Ask around any site in Kampala and you will hear that a 230 x 230 column with four Y16s is what you use. For a ground-floor column in a small G+1 house with short spans, that is often reasonable. Applied to a G+4 with 5m spans, it is not, and the reason is worth understanding.
Load accumulates downwards
A column carries everything above it. The top-storey column takes the roof and one floor. The ground-floor column of a G+4 takes five floors, the roof, all the walls sitting on those floors, and the live load of everything the building is used for.
So the axial load on the ground-floor column is roughly five times the load on the top one. Yet on many sites every column in the building is the same size, because it is simpler to set out and the steel order is easier. The result is either a top storey with far more concrete than it needs, or — much worse — a ground floor with far less than it needs.
What drives the number
Four things, in rough order of influence:
- Tributary area. The floor area each column supports. Double the span in both directions and you have quadrupled the load on that column. This is why spans matter more than storeys for column size.
- Number of storeys above. Straightforward accumulation.
- Use. Residential live loads are modest. A retail floor, a store room or an assembly space can be several times higher, and clients change the use after handover far more often than they tell the engineer.
- Slenderness. A tall storey height with a small section buckles before it crushes. A 3.0m storey and a 4.5m double-height entrance behave very differently even under the same load.
Practical consequences
On our G+4 work we usually end up with something like 400 x 400 or 300 x 500 at ground floor, reducing at upper levels, with the reinforcement percentage reducing before the section does. Reducing steel first and section second keeps the setting-out simple for the block layers, which matters more on site than it does on the drawing.
Where a client will not accept a large section in a room — a common conversation on retail ground floors — the options are a rectangular column oriented into the wall line, a higher concrete grade, or a column relocated with a transfer beam above. All three have costs; the last one has a large cost, and it is better discussed at design stage than after the walls are set out.
The eccentricity nobody draws
Columns in real buildings are rarely loaded purely axially. Beams frame in from one side, edge columns take load off-centre, and a corner column is eccentric in two directions at once. Bending combined with axial load reduces capacity substantially. Perimeter and corner columns therefore often need more steel than internal ones carrying a similar area, which looks wrong until you work it through.
What to ask for
If you are shown a structural drawing where every column on every floor is identical, ask whether the ground-floor columns have been checked for the full accumulated load, and whether the design allowed for the use the client actually intends. Neither question is unreasonable, and both are cheaper to ask now than to answer later.
OK Estate Construction and Supply Limited builds framed structures across Uganda and South Sudan. Talk to our team about your building.