Columns carry every floor above them down to the footings. They are the members where cutting corners is most dangerous, and where planning for the future costs least.
Typical Sizes, and the Steel IS 456 Requires
| Column | Typical use | Gross area | 0.8% minimum steel | Bars that meet it |
|---|---|---|---|---|
| 230 × 230 mm | Light single-storey work | 52,900 mm² | 423 mm² | 4 × 12 mm (452 mm²) |
| 230 × 300 mm | Single storey, smaller G+1 | 69,000 mm² | 552 mm² | 6 × 12 mm (679 mm²) or 4 × 16 mm (804 mm²) |
| 230 × 380 mm | G+1 | 87,400 mm² | 699 mm² | 4 × 16 mm (804 mm²) |
| 230 × 450 mm | G+1, G+2 | 103,500 mm² | 828 mm² | 8 × 12 mm (905 mm²) or 6 × 16 mm (1,206 mm²) |
| 300 × 450 mm | G+2 and heavier | 135,000 mm² | 1,080 mm² | 6 × 16 mm (1,206 mm²) |
The last column is the smallest bar arrangement that satisfies IS 456's 0.8% minimum for that size — a floor, not a design. These are common patterns, not a design. Spans, loads, soil and the position of each column change the answer, and corner, edge and interior columns often differ.
What Decides Column Size?
- Load: the floors, walls, roof and live load above.
- Span: longer beam spans put more load on each column.
- Height and slenderness: tall, thin columns can buckle.
- Soil: weak soil may change the column layout and footing design.
- Future floors: design for the final height.
- Wall thickness: 230 mm columns sit flush with 9 inch walls; with 8 inch AAC, the column may project.
IS 456 Rules for Columns
| Rule | Requirement |
|---|---|
| Minimum longitudinal steel | 0.8% of gross section |
| Maximum longitudinal steel | 6% (4% practical) |
| Minimum number of bars | 4 in rectangular columns |
| Minimum bar diameter | 12 mm |
| Cover to main bars | At least 40 mm, not less than bar diameter |
| Ties | At least ¼ of largest bar, ≥ 6 mm; pitch ≤ least dimension, 16 × smallest bar, 300 mm |
Example: a 230 × 300 mm column has 69,000 mm²; 0.8% is 552 mm², which four 12 mm bars (452 mm²) do not meet — so four 12 mm bars alone would be under the minimum for that size; six 12 mm (679 mm²) or four 16 mm (804 mm²) would. This is why the drawing, not habit, must set the bars.
Mistakes to Avoid
- Shrinking columns to fit a room or a wall.
- Using fewer or thinner bars than drawn.
- Wider tie spacing than drawn.
- Off-centre columns between floors — columns should line up vertically.
- Not designing for a future floor.
The Short Version
Expect 230 × 300 mm or larger columns for a G+1 house, with bars and ties exactly as drawn. IS 456 sets minimums the design must meet; your engineer sizes each column for the floors it will eventually carry.
