Module 16 · Lesson 16.1
Floor systems, and the multiplication
The decision that matters is the floor zone, not the beam depth — and whatever you decide, you will pay for it once per storey.
Why this matters
A multi-storey building is the same floor, repeated. That sounds obvious and it changes everything about where design effort belongs: a decision made once about how a floor is built is then paid for on every storey, in cladding, in services, in lift travel and in whether the building fits under a planning height. The single most consequential number in the whole design is one that is fixed before any member is sized — the floor zone — and getting it right requires comparing systems on a quantity most people do not compare them on.
By the end of this lesson you should be able to
- Compare floor systems on zone rather than depth
- Say why services through the structure change the answer
- Multiply a per-floor saving through a building
- Apply the imposed-load reduction correctly
What you should already know
- Composite beams and their construction stages (Module 11)
- Load combinations and the imposed-load family (Module 2)
- Cellular and castellated sections as a member type (Module 5)
The floor zone, not the beam depth
The question a floor system has to answer is not "how deep is the beam?" but "how much of the building's height does this floor consume?" That is the floor zone:
floor zone = structural depth + services below the structure + finishes
And the middle term is where systems differ most, because some let the services pass through the structure and some do not.
For a 12 m span, with 400 mm of services and 250 mm of finishes:
| System | Structure | Services below | Floor zone | Steel |
|---|---|---|---|---|
| Downstand composite | 500 mm | 400 mm | 1150 mm | 40 kg/m² |
| Slab on beams | 667 mm | 400 mm | 1317 mm | 55 kg/m² |
| Cellular beam | 600 mm | 0 | 850 mm | 45 kg/m² |
| Integrated (slim floor) | 429 mm | 0 | 679 mm | 55 kg/m² |
Read the third and first rows against each other:
The cellular beam is 100 mm deeper than the downstand composite beam, and its floor zone is 300 mm shallower.
That is the whole argument. A designer comparing beam depths would choose the downstand and lose 300 mm per storey. The ducts have to go somewhere, and a system that gives them a hole to go through has already paid for its extra depth several times over.
Try it
Floor zone, and what it is worth multiplied
Four floor systems, drawn as structure (solid) plus the services that have to go beneath it (hatched). The bar length is the floor zone — the quantity that actually consumes building height.
Compare against the downstand
- Downstand structural depth
- 500 mm
- Downstand floor zone
- 1150 mm
- Alternative structural depth
- 600 mm
- Alternative floor zone
- 850 mm
- Deeper structure?
- yes
- Shallower floor?
- yes
- Saving per floor
- 300 mm
- Height saved
- 3.00 m
- Façade saved
- 420 m²
- Cladding saved
- £294k
- Extra steel
- 60 t
- Steel cost
- £132k
- Net
- £162k
300 mm saved per floor becomes 3.00 m over 10 storeys — 420 m² of façade that need not be bought. Against that, 60 tonnes more steel. The net is in favour of the shallower option, and it is close enough that the real decision usually turns on something else — a planning height limit, a lettable-area calculation, or the programme.
Things worth trying
- Start with the cellular option at 12 m span. Its structure is 100 mm DEEPER than the downstand and its floor zone is 300 mm shallower, because the ducts go through it.
- That 300 mm is 3.0 m over ten storeys, 420 m² of façade and £294k — against 60 tonnes of extra steel at £132k. Net £162k in favour.
- Now switch to the integrated floor. It saves MORE height — 4.71 m — and delivers LESS net value, because its steel penalty is three times larger.
- That is the point: the quantity to optimise is the net, and it has two terms moving in opposite directions. The shallowest system is not automatically the right one.
- Take the service depth down to 150 mm. The downstand catches up sharply, because the thing it was losing on has shrunk. On a lightly serviced building the simple solution wins.
- Now take it to 700 mm — a heavily serviced laboratory or hospital. The gap widens dramatically and the through-structure systems become almost unarguable.
- Increase the storeys to 30 and watch every figure scale. The decision does not change, but its importance does: the same 300 mm is now 9 m of building.
- Try the slab-on-beams option. It is deeper AND has services beneath it, so it loses on both terms — which is why it is used for its other virtues, not for its depth.
Worked example
Choosing a floor system for a ten-storey office
Given
- Ten storeys, 1200 m² per floor, 140 m perimeter
- 12 m clear span between columns, 400 mm of services, 250 mm of finishes
- Cladding £700/m², steel £2200/tonne erected
- Compare downstand composite, cellular and integrated
Find
Which floor system to adopt, and on what grounds.
Assumptions
- Span-to-depth ratios and steel rates are engineering recommendations, not code values
- Cladding and steel rates are illustrative; a real comparison would be priced
- The comparison ignores fire protection, which favours systems with less exposed steel
Load take-down, and why the reduction exists
A column low in a building carries the floors above it. The permanent load simply accumulates — ten floors of concrete weigh ten times one floor of concrete.
The imposed load does not, and the reason is probabilistic. The chance that all ten floors are simultaneously at their full design imposed load is very small. The reduction factor acknowledges that:
| Storeys carried | αn | Imposed load |
|---|---|---|
| 1 | 1.000 | 90 kN |
| 3 | 0.900 | 243 kN |
| 5 | 0.820 | 369 kN |
| 10 | 0.760 | 684 kN |
For the worked column at 120 kN permanent and 90 kN imposed per storey, the lowest column takes 1884 kN against 2100 kN unreduced — 10% less.
Notice why the total reduction is only 10% when the factor is 0.76: the reduction applies to the imposed part only, and on this column the permanent load is the larger of the two. A column under a lightly loaded roof and heavy floors would gain much more; a column carrying mostly self-weight would gain almost nothing.
αn is a nationally determined parameter and is unverified in this course, so the calculation is teachable and not usable. What is worth carrying regardless is what it is FOR — it is a statement about probability, not about strength, and it is the one place in a load take-down where the answer is smaller than the sum of its parts.
Practice
A 12 m span cellular beam is 600 mm deep with services through it, plus 250 mm of finishes. What is its floor zone, in mm?
Practice
300 mm is saved per floor on a ten-storey building. How much building height is saved, in m?
Practice
That 3.0 m is saved on a building with a 140 m perimeter. How much façade is saved, in m²?
Practice
A column carries 10 storeys. With αn = (2 + (n − 2) × 0.7)/n, what is αn?
Check yourself
Why does the imposed-load reduction exist?
Summary
- Compare floor ZONE = structure + services below + finishes, not beam depth
- A 600 mm cellular beam gave 850 mm of zone; a 500 mm downstand gave 1150
- 300 mm per floor × 10 storeys = 3.0 m = 420 m² of façade = £294 000
- It cost 60 t of extra steel, so the net was £162 000 in favour
- The integrated floor saved MORE height and delivered LESS net value
- So the quantity to optimise is the net, which has two opposing terms
- Imposed-load reduction is a probability statement: αn = 0.76 over ten storeys
- It applies to the imposed part only — the worked column's total fell just 10%
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint