• Journal of Korean Society of Steel Construction
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• v.25 no.2
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• pp.165-178
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• 2013

This paper presents a research work for the evaluation of the nominal flexural strength of composite girders in positive bending region. Current predicting equations for the nominal flexural strength of composite girders in the 2012 version of the Korea Bridge Design Codes based on Limit State Design Method are able to apply for the composite girders with conventional structural steels. For applying composite girders with high yield strength steels of HSB800 as well as HSB600, there is a need for improving the current predicting equations. In order to investigate the nominal flexural strength of composite girders, previous research works are carefully reviewed and parametric study using a moment-curvature analysis program is conducted to evaluate the ultimate moment capacity and the ductility of a wide range of composite girders. Based on the results of the parametric study, less conservative nominal flexural strength design equations are proposed for conventional composite girders. In addition, new design equations for predicting the nominal flexural strength of composite girders with HSB600 and HSB800 high-performance steels are provided.

• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.5
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• pp.101-112
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• 2011

This paper shows a study carried out on the estimation of nominal flexural strength for CFRP-plated RC beams failed by intermediate crack debonding. A strength reduction factor is proposed to consider the effect of the intermediate crack debonding for the determination of nominal flexural strength. The proposed factor is derived from experimental data and utilizes the ratio of effective stress(or strain) in the CFRP plate to its ultimate strength(or strain) which is called effective strain model. An analytical equation for the estimation of the nominal flexural strength is formulated as a function of strength reduction factor. The validity, accuracy and efficiency of the proposed factor are established by comparing the analytical results with the experimental data, and the major design codes, as well as a number of factors given by researchers. The analytical results presented in this paper indicate that the proposed factor can effectively estimate the flexural nominal strength of CFRP-plated reinforced concrete beams failed by intermediate crack debonding.

• Advances in concrete construction
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• v.7 no.1
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• pp.1-9
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• 2019

The conventional ACI rectangular stress block is developed on the basis of normal-strength concrete column tests and it is still being used for the design of high-strength concrete members. Many research papers found in the literature indicate that the nominal strength of high-strength concrete members appears to be over-predicted by the ACI rectangular stress block. This is especially true for HSC columns. The general shape of the stress-strain curve of high-strength concrete becomes more likely as a triangle. A triangular stress block is, therefore, introduced in this paper. The proposed stress block is verified using a database which consists of 52 tested singly reinforced high-strength concrete beams having concrete strength above 55 MPa (8,000 psi). In addition, the proposed model is compared with models of various design codes and proposals of researchers found in the literature. The nominal flexural strengths computed using the proposed stress block are in a good agreement with the tested data as well as with that obtained from design codes models and proposals of researchers.

• Computers and Concrete
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• v.24 no.1
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• pp.85-94
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• 2019

The ACI building code is allowing for higher strength reinforcement and concrete compressive strengths. The nominal strength of high-strength concrete columns is over predicted by the current ACI 318 rectangular stress block and is increasingly unconservative as higher strength materials are used. Calibration of a rectangular stress block to address this condition leads to increased computational complexity. A triangular stress block, derived from the general shape of the stress-strain curve for high-strength concrete, provides a superior solution. The nominal flexural and axial strengths of 150 high-strength concrete columns tests are calculated using the proposed stress distribution and compared with the predicted strength using various design codes and proposals of other researchers. The proposed triangular stress model provides similar level of accuracy and conservativeness and is easily incorporated into current codes.

• Journal of Korean Society of Steel Construction
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• v.29 no.1
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• pp.1-12
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• 2017

This paper proposes nominal flexural strength considering strain-hardening effect of HSB600 high performance steel for compact composite I-girders in positive bending. Unlike conventional steels, HSB600 undergoes strain-hardening just after yielding without going through yield plateau. However, because the nominal flexural strength specified in domestic and foreign bridge design specifications has been developed for the conventional steel composite girders, the nominal flexural strength does not appropriately consider the strain-hardening of HSB600. Therefore, plastic moment considering a strain-hardening is proposed so as to consider effect of the strain-hardening of HSB600 on flexural strength and then moment-curvature analysis is performed to a wide range of cross-sections. From results of the analysis, a parameter representing the effect of the strain-hardening on the flexural strength of HSB600 composite girders is proposed. Furthermore, by using this parameter, the nominal flexural strength considering the strain-hardening effect for HSB600 composite I-girders in positive bending is proposed and then evaluated by comparing with the current AASHTO LRFD bridge design specifications.

