Combined footings transfer loads from two or more columns to soil when individual footings overlap or when a column lies close to a property line. These elements balance column loads, soil pressure, and geometry to prevent unequal settlement.
This article explains practical steps, key formulas, and a worked numerical example to show how to size a combined footing and check structural requirements. The language is plain and focused on common checks used on typical small- to medium-span buildings.
When a combined footing is the right choice
Combined footings are chosen when two columns are close enough that their individual square or rectangular footings would overlap, or when one column is near an edge and a single isolated footing would cause eccentric loading on the soil.
They are also used to equalize pressure under footings supporting different column loads, to reduce differential settlement, and when site constraints prevent separate footings.
Common scenarios
Typical situations include property line columns, columns with very different loads, and sites with low allowable bearing capacity that require wider spread of load.
- Two columns near a property boundary.
- Unequal column loads requiring a shared base to distribute pressure.
- Limited headroom where slab-on-grade needs a continuous foundation strip.
Basic calculation steps
Designing a combined footing follows a logical sequence: establish loads, choose soil bearing capacity, estimate dimensions, check bending and shear, then design reinforcement. Each step needs simple arithmetic and an understanding of equilibrium.
Start with column loads and position, then compute the total required footing area from allowable soil pressure. From that, select an initial shape and depth and refine checks for bending, shear, and development length.
Step 1: Obtain loads and geometry
Gather factored or service loads from columns, distances between column centers, and any eccentricities. Note whether columns are on a grid or near edges.
- Axial loads: P1, P2 (kN).
- Column spacing: center-to-center distance L (m).
- Column sizes and eccentricities relative to footprint.
Step 2: Footing area and initial plan size
Total area A required is total vertical load divided by allowable soil bearing pressure qa. Choose a length that spans between column extents and a width based on area/length.
Formula: A = (P1 + P2)/qa. If length between column centers plus overhangs is Lf, then width B = A / Lf.
Step 3: Check eccentricity and pressure distribution
Ensure resultant vertical load falls within the middle third of the footing plan to avoid tension at the base. For combined footings with unequal loads, the neutral axis may shift, producing a trapezoidal pressure pattern.
Compute eccentricity e = (Σ(P*x))/ΣP measured from a reference axis, then find pressure extremes: q_max = (P_total/A) (1 + 6e/Lf) and q_min = (P_total/A) (1 – 6e/Lf) for rectangular assumptions.
Design example: two columns sharing a footing
Below is a worked example that follows the previously listed steps. Numbers are illustrative but reflect typical unit choices used on small buildings.
Assume two columns with loads P1 = 600 kN and P2 = 300 kN, center-to-center spacing 3.0 m, and allowable soil bearing qa = 200 kN/m2. Column widths are small compared to footing dimensions.
Calculate required area and plan dimensions
Total vertical load P_total = 600 + 300 = 900 kN. Required area A = P_total / qa = 900 / 200 = 4.5 m2.
Choose footing length Lf as the distance between outer edges covering columns and reasonable overhangs. Use Lf = 3.0 m + 0.4 + 0.4 = 3.8 m (0.4 m overhangs each side). Then width B = A / Lf = 4.5 / 3.8 = 1.184 m, round to 1.20 m.
Check eccentricity and pressure spread
Take reference at midpoint between columns or at one end; compute centroid of loads along footing direction. Place x = 0 at left edge of combined footing plan, locate P1 and P2 positions, and compute e.
If left column is at x1 = 0.4 + 0.0 = 0.4 m and right column at x2 = 0.4 + 3.0 = 3.4 m, centroid x_c = (600*0.4 + 300*3.4)/900 = (240 + 1020)/900 = 1.4 m from left edge. Footing centroid is at Lf/2 = 1.9 m, so eccentricity e = 1.4 – 1.9 = -0.5 m (toward left). Absolute e = 0.5 m.
Average pressure q_avg = 900 / (3.8*1.2) = 900 / 4.56 = 197.4 kN/m2. For rectangular strip assumption, maximum pressure q_max = q_avg (1 + 6e/Lf) = 197.4 * (1 + 6*0.5/3.8) = 197.4 * (1 + 3.0/3.8) = 197.4 * (1 + 0.7895) = 197.4 * 1.7895 ≈ 353.4 kN/m2.
q_max exceeds allowable bearing 200 kN/m2, which means the chosen plan produces excessive local pressure. To reduce q_max, increase width or length until q_max ≤ qa.
Adjust dimensions to control pressure
Increase width B until q_max = qa. Since q_avg scales inversely with area A (A = Lf*B), choose B_new such that q_max_new = qa.
Rearrange: q_max = (P_total/A) (1 + 6e/Lf) = qa. So A = P_total (1 + 6e/Lf) / qa. Plugging numbers: A = 900 * 1.7895 / 200 = 8.05275 / 0.2? Careful: 900*1.7895=1610.55 then divide by 200 gives 8.05275 m2.
