Computational Engineering / Computation for low-carbon structural design
What actually moves the number
Four schemes, five spans, and the comparison that shows which decision matters more.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A computational engineering reviewer must check algorithms, convergence claims, statistical assumptions and numerical examples beyond the cases explicitly recorded.
- Verify quantity rates and factor set against current project-appropriate EPDs and whole-life methodology. Review biogenic carbon, exclusions, joint uncertainty, foundation effects and service integration before any project use.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
The illustrative comparison below varies both the structural scheme and its grid. Both affect carbon, so it is useful to examine them together before fixing the brief. Their relative influence depends on the building, quantities and factor set; the teaching model cannot establish a general ranking of design decisions.
By the end of this lesson you should be able to
- Read a four-scheme comparison and identify what drives the ranking
- Explain the span dependence assumed by the rate model
- Compare the spread from scheme choice against the spread from span
- Show that mass and carbon rank the same schemes differently
The assumed effect of span
The teaching model makes the composite steel rate proportional to span raised to 1.5. This is an illustrative rate relationship, not a general law for structural mass or total carbon. Other quantities in the scheme follow different assumptions.
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 total gross floor area across 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 about 11.7 tonnes 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
The two spreads are similar in this illustrative model. Discuss the grid while comparing materials, and state the rate assumptions before drawing project conclusions.
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 illustrative study shows that both grid and scheme affect embodied carbon. When should their effects be considered?
Summary
- The teaching model’s steel rate scales with span to the power1.5; total scheme carbon also includes other quantities
- Doubling the span cost 83 % more carbon; changing scheme saved up to 48 %
- Grid and scheme have comparable effects in this illustrative study; test their influence for the project
- 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