Question

Consider a balanced doubly-reinforced concrete section. If the material and other sectional properties remain unchanged, for which of the following cases will the section become under-reinforced?

• A. Area of tension reinforcement is increased.
• B. Area of compression reinforcement is increased.
• C. Area of tension reinforcement is decreased.
• D. Area of compression reinforcement is decreased.

Hint:

In under-reinforced sections, failure is ductile, providing ample warning before collapse, whereas over-reinforced sections lead to brittle failure.

Answer 1

The Correct Option is B

Step 1: Understanding balanced, under-reinforced, and over-reinforced sections: A balanced section means that both concrete and steel reach their ultimate strength simultaneously. An under-reinforced section means steel yields before the concrete reaches its maximum strength, ensuring ductile failure. An over-reinforced section means concrete fails before steel, resulting in brittle failure. Step 2: Evaluating the effects of changes in reinforcement: Option (A): Increasing the area of tension reinforcement shifts the balance towards over-reinforcement. Incorrect. Option (B): Increasing the area of compression reinforcement leads to under-reinforcement as it reduces the reliance on tensile steel. Correct. Option (C): Decreasing the area of tension reinforcement makes the section more under-reinforced as it reaches yield stress earlier. Correct. Option (D): Decreasing compression reinforcement does not make the section under-reinforced, but rather affects overall strength. Incorrect. Conclusion: The correct choices that lead to under-reinforcement are: \[ \text{Option (B) and Option (C)} \]