• Computers and Concrete
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• v.6 no.5
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• pp.357-376
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• 2009

In the design of concrete columns, it is important to provide some nominal flexural ductility even for structures not subjected to earthquake attack. Currently, the nominal flexural ductility is provided by imposing empirical deemed-to-satisfy rules, which limit the minimum size and maximum spacing of the confining reinforcement. However, these existing empirical rules have the major shortcoming that the actual level of flexural ductility provided is not consistent, being generally lower at higher concrete strength or higher axial load level. Hence, for high-strength concrete columns subjected to high axial loads, these existing rules are unsafe. Herein, the combined effects of concrete strength, axial load level, confining pressure and longitudinal steel ratio on the flexural ductility are evaluated using nonlinear moment-curvature analysis. Based on the numerical results, a new design method that provides a consistent level of nominal flexural ductility by imposing an upper limit to the axial load level or a lower limit to the confining pressure is developed. Lastly, two formulas and one design chart for direct evaluation of the maximum axial load level and minimum confining pressure are produced.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.587-592
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• 2002

This study attempts to calculate tile nominal flexural strength of reinforced concrete beam with carbon fiber sheets by tile 27 cases which have three steel ratios and four reinforcing ratios. Based on the result, application possibilities of strength design method to estimate the nominal moment is investigated and valuable data of carbon fiber sheets for reinforcing design will be discussed.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.11a
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• pp.323-326
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• 2005

Although many structures. with high strength concrete have been recently constructed, the flexural behavior of reinforced and prestressed concrete beams with high strength concrete is not exactly defined. This paper presents an experimental study on the flexural strength of the high strength concrete beams. Five large scale beams simply supported were tested and measured. Each beam was loaded by two symmetrical concentrated loads applied at 1.25m from the center of span. The concrete strength, the prestressed force and longitudinal tensile reinforcement ratio vary from beam to beam. From the experimental tests, the flexural strength from tests is larger than the nominal flexural strength of codes. Moreover, the initial crack-load is affected by the prestressed force and the crack width and spacing are controlled by the longitudinal tensile reinforcement ratio.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.593-598
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• 2002

Routinely, strength method for the determination of the nominal moment of reinforced concrete beam is assumed to also be suitable for strengthening beams with carbon fiber sheets since typically strengthening beams compromise 98% by volume of reinforced concrete. Flexural capacity of strengthening beam is absolutely dependent upon the type of reinforcement materials, amount of reinforcement, anchoring system, adhesion capacity between reinforcement material and concrete. Therefore, it might be incorrect to use strength method for analysis and design of strengthening beam without considering the differences in the load-deflection curves, mechanism of failure, state of stress distribution, failure strain of the reinforcement. An flexural analysis based on force equilibrium and strain comparability has been developed for strengthening beam. Systematic experimental investigations are compared with analytical results. Then, the adaptation of strength method for strengthening beam have also been discussed.

• Structural Engineering and Mechanics
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• v.21 no.3
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• pp.333-350
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• 2005

This paper presents results of an experimental study to evaluate a new retrofit technique for strengthening shear deficient short concrete beams and columns. In this technique a mortar jacket reinforced with expanded steel meshes is used for retrofitting. Twelve short reinforced concrete specimens, including eight retrofitted ones, were tested. Six specimens were tested under a constant compressive axial force of 15% of column axial load capacity based on original concrete gross section, $A_g$, and the concrete compressive strength, ${f_c}^{\prime}$. Main variables were the spacing of ties in original specimens and the volume fraction of expanded metal in jackets. Original specimens failed before reaching their nominal calculated flexural strength, $M_n$, and had very poor ductility. Strengthened specimens reached their nominal flexural strength and had a ductility capacity factor of up to 8 for the beams and up to 5.5 for the columns. Based on the test results, it can be concluded that expanded steel meshes can be used effectively to strengthen shear deficient concrete members.