Limit State Method MCQ with answer and solution

Concept

IS 456 gives the design bond stress τ_bd for plain bars in tension. For M40 and above it is 1.9 N/mm².

Formula

L_d = φ σ_s / (4 τ_bd)

Step-by-step solution

1. IS 456:2000 gives τ_bd for plain bars in tension (limit state method). 2. Values: M20 = 1.2, M25 = 1.4, M30 = 1.5, M35 = 1.7, M40 and above = 1.9 N/mm². 3. For M40, τ_bd = 1.9 N/mm². 4. Answer is (D).

Explanation

IS 456 lists the design bond stress for plain bars in tension for each concrete grade. For M40 and higher grades, the value is 1.9 N/mm². This value is used to find the development length L_d.

Why the other options are wrong

(A) 1.2 N/mm² is for M20. (B) 1.5 N/mm² is for M30. (C) 1.7 N/mm² is for M35.

Shortcut

Remember the series 1.2, 1.4, 1.5, 1.7, 1.9 for M20 to M40.

Common mistake

Adding 60% for deformed bars (1.9 × 1.6 = 3.04) or 25% for compression when the question asks for plain bars in tension.