Foundations
The Foundations module designs isolated spread footings with a pedestal to ACI 318 Chapter 13. Open it from New (Alt+N) → Foundations, or from the module button at the left of the top bar.
What you enter#
| Data | Notes |
|---|---|
| Footing B × L × h and depth Df | B along x, L along y; Df from grade to the bottom of the footing (backfill goes on top) |
| Pedestal bx × by and height hp | Loads are applied at the top of the pedestal; the shears add moment at the base with the arm hp + h |
| Bar mat | Bar size and count in x and in y, which layer is lowest, bottom and side cover |
| Soil | Allowable pressure (net or gross), backfill unit weight, friction coefficient, minimum safety factors |
| Loads | Factored Pu, Mux, Muy, Vux, Vuy for the concrete and service Ps, Mxs, Mys, Vxs, Vys for the soil. Leave the service values at zero to use factored ÷ 1.4 |
What is checked#
| Check | Method |
|---|---|
| Soil pressure | Rigid footing, linear pressure without tension. Inside the kern the classic N/A ± M/S; outside, the part of the base that lifts off is found by exact equilibrium over the contact polygon (biaxial moments included) |
| Overturning and sliding | Total weight (service load + footing + pedestal + backfill) about each edge; friction against the resultant horizontal force, no passive pressure |
| Two-way shear | Critical perimeter at d/2 from the pedestal (22.6.4), vc from Table 22.6.5.2 with λs = 1 (13.2.6.2), and the fraction γv of the pedestal moment transferred by eccentric shear (8.4.4.2) |
| One-way shear | At d from the pedestal face in each direction, with the actual (non-uniform) pressure; vc = 8·λs·λ·ρw^⅓·√f'c (Table 22.5.5.1(c)), λs = 1 |
| Flexure | Moment at the pedestal face in each direction from the net factored pressure; φMn with the bars provided and the required As |
| Reinforcement | As,min = 0.0018·b·h, maximum spacing, central band in rectangular footings (13.3.3.3) |
| Development | Straight development length (Table 25.4.2.3) against the distance from the pedestal face to the bar end |
| Bearing | Pedestal on footing (22.8.3) with the confinement factor √(A2/A1) ≤ 2 |
The plan and section sheet shows the bar mat with its call-outs, the pedestal, the backfill and grade, and exports to DXF like every other drawing.
Verification#
Wight, Reinforced Concrete Mechanics and Design, Example 15-2 (as solved by StructurePoint) — two-way shear stress and the three φvc equations, moment at the face, minimum steel, development length and bearing — and the ACI 318-19 one-way shear check with the ρw equation from Wight's ACI 318-19: What's New. The contact pressure solver is tested against the closed-form solutions inside and outside the kern and for exact equilibrium under biaxial uplift.
Not included yet#
Combined and strip footings, mats, pile caps, settlements, and hooked development. The soil bearing capacity comes from the geotechnical report.
Retaining walls#
New → Foundations offers a T wall, an L wall (no toe, for a property line in front), an inverted L (no heel, for a property line behind), a basement wall supported by the floor slab, and a gravity wall.
The wall opens in its section. Click any dimension or soil value in the drawing to edit it in place: stem and footing sizes, embedment, shear key, fill slope β, wall friction δ, water table, and γ, φ and c of the fill, of each stratum and of the foundation soil. Add strata with + in Operations; the last stratum continues downward.
What is checked:
- Earth pressure with Rankine, Coulomb, at rest (Jaky) or Mononobe-Okabe for seismic cases (kh, kv in the load table).
- Overturning, sliding (base friction, adhesion, reduced passive, shear key), soil pressure (allowable qa, Terzaghi or Meyerhof with B' = B − 2e) and the middle third.
- Basement walls: the stem is a vertical beam with the slab as a pin or a spring (k = E·t/L of the slab); a floor slab bearing against the footing takes the base shear.
- Concrete per ACI 318-19: stem, heel and toe flexure and shear, minimum steel, spacing and the stem hook in the footing.
Tools → Wall · diagrams and assistant shows the stem shear and moment, the quantities per meter, and proposes the cheapest wall that meets every case for where you can build and the wall types you allow.
Not checked: settlement and global (slope) stability, which belong to the geotechnical report, shear key reinforcement and counterfort walls.