Structure: The Biggest Chunk

Break a new building's up-front embodied carbon down element by element - foundations, frame, floors, facade, finishes, services - and one block towers over the rest: the structure. Substructure (the foundations) plus superstructure (the frame, beams, columns and floor slabs) together typically account for well over half of the up-front carbon of a new-build, and often much more for a tall or long-span building. No other single element comes close. The facade, the finishes and the services matter and add up, but the structure is the mountain the whole rest of the profile sits under.

The reason is simple once you see it, and it is worth stating plainly: carbon roughly follows mass, and the structure is where the mass is. A building's structure is, by weight, the overwhelming majority of what gets built - tonnes of concrete and steel, poured and rolled to carry the loads of gravity, wind and use over decades. Everything else - the plasterboard, the glazing, the carpet, the ductwork - is comparatively light. And the structure is not made of just any material; it is made of the two highest-carbon mainstream materials there are, concrete and steel (the subjects of the next two lessons). Heavy material times high carbon-per-tonne gives a large number, and that large number is the structural chunk.

There is a compounding reason too. Structure is sized to be safe, and the loads it must carry include its own weight - so a heavy frame needs bigger foundations to hold it up, which adds more mass, which is more carbon. Weight breeds weight. This is why a lean, efficient structure pays off twice, and why the single most powerful carbon lever most designers ever touch is how much structural material a building uses. Before any material substitution, the quantity of structure - driven by the grid, the spans, the depth of the floor system and how conservatively it is sized - sets the scale of the whole carbon problem. See it clearly: the up-front carbon of a building is, first and foremost, a story about its structure.

It helps to split the structure into its two halves, because they behave differently. The superstructure is everything above ground that carries load: the columns and beams (or load-bearing walls), and the floor slabs and the roof structure. The substructure is everything below: the foundations - pads, rafts, piles - and any basement or retaining structure that transfers the whole building's weight into the ground. In a typical building the superstructure is the larger carbon share of the two, but the substructure is far from trivial, and on difficult ground it can balloon.

Within the superstructure, the floors are usually the heaviest hitters, for a reason worth holding onto: the floor structure repeats on every single level. A ten-storey building has one roof and one set of foundations but ten floors, each a large horizontal plate of concrete or a steel-and-concrete deck spanning between supports and carrying the loads of everything on it. Multiply a heavy floor system by many storeys and the floors become the dominant line in the whole carbon account. Columns and beams matter, but they are slender compared with the great flat expanses of the floors.

The substructure is the part most governed by things outside the drawing - chiefly the ground. On good, firm soil, foundations can be modest; on soft, wet or made ground, they grow dramatically, demanding deep piles or thick rafts, and with them a large slug of concrete and steel that never shows in the finished building but sits permanently in its carbon total. A basement is especially carbon-expensive: it means excavation, retaining walls and waterproofing, all heavy. This is why site selection, ground conditions and the decision to dig down are genuine carbon decisions, not just cost ones. Understanding this split - superstructure led by the floors, substructure driven by the ground - tells you where within the structure to look, and it frames the rest of this module: the materials those elements are made from, and how design can use less of them.

If you had to point at one part of a multi-storey building and say 'the carbon is mostly there,' you would point at the floors. Understanding why sharpens every later decision. A floor is a large horizontal element that must resist gravity across a span while staying stiff enough not to bounce or sag; the further it has to span between supports, the deeper and heavier it must be, and heaviness rises faster than span, not in step with it. So the choices that set the floor - the column grid, the span, the floor system (flat slab, beam-and-slab, hollow-core, composite deck) - are among the most carbon-consequential a designer and engineer make, precisely because that choice is then repeated on every storey.

This leads to the module's most important early lesson: for structure, using less material usually beats swapping material. A shorter span or a more efficient floor system that removes concrete and steel from every floor plate cuts carbon on a scale no finish specification can match. A structural grid chosen for a car park below rather than for the best floor above, an over-cautious 'round it up' sizing culture, a deep transfer structure to carry columns that do not line up - each quietly adds mass across the whole building. The carbon lever here is discipline about quantity: right-sized spans, a rational and repetitive grid, floor systems that carry their loads with the least material, and structural engineers briefed early that reducing tonnage is a project goal, not just reducing cost.

None of this means material choice does not matter - it matters enormously, and the next lessons are about it. But sequence counts. Decide how much structure the building needs and how efficiently it is arranged first; then decide what that structure is made of and how low-carbon a version of that material you can get. A lean structure in ordinary concrete can easily beat a bloated one in 'green' concrete. Quantity first, then material: that is the order in which the structural carbon problem yields.

The practical upshot of this lesson is a priority rule for low-carbon design: look at the structure first, and look at it early. Because the structure is the biggest carbon chunk and because its scale is fixed at the concept and scheme stages - the form, the grid, the number of storeys, whether you dig a basement, how the loads come down - the largest carbon savings available on most projects are decided before a single finish is chosen, by the architect and structural engineer working together at the very start. Leave structural carbon until the design is fixed and you have already spent the biggest part of the building's carbon budget. This is the argument for bringing the structural engineer and, where warranted, a carbon specialist into the room at concept, not detailed design.

What you do with that priority is the rest of this module and the next: understand the two big materials (concrete in 4.2, steel and metals in 4.3), then design to use less of them and to choose lower-carbon versions (Modules 5 and 6). The order matters - reuse an existing structure if you can (Module 7), build less structure, arrange it efficiently, then lower the carbon of the material that remains.

As always in this course, the principle is yours but the binding numbers are not. That the structure is usually the biggest element is a robust, well-established pattern - but the actual share for your building, the tonnages, and the carbon per tonne of your specified concrete and steel are things only a proper embodied-carbon assessment, using verified EPD data for your actual products and a qualified structural engineer and LCA specialist, can tell you. The element split shown here is illustrative and varies widely by building type, height, span and ground. Use the principle to know where to look and what to prioritise; use the method, the data and the specialists to get the numbers that decisions are actually made on. Know this: get the structure right and you have won most of the embodied-carbon battle; get it wrong and no amount of careful finishing will save you.