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Timber structures: sustainability Material matters: low-carbon timber design

from Timber Industry Yearbook 2022

by Open Box Media & Communications

from Timber Industry Yearbook 2022, page 70

Article from: Timber Industry Yearbook 2022

Timber structures: sustainabil...

Material matters: low-carbon timber design

Will Arnold considers how the efficient use of timber in construction can contribute to a more sustainable future.

Millennium Line 2 – Brentwood Station. Photo: Nic Lehoux

“One of the unique properties of biogenic materials, such as timber, is that their growth causes carbon to be sequestered from the atmosphere and locked away within the material. This is a favourable factor that helps to combat climate change.”

With the Intergovernmental Panel on Climate Change (IPCC) report 2021 sounding a ‘code red for humanity’, the science is now unequivocally clear: the climate is changing, it is human induced and it will get considerably worse unless drastic action is taken.1 Predictions for a 2°C warmer world indicate that by 2100 (less than 79 years from now) future generations will experience significant increases in the occurrence of climate events: heatwaves (three times as frequent), extreme rainfall (1.5 times more often) and flooding (a 12-fold increase in economic damages).2

Significant changes must be made across every aspect of our lives to prevent catastrophe. So what role does timber in construction play?

Using carbon-sensitive timber

With embodied carbon responsible for around 10% of global emissions, the choice of how we construct buildings and what they are made from has never been more important. In recent years, there has been significant progress in the development of new timber technologies, allowing for longer spans, more dramatic architecture and quicker construction.

However, if timber construction is going to be part of the solution to the climate crisis, then it must be considered properly. To be carbon-sensitive, the timber used in construction must be: • used efficiently • detailed to last forever • sourced from a low-carbon supplier.

Low-carbon design

Simply switching out other construction materials for timber, without changing any other aspects of the design, can lead to gross inefficiencies. Timber has unique properties that dictate the manner in which it should be used – and if it is to be used efficiently, the design process must reflect the qualities of the timber products chosen for the particular application.

Structural designers must remember that the optimal form for a timber building has direct load paths with minimal >>

structural heroics and complexities. Transfer structures (e.g. due to changes in column spacings) add significant material to a scheme – and sub-optimal structural arrangements (e.g. due to complex architectural geometry) add more material still. This is all material that could be omitted by opting for a simpler, continuous design, which would be quicker and cheaper to build.

Loadbearing walls or columns spaced similarly to domestic construction is usually optimal; with 4m to 6m spans suiting most circumstances. Longer spans are possible, but deflection and vibration start to govern the design, quickly adding material at an exponential rate. In an open-plan office, having a column every 6m might seem closer than we are used to – but materiality matters, and many would argue that it is preferable to sit close to a natural material, rather than far away from one that is artificially made.

Circular economy

One of the unique properties of biogenic materials, such as timber, is that their growth causes carbon to be sequestered from the atmosphere and locked away within the material. This is a favourable factor that helps to combat climate change, and reforestation is on the agenda for most major world economies as part of their sustainability commitments.

However, we must not view sequestration in isolation – it is part of a carbon cycle, whereby carbon leaves the atmosphere (through sequestration into a tree) and later re-enters it (when the tree dies and rots, or is felled and burnt).

Similarly, this sequestration doesn’t mean that a timber structure absorbs carbon from the atmosphere. Instead, a timber structure locks away sequestered carbon for a prolonged period, until the decision is made to dismantle the structure and, for example, downcycle the timber into a more short-lived product (for example, chipboard or animal bedding) or burn it for fuel.

Millennium Line – Gilmore Station. Photo: Nic Lehoux

From an environmental perspective, the challenge is to lock that carbon away for as long as possible, meaning that good detailing to guard against water, rot and fire is important. Planning for a circular economy is essential, enabling the timber elements to be reused in several more structures in the future – though we mustn’t forget that the potential to save carbon in the future has to be balanced against the certainty of today’s emissions. Taking the decision to increase circularity at the expense of today’s emissions is a gamble and requires delicate consideration.

Timber sourcing

Finally, once a structure has been efficiently configured and detailed with the future in mind, one step remains – sourcing it.

It goes without saying that all timber used on every project should be FSC® or PEFCTM certified (so that felled trees are replaced with new saplings) – but how do carbon emissions feature in sourcing? >>

Remember, producing a timber product is not carbon-negative. While the timber has carbon locked away within it from decades of sequestration in the past, the actual process of felling trees, turning logs into timber products and then into structural frames will always require energy. The priority therefore is to find the lowest-carbon means to turn trees into structural frames, and there is a large variation in the emissions produced by different manufacturers of the same material.

For a timber project to minimise its emissions, it needs to prioritise those manufacturers working on technological developments to reduce production emissions. Many are looking at burning biomass as a way to minimise emissions, but even burning waste wood releases emissions,3 and so the challenge is to find ways to produce timber structures without burning anything at all, instead using renewable-powered energy to dry and process the wood.

Samuel Brighouse Elementary. Photo: SP

About the author

Looking ahead

Will Arnold Head of Climate Action The Institution of Structural Engineers (IStructE)

If designed well, detailed carefully and specified thoughtfully, timber structures can play a significant role in reducing the construction industry’s emissions. This requires the whole team – from client to contractor – to work together to prioritise sustainability. The industry has never been better equipped to start tackling emissions and it has never been more important than it is now. n