A pad footing is a block of concrete under a single column, wide enough that the soil can carry the load and deep enough that the block does not break doing it. It is the cheapest foundation there is, and the one most buildings in Nigeria sit on.
This page carries on from the load takedown: the same three-storey building, the same interior column, and the number that arrived at the bottom of it.
The one thing that trips everybody up
A footing is designed at two different limit states in the same calculation, and using the wrong load in the wrong place is the single most common mistake on a foundation sheet.
| What you are doing | Load to use | Why |
|---|---|---|
| Sizing the plan area | Service Gk + Qk |
Allowable bearing pressure already has a factor of safety of 2 to 3 inside it. Factoring the load as well would apply the safety factor twice. |
| Bending, shear, steel | Ultimate 1.4Gk + 1.6Qk |
This is reinforced concrete design, exactly like a beam or a slab, and BS 8110 designs concrete at ultimate. |
Size the pad on the service load. Reinforce it on the ultimate load. If a footing comes out enormous, check this first — sizing on the factored load makes every pad about 45% too big.
Step 1 — The two loads
From the takedown, the ground-floor interior column carries three storeys. Adding the same items up unfactored gives the service load alongside the ultimate one:
Floor Gk 6.04 x 16 = 96.6 kN, Qk 1.5 x 16 = 24.0 kN
Beams 8 m x 1.57 = 12.6 kN
Column 0.225² x 24 x 3.0 = 3.6 kN
= 136.8 kN
N (service) = 3 x 136.8 = 410 kN
N (ultimate) = 589 kN (from the takedown)
Materials: fcu 25, fy 460, column 225 × 225, allowable bearing pressure 150 kN/m², founding 1.0 m below ground.
That 150 kN/m² is a presumed value, not a measured one — fine for sizing a bungalow, not fine for a block of flats. Where the number comes from is a page of its own.
Step 2 — Size the pad
The soil carries the column load plus the footing itself and the earth backfilled on top of it. You do not know those until you have a size, so start with an allowance of about 15% and check it afterwards.
side = sqrt(3.14) = 1.77 m -> use 1.8 m x 1.8 m (A = 3.24 m²)
Now check the allowance instead of trusting it. Take the pad 400 mm thick, so 600 mm of backfill sits on it:
Backfill (3.24 - 0.05) x 0.6 x 18 = 34.5 kN
-------
65.6 kN (16% of N)
q = (410 + 65.6) / 3.24 = 146.8 kN/m² <= 150 PASS
The takedown page estimated 2.7 m² and a 1.65 m square by dividing the column load straight by the bearing pressure. That is the right first move, and it is why the real pad is a size bigger: the footing has to carry itself too.
Step 3 — The pressure that causes bending
Here is the second thing worth getting straight. The footing's own weight is carried by the ground directly underneath it — it goes straight down, bends nothing, and must be left out of the design pressure. Only the column load bends the pad.
Step 4 — Bending at the column face
The pad is a cantilever sticking out of the column on all four sides, loaded upwards by the soil. The critical section is the face of the column.
M = q l² / 2 per metre width
= 181.8 x 0.7875² / 2 = 56.4 kNm/m
Steel is needed both ways, so there are two layers. Design both on the upper layer's effective depth — it is the shallower of the two, and using the deeper one for both overstates half the footing.
K = M / (b d² fcu) = 56.4 x 10&sup6; / (1000 x 332² x 25) = 0.020
0.020 < 0.156, so no compression steel
z = d[0.5 + sqrt(0.25 - K/0.9)] = 0.98d -> capped at 0.95d = 315 mm
As = M / (0.87 fy z) = 56.4 x 10&sup6; / (0.87 x 460 x 315) = 447 mm²/m
As,min = 0.13% bh = 0.0013 x 1000 x 400 = 520 mm²/m governs
Provide T12 @ 200 c/c each way = 565 mm²/m
Minimum steel beating the calculated steel is normal in footings and is not a sign of an error — a pad is thick because of shear, not because of bending, and 0.13% of a thick section is a lot of steel. The same thing happens with minimum links in beams.
