This article walks through a clear, practical example of how to calculate a shallow footing for a typical column load. It focuses on the key numbers you need: loads, soil capacity, required footing area, and basic reinforcement considerations.
Each step is short and uses plain language so you can follow the math and adapt values to your own project. Numbers are realistic and explained so the same approach works with different inputs.
Basic principles and initial checks
Foundations transfer building loads to the ground safely. The main checks are bearing capacity, settlement, and structural strength of the footing itself. Start with accurate loads and reliable soil data.
Typical inputs include column axial load, any eccentricity, soil allowable bearing pressure, and concrete or steel properties. With those, you estimate footing area, dimensions, and reinforcement needs.
Key terms to know
Allowable bearing pressure: the safe pressure the soil can take without excessive settlement.
Net allowable: bearing pressure after applying factors and any groundwater effects.
Service loads: loads that the foundation must carry during normal use, often factored separately for ultimate design checks.
Step-by-step example calculation
We will size a square isolated footing supporting a column with a factored axial load. Values chosen are common for small buildings and can be adjusted.
Assumed data (example): Column axial load = 500 kN (service), Soil allowable bearing pressure = 150 kN/m2, Concrete = M25 (25 MPa), Steel = Fe500.
1. Convert loads and select design load
Use service loads for bearing checks unless a factor is specified. For structural checks use factored loads per local codes. Here we take the service axial load 500 kN for bearing.
- Service column load = 500 kN
- Soil allowable bearing = 150 kN/m2
2. Calculate required footing area
Required area = Column load / Allowable soil pressure.
Area = 500 kN / 150 kN/m2 = 3.333… m2.
For a square footing, side length = sqrt(area) = sqrt(3.333) = 1.826 m. Round up to practical size.
- Choose footing size = 1.9 m x 1.9 m (area = 3.61 m2)
3. Check footing thickness for bending and shear
Estimate effective depth based on span and loads. For a square footing, bending moment can be approximated at midspan under the column face.
Assume footing projects a/2 in each direction where a is column width. If column is 300 mm x 300 mm, projection = (1.9 – 0.3)/2 = 0.8 m each side.
Approximate factored moment for ultimate load (simplified approach): M = Pu * (e) where Pu is ultimate axial load and e is eccentricity. For this example, keep e small and do a bending check based on cantilever action: M ≈ q * l^2 / 2. Use conservative checks and local code formulas for final design.
As a practical initial depth, use depth d ≈ 0.1 * footing side = 0.19 m. Increase based on shear and bending checks.
4. Shear checks
Check one-way shear at critical section located d from the column face. Compute factored shear and compare with concrete shear capacity.
If shear is close to or exceeds capacity, increase depth or provide shear reinforcement. In small footings with modest loads, a depth of 250–350 mm is common.
5. Settlement estimate
Settlement depends on soil type. As a quick check, a cohesionless soil with good bearing pressure often gives settlement within acceptable limits for shallow footings.
If soil is compressible, reduce allowable bearing pressure or use deeper foundation options. A geotechnical report gives the best input for settlement estimates.
Load combinations and safety factors
Always apply the safety factors required by your local code for structural checks. For bearing checks, many codes allow using unfactored loads against allowable soil pressure, but check local practice.
For structural strength, apply load factors (for example 1.5 for dead+live) to obtain ultimate loads used in bending and shear design.
Load breakdown example
For a quick converted ultimate load: assume dead load + live load factor 1.5. If service load is 500 kN, ultimate Pu = 1.5 * 500 = 750 kN. Use Pu for bending and shear checks.
Compute design pressure under ultimate load: qd = Pu / footing area = 750 / 3.61 ≈ 208 kN/m2. This should be compared to allowable or factored soil strength as per code.
Eccentric loads and eccentricity effects
If the column load is eccentric, effective bearing pressure distribution becomes triangular or trapezoidal. Check that the resultant falls within the middle third to avoid uplift at one edge.
If eccentricity e leads to net uplift, adjust footing size or use tie beams to redistribute loads.
Reinforcement and detailing notes
Reinforcement resists bending and controls cracking. For the example footing, main reinforcement is placed in both directions near the bottom face, with distribution steel near the top.
Typical practical layout uses two layers of bars in larger footings or a single layer if depth is small and bending demand is low.
Minimum reinforcement
Most standards require a minimum percentage of steel to control shrinkage and temperature cracking. For concretes like M25, this often works out to around 0.15% of cross-section in each direction as a starting point.
- If footing thickness = 300 mm, effective depth d = 250 mm (allowing 50 mm cover).
- Required As,min ≈ 0.0015 * b * d. For b = 1900 mm: As,min ≈ 0.0015*1900*250 = 712.5 mm2.
Provide practical bar sizes that meet or exceed this area, such as 4 bars of 12 mm diameter in each direction (each 12 mm bar area ≈ 113 mm2, 4 bars = 452 mm2 — might be under, so use 6 bars or larger). Adjust to meet calculated needs.
Bar spacing and cover
Maintain minimum clear cover, typically 50 mm for footings cast against the ground. Ensure bars are spaced to allow proper concrete placement and compaction.
Anchor bars into the column or provide dowels to ensure load transfer between column and footing.
Common checks and practical tips
Simple calculations are a good start, but always cross-check with code formulas and practical constraints like excavation depth, groundwater, and constructability.
Small changes in soil pressure or load can change footing size significantly. Use conservative assumptions when soil data is limited.
When to increase footing size
If calculated pressure exceeds allowable, increase area. If punching shear or bending demand cannot be met without excessive depth, increase plan size instead of depth where possible.
Increasing side length by 10–15% often reduces bending and shear demands noticeably without large extra cost.
Documentation and drawing notes
Include a clear plan showing footing dimensions, reinforcement layout, concrete grade, bar marks, and cover. Note soil bearing value used and any allowances for water table or adjacent loads.
Supply a simple calculation sheet with inputs, formulas, and final checks so others can review or adapt the work easily.
Conclusion
A straightforward calculation path helps size a shallow footing: start with loads, divide by soil allowable, pick a practical plan size, then check bending, shear, and settlement.
This example gives a clear workflow that can be adjusted to different loads and soil values. Always validate with local codes and site soil information before finalizing design.
Frequently Asked Questions
How do I pick a safe soil bearing pressure?
Use values from a geotechnical report when available. If not, conservative tabulated values for soil types can be used with caution. Reduce values when groundwater or fill is present.
What margin should I use between calculated and allowable pressure?
Design practice often uses a safety margin inherent in allowable values. If you have uncertainties in loads or soil, reduce the allowable pressure or increase footing size to be safe.
Is it better to increase footing area or depth?
Increasing area reduces bearing pressure and bending demand, often the preferred first measure. Increasing depth is effective for shear and bending but raises excavation and concrete costs.
When is a shallow footing not suitable?
If soil is very weak or highly compressible, or if loads are very high, shallow footings may produce excessive settlement. In such cases, consider deeper foundations like piles or rafts based on geotechnical advice.
Can the same approach be used for rectangular footings?
Yes. Use the required area to set plan dimensions. For rectangular footings, consider different bending spans in the two directions and check both independently.