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?

Difficulty: Easy

To ensure compression failure of the concrete section
To avoid rupture of steel if flexural failure occurs
To hold stirrups in position only
To provide sufficient ductility and post-cracking capacity so that failure is not sudden and brittle
To eliminate shear cracks entirely

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

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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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