Structural footings (AS 3600)

The Footings workspace designs pad, combined, strip and pile-cap footings as model entities to AS 3600:2018. It also handles pedestal design, settlement checks, and manual footing creation from design actions without needing a frame. The schedule is the design surface: the engineer's real job is deciding how many footing types exist and where the boundaries fall, not sizing individual pads. Once solved, the workspace auto-designs every support, groups identical sizes into marks, and lets you iterate on type boundaries by adjusting the typing tolerance.

This page covers the structural footing types designed inside the model. For the standalone pad footing calculator (a single-pad check outside the model), see Design calculators. For the AS 2870 residential footing wizard (stiffened raft, waffle raft, residential strip), see Residential footings (AS 2870).

Design code selector

The Footings workspace supports three design codes via the Design code selector in the assumptions panel:

Code	What it covers	Key differences from AS
AS 3600	All checks, all footing types (default)	Stress block α₂ = 0.85, γ = 0.85; φ 0.85 flexure / 0.7 shear / 0.7 punching; k_v one-way shear; f_cv punching; min steel Cl 8.1.6.1; dev length Cl 13.1.2.2
ACI 318	All checks except deep-cap strut-and-tie	β₁ stepping (0.85 for f'c ≤ 28 MPa, −0.05 per 7 MPa); φ 0.90 flexure / 0.75 shear; V_c = 0.17λ√f'c one-way; punching least-of-three at d/2; min steel 0.0018 A_g; dev length 25.4.2.3 (2.1 divisor for #6 and smaller, 1.7 for #7+)
Eurocode 2	All checks except deep-cap strut-and-tie	γ_c = 1.5 / γ_s = 1.15 (no global φ); η = 1.0 / λ = 0.8 stress block; v_Rd,c depends on ρ_l; rounded punching perimeter (EN 6.4.2); min steel 0.26 f_ctm / f_yk; bearing f_cd √(A₂/A₁) cap 3.0

Selecting a code triggers a full re-design of every footing against that code's checks. The report names the actual design code (not a hardcoded AS 3600 reference). Bar callouts automatically switch between AU N-bars, US #-bars (ASTM Grade 60) and EU H-bars (B500) depending on the selected code.

Code currently applies to PAD, COMBINED, STRIP and shallow PILE-CAP

Two sub-models stay AS 3600 regardless of the selected code:

• Deep-cap strut-and-tie (AS 3600 Section 7) - ACI Appendix A / EC 6.5 are separate engines.
• Strip longitudinal grade-beam - the Winkler beam-on-elastic-foundation span stays AS 3600.

These are documented residuals, not silent mismatches. The report flags them.

The four types

The workspace recognises four footing types, each designed for a different situation:

Type	When it is used	Engine
Pad	One column, one footing. The default for a single support.	pad_footing.rs
Combined	Two columns whose pads would clash on a single axis. The workspace detects the clash and designs one rectangular pad carrying both columns.	combined_footing.rs
Strip	Three or more columns in a line, or a wall. A grade beam on an elastic foundation, spanning between its supports.	strip_footing.rs
Pile cap	A column supported by piles. The cap transfers load to the pile group via flexure, shear and strut-and-tie action.	pilecap.rs

Pad footings

A pad footing is the simplest case: a rectangular reinforced concrete pad under a single column. The engine sizes the pad from the service load against the allowable bearing pressure, then checks bending, one-way shear, punching shear, development length, bearing stress at the column face, and (minimally) overturning and sliding stability. If any check fails, the footing auto-grows until it passes or hits a size cap.

Combined footings

A combined footing is an inverted beam spanning between two columns. Soil pressure loads the pad upward; the columns are the supports. The engine computes exact beam statics by direct integration of the factored pressure line with the column loads, so the moment and shear diagrams are not approximated.

The workspace designs a combined footing when two pads clash on a single axis (the two columns are aligned along one direction and close enough that their pads overlap). Diagonal pairs or clusters that are really a raft are detected and refused, not approximated. The combined footing engine requires exactly two columns; three or more is a raft and is rejected with a clear message.

