Minimum tensile steel in an RCC beam as per code intent (IS 456 context): what is the primary purpose of specifying a minimum area of longitudinal reinforcement in flexural members?
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ATo ensure compression failure of the concrete section
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BTo avoid rupture of steel if flexural failure occurs
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CTo hold stirrups in position only
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DTo provide sufficient ductility and post-cracking capacity so that failure is not sudden and brittle
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ETo eliminate shear cracks entirely
Answer
Correct Answer: To provide sufficient ductility and post-cracking capacity so that failure is not sudden and brittle
Explanation
Introduction / Context:Reinforced concrete beams must maintain integrity after cracking of the tensile concrete. Design codes prescribe a minimum area of tensile reinforcement so that the beam can safely carry moments beyond first cracking and display ductile behavior, providing warnings before failure.
Given Data / Assumptions:
- Flexural member (RCC beam) designed per limit state concepts.
- Concrete is weak in tension; tensile zone cracks at service loads.
- Longitudinal steel bridges the cracked zone and provides ductility.
Concept / Approach:The minimum tensile steel ensures that the moment of resistance of the cracked section exceeds the cracking moment and that steel yields gradually rather than the member failing in a brittle manner. It also helps control crack widths and deflection under service loads. This is a safety and serviceability requirement, not merely a detailing convenience.
Step-by-Step Solution:
Recognize that concrete tension capacity is negligible in design; post-cracking capacity must come from steel.Minimum steel area criteria ensure the beam remains under-reinforced and ductile.Select the option emphasizing ductility and prevention of sudden brittle failure.Verification / Alternative check:
Load–deflection curves of beams with minimum steel show gradual yielding rather than abrupt loss of capacity at cracking.Why Other Options Are Wrong:
Compression failure is undesirable in flexural members; holding stirrups is a secondary detailing purpose; eliminating shear cracks is a separate shear reinforcement issue; steel rupture avoidance is a consequence of adequate ductility but not the primary code intent stated most directly.Common Pitfalls:
Confusing minimum tension steel (ductility) with maximum steel limits (over-reinforcement) or with minimum shear links.Final Answer:
To provide sufficient ductility and post-cracking capacity so that failure is not sudden and brittle