Residential footings (AS 2870)

The Residential footing (AS 2870) wizard (Tools ▸ Residential footing (AS 2870), or Cmd/Ctrl-K → "footing" / "residential") is a deemed-to-comply footing selector: it turns a site class, a wall construction type and a slab length into a buildable footing design (beam/footing sizes, reinforcement and slab mesh) straight from AS 2870:2011 Section 3. It supports three footing systems: stiffened raft (Figure 3.1), waffle raft (Figure 3.4) and strip footing (Figure 3.6).

This is a design aid, not an analysis of a structural model: it does not need a frame, a mesh or a solve. You give it the site and the wall type, and it hands back the footing you can put on a drawing.

Verification build - deemed-to-comply, not a rational analysis

This engine is marked verified: false. The Figure 3.1 / 3.4 / 3.6 table values (beam and footing sizes, reinforcement, slab mesh) have been transcribed from AS 2870:2011 by hand and every result is flagged pending an engineer's cross-check against the standard. It is a deemed-to-comply selector: it looks up a standard table row, it does not analyse a beam-on-mound or design to first principles. Confirm every result against the standard and your geotechnical report before use.

What it computes

Section 2 - site classification (Table 2.1). Given the characteristic surface movement ys (mm) and the design depth of suction change Hs (m), the engine assigns a reactivity class: A (sand/rock, ~0 mm), S (≤20 mm), M (20-40 mm), H1 (40-60 mm), H2 (60-75 mm) or E (>75 mm). When Hs ≥ 3.0 m (deep/dry climates), M/H1/H2/E are promoted to their -D (deep) variants (M-D, H1-D, H2-D, E-D). Class P (problem sites - soft, unstable, fill, landslip) is never auto-assigned; it always needs a specific engineering assessment.

ys itself can be computed from a soil profile per Appendix D:

ys = Σ (Ipt · Δu · h) / 100

summed over layers, where Ipt is the instability index (%/pF), Δu is the suction change (pF, decreasing linearly from a surface value to zero at Hs) and h is the layer thickness (mm).

Section 3 - deemed-to-comply footing. For a site class and a wall construction type, the engine looks up the footing from the standard's table for the chosen system. The five wall construction types are clad frame, articulated masonry veneer, masonry veneer, articulated full masonry and full masonry.

• Stiffened raft (Figure 3.1) - edge/internal beam depth D, bottom reinforcement (trench mesh with a bar alternative), top-beam reinforcement where required, and the maximum beam spacing. Slab mesh (SL72/SL82/SL92) from the slab length.
• Waffle raft (Figure 3.4) - rib (beam) depth D, edge-beam reinforcement (mesh with a bar alternative) and internal-beam reinforcement, over formed voids. The slab mesh here has its own per-row length bands (< 20 m and ≥ 20 and < 30 m). Full masonry is never permitted on a waffle raft.
• Strip footing (Figure 3.6) - footing depth D and width B, reinforcement, the minimum stump/pier depth Ds and spacing Ls for a suspended floor, and the infill-floor slab mesh (SL62/SL72/SL82). Figure 3.6 covers Classes A, S, M, M-D and H1 only.

Using it

1. Open Tools ▸ Residential footing (AS 2870).
2. Choose the footing system - stiffened raft, waffle raft or strip footing.
3. Set the site class from your geotechnical report (A, S, M, M-D, H1, H1-D, H2 or H2-D).
4. Choose the wall construction (clad frame, articulated masonry veneer, masonry veneer, articulated full masonry or full masonry).
5. Enter the max slab length (m) - this drives the slab mesh selection independently of the beam design.
6. Read the result panel: site class and description, then the footing for the chosen system (beam/footing sizes, reinforcement, and slab mesh). Copy exports the inputs and the selected footing design as tab-separated text.
7. If the combination isn't in the deemed-to-comply table (see below), the panel shows the reason instead of a design.

Scope and limitations

In scope: deemed-to-comply stiffened raft (Figure 3.1) and waffle raft (Figure 3.4) on site classes A, S, M, M-D, H1, H1-D, H2 and H2-D, and strip footings (Figure 3.6) on Classes A, S, M, M-D and H1, for the five wall construction types above, on slab lengths up to 30 m.

Not yet handled (shown in the wizard's warnings):

• Footing slabs (Figure 3.2 / 3.3, Class A) and the stiffened slab with deep edge beam (Figure 3.5) - not yet tabulated.
• Class E / E-D and Class P - the wizard returns an error for these; they sit outside the deemed-to-comply system and need a rational design under Section 4.
• Strip footings on Classes H1-D, H2 and H2-D - Figure 3.6 does not include them; use a raft or a rational design.
• Slab length > 30 m - outside the deemed-to-comply limits; also errors out.
• Some class/construction combinations are simply not permitted by the standard (e.g. Class H2 with full masonry on a stiffened raft, or full masonry on any waffle raft) and return an error rather than a design.
• Rational beam-on-mound analysis (Section 4) - no structural analysis of the raft itself, only table lookups.
• Piers, piles and Class P footings, articulation-joint design, and other Cl 3.1.1 exclusions (permanent slab joints, more than two storeys, concrete ≥ 32 MPa).
• The footing wizard takes the site class directly. To derive the class from a borehole profile instead, use Tools ▸ Site classification (AS 2870): enter each layer's thickness and instability index Ipt plus Hs and du, and it computes ys (Appendix D) and the class, then can push that class into the footing wizard. The Appendix D model there is simplified (verified: false).

Verification

The Table 2.1 class thresholds and the Figure 3.1 / 3.4 / 3.6 tables are transcribed directly from AS 2870:2011 and locked with unit tests; the module stays verified: false pending independent worked-example checks and an engineer sign-off, the same discipline as the wind engine. See Design checks for the same verification discipline applied to member design.