Design a combined footing — a single footing that supports two columns. Vary the two column loads, the column spacing and the footing dimensions to see the load eccentricity, the bearing-pressure distribution delivered to the soil and the kern check update in real time, and find a footing with a near-uniform pressure.
Parameters
Column 1 load P₁
kN
Vertical load delivered by the left column
Column 2 load P₂
kN
Vertical load delivered by the right column
Column spacing s
m
Centre-to-centre distance between the two columns
Footing length L
m
Footing dimension along the line of columns
Footing width B
m
Dimension perpendicular to the line of columns
Results
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Total load P (kN)
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Eccentricity e (m)
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Average pressure (kPa)
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Max pressure (kPa)
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Min pressure (kPa)
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Pressure verdict
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Combined footing side view — bearing pressure
Two columns are carried on one footing, with the soil bearing-pressure distribution drawn beneath it — a trapezoid within the kern, or a triangle with a lifted end outside it. ▼ marks the resultant load position, ◇ the footing centroid.
Load eccentricity e (s: column spacing, P₁, P₂: column loads) and the maximum bearing pressure p_max for a trapezoidal distribution (P: total load, L: length, B: width). The minimum pressure uses the same form with the factor replaced by 1-6e/L.
$$|e|\le\frac{L}{6}\;\Rightarrow\;\text{within the kern (full compression, trapezoid)}$$
If the eccentricity stays inside the kern (L/6) the whole footing base is in compression and the pressure is trapezoidal. Outside the kern one end lifts off, the distribution collapses to a triangle and the peak pressure rises sharply.
What is a Combined Footing?
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How is a "combined footing" different from an ordinary foundation? I only really picture one footing under each column.
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Good question. Usually you do use an "isolated footing" — one square pad placed directly under each column. But sometimes that just doesn't work. For example, two columns stand so close together that two isolated footings would overlap. Or an exterior column sits right at the property line, so you can't centre a footing symmetrically beneath it. In those cases you carry both columns on one long footing. That is a "combined footing".
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I see — both columns on one slab. But if the two columns carry different loads, won't one side of the footing get pushed harder?
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That is exactly the heart of the design. The force the footing presses on the soil with is the "bearing pressure", and the ideal is for the whole base to push down evenly. For that, the "resultant" of the two column loads should pass exactly through the centroid of the footing. But when the left column is 600 kN and the right is 1000 kN, the resultant shifts toward the heavier side. The distance it strays from the centroid is the "eccentricity e". Try raising P₂ on the left — you will see the eccentricity grow and the pressure verdict change.
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I did! As I raise P₂ the pressure becomes a sloping trapezoid and the maximum pressure climbs fast. Does that "trapezoid" shape mean something?
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When there is eccentricity the pressure is no longer a uniform rectangle but a trapezoid — higher under the heavy column, lower under the light one. It is the same as beam bending: an axial force plus a bending moment. The maximum pressure is p_max = P/(LB)·(1+6e/L). The bigger the eccentricity e, the more the 6e/L term bites and the higher the peak. So you want to keep the eccentricity as small as you can.
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When I push the eccentricity even further, the verdict switches to "part lifts off (outside kern)" and one end of the side view rises. Is that a bad thing?
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Yes — that is the "kern" story. The middle third of the footing length, the range ±L/6 about the centre, is called the "kern". As long as the eccentricity stays inside it, the whole base is in compression and the pressure stays trapezoidal. But once it leaves the kern, on paper one end of the footing would have to "pull" on the soil. Soil can't take tension, so that end lifts off and contact is lost. The load piles onto what is left, the pressure collapses into a triangle, and the peak shoots up. So the basic rule is "keep the eccentricity inside the kern". Going outside it is a signal to rethink the footing shape or column positions.
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That makes sense. So if the bearing pressure is too high, can I just make the footing bigger?
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Widening B increases the base area, so the average pressure drops. But when the eccentricity is already outside the kern, just adding area can leave you stuck with a triangular distribution. In that case you first adjust the footing length or its overhangs so the resultant passes through the centroid, bringing the eccentricity back inside the kern. If that still isn't enough, you switch to ground improvement or a pile foundation. The order matters — remember "eccentricity first, area second".
Frequently Asked Questions
First sum the two column loads to get the total load P, then find how far its resultant lies from the footing centroid (the eccentricity e). Next compute the average bearing pressure q = P/(L·B). If the eccentricity is within the kern (L/6), the pressure is trapezoidal, with maximum q_max = q·(1+6e/L) and minimum q_min = q·(1-6e/L). This tool performs the whole calculation automatically and also compares the peak against the allowable bearing pressure.
The kern is the middle third of the footing base — along the length, the range from -L/6 to +L/6 about the centre. If the resultant load acts inside the kern, the whole base stays in compression and the pressure is trapezoidal. If it falls outside, one end of the footing would have to pull down on the soil, which the soil cannot resist, so that end lifts off; the pressure collapses onto a triangular block and the peak value rises sharply. Keeping the resultant inside the kern is the basic goal of footing design.
