Special thanks to Corentin Fivet, Jan Brütting and team at EPFL, as well as the Institute of Structural Engineers’ Climate Task Force for precipitating much of my thinking on this issue and for my colleagues at BIG for helping me crystallize these thoughts into what has become this series
The process of estimating carbon, both operational but now especially also embodied, in building designs, known as carbon accounting, is becoming more and more the norm in the industry, and this is certainly a necessary step towards net zero construction. All worthwhile sustainability certification schemes already include a carbon assessment (generally via and LCA), and formal, national regulations are not far off in the future. All across Europe, committees and political action groups are gearing up to set carbon limits into law. In the UK, professional and political organizations (IStructE, RIBA, LETI) have preceded government in setting our carbon targets. In Denmark, the first legal requirements enter the building regulations in 2023, i.e. just over a year away. The Danish framework is set up as an incremental implementation of more and more strict limits for the total whole-life carbon magnitudes of construction projects, starting with 12 kgCO2eq/m2/year for projects over 1000m2, which will gradually reduce in value and increase in scope to include all new construction.
However, one thing becomes painfully clear once you begin to track embodied carbon, which is that we are currently very far from being able to hit the necessary targets to stay on path towards net zero by 2050. According to the assessment laid out by the IStructE in the development of the SCORS rating tool for embodied carbon, we should be hitting a global average for structure only of approximately 240 kgCO2eq/m2 in 2025, 140 in 2030, 50 in 2040 and net zero by 2050. Given what we know about the knowledge and performance level of the global construction industry, and the historical rate of change, those of us in Western Europe and North America should be aiming well below those averages.
As my colleagues and I seek to reduce embodied carbon in their projects, I see a common frustration resulting from a lack of perceived effect of such efforts. The traditional techniques and tricks in our toolboxes, design optimization, materials selection, geometry rationalization for lowest embodied carbon, simply don’t move the needle far enough. We know from examples that building zero-carbon and energy positive operationally is feasible already, but embodied, upfront carbon costs still pose a major hurdle to architects and engineers. The pathway to net-zero carbon construction is only partially illuminated, i.e. the knowledge, technology and system of thinking we have at this moment in time will only take us partways to our destination (figure 1).
This points to Einstein’s well known adage, “The thinking that got us to where we are is not the thinking that will get us to where we want to be”. Pursuing net zero embodied carbon will require questioning traditional thinking and rearranging the profession’s priorities such that carbon is equally as important as safety, which previously held a solo position on the top shelf of the altar of engineering principles. There is already a recent bloom of such thinking, where traditional thinking on applied loading, materials safety factors and performance-based design are being discussed. The battle between resilience, flexibility as pertains to structures longevity, and sustainability is hotly debated. These are important conversations and must continue to spread within the engineering community.
To get to net-zero we will also have to engage in the discussion around reuse, circularity, buildings as material banks (BAMB), urban mining and material flows. All these labels converge within the umbrella of a total paradigm shift towards viewing buildings (or any other object subject to design as an intentional process) as temporary states in a continual process of change rather than final products. Reuse is interesting because it is not a newly developed field of science or thinking, but a very old one, based on necessity and lack of new supply. In this way it harkens back to many other examples where modern-day fields of science perform lengthy, rigorous, incremental examinations only to conclude in agreement with the age-old, time-tested wisdom of cultures, i.e. with that which was already known but perhaps forgotten.
However, it is clear that, almost from inspection, reusing structural materials directly can save enormously on the upfront carbon cost, theoretically up to 100%. Recent studies by Fivet, Brütting et al. show carbon savings around 50% when comparing optimized reuse-scenarios versus new-design-scenarios, however this comparison considers the higher efficiency possible for a new-design scenario, and penalizes the reuse-scenario for this. This is a prudent approach, especially when you consider impending (current) resource scarcity, but it does perhaps undeservedly handicap the reuse case, adding theoretical, invented emissions to the equation, when direct reuse has no actual emission apart from those associated with construction and transport. However, I think these are details that shouldn’t detract from the overwhelmingly positive effect widespread reuse could have - reusing structural elements directly has potential to significantly reduce embodied carbon, which is rare in our world.
