Limit State of Collapse — Short Column Under Axial Load
Clause 39.3 gives the design formula for short columns subjected to axial compression. A column is 'short' when the effective length to least lateral dimension ratio (le/b or le/D) does not exceed 12. The formula accounts for the minimum eccentricity that always exists due to construction imperfections.
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Key Requirements
•Column is short if le/b ≤ 12 AND le/D ≤ 12 (both directions)
•If emin ≤ 0.05D, the column may be designed as axially loaded using Clause 39.3
•Pu = 0.4 fck Ac + 0.67 fy Asc — for short column with axial load
•Minimum longitudinal steel: 0.8% of gross area; Maximum: 6% (4% at laps)
Reference Tables
Axial Load Capacity Pu (kN) — Common Column Sizes
Size (mm)
Steel %
M20
M25
M30
230 × 230
1.0%
504
610
716
230 × 230
2.0%
603
709
815
230 × 450
1.0%
989
1196
1404
230 × 450
2.0%
1183
1391
1598
300 × 300
1.0%
862
1043
1224
300 × 300
2.0%
1033
1214
1395
300 × 600
1.0%
1724
2086
2448
300 × 600
2.0%
2066
2428
2790
450 × 450
1.0%
1940
2347
2754
450 × 450
2.0%
2324
2731
3138
Fe500 steel. Pu = 0.4 fck Ac + 0.67 fy Asc. Values in kN.
Formulas
Pu = 0.4 fck Ac + 0.67 fy Asc
Axial load capacity of short column (accounts for minimum eccentricity)
Pu = Ultimate axial load capacity (N)fck = Characteristic compressive strength (MPa)Ac = Area of concrete = Ag − Asc (mm²)fy = Yield strength of steel (MPa)Asc = Area of longitudinal reinforcement (mm²)
emin = max(l/500 + D/30, 20 mm)
Minimum eccentricity
emin = Minimum eccentricity (mm)l = Unsupported length of column (mm)D = Lateral dimension in the direction under consideration (mm)
Practical Notes
✓A 230 × 230 column with 1% steel in M25 carries ~610 kN (≈ 62 tonnes). For a typical G+3 residential building with 25 kN/m² total load per floor, this serves about 6 m² tributary area — tight for central columns.
✓The 0.4 and 0.67 factors (instead of 0.45 and 0.75 for pure axial) already account for the minimum eccentricity. If emin > 0.05D, use the interaction diagram (SP:16 charts).
✓At laps, maximum steel is 4% (not 6%) — this is a practical constraint on bar scheduling that's often missed.
Common Mistakes
⚠Forgetting to check le/b ≤ 12 before using the short column formula — if le/b > 12, the column is slender and needs moment magnification (Clause 39.7).
⚠Not checking minimum eccentricity — if emin > 0.05D, the axial load formula doesn't apply. Must use biaxial bending interaction diagram.
⚠Using 6% steel at lap zones — maximum at laps is 4%. This means if you detail 4% steel with 100% lapped, the actual steel at the lap section is 8%, which is non-compliant.