A combined footing gets top steel between the columns (hogging) and bottom steel at the outboard cantilevers and midspan when the resultant sits outside the columns. Both faces are designed independently.

Strip footings

A strip footing is a grade beam on an elastic foundation. The transverse direction (across the strip width) is designed for the peak column load as a cantilever from the wall face, exactly like a pad footing per metre. The longitudinal direction (along the strip) is designed once a soil stiffness (ks) is supplied: the strip spans between its columns as a beam on elastic supports, and the engine produces top and bottom steel for the longitudinal moment.

Without a soil stiffness value, the longitudinal direction is refused rather than guessed. The transverse direction is always designed. A strip footing shows a pressure profile across its width in the detail drawing.

Clash detection and shared footings

When you click Design all footings, the workspace first designs one pad per support, then checks whether any pads overlap. Overlapping pads are grouped:

• Two pads on a single axis become a combined footing (if the combined engine is available).
• Three or more on a single axis become a strip footing.
• Diagonal pairs, clusters, or groups held apart by a manual override are flagged as unresolved clashes and refused, not approximated.

A clash is not a failed check. It is a design that cannot be built as drawn: two slabs occupying the same ground, both reporting OK individually. The honest answer is one shared footing, which needs its own engine run. Until that engine produces a result, the clash is reported, not designed around.

Loss of contact

The bearing pressure check uses the statics core in footing_core.rs, which implements the kern test and partial-contact redistribution. The key behaviours:

• Full contact: the resultant is inside the kern (6e/bx + 6e/by <= 1). The elastic trapezoid gives the corner pressures.
• Uniaxial partial contact: the resultant is outside the kern about one axis. The base lifts and the pressure redistributes over a triangular block on the reduced contact length. The minimum pressure is clamped to exactly zero (soil carries no tension, never a fictitious negative corner pressure).
• Biaxial partial contact (outside the kern about both axes): detected and refused. No utilisation is reported. The elastic result would understate the peak pressure because it spreads load over lifted soil, so no honest number exists. The engineer must reduce one eccentricity, enlarge the footing, or check the case by hand.
• Overturning: the resultant is outside the base entirely. The check is refused with a clear message.

A refused check is never a silent zero. The footing's status is incomplete or error, and the refused check is named.

Service vs factored split

The workspace enforces a distinction between bearing (service) and strength (factored) checks:

• Bearing is a working-stress check against the geotechnical allowable pressure. It runs on the unfactored service loads. When no service combination is available, the engine falls back to the factored load and says so in its note, rather than silently treating a factored load as service.
• Bending, shear, punching, development are strength checks. They run on the factored (ultimate limit state) loads.
• A missing combination is reported as null, not substituted. If a footing has no service load, the bearing utilisation is not invented.

The split is enforced at the design level: designFooting in packages/footings/design.js passes separate service and ultimate payloads to the engine, and the engine uses the correct load level for each check.

Sizing outputs

The auto-design produces several sizing outputs:

• Required plan area: the concentric area needed to keep bearing pressure within the allowable, from footing_core.rs::required_area.
• Required plan size: required_size iterates the real bearing pressure solution (including eccentricity) to find the smallest footing of a given aspect ratio that satisfies the allowable.
• Required depth: when a shear check fails, the engine reports the thickness that would pass (thickness_required). The workspace can then grow the footing to that depth rather than arbitrarily increasing the plan.

The footing auto-grows when a check fails: shear or punching drives depth growth, while bearing, development length, overturning and sliding drive plan growth. Growth that still fails at the cap is reported honestly as OVER, and the design panel shows where the auto-design started so growth is never silent.

Punching shear

Punching shear is checked to AS 3600:2018 Cl 9.3, with the critical perimeter at d/2 from the column face (Cl 9.3.1.3). The engine is aware of column position:

Position	Perimeter treatment
Interior	Full rectangular perimeter: 2(bx + d) + 2(by + d)
Edge	Perimeter shortened by the column's proximity to the pad edge
Corner	Perimeter truncated on two sides

Moment transfer reduces the capacity per Cl 9.3.4 Eq 9.3.4(1) when the column transfers a moment to the footing.

