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Column Offset on an Isolated Footing: Why Size Alone Is Not Enough

By 8 min readUpdated

A common belief on site: if the footing is big enough, it is safe. We divide the load by the soil pressure, check that the footing area is larger, and move on.

That check counts only the load. It never asks where the column stands on the footing. Here is why the position (pwesto) of the column matters just as much, with one footing checked to NSCP 2015.

Why an off-centre column matters

When the column sits at the centre of the footing, every square metre of soil carries the same pressure.

Move the column away from the centre and the footing wants to rotate toward that side. The soil under the near edge is pressed harder, and the soil under the far edge carries less. The pressure becomes a slope instead of a flat block.

The usual design rule, and the one our Isolated Footing Designer sheet uses, is that the pressure at the most heavily loaded edge must stay within the allowable. That edge also settles more, so the footing can tilt.

Columns end up off-centre for ordinary reasons: a footing moved away from a boundary or an old foundation, a small error in setting out, or a column line that changed after the pits were dug.

The test footing

I used the same footing as in the isolated footing worked example, with the column moments set to zero so that only the offset acts:

  • Footing 2.4 × 2.4 m, 600 mm thick, base 1.5 m below ground
  • Column 400 × 400 mm, f'c = 21 MPa, fy = 420 MPa, 16 mm bottom bars
  • Service loads PD = 700 kN and PL = 350 kN, so Ps = 1,050 kN
  • Soil: allowable bearing qa = 250 kPa, 18 kN/m³; concrete 24 kN/m³; surcharge 5 kPa

First, the net allowable pressure, what is left of qa after the footing, the soil on it and the surcharge:

qnet = 250 − 0.6 × 24 − (1.5 − 0.6) × 18 − 5 = 250 − 14.4 − 16.2 − 5 = 214.4 kPa

A quick first check divides the load by qnet: A,req = 1,050 / 214.4 = 4.897 m², against 2.4 × 2.4 = 5.76 m² provided. It gives the same answer wherever the column is. But NSCP 413.3.1.1 sizes the base from the service forces and moments, so the offset must be included. That is the edge-pressure check below.

With the column at the centre, the pressure is uniform: 1,050 / 5.76 = 182.3 kPa everywhere, under the 214.4 kPa limit. PASS.

Case A: the same footing, column 300 mm off-centre

Only the column moves, 300 mm along one axis (eo,x = 300 mm).

Plan of the 2.4 m footing with the column centred and 300 mm off-centre (to scale) Two plans of the same 2.4 by 2.4 m footing with a 400 by 400 mm column, drawn to scale. Left: the column at the centre of the footing. Right, case A: the column moved 300 mm along x, leaving 0.70 m from the column face to the near edge. The kern (middle third) is drawn as a dashed diamond reaching L/6 = 400 mm from the footing centre; the 300 mm offset is still inside it. The edge the column moved toward is the heavy edge, the opposite edge is the light edge. Footing 2.4 × 2.4 m, column 400 × 400 mm (plan, to scale) Column at the centre 2.4 m L/6 = 400 mm load at the footing centre Case A: column 300 mm off-centre 2.4 m 300 mm offset 0.70 m light edge heavy edge Kern (middle third): full contact while the load stays within L/6 = 400 mm of the centre Column load Ps = 1,050 kN, 300 mm from the footing centre: still inside the kern
Figure 1. Plan of the 2.4 × 2.4 m footing, drawn to scale: the column at the centre, and case A with the column 300 mm off-centre. The dashed diamond is the kern. It reaches L/6 = 400 mm from the footing centre, so the load is still inside it.
Check (service loads) Needed Case A Result
Footing area, axial load only≥ 4.897 m²5.76 m²PASS
Load inside the middle third (kern), e ≤ L/6≤ 0.400 m0.300 mPASS (full contact)
Pressure at the heavy edge (413.3.1.1)≤ 214.4 kPa319.0 kPaFAIL, about 49% over
Pressure at the light edge≥ 045.6 kPaPASS

The soil check fails, although the footing is exactly as big as before.

While the load stays inside the middle third, the pressure is a straight line from edge to edge:

q = (P / A) × (1 ± 6e / L)

Hand check: 6 × 0.300 / 2.4 = 0.75. So q,max = 182.3 × 1.75 = 319.0 kPa and q,min = 182.3 × 0.25 = 45.6 kPa.

Another way to see it: the offset adds a moment of 1,050 × 0.300 = 315 kN·m. The footing's section modulus is 2.4 × 2.4² / 6 = 2.304 m³, and 315 / 2.304 = 136.7 kPa. That is added at one edge and taken away at the other: 182.3 ± 136.7.

The heavy edge went from 182.3 to 319.0 kPa, 75% more, from a shift of one-eighth of the footing width (300 / 2,400).

Case B: column still 300 mm off-centre, footing 2.9 × 2.9 m

This is what it takes if the footing simply grows on every side, keeping its old centre. Thickness and depth stay the same, so qnet stays 214.4 kPa.

Check (service loads) Needed Case B Result
Footing area, axial load only≥ 4.897 m²8.41 m²PASS
Load inside the kern, e ≤ L/6≤ 0.483 m0.300 mPASS
Pressure at the heavy edge (413.3.1.1)≤ 214.4 kPa202.3 kPaPASS (0.94)
Pressure at the light edge≥ 047.4 kPaPASS

Hand check: q,avg = 1,050 / 8.41 = 124.85 kPa, and 6 × 0.300 / 2.9 = 0.6207. So q,max = 124.85 × 1.6207 = 202.3 kPa and q,min = 124.85 × 0.3793 = 47.4 kPa.

