Limit State Method MCQ with answer and solution

Concept

IS 456:2000 allows a 60% higher design bond stress for deformed bars (IS 1786) than for plain bars.

Formula

τ_bd (deformed) = 1.6 × τ_bd (plain); L_d = φ σ_s / (4 τ_bd)

Step-by-step solution

1. IS 456:2000 gives design bond stress τ_bd for plain bars in tension (for example 1.2 N/mm² for M20). 2. For deformed bars conforming to IS 1786, these values are increased by 60%. 3. So τ_bd for deformed bars = 1.6 × plain bar value (for example 1.6 × 1.2 = 1.92 N/mm² for M20). 4. Answer = 60%.

Explanation

Deformed bars have ribs on the surface, so they grip the concrete better than plain bars. IS 456:2000 therefore allows a 60% increase in design bond stress for them. This reduces the required development length.

Why the other options are wrong

A: 25% is the extra increase for bars in compression, not for deformed bars. B: 40% is not given in IS 456 for bond stress. D: 10% is not the code value.

Shortcut

Deformed bar: +60%. Bar in compression: further +25%.

Common mistake

Mixing up the 25% increase for bars in compression with the 60% increase for deformed bars.