There are two main reasons. One is when two columns stand so close together that separate isolated footings would overlap. The other is when an exterior column sits right at a property line, so an isolated footing cannot be centred symmetrically beneath it. A combined footing lets you carry close-spaced columns on one slab, or balance the eccentric load of a boundary column against an interior column, so the pressure delivered to the soil is kept as uniform as possible.
If the maximum bearing pressure exceeds the allowable bearing capacity of the soil: (1) enlarge the footing width B or length L to increase the base area, (2) adjust the column positions or footing shape to bring the resultant closer to the centroid and reduce the eccentricity, then (3) consider ground improvement or a pile foundation. When the eccentricity is outside the kern, bringing it back inside is the first priority — simply enlarging the area may not restore a trapezoidal distribution.
Real-World Applications
Buildings on a property line: In urban buildings the perimeter columns often stand just inside the property line. An isolated footing centred under a boundary column would spill onto the neighbouring lot. So the boundary column and the next interior column are tied together on one combined footing — or a "strap footing" — so the interior column's load cancels the boundary column's eccentricity. The eccentricity calculation in this tool is a quick way to check the resultant position of such a footing.
Closely spaced columns: Columns flanking an expansion joint, or the cluster of columns around a stair core, sit close enough that isolated footings interfere. Combining two columns on one footing secures the required base area while avoiding interference. The design must always check the eccentricity caused by the load difference and the resulting tilt of the pressure distribution.
Machine and equipment foundations: Machine foundations that carry two machines or two support legs on one slab also use the combined-footing idea. When the loads differ left and right, the pressure tilts and can cause differential settlement. As this tool does, the maximum and minimum pressures are checked and the tilt is kept within an acceptable range.
Preliminary foundation design and CAE: Before building a detailed soil-structure FEM analysis or a Winkler spring model, a rigid-footing / linear-distribution hand calculation like this tool gives a first read on how much the pressure tilts and whether the resultant is outside the kern. If the estimate already shows an out-of-kern case, the dimensions or layout can be revised before running FEM. Conversely, if the FEM result differs greatly from this estimate, it is a sanity check pointing to an error in the soil-spring setup or boundary conditions.
Common Misconceptions and Pitfalls
The first big misconception is "bearing pressure is uniform under the footing". The trapezoidal and triangular distributions in this tool treat the footing as a sufficiently rigid plate and the soil as a linear spring — a "linear pressure distribution" model. Real bearing pressure changes shape with the soil type. On sandy soil the edge restraint is weak, so it tends toward a convex shape with a higher centre. On clay soil, stress concentrates at the edges, so it tends toward a concave (saddle) shape with higher ends. The linear distribution is only a design simplification; use the peak value and its location as a conservative guide.
Next, "if it is within the kern it is safe" is a hasty conclusion. Even with a trapezoidal distribution inside the kern, if the maximum bearing pressure exceeds the allowable capacity of the soil the design fails. The kern check (trapezoid vs triangle) and the allowable-capacity check (is the strength enough) are two separate things — you only pass when both are satisfied. This tool reports the kern status and the allowable-value exceedance separately, so always check both. Note too that bearing capacity has two sides — strength (shear failure of the soil) and settlement — and even with enough strength, excessive settlement is unacceptable.
Finally, "only the vertical column loads matter" is an oversimplification. This tool handles only two vertical column loads, with the eccentricity arising from the load difference. But a real footing also carries horizontal forces from earthquake or wind, column-base bending moments, earth pressure and unbalanced earth pressure. These add further eccentricity and tilt to the pressure, and in combination they can push the resultant outside the kern. The result here is the basic case for vertical loads only — real design must include horizontal forces, moments and load combinations.
How to Use
Enter Column 1 Load (kN) and Column 2 Load (kN) in the P1 and P2 fields; typical office building columns range 500–2000 kN each.
Set Column Spacing (m) to define distance between centerlines; standard spacing is 4–8 m for combined footings.
Adjust Footing Length (m) to control the overall bearing plate dimension; start at 1.5× the column spacing.
Read Total Load P, Eccentricity e, and pressure distribution (Average, Max, Min) to verify the footing remains stable within allowable soil bearing capacity (typically 150–300 kPa for medium clay).
Worked Example
Design a combined footing for an industrial warehouse. Column 1 carries 800 kN, Column 2 carries 1200 kN, spaced 6 m apart. Set footing length to 8.5 m. Total load P = 2000 kN. Eccentricity e = 0.4 m (resultant load shifted toward the heavier column). Average pressure = 95 kPa. Maximum pressure = 142 kPa (at edge of heavier column). Minimum pressure = 48 kPa. Verdict: Safe for soil with 200 kPa allowable capacity; adjust footing width if minimum pressure falls below zero (tension zones).
Practical Notes
Unequal column loads create eccentricity; negative minimum pressure indicates uplift—extend footing length or widen it to redistribute loads.
Combined footings are economical when columns are spaced 4–7 m; beyond 8 m, consider separate footings with tie beams.
Verify footing thickness (typically 0.8–1.2 m) using punching shear around each column stub per ACI 318 or Eurocode 2.
Always check local soil investigation reports; bearing capacity varies 100–400 kPa between sandy and clayey strata.