Question

In a reinforced concrete beam, if the stress in steel reinforcement attains its yield stress before the concrete fails due to crushing, the beam is said to be

• A. balanced
• B. under reinforced
• C. over reinforced
• D. non-homogeneous

Hint:

Remember the failure modes in RC beams:
- Under-reinforced: Steel yields first $\rightarrow$ Ductile failure (gradual, with warning) $\rightarrow$ GOOD/DESIRABLE.
- Over-reinforced: Concrete crushes first $\rightarrow$ Brittle failure (sudden, no warning) $\rightarrow$ BAD/AVOID.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question describes a failure mode for a reinforced concrete beam and asks for the name of this type of design.

Step 2: Detailed Explanation:
The design of reinforced concrete beams is classified based on the sequence of failure. The goal is to ensure a safe failure mode.

• (B) Under-reinforced Section: This is a design where the amount of steel reinforcement is relatively low. When the beam is overloaded, the steel is the first component to reach its yield limit. Since steel is a ductile material, it will elongate significantly after yielding. This causes large deflections and wide cracks to appear in the concrete on the tension side of the beam, giving a clear and ample warning of impending failure long before the concrete on the compression side crushes. This is a

ductile failure and is the preferred, safe design method. This matches the question's description.

• (C) Over-reinforced Section: This is a design where a large amount of steel reinforcement is used. When the beam is overloaded, the concrete in the compression zone reaches its ultimate crushing strain before the steel has a chance to yield. Concrete is a brittle material, and its failure is sudden, explosive, and occurs without any prior warning. This is a

brittle failure and must be avoided in design.

• (A) Balanced Section: This is a theoretical state where the steel starts yielding at the exact same moment that the concrete reaches its ultimate crushing strain. It marks the boundary between under-reinforced and over-reinforced behavior.



Step 3: Final Answer:
If the steel yields before the concrete crushes, the beam is designed as an under-reinforced section.