RCC Flexure – Under-Reinforced Simply Supported Beam (Concept Check) Consider a simply supported, under-reinforced RCC beam. Evaluate the following statements about its flexural failure and neutral-axis shift as load increases: I) Failure occurs by crushing of concrete before the tensile steel has yielded. II) As the load is increased, the neutral axis moves upward (toward the compression face).

Introduction / Context:
Under-reinforced reinforced-concrete (RCC) beams are intentionally detailed so that the tension steel yields before the concrete in compression crushes. This ensures ductile behavior, wide warning cracks, and energy dissipation. The question probes two key ideas: the sequence of failure in an under-reinforced section and the direction of neutral-axis (NA) movement as load escalates and tension concrete cracks.

Given Data / Assumptions:

• Beam type: simply supported RCC beam.
• Reinforcement condition: under-reinforced (steel ratio below balanced value).
• Material behavior: linear-elastic to cracking for concrete in tension, followed by tension stiffening reduction; elastic-plastic response for steel near yield.

Concept / Approach:

In under-reinforced sections, design ensures steel yields first. After cracking in tension, the tension zone of concrete becomes ineffective in flexure, and the resultant compression block becomes comparatively smaller but more intense. As load increases beyond first cracking, the NA shifts toward the compression face because a smaller compression zone must carry the internal compressive force in equilibrium with the increasing tensile force in yielded or near-yield steel.

Step-by-Step Solution:

Verification / Alternative check:

Moment–curvature curves for under-reinforced sections show a clear yield plateau prior to ultimate, with concrete crushing marking the final limit state. Strain profiles confirm the NA shifting upward post-cracking and as curvature increases.

Why Other Options Are Wrong:

• Both false / both true: contradict the ductile design target and observed NA migration.
• I true, II false: corresponds to over-reinforced behavior, not under-reinforced.

Common Pitfalls:

Confusing under-reinforced with over-reinforced behavior; assuming NA remains fixed at the uncracked transformed-section location; ignoring tension-stiffening effects that still do not reverse the overall upward NA trend.

Final Answer:

I is false but II is true.