Distribute the piles. Check the cap.
Apply explicit factored load combinations, recover every pile-head demand with the Saul/CPGA rigid-cap method, then check the concrete cap for flexure, one-way shear, loaded-area punching, pile punching, and reinforcement adequacy.
Given a cap under combined loads, the question is how much each pile carries. The classical answer is the rigid-cap stiffness method — Saul (1968), implemented by the US Army Corps as CPGA (EM 1110-2-2906): assemble each pile's stiffness at its coordinates, sum into a 6×6 cap stiffness, solve for the cap's rigid-body displacement, and recover every pile's demands from its own displacement.
The structural mode carries those pile reactions into explicit cap geometry, column or pedestal dimensions, concrete and reinforcing-steel properties, and four physical reinforcement mats. It evaluates the sectional checks of the AASHTO LRFD 2nd edition through the 2002 interims, benchmarked against the public FHWA NHI-04-041 pile-footing examples — the code edition is stamped on every result, so a newer edition or local amendments are never silently implied.
Explicit load combinations
Named, already-factored combinations are evaluated independently, including optional cap self-weight factors; no hidden load factors are applied.
Arbitrary layouts
Piles at any coordinates; the rigid-cap stiffness matrix is assembled from the layout you define, not from a template grid.
Structural cap checks
Flexure and required/provided reinforcement, one-way shear, column or pedestal punching, pile punching, and the governing utilization are reported by combination.
Explicit applicability
Unsupported tension, horizontal structural loading, overlapping footprints, impossible bar layouts, and out-of-domain materials are rejected instead of silently extrapolated.
Traceable reports
Reports stamp the method and edition, retain each load combination, and show pile reactions and sectional checks — every number traceable to its input.
Validated against published benchmarks
Every figure below is produced by the engine on the cited published problem — closed-form solutions, design-manual tables, or independent codes — and reproduced by the test suite on every release.
Saul/USACE CPGA distribution + AASHTO LRFD 2nd edition with 2002 interims structural checks
| Quantity | PileCalc | Reference | Agreement |
|---|---|---|---|
| Pure moment on a 2×2 group: axialᵢ = M·kz·xᵢ / Σ(kz·x²)Saul (1968) / USACE CPGA closed form (My = 500 kN·m, kz = 100 MN/m) | ±125 kN | ±125 kN | exact |
| 3×1 line group under Fx + My: axial −150/0/+150 kN, lateral 100 kN eachElastic-center hand calculation | −150/0/+150; 100 kN | −150/0/+150; 100 kN | exact |
| Global 6-DOF equilibrium, asymmetric 5-pile layout with mixed stiffnessesStiffness-method identity, all six load components | Σ reactions ≡ applied loads | exact equilibrium | within 1e-9 (relative) |
| FHWA NHI-04-041 Pier Step 8.11 — Pile 1 under Pu + biaxial momentFHWA NHI-04-041, Design Step 8.11, published 20-pile geometry and pile-load equation | 290.44 kip | 290.45 kip | within 0.01% (published rounding) |
| FHWA column punching demand — 14 pile reactions outside the dᵥ/2 perimeterFHWA NHI-04-041, Design Step 8.11 (alternate equal-load check: 2508 kip) | 2508.1 kip | 2509 kip | within 0.04% (published rounded inputs) |
| FHWA column punching resistance (b₀ ≈ 625.61 in, dᵥ ≈ 36.40 in)FHWA NHI-04-041, AASHTO LRFD 5.13.3.6.3 worked calculation | 4081.94 kip | 4082 kip | within 0.01% |
The numbers, published
PileCalc's engine is checked term-by-term against the reference codes. A representative sample of the benchmarks — every intermediate value is visible in the app so you can reproduce them yourself.
Rigid and structural pile cap: common questions
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