Where the carbon is, and when it is decided

Carbon = quantity × factor

Two terms, and an engineer's influence over them is very unequal.

The quantity — how many tonnes of what — follows from the form, the grid, the span, the depth and how efficiently the material is used. All of that is structural design, and all of it is yours.

The factor — kilogrammes of CO₂e per tonne — follows from how the material was made: the recycled content, the production route, the cement replacement, the transport. Some of that is specifiable and much of it is a procurement decision taken later by someone else.

So the reliable lever is the quantity, and the quantity is set by decisions taken at concept stage.

Rate-based comparison

At concept stage there is no model to take quantities off. What there is: a floor area, a grid, a number of storeys, and experience of what schemes of that kind weigh.

A rate-based comparison uses that — kilogrammes per square metre of floor, by material, as a function of span — and it is the right precision for the stage. More precision would be false: the quantities are estimates, the factors are uncertain, and a figure to three significant figures would imply a confidence nobody has.

What it needs to be honest about is what it is for. It is for ranking options, not for reporting a building's carbon. Those are different jobs and the second needs a model and a take-off.

What a carbon figure needs beside it

A number on its own is not usable, and the course enforces this in the library: every result the carbon tool returns carries its provenance, so no interface can display the number without it.

Four things:

The scope. Cradle to gate, cradle to practical completion, whole life? These differ substantially and are routinely compared as though they did not.

The factors, with source and date. A factor without a source is not checkable, and factors move.

The quantities, and where they came from. A rate, a take-off, or a guess.

What is excluded. Foundations, substructure, the effect of a lighter frame on everything below it. A timber scheme's foundations are smaller, and a comparison that stops at the frame understates it.

The carbon factors built into this course's tool are illustrative teaching values. They are of the right order and they make the comparison behave correctly, and they have not been verified against a current published dataset. The tool says so on its own face, the currency register lists them as requiring verification, and no figure from them should be quoted outside the course.

Depth is a carbon quantity

The one most often missed. Structural depth does not appear in a frame's carbon total at all, and it drives:

• Building height — depth times the number of storeys. A 100 mm difference over twelve storeys is 1.2 m of building.
• Cladding area — which is expensive and carbon-intensive per square metre, often more so than the frame.
• Core and riser height, and everything vertical.
• Sometimes a planning limit, where the choice is a storey rather than a metre.

So a scheme comparison that ranks on frame carbon alone is answering a narrower question than the one being asked. Module 17's Pareto front used carbon and depth as the two axes for exactly this reason — and on those axes the composite steel frame turned out to be dominated by the concrete band beam.

Re-use, adaptability and standardisation

Three levers that no single-building calculation captures, and which are often larger than anything inside one:

Re-use. Keeping an existing frame avoids essentially all of its embodied carbon. No new-build option competes with that, and the comparison is usually not run because the existing frame is assessed against a brief written for a new one.

Adaptability. A structure that can be changed rather than demolished defers or avoids a future rebuild. It usually costs a little more now and the accounting rarely reaches far enough forward to see the return.

Standardisation. Fewer distinct components means less waste, less over-specification and simpler re-use later. It also costs a little structural efficiency, which is Module 15's trade appearing again in a different currency.