Punching is verified:false

AS 3600 has no clause governing punching shear in a reinforced pad footing. Cl 9.3 is scoped to flat slabs. The Cl 9.3.3/9.3.4 formulas are applied by analogy, with biaxial moment taking the governing axis. This has not been independently verified against a known-good reference implementation. Treat punching results as indicative, not certified.

For a combined footing, overlapping punching perimeters between the two columns are detected and refused rather than double-counted. The shared concrete would be counted once per column on capacity and the shared relief zone once per column on demand, which is unconservative.

Reinforcement

The engine reports required steel areas (mm2/m) in each direction, and selectBars in packages/core/rebar.js converts them to buildable layouts ("N bars @ spacing"). The selection prefers the smallest bar that can be spaced at or under the maximum permitted spacing (Cl 9.5.1(b)), because closer-spaced smaller bars give better crack control.

When no standard layout satisfies the required area, the engine refuses rather than emitting a bare mm2/m figure. A refused check is never a silent zero: the footing's status is incomplete and the panel says why.

An engineer can override the auto-selected bars by forcing a specific bar size and spacing. The engine then checks that bar and spacing against the loads, rather than deriving its own. One shared bar and spacing both directions ("N16 @ 200 EW") matches the conventional pad footing callout on a real drawing.

Dowels

Dowels are drawn only when the engine actually requires them (net uplift across the column-footing interface). AS 3600 has no blanket dowel-area provision (the 0.005Ag rule is ACI 318, not AS), so no dowels are invented when the interface stays in compression.

Detail drawings

The Detail tab in the design panel shows a section drawing for the selected footing type:

• Pad: a cross-section showing the bottom reinforcement in both directions, the column stub, the founding depth, and (when required) dowels.
• Combined: an elevation along the long axis showing the column positions, bottom steel at the outboard cantilevers and top steel between the columns (hogging).
• Strip: a transverse section at the strip width, with the transverse bars as the main steel and the longitudinal bars as the code minimum (or the designed value when ks is supplied).

All diagrams are click-to-zoom: clicking a diagram re-renders it large in a lightbox.

The schedule

The schedule in the results drawer lists footing marks, not individual supports. A building with 18 columns might have four or five footing types, and the schedule shows each type once with its count, size, thickness, governing check, utilisation, and the supports it covers.

The schedule appears in the PDF report as a dedicated section. It names the footing type (pad, combined or strip) and counts the supports each footing carries, so the reader knows exactly what is being built and where.

Rows are sorted worst-first: a footing needing attention appears at the top, not buried under passing rows. A footing with no honest utilisation sorts to the top because it needs a decision more than a passing one does.

The design panel

The design panel (the Inspector) has three tabs:

• Checks (default): demand/capacity pairs with units and clause references, never a bare utilisation. A number like "141 / 150 kPa" can be cross-read against the pressure diagram; "0.94" cannot. Where the engine reports no capacity pair (development length, bearing stress), the demand value is shown with the utilisation instead of inventing a capacity.
• Pressure: the bearing pressure diagram for the selected support, drawn from the engine's contact state. For a strip, this is the transverse pressure profile. For a combined footing, the engine reports the peak pressure but not the along-axis distribution, so no profile is drawn and the panel says why.
• Detail: the section drawing and bar callouts.

The support chips at the top of the panel let you switch between supports within the same type. The governing support (the one the type was designed for) is highlighted.

