The 'minimum reinforcement' in RC beams (IS 456) ensures:
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Maximum strength is achieved
Ductile behaviour after cracking — minimum steel carries tensile force from concrete at cracking, preventing brittle sudden failure
No deflection under service loads
Compressive strength governs design
Correct Answer
B. Ductile behaviour after cracking — minimum steel carries tensile force from concrete at cracking, preventing brittle sudden failure
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IS 456 Cl. 26.5.1.1: Minimum Ast = 0.85/fy × bw × d (in mm²) where fy in MPa. Purpose: prevent sudden (brittle) fracture if concrete cracks — minimum steel takes over tensile force. If Ast = 0, a cracked beam fails suddenly (no ductility). Minimum ensures: Ast_steel can carry at least the tensile force present in concrete just before cracking (M_cr = 0.7√fck × Z). Avoids over-reinforced sections too (Ast_max = 0.04 bD).
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