Reinforced Concrete Beams — Under-Reinforced Section and Neutral Axis For an under-reinforced reinforced concrete (RC) beam section, how does the depth of the actual (neutral axis at failure) compare with the critical (balanced) neutral axis depth?

Introduction:
RC beam behavior depends on the reinforcement ratio relative to a balanced (critical) state. Under-reinforced beams are preferred in design because they provide ductile tension failure. This question probes the relative position of the neutral axis in such sections.

Given Data / Assumptions:

• Beam is singly reinforced and behaves per standard RC theory.
• Plane sections remain plane; concrete carries no tension at ultimate.
• Balanced (critical) section has simultaneous concrete compression and steel tension at their limiting strains.

Concept / Approach:
In an under-reinforced section, steel yields before concrete reaches its crushing strain. To satisfy strain compatibility, the neutral axis lies shallower (closer to the tension face) than in the balanced case, meaning its depth is less than the critical neutral axis depth.

Step-by-Step Solution:
1) Define balanced depth: both materials at limit strains.2) With less steel than balanced, tensile steel yields first.3) Strain profile requires a smaller compression block depth.4) Therefore, actual neutral axis depth < critical neutral axis depth.

Verification / Alternative check:
Design codes and textbook stress–strain blocks show shallower neutral axis for under-reinforced sections, matching observed ductile failure modes.

Why Other Options Are Wrong:

• same as / greater than: Contradict the fundamental definition of under-reinforcement.
• depends only on cover / indeterminate: Neutral axis location depends on reinforcement ratio and section geometry, not just cover.

Common Pitfalls:
Confusing under-reinforced with over-reinforced (which has deeper neutral axis and brittle compression failure).

Final Answer:
less than