A short column of size 300 mm × 300 mm is subjected to an axial factored load of 1200 kN. Using M20 concrete and Fe415 steel, the minimum area of longitudinal reinforcement (in mm2) as per IS 456 is
- A360
- B480
- C600
- D720Correct
Solution
Concept
Minimum steel percentage in reinforced concrete columns
Formula
Asc,min = 0.008 × Ag
Step-by-step solution
1. As per IS 456:2000 Clause 26.5.3.1, the cross-sectional area of longitudinal reinforcement in a column must be at least 0.8% of gross area Ag. 2. Column dimensions: b = 300 mm, D = 300 mm. 3. Gross cross-sectional area Ag = 300 mm × 300 mm = 90,000 mm2. 4. Minimum area of longitudinal steel Asc,min = 0.008 × 90,000 mm2 = 720 mm2. Answer: D
Explanation
Clause 26.5.3.1 of IS 456:2000 mandates that the minimum area of longitudinal reinforcement in a column shall not be less than 0.8% of the gross cross-sectional area. For a 300 mm x 300 mm column, Asc,min = 0.008 × (300 × 300) = 720 mm2.
Why the other options are wrong
A) 360 mm2 corresponds to 0.4%, which is the minimum steel percentage for beams. B) 480 mm2 corresponds to approximately 0.53%. C) 600 mm2 corresponds to approximately 0.67%.
Shortcut
Min steel in column = 0.8% of Ag = 0.008 × 90,000 = 720 mm2.
Common mistake
Using 0.4% (beam minimum) or 0.12% (slab minimum) instead of column minimum of 0.8%.
Correct answer: D) 720