Foundations · Pile groups & caps
Pile cap for 2 to 9 piles calculator
Standard pile cap layouts from 2 to 9 piles (line, triangle, square, square with centre pile, 2 × 3, hexagon with centre, ring of 8, 3 × 3). Pile reactions follow from the axial load and biaxial moments on a rigid cap including its self-weight. The cap is designed for bending at the column face in both directions, one-way shear at the face with the short-span (av < 2d) enhancement, punching around the column and around the most loaded corner pile.
Standard: Rigid-cap pile reactions · AS 3600 Cl 8.1 flexure at the column face, Cl 8.2 one-way shear with short-span enhancement, Cl 9.3 punching around the column and the corner pile
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Number of piles | 4 piles | |
| Pile spacing | 1.8 | m |
| Pile diameter | 0.6 | m |
| Edge distance (pile centre to cap edge) | 0.6 | m |
| Cap depth | 1000 | mm |
| Column size (square) | 600 | mm |
| Design axial load N* | 6000 | kN |
| Design moment Mx* (about x) | 300 | kNm |
| Design moment My* (about y) | 200 | kNm |
| f′c | 40 | MPa |
| fsy | 500 | MPa |
| Cover | 75 | mm |
| Pile design capacity | 2200 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Cap plan size | 3.00 × 3.00 | m |
| Cap self-weight (× 1.2) | 270 | kN |
| Pile reactions max / min | 1,706 / 1,429 | kN |
| Moment at the column face, x / y | 1,948 / 1,981 | kNm |
| Bottom steel, x / y bars | 5,170 / 5,260 | mm² |
| Shear at the face x: V* / φVuc | 3,246 / 6,455 | kN |
| Shear at the face y: V* / φVuc | 3,302 / 6,455 | kN |
| Column punching V* / φVuo | 6,270 / 8,128 | kN |
| Corner pile punching Rmax / φVuo | 1,706 / 3,222 | kN |
- ✓ Pile reaction ≤ pile capacity — 78 %
- ✓ No pile in tension
- ✓ Flexure (section adequate)
- ✓ One-way shear
- ✓ Punching at the column
- ✓ Punching at the corner pile
Method, formulas and assumptions
- Rigid cap: Ri = (N* + 1.2Gcap)/n + Mx*·yi/Σy² + My*·xi/Σx².
- Flexure at the column face: M = ΣRi(xi − c/2) for piles beyond the face; As from the AS 3600 stress block over the full cap width (φ = 0.85).
- One-way shear at the face: Vuc = 0.15·b·dv·√f′c × 2dv/av (1 ≤ factor ≤ 4) for piles within 2d of the face; φ = 0.7.
- Punching: column perimeter 4(c + d) with fcv = 0.34√f′c (piles inside d/2 relieve it); corner pile: quarter perimeter π(D + d)/4 plus the two edges. Strut-and-tie design is preferred for deep caps (av/d < 0.5).
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.