Let’s start with a typical four-story office building. We’ll begin with a steel composite frame on a 7.5m grid, business as usual as far this typology goes. With piled foundations, the embodied carbon is 231 kgCO₂e/m² with a 52% / 48% split between the superstructure and substructure, which is easy to deduct on the displayed graphs.

A 7.5m grid is unlikely to be optimal, but we can quickly check this. By adjusting the sliders, we can vary the primary and secondary grid spacing. We can see that by reducing the grid size to 6m results in both reduced embodied carbon and structural depth.

I promised we’d come back to structural depth. At the moment, the timer beams are 760mm deep which, depending on how generous the floor to floor dimensions is, could impact the clear height. The default behaviour of Cactus is to choose the lowest weight sections that work for the parameter set by the users. This is helpful for choosing efficient solutions however, it often means it favours a deeper section over a slightly heavier shallower section. Fortunately, we can change this by moving the “optimise” slider to the left to “Depth” to search for a shallower option. In this instance, a 520mm deep timber also works with a negligible uplift in carbon due to the low carbon nature of the timber.