Strip foundation design

A strip footing is the foundation under a wall rather than under a column: a continuous band of concrete that takes the line load from blockwork and spreads it wide enough for the ground to carry. It is the most common foundation in Nigeria by a very long way, because most bungalows are load-bearing blockwork rather than frames.

It is also the one most often designed by copying the last job. That usually works, and the reason it works is worth understanding — because when it stops working, nothing warns you.

The pad footing example is the column version of this page. If your building is a frame, that is the one you want.

What is different about a strip

Everything is per metre run. The load arrives as kN/m along the wall, not kN at a point, so the footing is designed as a one-metre slice and the answer is repeated the whole length of the wall. That has two consequences:

• It bends one way only — transversely, across its own width, as a cantilever off each side of the wall. There is no second direction to design, unlike a pad.
• There is no punching shear. Nothing is trying to push a plug through it; the load arrives spread along a line.

So the design is four steps: width, bending, shear, and the steel along the length that no calculation asks for.

Step 1 — width, on the service load

This is the same distinction that catches people on pads, and it is worth stating twice. The width comes from the unfactored, working load. Allowable bearing pressure already has a factor of safety of two or three inside it; applying 1.4 and 1.6 on top of that would be a factor of safety on a factor of safety, and you would buy a footing half as wide again as you need.

The bending steel, further down, does use the ultimate load. Two loads, two purposes.

The example

A three-bedroom bungalow, 225 mm sandcrete blockwork, rendered both sides, on firm sandy laterite. Taking the wall down:

Read that again: the bearing requirement was 210 mm and the footing is 550 mm. On firm ground a bungalow's strip is not sized by the soil at all — it is sized by the need for a projection you can actually dig, form and stand on. This is why the standard 450–600 mm strip works on most sites, and why it is dangerous to assume it always will. The rule holds until the ground gets soft, and then it stops holding suddenly.

The 10% allowance covers the footing's own weight and the backfill over it. It is a shortcut; on a deep or thick footing, work it out properly.

The same wall on soft ground

Change nothing but the soil — soft clay at 50 kN/m² instead of firm laterite at 100:

Still governed by buildability. Now put two storeys of blockwork on it — gk 65 kN/m, qk 22 kN/m, on ground worth 75 kN/m²:

A 1.3 m wide strip is a different animal from a 550 mm one, and the rest of this page is about what changes when it gets that wide.

Step 2 — bending, and the rule that usually cancels it

The footing cantilevers off each face of the wall under the upward soil pressure. The projection is:

Here is the rule worth carrying around:

The bungalow, checked

The bending moment here is about 0.7 kNm/m, which needs under 10 mm²/m of steel against a minimum of 325. The minimum is not a rounding-up of the calculation — it is a different requirement, about shrinkage and about the ground not being as uniform as the arithmetic pretends.

The two-storey wall, where it does bend

Minimum steel governs again — at four times the calculated area. If you take one thing from this page, take that: strip footings are almost never governed by their bending calculation. They are governed by minimum steel, by the 150 mm projection, and by the thickness needed to keep the projection inside the 45° spread. Doing the moment properly is still worth it, because it tells you which of those three you are actually leaning on.

Step 3 — shear

Checked as a beam, at a distance d out from the wall face — the concrete within d of the support carries its load directly into it and is not part of the shear span.

It passes by a factor of nearly five, and it will on nearly every strip footing, for the same reason the bending did: the thickness rule that kills the cantilever kills the shear span too. A footing that fails shear is telling you it is too thin for its width, and the fix is thickness, never links — nobody puts links in a strip footing.

Step 4 — the steel along the length

No calculation on this page asks for a single bar running along the wall. Put them in anyway, and understand why: the transverse steel resists bending from the load, and the longitudinal steel resists the ground.

A footing 15 m long crosses soft spots, old drains, backfilled trenches and tree roots. It shrinks as it cures. It gets built in sections on different days. None of that appears in a bearing pressure, and all of it cracks an unreinforced strip. Two or three T12s top and bottom, lapped properly and carried right through corners and junctions, is what turns the footing into a beam that can bridge a metre of bad ground.

This is the steel most often left out on site because it is not on the bar bending schedule for the bending calculation. Put it on the schedule. See the bar bending schedule example for how to write it up so it gets bought and bent.

When a strip is the wrong answer

The width sizing is also the warning system. Watch for these:

• The strips start to touch. When adjacent footings are wide enough to nearly meet, you are already paying for a raft in strips, and the strips settle less predictably. Compare the two — the raft example shows what the alternative involves.
• The allowable pressure is under about 50 kN/m². Below that, widths grow faster than the thickness rule allows and every footing needs a real bending design. That is also the range where settlement, which none of this calculates, starts to be the thing that matters.
• The building is a frame. Columns want pads. A strip under a frame only makes sense as a combined footing when pads would overlap, and that is a different calculation from this one.
• You do not know the bearing value. Guessing it is the single largest error available in foundation design. See soil bearing capacity for what presumed values are worth and when a test stops being optional.

What this example leaves out

• Settlement. Bearing pressure asks whether the ground fails. Settlement asks how far it moves before it does not, and they are different questions with different answers. Neither this page nor Structura calculates it.
• Depth. The footing has to sit below topsoil, below fill, below any seasonal moisture change, and below the influence of nearby trees. That is a site decision, not a calculation.
• Steps. On a sloping site the strip steps down; each step needs a lap length of overlap and the vertical face needs to be formed, not just cut into the trench side.
• Ground water and sulphates. Both change the concrete specification. See cover, grades and mixes.

Next

• Which foundation?
• Ground beam design
• Pad footing design
• Raft foundation design
• Soil bearing capacity
• Load takedown
• Bungalow or duplex
• All worked examples