Question

Read the following statements relating to flexure of reinforced concrete beams:
I. In over-reinforced sections, the failure strain in concrete reaches earlier than the yield strain in steel.
II. In under-reinforced sections, steel reaches yielding at a load lower than the load at which the concrete reaches failure strain.
III. Over-reinforced beams are recommended in practice as compared to the under-reinforced beams.
IV. In balanced sections, the concrete reaches failure strain earlier than the yield strain in tensile steel.
Each of the above statements is either True or False.
Which one of the following combinations is correct?

• A. I (True), II (True), III (False), IV (False)
• B. I (True), II (False), III (True), IV (False)
• C. I (True), II (True), III (True), IV (False)
• D. I (False), II (True), III (True), IV (False)

Hint:

In reinforced concrete, under-reinforced beams are preferred because they ensure ductile failure, where steel yields before concrete crushing.

Answer 1

The Correct Option is A

Let's analyze the statements:
- Statement I: In over-reinforced sections, concrete fails before the steel reaches its yield strain. This is true because in over-reinforced sections, the amount of steel is excessive, leading to concrete failure first.
- Statement II: In under-reinforced sections, the steel yields before the concrete reaches failure strain. This is true, as under-reinforced beams are designed such that the steel yields first, providing a ductile failure mode.
- Statement III: Over-reinforced beams are not recommended in practice as they lead to brittle failure due to premature concrete crushing. So, this statement is false.
- Statement IV: In balanced sections, the concrete and steel fail simultaneously, so concrete reaches failure strain at the same time or before the steel yields. This statement is false because concrete failure is typically before the yield of steel in a balanced section.
Thus, the correct combination is (A).