Ready-reference design charts for isolated and combined footings. All values assume safe bearing capacity (SBC) of 150 kN/m², M20 concrete (fck = 20 MPa), Fe500 steel (fy = 500 MPa), and a minimum cover of 50 mm. Sizes are rounded up to the nearest 50 mm or 100 mm for practical site execution. Always verify one-way shear, two-way (punching) shear, and bending moment at the critical section.
Isolated Square Footings (M20, Fe500)
SBC = 150 kN/m². Column size assumed 300 mm × 300 mm for loads ≤ 500 kN, 400 mm × 400 mm for 600–1000 kN, 500 mm × 500 mm for > 1000 kN. Self-weight of footing included at 10% of column load.
| Load | SBC | Size | Depth | Steel |
|---|
| 200 kN | 150 kN/m² | 1.2 m × 1.2 m | 300 mm | 6Y10 both ways |
| 300 kN | 150 kN/m² | 1.5 m × 1.5 m | 350 mm | 7Y10 both ways |
| 400 kN | 150 kN/m² | 1.7 m × 1.7 m | 400 mm | 7Y12 both ways |
| 500 kN | 150 kN/m² | 1.9 m × 1.9 m | 450 mm | 8Y12 both ways |
| 600 kN | 150 kN/m² | 2.1 m × 2.1 m | 450 mm | 9Y12 both ways |
| 800 kN | 150 kN/m² | 2.4 m × 2.4 m | 500 mm | 10Y12 both ways |
| 1000 kN | 150 kN/m² | 2.7 m × 2.7 m | 550 mm | 9Y16 both ways |
| 1200 kN | 150 kN/m² | 3.0 m × 3.0 m | 600 mm | 10Y16 both ways |
| 1500 kN | 150 kN/m² | 3.3 m × 3.3 m | 650 mm | 12Y16 both ways |
Isolated Rectangular Footings
Used when property line or adjacent footings restrict one direction. Length-to-breadth ratio should not exceed 2:1. SBC = 150 kN/m², M20, Fe500.
| Load | SBC | Size | Depth | Steel |
|---|
| 300 kN | 150 kN/m² | 1.8 m × 1.2 m | 350 mm | Long: 7Y10, Short: 6Y10 (central band) |
| 500 kN | 150 kN/m² | 2.2 m × 1.5 m | 450 mm | Long: 8Y12, Short: 7Y12 (central band) |
| 800 kN | 150 kN/m² | 2.8 m × 1.8 m | 500 mm | Long: 10Y12, Short: 9Y12 (central band) |
| 1000 kN | 150 kN/m² | 3.2 m × 2.0 m | 550 mm | Long: 9Y16, Short: 8Y16 (central band) |
| 1500 kN | 150 kN/m² | 3.8 m × 2.4 m | 650 mm | Long: 12Y16, Short: 10Y16 (central band) |
Combined Footings — Thumb Rules
Used when two columns are close (< 1.0 m apart) or when individual footings overlap. Design as a beam in the longitudinal direction.
| Load | SBC | Size | Depth | Steel |
|---|
| 300 + 400 kN (2 cols) | 150 kN/m² | 3.2 m × 1.5 m | 500 mm | Top: 6Y12, Bot: 8Y12 long + 7Y10 short |
| 500 + 500 kN (2 cols) | 150 kN/m² | 3.6 m × 1.8 m | 550 mm | Top: 7Y12, Bot: 9Y12 long + 8Y10 short |
| 600 + 800 kN (2 cols) | 150 kN/m² | 4.2 m × 2.0 m | 600 mm | Top: 8Y12, Bot: 10Y16 long + 9Y12 short |
| Strap footing rule | — | — | — | — |
Notes
• Minimum depth of footing: 150 mm for unreinforced footings (Cl 34.1.2). For reinforced footings on soil, practical minimum is 300 mm.
• Critical section for bending moment: at the face of the column/pedestal (Cl 34.2.3.2).
• Critical section for one-way shear: at a distance d (effective depth) from the face of the column (Cl 34.2.4.1).
• Critical section for two-way (punching) shear: at d/2 from the column face all around (Cl 31.6.1). Punching shear often governs the depth of the footing.
• Permissible shear stress in M20 concrete (τc) ranges from 0.28 to 0.62 MPa depending on steel percentage (IS 456 Table 19).
• For rectangular footings, reinforcement in the short direction must be distributed: fraction (2/(β+1)) placed in a central band equal to short side width, where β = L/B (Cl 34.3.1).
• Self-weight of footing is typically assumed as 10–15% of column load for initial sizing.
• Minimum reinforcement in each direction: 0.12% of bD for Fe500 steel (Cl 26.5.2.1).
• Net upward soil pressure = P/(L×B) for concentric loading; ensure it does not exceed SBC.
• All values are for preliminary sizing. Final design must include detailed one-way shear, two-way shear, and bending checks.
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