A solid footing keeps a building steady by spreading loads to soil below. This article breaks down how to size and check shallow footings in plain language, with numbers and common pitfalls to watch for.
The focus is on simple, practical methods you can follow on paper or in basic spreadsheets. No heavy theory—just clear steps to get reliable results.
Key concepts to know before sizing
Footings transfer vertical loads from columns and walls into the ground. The main checks are bearing capacity, settlement limits, bending, shear and reinforcement needs.
Keep these terms in mind: safe bearing capacity, service loads, eccentricity, effective depth, and development length. Understanding them makes calculations faster and safer.
Essential inputs and soil checks
Good input data leads to reliable outcomes. Start with accurate loads, soil properties and clear geometry. Small errors in inputs often cause the biggest design mistakes.
Collect these items early: column loads (dead + live), soil allowable bearing pressure, depth to bearing stratum, groundwater level, and concrete/steel grades.
Loads and combinations
List the factored or service loads you will use. For simple checks use service loads first, then apply factors if strength checks are needed.
- Dead load (self-weight of structure)
- Live load (occupancy)
- Any point loads from machinery or concentrated items
- Load eccentricity if column is not centered
Soil strength and settlement
Obtain allowable bearing capacity from a site report if possible. If not available, conservative values based on soil type can be used with caution.
- Sandy soils: higher bearing than soft clays
- Loose fills: treat as weak unless compacted
- High groundwater: reduce allowable capacity
Step-by-step sizing and checks
Work through a logical sequence: preliminary size, bearing check, bending and shear checks, then reinforcement. Each step has straightforward calculations.
Below is a compact workflow you can follow with a simple example.
1. Preliminary footing size
Start by estimating the footing area from service load and allowable soil pressure:
- Required area = Total vertical load / Allowable bearing pressure
Example: A column carries 300 kN. Allowable soil pressure = 150 kN/m2. Area = 300 / 150 = 2.0 m2. For a square footing side = sqrt(2.0) ≈ 1.414 m. Round to a practical size, say 1.45 m.
2. Check bearing and eccentricity
Place column centrally if possible. If there is eccentricity, check pressure distribution. For eccentric loads the pressure becomes triangular; ensure maximum edge pressure ≤ allowable.
- For eccentricity e: maximum pressure p(max) = (P/A) (1 + 6e/B) for a rectangular strip (approx)
- Keep e less than B/6 to avoid uplift at one edge
Example: If eccentricity is 0.05 m and footing width B = 1.45 m, then check that e < B/6 ≈ 0.242 m. No uplift expected.
3. Bending moment and depth
Calculate bending moment at critical sections. For column footings, treat pressure as uniformly distributed to find moment at column face.
- For a central column on a square footing, worst moment at mid-span between column and edge: M = q·(L1)^2/2 where q = soil pressure and L1 = distance from column center to face.
Example: Using q = 150 kN/m2 and half-span L1 = 0.725 m, M ≈ 150 × 0.725^2 / 2 ≈ 39.4 kN·m. Add moments from column eccentricity if present.
Estimate effective depth d using M = 0.87 f_y A_s z (or using simplified rectangular stress block). A quick check uses d ≈ sqrt( M / (0.125 f_ck b) ) as a conservative start where f_ck is concrete strength and b is footing width.
4. Shear checks
Check one-way shear around the column face and punching shear under the column. For shallow footings, punching shear is often the governing check.
- One-way shear: V = shear force at section; compare V with v_c × b × d
- Punching shear: evaluate v_ed at perimeter 2d from loaded area; ensure v_ed < v_rd,c unless shear reinforcement is provided
Use conservative v_c values if exact soil-composite action is uncertain. Increase depth rather than rely on shear reinforcement in many cases.
5. Reinforcement layout
Provide top and bottom steel as needed to resist bending and to control cracking. Use minimum reinforcement to control shrinkage and temperature cracking.