Getting to a place where we can actually reuse materials at scale, requires a whole-sale reshuffling of both the industry and market, and also of our traditional way of designing, our philosophy of design even. A map of this pathway is shown below, the arrow representing a guess at the order in which these topics could be considered and addressed. As I continue to discuss this mapping in what will follow, my focus will lie on reuse for structures specifically, although there will be locations of overlap with reuse for architecture.
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In a construction industry that is a) responsible for an outsized portion of global carbon emissions, b) facing imminent material scarcity issues and c) fundamental to questions of global equity, the emerging middle class, and increasing urbanism, and d) is increasingly being subjected to higher and more complex demands, it is only entirely expectable that conflicts would arise. The right to shelter, along with the growing global middle class, is pulling the industry fast in the direction of cheap and dirty construction. Much like in the discussions of general emissions reductions, any concept of equity demands that western, affluent society places far heavier restrictions on itself before the developing world, and leads the way towards low- and zero-carbon construction. Referring back to figure 1, this cannot be done without significant changes to the mindset of all participants in the design and construction industries. One could call this sacrifice or compromise, but I believe in calling it opportunity and making room for creativity.
Embracing reuse means inverting the traditional design process. What is today the ‘last’ step in the design process, namely finding the materials that fit the concept the designer has envisaged, must become the starting point. Rather than inventing a building and finding the materials and elements that fit the conceived geometry, we must consider the characteristics, both material and geometrical, of an available stock and let these properties inform and define the design. Varying degrees of adherence to the purity of the stock can be imagined, where more or less buffering is allowed, through dimensional alteration and new-material use, is tolerated, thus providing designers with some lee-way which could be seized as a pathway for artistic expression and design control. However, already now, computational methods are being used to encode this flexibility in design-for-stocks, enabling designers to select not specific geometries but rather characteristics of geometries, such as minimum weight, carbon, or minimum alteration, for which the algorithms will optimize. While this might sound an awful lot like automated design and/or an inversion of the design lead hierarchy between architects and engineers (i.e. an architect’s nightmare), it is more likely that this will simply become the latest method of discovering optimal form. Optimal forms have been derived before, for cost and structural efficiency, and yet architects and engineers continue to pour all their creativity and effort into diverging from said forms, a tradition that will likely remain. Instead, designing for reuse will simply enable designers to have a starting point for their design based in a paradigm of lowest-carbon.
Although this essay series is about reuse, I would be remiss if I seemed to imply that reuse is the only tactic available for reducing embodied carbon. It is not, and should certainly be combined with a number of other pathways. In fact, it should not even be your first choice. Whenever possible, starting now, we must must question whether a building in fact needs to be built at all. We need to be much more diligent in demonstrating how existing buildings can be directly reused and refurbished, and active in convincing clients and decision-makers that this is necessary and even beneficial. Deconstruction and onsite reuse, followed by deconstruction and off-site reuse, should only be considered once refurb options have been investigated and discarded.
However, for most designers today, the logistical infrastructure and contract framework is not yet in place to enable reuse of structure, and so when the cat is out of the bag, when new-build is the order, or when the location is an empty site, a number of other topics should be considered. These could be lumped into two categories: materials and design. Materials means specifying low-carbon materials and designs that enable them, supporting the development of new natural products to scale up and preferring recycled or upcycled products. Design means working to make sure your spans and grid bias low-carbon design, that your calculations and analysis are precise, careful and extensive enough to provide you the confidence to approach 100% utilization, and an increasing adoption of performance based design.
The question of utilization ratios and the closer related topic of conservatism deserve extra discussion. Codes and standards are based in tradition and historic experience of what has worked. The strengths and properties of established construction materials are well-documented, and the remaining uncertainty is dealt with via partial safety factors on materials and loading. It is typical for engineers to limit utilizations in early design phases in order to account for the almost inevitable increase due to higher detail levels as projects progress, but we should be diligent in bringing these ratios as close 100% as possible by the end of detailed design. There is no need, and most likely no benefit, in an additional, arbitrary safety factor. Building failure is very rarely a question of marginal overstepping of utilizations, but rather wholesale misunderstanding of load paths and a lack of the required imagination to discover all failure modes. Reducing utilization ratios is not a good replacement for understanding your structure, its characteristics and its failure modes, i.e. skilled engineering practice.
