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Columns should preferably fail in under-reinforced condition – myth or reality

By: Contributor(s): Publication details: Thane ACC LTD 2024Edition: Vol.98(11), NovDescription: 14-25pSubject(s): Online resources: In: Indian Concrete Journal - ICJSummary: Ductile failure of reinforced concrete (RC) sections can be ensured only in flexure as all other modes of failure are brittle in nature. Flexure design as per IS: 456 (2000) can be performed within the domain of maximum under-reinforced and balanced conditions as over-reinforced sections are not permitted. Curvature ductility values are maximum for maximum underreinforced sections and minimum for balanced sections. The amount of reinforcements provided in beams define the type of section i.e., under-reinforced, balanced or over-reinforced. As per IS: 456 (2000) all types of sections are permitted in compression. However, for columns with a certain amount of reinforcement, failure can occur in balanced, under-reinforced or over-reinforced conditions depending upon the loading on the section. It is a common notion amongst the designers that similar to beams, columns should also fail in under-reinforced condition to ensure ductile failure. In the present work, using the limit state methodology of IS: 456 (2000), curvature ductility of rectangular RC columns with 0.8 and 4 % reinforcement (distributed on two sides) have been evaluated for three grades of concrete (M20, M40 and M60) with four grades of steel (Fe415, Fe500, Fe550 and Fe600) and different effective covers ( d'/D = 0.05, 0.1, 0.15 and 0.2). Curvature ductility values of sections with identical configurations acting as beams have also been determined. The variation of curvature ductility values of columns failing in highly under-reinforced, balanced and highly over-reinforced conditions are not significant, whereas the corresponding variation is significantly high for beams. Thus, beams can be made considerably more ductile than columns. Hence, the notion amongst the designers that columns should preferably fail in under-reinforced condition does not seem to be justified.
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Ductile failure of reinforced concrete (RC) sections can be ensured only in flexure as all other modes of failure are brittle in nature. Flexure design as per IS: 456 (2000) can be performed within the domain of maximum under-reinforced and balanced conditions as over-reinforced sections are not permitted. Curvature ductility values are maximum for maximum underreinforced sections and minimum for balanced sections. The amount of reinforcements provided in beams define the type of section i.e., under-reinforced, balanced or over-reinforced. As per IS: 456 (2000) all types of sections are permitted in compression. However, for columns with a certain amount of reinforcement, failure can occur in balanced, under-reinforced or over-reinforced conditions depending upon the loading on the section. It is a common notion amongst the designers that similar to beams, columns should also fail in under-reinforced condition to ensure ductile failure. In the present work, using the limit state methodology of IS: 456 (2000), curvature ductility of rectangular RC columns with 0.8 and 4 % reinforcement (distributed on two sides) have been evaluated for three grades of concrete (M20, M40 and M60) with four grades of steel (Fe415, Fe500, Fe550 and Fe600) and different effective covers ( d'/D = 0.05, 0.1, 0.15 and 0.2). Curvature ductility values of sections with identical configurations acting as beams have also been determined. The variation of curvature ductility values of columns failing in highly under-reinforced, balanced and highly over-reinforced conditions are not significant, whereas the corresponding variation is significantly high for beams. Thus, beams can be made considerably more ductile than columns. Hence, the notion amongst the designers that columns should preferably fail in under-reinforced condition does not seem to be justified.

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