- Compute required As from bending moment: As = M / (0.87 f_y z)
- Check spacing and bar sizes against minimums in codes
- Provide extra steel near columns if moment concentrations occur
Place main bars in both directions in a mat. Keep clear cover as per exposure conditions (commonly 50 mm for in-ground footings).
Common mistakes and practical tips
Many problems come from overlooked site conditions or rounding errors. A few common issues are easy to prevent with a checklist.
Use conservative assumptions when data is limited, and verify key numbers like bearing capacity and groundwater depth before finalizing sizes.
Typical errors to avoid
- Using undrained shear strength as allowable pressure without factor adjustments.
- Ignoring soft layers near surface that cause settlement even with adequate bearing pressure.
- Underestimating the effect of groundwater on bearing pressure and concrete cover needs.
- Failing to check eccentric loads and their effect on edge pressures.
Practical tips
Keep drawings simple and note assumed soil values. If footing size grows unusually large, consider ground improvement or deep foundations instead.
- Prefer slightly deeper footings over relying on marginal shear strength.
- Use standardized bar layouts to simplify construction and reduce errors.
- Label all assumptions clearly on calculation sheets so others can review quickly.
Example summary with quick checks
Here is a compact checklist and a worked mini-example to tie all steps together. The numbers are illustrative and should be verified on each site.
Checklist: load list, soil allowable, preliminary size, eccentricity check, bending moment, shear checks, reinforcement, cover and details.
Worked mini-example
Given: Column load = 400 kN (service), allowable soil = 200 kN/m2, concrete grade 25 MPa, steel f_y = 500 MPa.
- Area needed = 400 / 200 = 2.0 m2 → choose square 1.45 × 1.45 m.
- Assume uniform q = 400 / 1.45^2 = 190 kN/m2. If this exceeds allowable, increase size; here q > allowable so increase to 1.6 m side (area 2.56 m2). New q = 156 kN/m2 < 200 kN/m2.
- Half-span = 0.8 m. Moment M ≈ q × L1^2/2 = 156 × 0.8^2 /2 ≈ 49.9 kN·m.
- Assume effective depth d ~ 300 mm. Compute required As ≈ M / (0.87 f_y z). With z ≈ 0.9d = 270 mm, As ≈ 49.9e6 / (0.87 × 500e6 × 0.27) ≈ 420 mm2. Provide 4 bars of 12 mm diameter per direction (area ≈ 452 mm2).
- Check shear: one-way shear V ≈ shear from half-width; compare to v_c b d. If close, increase d to 350 mm or add shear reinforcement.
This quick run shows how footprint, pressures, moments and reinforcement link together. Adjust dimensions if any check fails.
Conclusion
Sizing and checking shallow footings is a sequence of logical steps: get good inputs, estimate area, verify bearing and eccentricity, check bending and shear, and then design reinforcement.
Careful assumptions, conservative checks where data is weak, and clear notes on calculations keep work reliable and easy to review.
Frequently Asked Questions
What is the simplest way to estimate footing size?
Divide the total vertical service load by the allowable soil pressure to get area. Then choose a practical shape and check eccentricity, bending and shear.
How does groundwater affect footing sizing?
Groundwater can reduce the effective bearing capacity and increase settlement risk. It also affects concrete cover due to corrosion risk. When high groundwater is present, reduce allowable pressure or improve drainage.
When should punching shear be a concern?
Punching shear matters when a column load is concentrated over a small loaded area. Check perimeter stresses at about 2d from the column face; if v_ed exceeds v_rd,c consider increased depth or shear reinforcement.
Is it better to increase depth or area to resist bending?
Depth increases section modulus more effectively than area increase, but deeper footings cost more excavation and concrete. Often a balanced change to both size and depth is most economical.
What reinforcement is typical for small footings?
A two-way mat of main bars in both directions with distribution bars is common. Minimum reinforcement should also be provided to control cracks—follow local code limits on bar spacing and cover.
How to handle uncertain soil data?
If soil data is lacking, use conservative allowable pressures and limit bearing stresses. For any critical or large structure, obtain at least basic site investigations before final design.