Foundations · Pile groups & caps
Pile cap (2/3/4 piles) calculator
Truss-method tie force and steel, strut angle, pile load and column punching.
Standard: AS 3600 Ch 7 strut-and-tie · Cl 9.3 punching · AS 2159
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 | |
| Column load N* | 4000 | kN |
| Pile spacing | 1.8 | m |
| Pile diameter | 600 | mm |
| Cap depth | 1000 | mm |
| Column size | 500 | mm |
| Cover | 75 | mm |
| Tie bar diameter | 20 | mm |
| f'c | 32 | MPa |
| fsy | 500 | MPa |
| Pile design capacity φRd,g | 1200 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| d (to tie) | 895 | mm |
| Load per pile | 1,000 | kN |
| Tie force T* | 1,006 | kN |
| Tie location | each direction (over the piles) | |
| Ast per tie (φ = 0.8) | 2,514 | mm² |
| Bars per tie | 9 N20 | |
| Strut angle | 35 | ° |
| Column punching φVuo | 6,724 | kN |
- ✓ Pile load ≤ pile capacity
- ✓ Strut angle ≥ 30° (Cl 7.2.2)
- ✓ Column punching V* ≤ φVuo
Method, formulas and assumptions
- Truss analogy, lever arm z ≈ d
- 2 piles: T = N s/(4d); 3 piles: T = N s/(9d) per side; 4 piles: T = N s/(8d) each way
- Ast = T*/(φ fsy), φ = 0.8 for ties
- Punching at d/2: u = 4(c + d), fcv = 0.34√f'c (βh = 1), φ = 0.7; all pile reactions taken outside the perimeter (conservative)
- Also check pile punching, bursting and anchorage of ties beyond pile centres.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.