APSC AE WRD 2019 PYQ: Beam Moment of Resistance

Beam Moment of Resistance

Question: If the area of tensile steel reinforcement is doubled, the moment of resistance of the beam increases only by about

(A) 12%

(B) 22%

(C) 32%

(D) 42%

✅ The correct answer is: (B) 22%

📌 Explanation

In a reinforced concrete beam, doubling the area of tensile steel reinforcement does not double the moment of resistance due to the shift in the neutral axis and limitations imposed by the concrete's compressive strength. For a typical beam section, the moment of resistance increases by approximately 22%, as the increased steel area reduces the lever arm slightly, capping the capacity gain.

📌 Related Information

• The moment of resistance in under-reinforced beams is primarily governed by the steel's tensile strength, but in balanced or over-reinforced sections, concrete's compressive strength limits the increase.
• Design codes like IS 456 or ACI provide guidelines for calculating moment capacity, accounting for lever arm reduction when steel area increases.

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