A pile cap's one-way shear check has a special rule most beam or footing intuition doesn't prepare you for — a pile reaction only counts in full if its centre sits far enough beyond the critical shear section, and is credited only partially, or not at all, if it's close to or inside it.
Every check above runs automatically inside the Structyze Pile Cap module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- 4 piles, 400 mm diameter, at 1.2 m × 1.2 m spacing
- Cap 1.8 × 1.8 m, thickness 800 mm, cover 75 mm
- Column 400 mm square
- Axial load: dead 1200 kN, live 600 kN
- Allowable pile load 500 kN
Working Through It
Each pile's service reaction R = (270+135)/4 ≈ 101 kip against the 112 kip allowable — passing. Factored Pu = 1.2(270)+1.6(135) = 540 kip, so Ru = 135 kip per pile.
Flexure at the column face totals 360 kip·ft across the two piles beyond it, but the ACI minimum steel requirement (0.0018bt on the full cap width) actually exceeds what that moment alone would need — the provided 8–#8 bars clear both with utilisation 0.66.
The interesting check is one-way shear: ACI 318 Cl 13.2.7.4 says a pile reaction counts in full only if its centre is at least d/2 beyond the one-way critical section, zero if at least d/2 inside it, and linearly in between. For this compact 4-pile cap, the piles sit close enough to the column that only about 8% of their reaction actually counts — one-way shear demand ends up tiny (utilisation 0.12) compared to what a naive full-credit calculation would suggest.
What Governs, and Why
Two-way (punching) shear governs at utilisation 0.59, well ahead of one-way shear's 0.12 — a direct consequence of the pile-credit rule stripping most of the one-way shear demand away for a compact cap. A wider pile spacing would shift that balance and could make one-way shear govern instead.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Pile Cap module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Pile Cap and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does a pile's position matter so much for one-way shear?
The credit rule reflects how a pile load actually travels to the cap — a pile close to the column delivers its load through a strut-like path that doesn't cross the one-way critical section in the same way a pile further out does; the sectional method's linear credit is a simplified stand-in for that behaviour.
What about a deep pile cap (span-to-depth under about 2)?
The sectional (beam) method used here becomes conservative for a truly deep cap — a strut-and-tie analysis usually permits less bottom steel, and is a good idea to doing separately for large pile groups.
Final Thoughts
The pile-reaction credit rule is the single easiest thing to get wrong hand-calculating a pile cap — it's not obvious until you've seen it fail once that a compact cap's one-way shear demand is nowhere near what a full-credit calculation implies.