In flexural design terminology, an “under-reinforced” reinforced concrete section means which of the following? Select the statement that best captures the behavior at ultimate.

Introduction / Context:RC beams are categorized as under-reinforced, balanced, or over-reinforced based on the relative amounts of tension steel and concrete compression capacity. This classification affects the mode of failure and ductility.

Given Data / Assumptions:

• Flexural member with tension reinforcement.
• Ultimate limit state behavior considered.

Concept / Approach:An under-reinforced section has less tension steel than the balanced amount, so the steel yields before the concrete compression zone crushes. This produces a ductile failure with significant warning (deflection/curvature) prior to collapse—highly desirable in design practice.

Step-by-Step Solution:Compare actual steel ratio ρ with balanced ratio ρ_bal.If ρ < ρ_bal → steel yields first at ultimate → under-reinforced behavior.This ensures tension-controlled failure with greater rotational capacity.

Verification / Alternative check:Strain compatibility shows steel reaches yield strain while concrete compression strain is below crushing strain when ρ < ρ_bal. Moment–curvature diagrams corroborate the ductile response.

Why Other Options Are Wrong:

• Steel on underside only / one face only: A geometric placement statement, not a behavioral definition.
• Steel provided is insufficient: Vague and potentially unsafe wording; the intent is not “inadequate for strength” but “less than balanced” so that yielding governs.

Common Pitfalls:Equating under-reinforcement with unsafe capacity; ignoring serviceability; overlooking compression reinforcement benefits for ductility and long-term deflection.

Final Answer:Steel will yield first