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Civil Engineering QUESTION #2121
Question 1
In the design of RC beams using the limit state method (IS 456 / BS 8110), a beam is classified as 'under-reinforced' when:
  • The area of steel provided exceeds the balanced steel area
  • The neutral axis depth is less than the limiting neutral axis depth, ensuring that steel yields before concrete crushes — providing ductile failure with adequate warning✔️
  • The shear reinforcement is insufficient for the applied shear force
  • The beam deflects more than span/250 under service loads
Correct Answer Explanation
Under-reinforced sections have \(x_u < x_{u,max}\) (actual NA depth < limiting NA depth). Steel reaches its yield strain before concrete reaches ultimate strain (0.0035), causing gradual ductile failure with large deflections — giving adequate warning. Over-reinforced beams fail suddenly in compression without warning. Codes mandate under-reinforced design for safety.