Step 5 — Vertical shear
The first shear check is the ordinary beam one, taken on a plane a distance d out from the column face.
V = 181.8 x 0.455 = 82.8 kN/m
v = V / (b d) = 82.8 x 10³ / (1000 x 332) = 0.249 N/mm²
100As/bd = 100 x 565 / (1000 x 332) = 0.170
vc = (0.79/1.25)(0.170)1/3(400/332)1/4 = 0.367 N/mm²
0.249 < 0.367 PASS
Step 6 — Punching shear
The second is the one that decides the thickness: the column trying to punch a plug straight through the pad. BS 8110 checks it on a square perimeter 1.5d out from the column face, against the load outside that perimeter.
inside the pad and applies)
u = 4 x 1221 = 4884 mm
V = q (A - area inside u) = 181.8 (3.24 - 1.49) = 318 kN
v = V / (u d) = 318 x 10³ / (4884 x 332) = 0.196 N/mm²
0.196 < vc = 0.367 PASS
And the absolute limit, at the column face itself, where the concrete would simply crush:
v = 589 x 10³ / (900 x 332) = 1.97 N/mm²
limit = 0.8 sqrt(fcu) = 0.8 sqrt(25) = 4.0 N/mm² PASS
If punching fails, thicken the pad — do not add more steel. Links in a footing are awkward to fix, easy to get wrong on site, and cost more than the extra 50 mm of concrete that would have solved it.
Step 7 — Anchorage, and what to buy
The bars have to be developed past the section where they are needed, or the steel is there and the bond is not.
required anchorage, T12 in fcu 25 = 40 x 12 = 480 mm
737 > 480 straight bars, no bobs needed
The pad in full: 1800 × 1800 × 400 deep, 9 no. T12 × 1700 long each way in the bottom, on 50 mm blinding, founded 1.0 m down.
| Item | Working | Quantity |
|---|---|---|
| Excavation | 1.8 × 1.8 × 1.0, plus working space | 3.24 m³ |
| Blinding | 50 mm under, 1.9 m square | 0.18 m³ |
| Concrete | 1.8 × 1.8 × 0.4 | 1.30 m³ |
| Formwork | 4 × 1.8 × 0.4 | 2.88 m² |
| Reinforcement | 18 × 1.70 m × 0.889 kg/m | 27.2 kg |
Sense check: 27.2 kg in 1.30 m³ is 21 kg/m³. Pads normally land between 20 and 40 kg/m³ — an order of magnitude less than a beam, because a footing is mostly concrete doing nothing but spreading load. A pad that comes out at 90 kg/m³ has been designed as if it were a beam. See the rebar weight chart for the 0.889 kg/m and what a tonne of T12 actually is.
When a pad will not do
Pads stop working for two reasons, and they look completely different on a drawing:
- They run into each other. Once the pads cover much more than about half the building footprint, the gaps between them are doing no work and a raft — one slab under everything — is usually cheaper, stiffer and faster. A raft is an inverted slab: the span steel goes in the top face and the steel over the columns goes in the bottom. Getting that upside down on site is a serious failure, not a snag.
- The wall is continuous, not a column. A load-bearing blockwork wall is carried on a strip footing, designed per metre run rather than per column — same checks, one-way cantilever.
- The soil cannot take it at any sensible size. Soft clay, deep fill, high water table — that is piles, and piles need a soil model that comes out of a ground investigation, not out of a table.
Structura designs pads, strips and rafts, and stops at piles deliberately — see what it will not do. Settlement is not calculated for any foundation type; bearing pressure is a strength check, and a footing can pass it while still settling more than the building can tolerate.
Run this footing yourself
Structura designs pad, strip and raft foundations and shows every step above — service sizing, ultimate bending, both shear checks, the bar bending schedule and the quantities — with a PASS or FAIL on each. Single members, footings included, are free to run, as many as you like.
Run the whole building instead and the takedown feeds the footing for you, so the 410 kN never gets copied across by hand.
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