Concrete Footing to AS3600

This design guide explains the principles behind the structural foundation design of reinforced spread footing and slab-on-grade as per AS3600-2018. The key difference between these two common footing types is that spread footings are embedded in soil whereas a slab-on-grade sits on top of the founding material. This guide explains the various checks that must be conducted to ensure that the foundation is adequate and stable to support nominated loads. 
ExplanationBoth embedded footings and slab-on-grade are structural elements which transfer load imposed by the superstructure to the soil underneath. They typically support columns, walls, or other vertical members. The geotechnical and structural engineer must work together to ensure the footing is adequate against bearing failure (geotechnical checks) and ultimate limit state (structural checks).
Bearing Pressure and Overturning MomentsThe engineer must ensure that the bearing pressure of the foundation onto the soil from applied loading does not exceed the soil bearing capacity. Distribution of the bearing pressure depends on the eccentricity of the loads - a concentric load results in even distribution, while an eccentric load leads to a greater pressure on one side than the other. Eccentric loads lead to overturning moments, which are significantly more dangerous than concentric loads as they cause rotation and differential settlement.
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Concentric vs. eccentric loads
For eccentric loading on a footing, the ratio between the resultant overturning and resisting moment is called the factor of safety (FoS). Different standards and codes recommended varying FoS values, generally greater than 1.5. 
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Factor of safety=Resultant resisting momentResultant overturning moment=ΣMRΣMO\text{Factor of safety}=\frac{\text{Resultant resisting moment}}{\text{Resultant overturning moment}} = \frac{\Sigma M_R}{\Sigma M_O}Factor of safety=Resultant overturning momentResultant resisting moment​=ΣMO​ΣMR​​
Bearing failure is the most common failure mode for footings. There are also other failure modes such as sliding and uplift but these are rare. Geotechnical investigations are conducted to ensure soil parameters (friction angle, cohesion, etc.) are adequate prior to construction and footings are almost always subject to compressive loads, hence no uplift.
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Bearing failure
Check out this CalcTree article to learn more about different foundation bearing failure modes and how to calculate soil bearing capacities.
👀More info about eccentric loading
Strength ChecksThere are three ULS strength checks to be conducted for design of footings:
One-way shear check (beam shear) - AS3600 Section 8.2
Two-way shear check (punching shear) - AS3600 Section 9.3
Flexural check (bending) - AS3600 Section 8.2
The section shear and flexural capacities, ﻿ϕVu​
 and ﻿ϕMu​
, calculated as per the referenced AS3600 sections, must be greater than the imposed loads i.e.
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V∗ < ϕVuM∗ < ϕMuV*\ <\ \phi{V_u}\\M*\ <\ \phi{M_u}V∗ < ϕVu​M∗ < ϕMu​
For a deep-dive into the section design procedures, check out CalcTree's Concrete Beam Design Calculator. Here we will explain how to calculate the design actions on footings.
One-Way Shear
As per AS3600 Cl. 8.2.3.2, the maximum transverse shear near the support is taken at a distance dₒ away from the face of the support. Footings typically don't have shear reinforcement (also known as links or ligs) and so the design shear should be resisted by the concrete itself.
The critical one-way shear V* can be calculated by:
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V∗ = q(x−do)where:x=L−Lc2  or  B−Bc2do=the distance from the extreme compressive fibre of a concrete member to the centroid of its tensile reinforcementV^*\ =\ q(x-d_o)\\\text{where:}\\x=L-\dfrac{L_c}{2}\text{\ \ or\ \ }B-\dfrac{B_c}{2}\\d_o=\text{the distance from the extreme compressive fibre }\\\text{of a concrete member\ to the centroid of its tensile reinforcement}V∗ = q(x−do​)where:x=L−2Lc​​  or  B−2Bc​​do​=the distance from the extreme compressive fibre of a concrete member to the centroid of its tensile reinforcement
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Critical shear plane in a footing
Two-Way (Punching Shear)
Punching shear failure is sudden and signs of failure cannot be observed (like flexural or shear cracks). Longitudinal reinforcement does not provide protection against punching shear, however it can be designed for by introducing shear reinforcement (also known as links or ligs) or local thickening of the concrete. Footings are typically designed thick enough as to avoid shear reinforcement.
AS3600-2018 deals with punching shear in the slabs section (section 9) of the code.
The punching shear design force can be calculated by:
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V∗ = q (Atotal − Ashear)where:Atotal=L × BAshear=(Lc+dom)(Bc+dom)=area bound by shear perimeterdom=mean value of do considering tensile reinf in both directionsV^*\ =\ q\ (A_{total}\ -\ A_{shear})\\\text{where:}\\A_{total}=L\ \times\ B\\A_{shear}=(L_c+d_{om})(B_c+d_{om})=\text{area\ bound\ by\ shear\ perimeter}\\d_{om}=\text{mean\ value\ of}\ d_o\text{\ considering\ tensile\ reinf\ in\ both\ directions}V∗ = q (Atotal​ − Ashear​)where:Atotal​=L × BAshear​=(Lc​+dom​)(Bc​+dom​)=area bound by shear perimeterdom​=mean value of do​ considering tensile reinf in both directions
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Punching shear perimeter in a footing
Flexure
The critical bending moment, M*, occurs at the column face. The calculators output the bending moment and capacity per meter strip of the footing in the direction considered:
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M∗ = qx22where:x=L−Lc2  or  B−Bc2M^*\ =\ \dfrac{qx^2}{2}\\\text{where:}\\x=L-\dfrac{L_c}{2}\text{\ \ or\ \ }B-\dfrac{B_c}{2}M∗ = 2qx2​where:x=L−2Lc​​  or  B−2Bc​​
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Critical section for bending in a footing
Other ConsiderationsOther than satisfying the limit state designs (strength checks) explained above, there are other factors which must be considered for the footing and slab-on-grade design:
Soil condition - weaker soil tend to induce larger differential settlement and rotation in structure, reducing stability. This may require deeper embedment for a footing, or the use of engineering fill for a slab, or even a complete change of foundation structure. Chemical properties of the soil can affect the design of the footing and slab, hence geotechnical assessment is essential prior to design.
Waterproofing - constant exposure to moisture causes hydrostatic pressure and continuous embedment in moist soil causes deterioration of the concrete surface. Typically, a waterproofing membrane is laid under the footing or slab to achieve watertightness. Ensure that there are sufficient drainage measures. 
Crack control and cover - minimum reinforcement must be met for serviceability design and to reduce cracking. Cover may need to be increased depending on how the footing or slab is cast (e.g. with formwork, against ground surface, pre-cast, etc.) and the soil classification. 
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Related ContentConcrete Slab-on-Grade Designer to AS3600
Rectangular Footing Designer to AS3600
Rectangular Spread Footing Design to ACI-384
Foundation Bearing Failure Modes and Capacities
Slab Thickness Calculator to ACI 360R-10
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