudinal shunt-series coupling slot at the center of broad wall shared by two crossed rectangular waveguides and an asymmetric compound iris that excites the coupling slot. We first have obtained scattering parameters for the proposed coupler by use of EM S/W tool HFSS and then extracted the parameters of T- network equivalent circuit for the coupling slot. We also have analyzed the resonant properties such as resonant length and normalized admittance by changing the geometrical dimensions. The measured results for the fabricated coupler with short-circuited block ${\lambda}_g/4$ away from the coupling slot are well agreed with the simulated ones.