Pile cap design to SANS 10100
A pile cap is deep compared with the pile spacing, so it does not bend like a slab. The load runs from the column down to each pile in sloping concrete struts, and the bottom bars between the piles act as ties that stop the struts pushing the piles apart (the truss method). The tie steel goes in bands over the piles. Shear across the cap, punching and anchorage of the ties finish the design. Every number below comes from the free STATIX pile cap calculator with the inputs in step 1.
1. The problem
| Input | Value |
|---|---|
| Design code | SANS 10100 |
| Axial N ULS | 2500 kN |
| Layout | Preset (2 to 6 piles) |
| Piles | 4 — square |
| Diameter | 450 mm |
| Spacing c/c | 1350 mm |
| Pile capacity Rall | 800 kN |
| Column cx | 400 mm |
| Column cy | 400 mm |
| Depth h | 900 mm |
| Edge overhang past pile | 150 mm |
| Concrete fcu | 30 MPa |
| Rebar fy | 450 MPa |
| Cover | 75 mm |
| Exposure | In contact with soil |
| Bar size | 20 mm |
| Layers | 2 |
2. Loads and pile reactions
The column load plus the cap's own weight is shared between the piles; a column moment makes the piles on one side take more. Each pile reaction must stay within the pile's capacity.
3. The truss: struts and ties
From the column, a strut runs to each pile. Where a strut meets a pile it pushes outward; the tie across the bottom holds it. The tie force follows from the pile reaction and the slope of the strut (the pile spacing against the lever arm).
4. Tie steel in bands over the piles
The tie bars are concentrated in bands over the piles, where the struts land. Each band needs T / f_s of steel; the bars are then chosen.
5. Bending cross-check
The cap is also checked as a wide beam bending at the column face, as a cross-check on the truss: the larger steel governs.
6. Shear across the cap
A section across the cap near the column face must carry the pile reactions outside it. Because the piles are close to the column, the shear strength is enhanced (2d / a_v), as in a corbel.
7. Anchorage, cover and the rest
The tie bars must be anchored beyond the piles (usually bent up), the cover must suit the soil, and starter bars, side links and a top mat complete the detailing.
Two-, three- and five-pile caps follow the same truss; the calc sheet shows the geometry of each.
Open the pile cap calculator
A worked example for learning, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.
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