The 'balanced section' in RC beam design occurs when:
Choose the correct answer
Steel yields only, concrete is not stressed
Extreme fibre concrete strain = εcu and tensile steel strain = εy simultaneously — balanced reinforcement ratio
Applied moment equals zero
No reinforcement is needed in compression
Correct Answer
B. Extreme fibre concrete strain = εcu and tensile steel strain = εy simultaneously — balanced reinforcement ratio
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Balanced section: concrete reaches εcu (extreme fibre) simultaneously when steel reaches εy (yield). In LSM: xu_b/d = εcu/(εcu+εy) = 0.0035/(0.0035+fy/(1.15×Es)) = 0.0035/(0.0035+0.87fy/Es). For Fe 415: xu_b/d = 0.479. Above this ratio: over-reinforced (concrete controls, brittle). Below: under-reinforced (steel yields first, ductile). IS 456 limits xu_max/d to ensure ductile failure.
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