A bungalow and a duplex look like the same building with a staircase added. Structurally they are two different animals: one carries its load down through walls, the other through a frame, and almost everything else follows from that one difference.
This page puts numbers on it — the same 4 m × 4 m grid and the same loads used in the rest of these worked examples.
The bungalow: walls carry it, and the numbers are small
A single-storey house has no suspended floor. The roof lands on the walls, the walls land on a strip footing, and the ground floor slab sits on compacted fill carrying nothing but itself. There is often no frame at all.
3.0 kN/m² x 3.0 m = 9.0 kN/m
Roof 2 m tributary, Gk 0.5 + Qk 0.6 = 2.2 kN/m
Ring beam = 1.5 kN/m
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Service 12.7 kN/m
Width needed = 1.15 x 12.7 / 150 = 0.10 m
One hundred millimetres. No one builds a 100 mm footing, which is the whole point: a bungalow's strip footing is not sized by bearing capacity on decent ground. It is sized by what can actually be built — a wall plus a working margin each side, so 450 to 600 mm wide and 225 to 300 mm deep.
Pressure = 12.7 / 0.6 = 21 kN/m² (14% of the allowable)
That margin is why bungalows are forgiving and why bad ground catches people out anyway. At 21 kN/m² the soil is barely being asked anything — but if the footing sits on fill, or on expansive clay that moves seasonally, the load was never the problem. See soil bearing capacity.
The duplex: one suspended floor, and a frame appears
Add a floor and the load goes up by roughly an order of magnitude per support, because a suspended concrete slab weighs far more than a roof and carries people as well.
| Bungalow | Duplex | |
|---|---|---|
| Suspended floors | 0 | 1 |
| Load path | Wall → strip | Slab → beam → column → pad |
| Load at ground level | 13 kN/m of wall | 167 kN per column |
| Foundation | 600 wide strip | 1.2 m square pad |
| Columns | Usually none | 225 × 225 typical |
| Staircase | — | Designed member |
| What governs | Buildability | Slab deflection, then everything else |
Sizing that pad, from the takedown's per-storey figures plus a roof:
Roof = 30 kN service
N = 167 kN (ultimate 238 kN)
A = 1.15 x 167 / 150 = 1.28 m² -> 1.2 m square
check (167 + 28) / 1.44 = 136 kN/m² <= 150 PASS
And the column itself, checked the way the column example does it:
~ 0.4 fcu Ac = 0.4 x 25 x 50 625 = 506 kN
Demand = 238 kN under half of it
So a duplex column is not sized by its axial load either. 225 × 225 gets used because it matches the block wall it sits in and the beam it supports, and because a slimmer column becomes slender and starts attracting additional moments. Load only begins to drive column sizes at three or four storeys.
What genuinely gets harder with the second floor
- Deflection of the suspended slab. The first real design constraint in the building, and the one that decides the floor thickness. The slab example is a 150 mm slab that fails by a hair and a 160 mm one that works.
- The staircase becomes a designed member, with its own loads, its own deflection check and a detailing trap at the landing. Stair design covers it.
- Continuity. Beams over several bays are continuous, so there is hogging over the supports and top steel that a simply supported analysis will not tell you about.
- Differential settlement between heavily loaded internal pads and lightly loaded external ones. Keeping bearing pressures similar across the building matters more than keeping every one of them low.
- Robustness ties and fire — two requirements that do not fall out of any structural calculation and therefore get skipped. See cover, grades and mixes.
Wind is not usually one of them. At one or two suspended floors a framed concrete building in Nigeria is stiff and heavy enough that gravity governs everywhere. It starts to matter from about three storeys up, and it becomes the thing that decides the frame not long after.
"Can I just deck my bungalow later?"
The most common structural question asked in Nigeria, and the answer is: only if the bungalow was designed for it, and usually it was not.
Adding a suspended floor multiplies the load at the base of every support by something like ten. Every one of these has to be checked before a single block goes up, by an engineer who has seen the existing structure:
- The foundations. A 600 mm strip under a load-bearing wall was never meant to carry a slab. This is the check that most often ends the conversation, and it is the one that cannot be fixed afterwards without underpinning.
- Is there a frame at all? Many bungalows have columns cast within the walls, and many have decorative ones that are not tied into anything. A column that does not continue into a footing is not a column.
- Starter bars. If none were left projecting, the new columns cannot be connected to the old ones in any way the calculation can rely on.
- What the existing concrete actually is. The grade on a twenty-year-old drawing is not evidence. Core tests or a rebound hammer, and a cover meter to find the steel that is there.
- The stair opening, which usually means cutting the existing roof structure and possibly a wall that was carrying it.
Designing a bungalow with the extra floor already in mind is cheap — bigger pads, starter bars, a frame that goes to the foundation. Retrofitting all three afterwards is not, and doing none of them is how a building collapses during construction rather than after it.
Design either one
Structura runs both cases: single members for a bungalow's strip footing or lintel, and a whole-building takedown for a duplex — slab to beam to column to footing, with the stair and the quantities. Single members are free to run, as many as you like.
Whole buildings go up to three suspended floors on gravity alone, and up to seven once wind and sway are designed. Above that the engine stops rather than producing a number that looks fine.
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