Pile cap design

A pile cap is a thick reinforced concrete pad transferring column loads to a group of piles. The engine designs pile caps for 1 to 9 piles in regular rectangular grids, checking:

• Flexure (AS 3600 Cl 8.1): critical section at the column face for each direction.
• One-way shear (AS 3600 Cl 8.2): beam shear across the full width, checked in both directions.
• Punching shear (AS 3600 Cl 9.3): perimeter at d/2 from the column face (interior, edge or corner), with edge/corner truncation where the perimeter extends beyond the cap.
• Deep-cap strut-and-tie (AS 3600 Section 7): for caps where the pile-to-column distance is less than 2d, the engine checks the strut capacity from column to pile. Pile bearing area is π/4·d² for circular piles. The a_v distance is measured to the pile face, not the pile centre.

Pile reactions are distributed linearly from the column axial + biaxial moment across the group. The worst-case pile (compression or uplift) drives the design.

The pile cap calculator (Tools → Pile cap calculator) is a standalone version for quick checks outside the model. Within the model, pile caps are designed as footing types when three or more supports are detected as a pile group.

Settlement

Settlement is computed from the footing geometry and soil parameters using standard soil mechanics:

• Immediate (elastic) settlement: Steinbrenner/Schleicher influence factors at the footing centre (flexible), verified against the 1.122 square-foot analytical solution. Fox embedment factor I_F is clamped to ≤ 1.0.
• Consolidation settlement: Terzaghi 1D primary consolidation with the 2:1 stress spread. Handles normally consolidated (Cc), overconsolidated-recompression (Cr), and overconsolidated-crossing (Cr then Cc) states. Missing soil parameters return NotComputed rather than fabricating a zero.

The settlement calculator (Tools → Settlement calculator) is the primary interface for shallow-foundation settlement checks. Within the model, settlement results are attached to footing supports when soil parameters are available.

Pedestal design

A pedestal is a short reinforced concrete column between a steel column base plate and the footing top. The engine reuses the AS 3600 Clause 10 column M-N interaction engine and adds pedestal-specific checks:

• M-N interaction (Cl 10): full biaxial interaction surface.
• Short-column classification (Cl 10.3.1): slenderness ratio L_e/r vs the limit; slender pedestals (needing the Cl 10.4 moment magnifier) are refused.
• Steel ratio limits (Cl 10.7.1): minimum and maximum longitudinal steel.
• Fitments (Cl 10.7.4.3): minimum tie diameter and maximum spacing.
• Interface bearing (Cl 12.6): bearing stress at the pedestal-base plate interface.

The pedestal calculator (Tools → Pedestal calculator) is a standalone version for quick checks outside the model.

Manual footing design

The manual footing designer (File → Footing from typed loads) creates a footing from scratch without needing a frame or a solve. You enter the column size and the service/factored design actions, and the engine sizes the footing, runs the AS 3600 checks, and adds it as a model entity.

A manual footing flows into the schedule, the reinforcement drawing, the PDF report and the Winkler FE soil-pressure analysis identically to a model-integrated footing. The source is tagged 'manual-load' so it survives Re-design all. Soil bearing pressure and material grades are taken from the Footings tab assumptions panel.

What is not covered yet

• Biaxial partial contact is detected and refused, not iteratively solved. The engineer must reduce one eccentricity, enlarge the footing, or check by hand.
• Shear reinforcement in footings is not designed. The engine checks one-way shear against the unreinforced concrete capacity only.
• Punching shear is applied by analogy (Cl 9.3 from flat slabs) and has not been independently verified. Treat results as indicative.
• Non-AS standards (ACI 318, EN 1992) are supported for pad, combined, strip and shallow pile-cap design via the Design code selector. Deep-cap strut-and-tie stays AS 3600 (ACI Appendix A / EC 6.5 are future scope). Strip longitudinal grade-beam stays AS 3600. ACI/EC model-wide load-combination sets are not yet implemented (load factors come from the model's combos).
• Three or more columns on a combined footing is a raft and is refused. No raft design engine exists yet.
• The bar schedule gives lengths and counts, not bending shapes or laps. It is not yet a bar-bending schedule.
• Strap footings and eccentrically loaded footings beyond biaxial moment are not designed.
• Multi-column mat/raft FE (SAFE-style) is not implemented. The Winkler FE solver handles single-pad soil-structure interaction only.