With Lf = 3.8 m, B_new = A / Lf = 8.05275 / 3.8 = 2.1197 m. Round to 2.15 m. That is a much wider footing than initial estimate but needed to keep soil pressure within limits.
Depth and bending checks
Once plan size is set, estimate effective depth d to resist bending under unfactored or factored loads. Use strip method: take a unit meter strip perpendicular to span where bending moment is highest.
Maximum bending moment per meter M_u can be found from pressure distribution and reaction forces. For a more conservative, uniform pressure q = qa over area, bending about the critical section at midspan is approximated by M = q * Lf^2 / 8 (for simply supported strip). Use design codes to apply factors.
- Assume design pressure q_d = qa = 200 kN/m2.
- For one meter strip, load w = q_d * B = 200 * 2.15 = 430 kN/m per meter length along footing.
- Span considered Lf = 3.8 m, M = w*Lf^2/8 = 430 * 3.8^2 / 8 ≈ 430 * 14.44 / 8 ≈ 7774 / 8 ≈ 971.8 kN·m per meter.
Convert to bending moment per meter of strip and choose required steel using standard reinforced concrete formulas: Mu = 0.87 fy As d (1 – 0.416 x) or use tabulated charts. For fy = 415 MPa, compute needed As accordingly.
Example quick check: Using simplified rectangular stress block, approximate required As = Mu / (0.87 fy jd). With d approximated as overall depth minus cover and bar diameters, iterate depth to get workable reinforcement.
Common checks and detailing points
After initial sizing, several checks ensure safety and serviceability: shear near columns, punching shear for concentrated column loads, development length, and differential settlement checks between columns.
Detailing must ensure proper bar anchors, adequate cover, and distribution bars across the width to control crack widths and distribute loads evenly.
Punching shear
Columns impose concentrated loads; verify punching shear around column perimeter. Use critical perimeter at 0.5d from column face and compare shear demand with shear capacity of section.
- Compute shear force V = column load – soil reaction within critical perimeter area.
- Check against concrete shear resistance VRd,c and add shear reinforcement if needed.
Shear along beam-like sections
For longitudinal shear between columns, check one-way shear near column faces using V = shear from strip loads. Provide stirrups or bent-up bars if shear exceeds concrete capacity.
Reinforcement layout
Place main steel along the longer dimension to resist bending; provide distribution bars perpendicular to mains. Maintain minimum reinforcement ratios to control cracking and ensure ductility.
- Top bars where negative moment occurs near column supports.
- Bottom bars at midspan for positive bending.
- Provide extra bars under columns if punching checks approach limits.
Practical tips on construction and soil interaction
Site practices influence final performance. Proper compaction, uniform bearing surface, and consistent concrete quality reduce the risk of differential settlement and cracking.
Ensure accurate excavation and use blinding concrete to create a level base. Apply a thin layer of lean concrete to avoid point loads from stones and to provide a clean form for reinforcement placement.
Handling weak spots and variable soil
If part of the footing sits on weaker soil, redistribute load by increasing width locally or by using stepped footing profiles. Alternatively, improve soil using compaction, aggregates, or geotextile reinforcement.
Monitoring settlements
Measure settlement periodically during construction if significant loads are applied quickly. This helps identify uneven settlement early and take corrective action, such as underpinning or grouting, if needed.
Conclusion
Combined footing design balances column loads, geometry, and soil capacity to produce an efficient foundation that limits settlement and keeps soil pressure within safe limits. A clear calculation path—load assembly, area estimate, eccentricity check, bending and shear checks—keeps the process straightforward.
Iterate plan dimensions and depth until all checks pass and detailing meets code requirements. Practical site measures and proper reinforcement placement complete a reliable footing system.
Frequently Asked Questions
What is the primary reason to use a combined footing?
When columns are so close that separate footings would overlap, or when a column near a property line requires a shared base to avoid eccentric soil pressure, a combined footing is used to distribute loads safely.
How do you account for unequal column loads?
Calculate the centroid of the load system and the resulting eccentricity. Adjust plan dimensions so the maximum soil pressure does not exceed allowable values; this often means increasing width or length.
When is punching shear critical?
Punching shear is critical around column bases where concentrated loads act. Check using a critical perimeter at 0.5d from the column face and provide shear reinforcement if demand exceeds concrete capacity.
Can combined footing be stepped or tapered?
Yes. Stepped or tapered footings can economize concrete and fit site constraints while keeping pressures within allowable limits, but checks must be done for each step to ensure even load transfer.
How to reduce footing size when soil is weak?
Use wider footings to lower pressure, improve soil strength by compaction or stabilization, or consider deep foundations if spreading is impractical. Each option affects cost and construction time differently.