Soil pressure under the footing: centred, case A and case B on one scale Three elevations on one scale with the service soil pressure drawn under each footing and a dashed line at the net allowable pressure qnet = 214.4 kPa. Centred column on the 2.4 m footing: uniform 182.3 kPa, pass. Case A, same 2.4 m footing with the column 300 mm off-centre: 45.6 kPa at the light edge rising to 319.0 kPa at the heavy edge, above the allowable, fail, about 49 percent over. Case B, 2.9 m footing with the same 300 mm offset: 47.4 kPa to 202.3 kPa, under the allowable, pass at 0.94. Service soil pressure under the footing (kPa), one scale Centred PASS 2.4 m footing 182.3 everywhere Case A FAIL 2.4 m, column 300 mm off 45.6 319.0 Case B PASS 2.9 m, column 300 mm off 47.4 202.3 dashed: qnet = 214.4 182.3 is under 214.4 319.0 is about 49% over 214.4 202.3 is under 214.4 (0.94)
Figure 2. Service soil pressure under the three footings, on one scale. Centred: 182.3 kPa everywhere. Case A: 45.6 to 319.0 kPa, over the 214.4 kPa allowable at the heavy edge. Case B (2.9 m): 47.4 to 202.3 kPa, under it.

A 2.8 m footing is not enough: its heavy edge is at 220.0 kPa, still over 214.4. So 2.9 m is the first 100 mm step that passes. That is 46% more footing area than 2.4 m (8.41 / 5.76), for the same column load.

Growing on every side also pushes the footing 250 mm toward whatever caused the offset. Often a better option is to extend the footing only on the side the column moved toward, so the column ends up at the footing centre again, or to tie it to the next footing with a beam. Which option fits is the engineer's decision.

The lesson

The quick P/qnet check passed in every case. Only the edge-pressure check caught the problem. Size is not the whole answer; position is part of it.

Three more things are worth knowing:

  • Centring is the cheapest fix. If the footing can be centred under the column, the 2.4 m footing goes back to a uniform 182.3 kPa.
  • The kern has a limit. For the 2.4 m footing, L/6 = 400 mm. At an offset of 400 mm, q,min reaches zero and q,max doubles the average: 2 × 182.3 = 364.6 kPa. Beyond that, part of the footing lifts off the soil and the rest is pressed even harder. A column on the property line is far past this point. That is the job of a strap footing.
  • The short side gets tight. In case A the column face is only 2.4 / 2 − 0.300 − 0.200 = 0.70 m from the near edge. The bottom bars have 700 − 75 = 625 mm to develop there, less than the full 698 mm that 16 mm bars need in the worked example. Hooks, or a closer check by the engineer (for example, allowing for extra steel provided), may be needed on that side.
Edge pressures of the 2.4 m footing against column offset Line chart for the 2.4 by 2.4 m footing under 1,050 kN. Horizontal axis: column offset from 0 to 400 mm, the kern limit L/6. The heavy-edge pressure rises in a straight line from 182.3 kPa at no offset to 319.0 kPa at 300 mm and 364.6 kPa at 400 mm. The light-edge pressure falls from 182.3 kPa to 45.6 kPa at 300 mm and to zero at 400 mm. A dashed line marks the net allowable pressure, 214.4 kPa. Beyond 400 mm part of the footing lifts off the soil. 2.4 m footing, Ps = 1,050 kN: edge pressure against column offset Beyond L/6: part of the footing lifts off the soil 0 100 200 300 400 0 100 200 300 400 Column offset from the footing centre (mm) Soil pressure at the edge (kPa) = L/6 qnet = 214.4 182.3, centred 319.0 Case A, 300 mm 45.6 364.6 0 heavy edge light edge
Figure 3. Edge pressures of the 2.4 m footing as the column moves off-centre. The heavy edge climbs from 182.3 kPa to 319.0 kPa at 300 mm and 364.6 kPa at 400 mm (L/6), where the light edge reaches zero.

Moments matter, too. The total eccentricity is e = eo ± M / P: the moment can add to the offset or reduce it, so design for the case where they add. In the worked example the moments gave only e = 0.024 m along x (25 kN·m / 1,050 kN); this offset is more than 12 times that.

Elevation X from the Sketches sheet of the Isolated Footing Designer (NSCP 2015), from the shipped v1.1 sample. This is the worked example with its moments and the column centred, not case A or B: the pressure runs from 178.0 to 199.7 kPa, under the dashed qnet = 214.4 kPa line.
Figure 4. Elevation X from the Sketches sheet of the Isolated Footing Designer (NSCP 2015), from the shipped v1.1 sample. This is the worked example with its moments and the column centred, not case A or B: the pressure runs from 178.0 to 199.7 kPa, under the dashed qnet = 214.4 kPa line.

This example uses gravity service loads only. The engineer of record also checks the NSCP load combinations with wind and earthquake, where moments can be much larger.

What to check on site

  1. Set out each footing from the gridlines on the plans (batter boards and string lines), not from the edge of the pit, a fence or an old wall. Check the column centre against the footing centre on the plan.
  2. After the footing bars and column starter bars are placed, measure from the column centre to each footing edge and compare with the plan.
  3. If a column has to be off-centre (a boundary, an existing footing, a drainage line), ask the engineer before the pour. The footing size or type may have to change.
  4. If a footing was already poured off-centre, measure the offset and report it to the engineer of record. Do not bend the starter bars to shift the column. Wait for the engineer's fix before forming the column.
  5. Watch the side the column moved toward. It has the highest soil pressure and the least room for bar anchorage.

This is not about any one trade. A 300 mm shift is easy to lose when lines are moved on site. The fix is a tape measure and a quick check before the pour.

General information only. Use the soil data from your own geotechnical investigation and follow the approved plans and the engineer of record.