Module 20 · Lesson 20.2
What actually moves the number
Four schemes, five spans, and the comparison that shows which decision matters more.
Why this matters
'Which frame material is lowest-carbon?' is the question that gets asked. It is not the most useful one, and the comparison in this lesson is designed to show why: across ordinary office grids, changing the span moves carbon more than changing the scheme does.
That is not an argument against choosing materials carefully. It is an argument for noticing that the grid — usually set by the architect, early, for reasons that have nothing to do with carbon — is a bigger structural lever than the material, and that a structural engineer who does not get into that conversation has missed the larger decision.
By the end of this lesson you should be able to
- Read a four-scheme comparison and identify what drives the ranking
- Show that carbon rises faster than linearly with span
- Compare the spread from scheme choice against the spread from span
- Show that mass and carbon rank the same schemes differently
Why span costs more than linearly
A longer span needs a deeper section, and a deeper section weighs more per metre. Both effects push the same way, so the material rate grows faster than the span does — for a framed scheme, roughly with the power 1.5.
The library's composite steel frame carries a rate of 32 kg/m² of steel at a 9 m grid. At 12 m it is 49 kg/m²; at 6 m it is 17 kg/m². Same building, same load, same material — a factor of nearly three, from the grid alone.
A flat slab behaves worse still, because it is a constant thickness: most of the concrete is under-utilised, and getting thicker to span further makes all of it heavier.
Four schemes at one grid
At a 9 m grid, 8 000 m² of floor, six storeys, with the course's illustrative factors:
| Scheme | Carbon (tCO₂e) | Intensity (kg/m²) | Mass (t) | Depth (mm) |
|---|---|---|---|---|
| CLT and glulam hybrid | 447 | 55.9 | 1 754 | 465 |
| Concrete band beam | 666 | 83.2 | 3 798 | 374 |
| Composite steel frame | 782 | 97.8 | 2 864 | 480 |
| Reinforced concrete flat slab | 866 | 108.2 | 4 963 | 318 |
Worst to best is a factor of 1.94 — a real difference and worth having.
Mass does not rank them the same way
Read the two middle columns against each other and the point of the module's second summary line appears.
By mass, the ranking is CLT (1 754), composite steel (2 864), band beam (3 798), flat slab (4 963).
By carbon, it is CLT (447), band beam (666), composite steel (782), flat slab (866).
The steel frame and the band beam swap. Steel is lighter by nearly a tonne per 100 m² and has higher carbon, because steel's factor per tonne is more than ten times concrete's. A comparison run on mass would have picked the wrong one of those two.
Structural mass alone does not determine environmental performance. It is one term of a product, and the other term varies by more than an order of magnitude between materials.
Span against scheme
Run all four schemes across spans from 6 m to 12 m and compare the two spreads:
- Worst-over-best across schemes, averaged over the spans: 1.93
- Worst-over-best across spans, averaged over the schemes: 2.09
Span wins. Not overwhelmingly — they are the same order — and that is itself the finding: the grid is as large a carbon decision as the material, and it is usually made by someone else, earlier, without the question being asked.
The practical consequence is not to stop comparing materials. It is to be in the room when the grid is set, with a number.
Try it
Computational carbon explorer
Four schemes across any grid and building size. Every figure carries its provenance, because the factors are illustrative teaching values.
Each factor is moved up and down by this, one at a time, and the ranking re-run.
| Scheme | Carbon (t) | kg/m² | Mass (t) | Depth (mm) |
|---|---|---|---|---|
| CLT and glulam hybrid | 447 | 55.9 | 1754 | 465 |
| Concrete band beam and slab | 666 | 83.2 | 3798 | 374 |
| Composite steel frame | 782 | 97.8 | 2864 | 480 |
| Reinforced concrete flat slab | 866 | 108.2 | 4963 | 318 |
- Lowest carbon
- CLT and glulam hybrid
- Lightest by mass
- CLT and glulam hybrid
- Worst over best
- × 1.94
- Spread from scheme choice
- × 1.93
- Spread from span (6 to 12 m)
- × 2.09
- Which decision moves it more
- the grid
- Ranking survives ±30 %?
- yes
The ranking survives a ±30% change to any single factor, so the comparison is worth acting on even though the factors are not verified.
Note the mass ranking
- At most spans the lightest scheme by mass is not the lowest-carbon one. Change the span and watch the two rankings separate — carbon is quantity times factor, and the factor varies by more than an order of magnitude between materials.
Provenance
Illustrative teaching values, not verified against a current published dataset. Replace with dated factors from a current source before quoting any figure outside this course. As at 2026-07-26. Status: requires-verification.
What is not in these figures
- Foundations and substructure — which would widen the lightest scheme's advantage.
- Cladding, which follows from the structural depth and is often more carbon-intensive per square metre than the frame.
- Cost, programme and buildability, none of which correlates reliably with carbon.
- Sequestration in the timber options, which is excluded — the conservative choice, and it is stated rather than assumed.
What this shows: Span moves carbon as much as scheme choice does — and a ranking is worth acting on only if it survives the uncertainty in its own factors.
Worked example
The grid decision, priced
Given
- A composite steel frame over 8 000 m², six storeys
- The grid under discussion: 6 m, 9 m or 12 m
- Illustrative factors as built into the course's library
Find
What the grid choice costs in embodied carbon
Practice
A scheme uses 447 tCO₂e over 8 000 m² of floor. What is its carbon intensity in kg CO₂e per square metre, to one decimal place?
Practice
Two schemes have masses of 2 864 t and 3 798 t, and carbon of 782 and 666 tCO₂e. Which has the higher carbon per tonne of material, and what is that figure for the lighter scheme, in kg CO₂e per tonne?
Check yourself
The grid moves embodied carbon more than the material choice does. What follows for when the decision should be made?
Summary
- Carbon rises faster than linearly with span, because deeper sections also weigh more per metre
- Doubling the span cost 83 % more carbon; changing scheme saved up to 48 %
- Span and scheme are decisions of comparable size, and the grid is usually set by someone else
- Mass and carbon rank the same four schemes differently — steel and band beam swap
- Steel carries 273 kg CO₂e per tonne of material here against concrete's 175
- Compare figures computed the same way, or you are comparing the accounting
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