Reinforced Concrete – Beam–Column Interaction (P–M curve) In a reinforced concrete beam–column, when do flexural (moment) capacity and axial (compressive) capacity increase together as axial load rises?

Introduction / Context:
Beam–column elements resist combined axial load and bending moment. Their strength interaction is described by the axial load–bending moment (P–M) curve. This question asks where on that curve both axial strength and flexural strength rise together as axial load increases.

Given Data / Assumptions:

• Reinforced concrete rectangular section under combined compression and uniaxial bending.
• Usual strain compatibility and stress block assumptions for design.
• Definitions: “tension-controlled” when steel yields first; “compression-controlled” when concrete crushes first; “balanced” point separates these regimes.

Concept / Approach:

Starting from pure bending (P ≈ 0), adding modest axial compression generally increases moment capacity because additional compression shifts the neutral axis and enhances the compressive block without sacrificing tension steel effectiveness. This continues up to the balanced point. In that regime, the governing failure is steel yielding in tension (tension-controlled). Beyond the balanced point, further increases in axial load reduce available lever arm and the governing failure transitions to concrete crushing; moment capacity then begins to decrease.

Step-by-Step Solution:

Verification / Alternative check:

Design charts and software-generated P–M interaction diagrams show the rising limb (tension-controlled) where both P and M increase together, peaking near the balanced region, followed by a descending limb as concrete crushing dominates.

Why Other Options Are Wrong:

Option A: The phenomenon is not tied to “elastic limit” wording; design is based on ultimate limit states and strain compatibility. Option C: In compression-controlled regions, moment capacity trends down with increasing P. Option D: The P–M curve clearly shows a region where both increase together.

Common Pitfalls:

Confusing the balanced point with maximum moment under zero axial load, or assuming axial compression always decreases moment capacity.

Final Answer:

When yielding of the tension reinforcement governs the section (tension-controlled region)