This calculator allows the user to analyse a rigid rectangular spread footing with piers for uniaxial or biaxial resultant eccentricities. The following checks are performed: overturning, sliding, uplift and soil bearing at the four corners of the footing.
❗This calculation has been written in accordance with ACI (American Concrete Institute) guidance.
Rectangular Spread Footing Design Elevation & Plan
Calculation
Inputs
Refer to image above for the nomenclature and sign convention used in this calculator.
Footing Properties
γc, kcf
:0.15
L
:8ft
B
:5ft
T
:2ft
Soil Properties
D
:2ft
γs kcf
:0.12
Kp
:3
μ
:4
Factor of Safety
Use the same factor of safety for all checks?
:No
Factor of safety
:1.5
FS (overturning)
:1.5
FS (sliding)
:1.5
FS (uplift)
:1.5
FS (bearing)
:1.50
Allowable bearing pressure, kips-sf
:150
Loads
Output
Summary of Applied Forces
Overturning Check
Resisting moments:
ΣMrx, ft-kips
:291.84ft kips
ΣMry, ft-kips
:291.84ft kips
Overturning moments:
ΣMox, ft-kips
:412.25ft kips
ΣMoy, ft-kips
:-410.25ft kips
Factors of safety:
FSx (overturning)
:0.71
FSx (overturning) Safe/Fail?
:Fail
FSy (overturning)
:0.71
FSy (overturning) Safe/Fail?
:Fail
Sliding Check
Resisting forces:
Ffx
:0.0kips
Ffy
:0.0kips
Pushing forces:
Fpx
:46.08kips
Fpy
:46.08kips
Factors of safety:
FSx (sliding)
:4.61
FSx (sliding) Safe/Fail?
:Safe
FSy (sliding)
:4.61
FSy (sliding) Safe/Fail?
:Safe
Uplift Check
Total downward load, ΣPz
:-76.71kip
Total upward load, ΣPz
:100.0kip
Factor of safety:
FS (uplift) (1)
:0.7671
FS (uplift) Safe/Fail?
:Fail
Bearing Check
Pressures at four corners:
P1 (kips-sf)
:0.00
P2 (kips-sf)
:0.00
P3 (kips-sf)
:0.00
P4 (kips-sf)
:0.32
Factor of safety:
Mobilised FS (bearing)
:0.0021067941878887
FS (bearing) Safe/Fail?
:Fail
Bearing Distribution Geometry
Biaxial eccentricity case
:Case 1
Different cases of biaxial bearing pressure
Dist. x =
:12.6947885654948ft
Dist. y =
:11.7721043972002ft
Brg. Lx =
:4.06775765434247ft
Brg. Ly =
:4.35356137131792ft
%Brg. Area
:88.7979059480117sqft
Explanation
Rectangular footings, also known as spread footings, are structural elements which transfer load imposed by the superstructure to the soil underneath. Spread footings are typically wider than they are deep and are used to support columns, walls, or other vertical members.
The engineer must ensure that the bearing pressure does not exceed the soil bearing capacity. Distribution of the bearing pressure depends on the eccentricity of the loads - a concentric load result 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.
Concentric load
Eccentric load
There are two types of eccentricities: uniaxial and biaxial. When the imposed loads on a footing produces a moment along only one axis (say, the x- or y-axis), it is said to have a uniaxial eccentricity. Loads which produce moments in both directions (x- and y-axis) have biaxial eccentricity.
Uniaxial eccentricity
Biaxial eccentricity
For a footing subject to only one load, the resultant moment due to eccentricity is fairly easy to calculate by resolving it to the footing centroid:
Moment about the x-axis: Mx=P×eyMoment about the y-axis: My=P×ex
When there are multiple loads on a footing (e.g. a strip or mat footing with multiple columns), the same approach is taken:
Resultant moment about the x-axis: Mx=Σ(P×ey)Resultant moment about the y-axis: My=Σ(P×ex)
Loads which are eccentric in both x- and y-axis induce biaxial bending. Depending on the location of these, induced moments may be acting in opposing directions. If the moment in one direction is greater than the other, it leads to an uneven bearing pressure distribution. In such cases, these moments are said to be either overturning or resisting - the convention is up to the designer.
Example of a footing subject to biaxial bending from multiple loads
In the example above, P1 and P2 are acting against each other but since P2 has a greater eccentricity, the bearing distribution will be greater under it than under P1. 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.
Factor of safety=Resultant overturning momentResultant resisting moment=ΣMOΣMR
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.
Even more explanation
Time to get math-y.
Let's consider a footing with the following load configuration:
Based on the location of the loads, we notice the following:
P1 and P2 acts against P3 and P4 about the x-axis
P1 and P4 acts against P2 and P3 about the y-axis
This can be verified by calculating the moment due to each load relative to footing centroid:
If we assume positive moment to be overturning and negative to be resisting, then:
ΣMx=Mx1+Mx2+Mx3+Mx4ΣMy=My1+My2+My3+My4Resultantoverturningmoment about x-axis: ΣMox=Mx1+Mx2Resultantoverturningmoment about y-axis: ΣMoy=My1+My3Resultantresistingmoment about x-axis: ΣMrx=Mx3+Mx4Resultantresistingmoment about y-axis: ΣMry=My2+My4
Now, we can also find the resultant eccentricity by summing the vertical loads and rearranging equations:
ΣP=P1+P2+P3+P4Resultant eccentricity from the x-axis: ex=ΣPΣMxResultant eccentricity from the y-axis: ey=ΣPΣMy
Why does this matter in practical design? A footing is most stable when it is in full compression i.e. the entire soffit of the footing is in bearing against the soil. Studies have shown that a footing is deemed to be in full compression if the resultant eccentricity is within an area within a sixth of its width/length from the centre. When the eccentric load is within the highlighted zone, the maximum and minimum bearing pressure can be calculated as:
If a footing is in partial compression or there is loss of contact with soil, there is a higher risk of failure due to overturning and the development of differential settlement. This occurs when the resultant eccentric load does not lie in the highlighted zone, leading to an asymmetric bearing pressure distribution: