Shallow footings
The general bearing-capacity equation with every correction factor, elastic settlement, base sliding, and eccentric tilt.
The strict footing analysis keeps bearing resistance, contact, settlement, rotation, and sliding on explicit load and unit bases. It is designed to make a scientifically invalid request fail before a result is shown. The same request and response schemas are used by the footing tool, API, Excel integration, saved reruns, and reports.
Method
Every response identifies method.footing.fhwa-bearing-settlement, the exact component models, and the edition string FHWA-SA-02-054 (2002) / PileCalc applicability envelope 2026-07. The public reference is the FHWA Geotechnical Engineering Circular No. 6: Shallow Foundations.
One vertical-load basis
verticalLoad is the gross compression load at the footing base, including the footing, pedestal, and overburden weight. PileCalc does not guess whether an entered load is net, column-only, or gross.Bearing and contact
The bearing equation uses Vesić factors with the FHWA GEC 6 shape corrections and Meyerhof effective area. Because the request does not establish durable compacted granular cover, the depth factor is conservatively 1.0. FHWA load-inclination factors are omitted when shape factors are used; horizontal demand is checked separately as sliding. Ground surface and footing base must both be level.
Biaxial moments create eₓ = My/V and eᵧ = Mx/V. Following FHWA GEC 6 Equations 6-6 and 6-7, capacity is evaluated on the Meyerhof effective area:
The full rectangular base must remain in compression. The request is rejected at or beyond6(|eₓ|/B + |eᵧ|/L) = 1. It is also rejected when the least-loaded corner pressure drops below the removed overburden, because the released settlement model does not represent partial unloading, heave, or soil tension.
Gross, net, and design bases
The output reports gross and net pressures and loads separately. The actual bearing factor of safety is q_net,ult / q_net,applied; it does not divide gross resistance by a net or column-only demand.
Layered settlement
Settlement layers start at zero below the footing base, are ordered and contiguous, and extend through at least four times the larger plan dimension. The stress increase follows FHWA GEC 6 Equation 5-17's 2-on-1 distribution. PileCalc integrates that distribution exactly through each declared layer instead of sampling only one midpoint.
Immediate settlement is the stress integral divided by the layer constrained modulus, formed from the entered Young's modulus and Poisson ratio. A layer with explicit compression parameters also receives primary one-dimensional e-log settlement, split at its preconsolidation stress when needed.
The response exposes every layer's average stress increase, initial and final effective stress, immediate component, and consolidation component. It also reports the remaining 2-on-1 stress ratio at the declared truncation depth.
Biaxial rotation
Signed linear contact pressures are evaluated at the two x edges and two y edges. The same layered settlement model runs at each edge; the settlement difference divided by the corresponding plan dimension gives signed rotation about that axis. This keeps rotation consistent with the settlement assumptions instead of applying an unrelated empirical tilt expression.
Sliding resistance
A nonzero horizontal load requires a shear direction, an explicit base-interface model, and a passive-resistance reduction factor. FHWA GEC 6 Equations 5-38 and 5-39 are alternatives: drained base friction and undrained base adhesion are never added together.
The undrained-adhesion option requires φ = 0. Reduced passive resistance on the leading embedded face is then reported as a separate auditable component. The face width is chosen from the declared shear direction, groundwater affects effective vertical stress, and the user-entered reduction factor remains visible in the request.
Inputs and units
width is the short plan dimension and must not exceed length. Layer depths and groundwater depth use the same declared SI or US length unit.
Enter the strength state appropriate to the declared analysis. The released friction angle range is 0–45 degrees. The program does not infer cohesion from qu.
If groundwater is present, enter its depth, water unit weight, and saturated unit weights for every affected settlement layer. There is no hidden 9.81 or 62.4 default.
These form the constrained modulus for immediate strain. Primary consolidation is included only when Cc, Cr, e0, and preconsolidationStress are all explicit.
Applicability and evidence
- Level ground, horizontal rectangular base, and full compression only.
- No secondary compression, creep, cyclic degradation, liquefaction, rocking, or construction-disturbance model.
- No automatic soil-parameter selection, load-combination generation, or structural footing design.
- Passive resistance is only as reliable as the entered reduction factor and the assumed intact leading soil.
The validation dossier includes public FHWA effective-area worked values and equation identities, reproduced by the contract, unit-invariance, and public worked-example test suites and exercised by the frozen public benchmark manifest in CI. A checksum-frozen copy of the Briaud–Gibbens full-scale footing measurements is included for field-performance comparison.
Settlement is still site-specific
Reading the results
- Compare actual bearing FS to the requested FS; keep gross and net values on their labeled bases.
- Review minimum contact pressure and kern utilization before interpreting the effective-area capacity.
- Review immediate and consolidation components by layer, not only the total settlement.
- Confirm the reported sliding interface mode, passive component, and reduction factor match the intended drained or undrained design condition.
Immutable provenance