20 February 2020

The low-carbon diet

Is frame-flexibility the key to avoid the life-cycle carbon yo-yo effect?

The built environment is responsible for a third of the UK’s carbon emissions, and radical change in building and infrastructure design is urgently required if we are to truly make an impact on climate change. It is widely recognised that embodied carbon makes up a fifth of this sector’s overall footprint.

Through the Climate Change Act, passed in May 2019, the UK government has legislated to achieve net zero carbon by 2050. To help deliver this, both the UK Green Building Council (UKGBC) and London Energy Transformation Initiative (LETI) have challenged our industry to strive towards delivering net zero carbon buildings by 2030.

UKGBC’s research advises that a third of a building’s carbon footprint results from operational energy consumption, whilst the remaining two-thirds are associated with the building’s initial design, procurement and construction, together with its ongoing maintenance and adaptation. Whilst the engineering community have made good progress in technological advances to reduce operational carbon emissions, tackling a building’s embodied carbon remains a major challenge.

To pave the road to zero carbon, embodied carbon metrics must be given priority over traditional measures of project viability, such as cost and efficiency. Developers are responding to this, recognising that occupiers are increasingly demanding more sustainable buildings which accord with their corporate and social responsibilities, alongside the values of their customers and employees. These identity markers are changing the way the buildings are valued, with evidence suggesting sustainable assets can yield a higher return on investment.

Spurred-on by this challenge, our multidisciplinary team, Chloe Souque, Richard Whitehead and Mark Terndrup, investigated the comparative embodied carbon of traditional structural frame solutions with that of timber-based alternatives. Through a thought-provoking high-level study, we explored four common structural options: steel frame with a holorib deck; steel frame with lightweight concrete holorib deck; traditional reinforced concrete frame; and reinforced concrete frame with post-tensioned (PT) slab.

We then compared these with more recognised sustainable solutions such as cross-laminated timber (CLT) with glulam timber beams and, finally, a hybrid of CLT with steel, often used to bridge the gap created by the structural limitations of timber frame construction.

We modelled a hypothetical seven-storey office block considering the substructure and superstructure frame for each option. Taking an industry norm of steel frame with holorib decking as a baseline, we compared the ‘cradle-to-gate’ greenhouse gas (GHG) emissions of each of the design solutions. Concrete options incorporate 50% ground-granulated blast furnace slag (GGBS).

Perhaps unsurprisingly, the results demonstrated that the carbon emissions for the traditional reinforced concrete frame were 17% higher than the baseline. What became clear is that, despite the carbon intensive process of steel manufacturing, the relative amount of material in traditional concrete framed buildings and their foundations results in significantly higher embodied carbon. However, the introduction of PT solutions dramatically lowers the volume of concrete used, resulting in 13% lower embodied carbon than the steel frame baseline.

CLT with glulam timber beams represent an 82% reduction in embodied carbon compared to traditional steel frame. The pure timber option performs so well because both CLT and glulam beams are inherently carbon-absorbing, with the raw materials acting as a natural carbon sink. The hybrid solution – using steel frame and CLT deck – lowers the embodied carbon by 29%. Whilst being more carbon-intensive, it sidesteps the complexities of full timber framed construction.

For the purposes of the research, we accounted for the benefit of carbon sequestration, assuming that all CLTs and glulam beams are dismantled and re-used at the end of the building’s life. Should these be landfilled, our calculations suggested that the embodied carbon could exceed that of the PT concrete frame or steel-frame buildings. This emphasizes the importance of adopting whole life thinking in our approach to building designs.

Our analysis considers only ‘cradle to gate’ – the route from material extraction to when it leaves the manufacturing facility. However, this isn’t the full picture in terms of the true whole-life carbon of the materials. Structural solutions need to help future proof our building stock and not become quickly obsolete. When it’s no longer possible to repurpose them, the ability for our buildings to be efficiently deconstructed before re-use is equally important, when selecting a structural solution. This is particularly relevant for CLT structures as they will release the absorbed carbon if sent to landfill – negating the original benefit.

This high-level study was designed to spark a discussion among our engineers, challenging perceptions rather than representing one ultimate design solution. Richard Whitehead, Waterman Structure’s Board Director, said; “It remains vital that our designs optimise the type and amount of materials required to suit a given function and location. It is essential that we take embodied carbon into account in our designs, while also making sure buildings are fit for purpose for generations to come. Looking at the entirety of a building’s lifespan is vital to selecting the right materials to maximise future re-use. Steel frame buildings are inherently more flexible than those with timber frames, for example, and ease of future adaptability is crucial when considering the whole-life carbon of a building.”

Richard continues; “Hybrid CLT and steel solutions are just one example of how we can create a low carbon commercial building which is likely to be more readily adaptable than an all-timber frame. While the appraisal of whole-life cycle carbon becomes subjective over the period of building use and occupation, our experience allows us to establish those attributes which make buildings flexible, avoiding those which can become prohibitive to later adaptation and repurposing.”

Responding to Richard’s observations, Chloe Souque, Waterman’s Associate Director for Sustainability, said; “Sustainability needs to be approached holistically, embodied carbon is just one amongst many considerations. For example, installing a living roof could potentially increase the overall embodied carbon of the structure, but how do you adequately factor-in its carbon sequestration benefit or the potential operational savings associated with reduced solar gains? What about the positive impacts on biodiversity, attenuation and the well-being of the occupants? These are the trade-offs we have grappled with for years as sustainability professionals. We certainly need more comprehensive data and holistic research to make fully informed decisions and not simply displace the problem. In the meantime, to paraphrase a famous scientist, we need to remember that not everything that can be measured matters and not everything that matters can be adequately measured.”

As part of publishing our ongoing research, look out for our next article where Mark Terndrup, Waterman’s Director for Building Services, explores embodied carbon in mechanical and electrical services. You will learn how different strategies can dramatically impact life carbon footprint, and how Soho Estates are taking this forward as they strive towards a net-zero carbon portfolio.

For more information on the study, or for advice on how to embed sustainability into your designs, contact
Chloe Souque – chloe.souque@watermangroup.com.

The low-carbon diet

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