Exploring the synergy between structural engineering design solutions and life cycle carbon footprint of cross-laminated timber in multi-storey buildings

ABSTRACT

Low-carbon buildings and construction products can play a key role in creating a low-carbon society. Cross-laminated timber (CLT) is proposed as a prime example of innovative building products, revolutionising the use of timber in multi-storey construction. Therefore, an understanding of the synergy between structural engineering design solutions and climate impact of CLT is essential. In this study, the carbon footprint of a CLT multi-storey building is analysed in a life cycle perspective and strategies to optimise this are explored through a synergy approach, which integrates knowledge from optimised CLT utilisation, connections in CLT assemblies, risk management in building service-life and life cycle analysis. The study is based on emerging results in a multi-disciplinary research project to improve the competitiveness of CLT-based building systems through optimised structural engineering design and reduced climate impact. The impacts associated with material production, construction, service-life and end-of-life stages are analysed using a process-based life cycle analysis approach. The consequences of CLT panels and connection configurations are explored in the production and construction stages, the implications of plausible replacement scenarios are analysed during the service-life stage, and in the end-of-life stage the impacts of connection configuration for post-use material recovery and carbon footprint are analysed. The analyses show that a reduction of up to 43% in the life cycle carbon footprint can be achieved when employing the synergy approach. This study demonstrates the significance of the synergy between structural engineering design solutions and carbon footprint in CLT buildings.

KEYWORDS:

• Cross-laminated timber
• multi-storey buildings
• life cycle analysis
• climate impact
• structural engineering design

1. Introduction

The buildings and construction sector is crucial for transition to a low carbon society (IPCC 2014). Worldwide, the sector accounts for 36% of the total final energy use and 38% of the energy-related carbon dioxide (CO₂) emissions (Dossche et al. 2017, Global Alliance for Buildings and Construction 2019, Ürge-Vorsatz et al. 2020). To keep the global average temperature rise well below 2°C in line with the Paris Agreement, global carbon emission is suggested to be reduced by 50% by mid-century, relative to the level in 1990 (European Commission 2016). This will require concerted efforts, and the buildings and construction sector is a key contributor to the current climate destabilisation (Churkina et al. 2020).

Increasingly, stringent regulations are being deployed to improve the sustainability of buildings in Sweden, where the buildings and construction sector represents one-fifth of the total greenhouse gases (GHGs) emissions (Boverket 2020). A new regulation on climate declaration of buildings will go into effect in January 2022, stipulating documentation of climate impact of new buildings in a life cycle perspective (Boverket 2018, Boverket 2020). The climate declaration will be based on the carbon footprint of buildings’ production and construction stages initially. The regulation is to specify carbon footprint limits for new buildings and is to be further expanded to cover other life cycle stages of buildings when the understanding on these increases. This will be a driving force for the deployment of low carbon building products and creates strategic opportunities for optimising synergies between construction solutions and carbon footprint.

Life cycle studies of modern buildings show that the production stage can represent a large share of the total environmental impact (Stephan et al. 2013, Liljenström et al. 2015, Satola et al. 2021), and the material choice for a structural frame has a significant effect on the overall climate impact of a building (Aye et al. 2012, Dodoo et al. 2014, Jayalath et al. 2020, Balasbaneh and Sher 2021). Literature demonstrates the climate benefits of timber-based building materials in contrast to alternative non-renewable materials (IPCC 2007, Dodoo et al. 2009, Sathre and O’Connor 2010, Lippke et al. 2011, IPCC 2014, Hill 2019). A recent study (Peñaloza et al. 2018) on long-term strategy for climate change mitigation through new construction concluded that the increased use of timber-based construction systems is an effective approach to reduce the carbon emission of the Swedish building stock.

Cross-laminated timber (CLT) is a relatively new building technology revolutionising the use of timber in structural applications and has facilitated the construction of large-scale and tall multi-storey buildings. The CLT industry is growing around the world (Muszynski et al. 2017), especially in Europe which produced about 70% of the global CLT in 2017 (United Nations Publications 2019). Many of the CLT buildings are the firsts of their kind and are yet to reach mid-service life. Hence, CLT is a subject of growing research to address open issues related to material efficiency, construction, long-term structural performance and life cycle climate impacts. An approach, which addresses the synergies between these open issues, is vital to improve the competitive advantage of CLT.

A few comprehensive studies have been reported on the life cycle climate impacts of CLT buildings. In a recent review of life cycle analyses of CLT in buildings, Cadorel and Crawford (2018) identified only nine detailed studies in the literature and most of these focused on the cradle-to-gate stages, with simplified modelling of the impact of the end-of-life stage. For example, Balasbaneh and Sher (2021) evaluated the life cycle environmental implications of hypothetical single-family building designed, using CLT or glue-laminated timber structural elements. Pierobon et al. (2019) compared the environmental impacts of mid-rise CLT building variants to functionally equivalent reinforced concrete building in a cradle-to-gate life cycle analysis. In a similar analysis, Jayalath et al. (2020) studied the life cycle performance of CLT mid-rise residential buildings, assuming the end-of-life CLT to be landfilled or incinerated. In the assessments of life cycle environmental impact of buildings with CLT structural systems, Durlinger et al. (2013) assumed the CLT elements are landfilled, owing to the unavailability of evidence regarding the fate of CLT products, while Liang et al. (2020) did not consider the end-of-life fate of the CLT elements. Dodoo et al. (2014) and Tettey et al. (2019) explored the climate impacts of timber frame multi-storey buildings, including CLT construction systems, considering the production, operation and end-of-life stages. However, these studies did not analyse the implications of maintenance and potential repair or replacement during the service life. Hence the few studies reported in the literature have not accounted for the complete life cycle activities of CLT buildings. Different synergies and trade-offs exist in the distinct life cycle stages of buildings and all activities need to be considered in a full characterisation of the climate impact of a building. While studies on carbon footprint of CLT buildings are reported in the literature (e.g. Robertson et al. 2012, Darby et al. 2013, Durlinger et al. 2013, Dodoo et al. 2014, Guo et al. 2017, Lolli et al. 2019, Tettey et al. 2019, Liang et al. 2020), to our knowledge, there are no studies focusing on the synergy between the engineering aspects and life cycle carbon footprint of CLT buildings.

This study analyses the carbon footprint of a CLT multi-storey building from a life cycle perspective, and explores strategies to optimise the building’s carbon footprint through a synergy approach. The life cycle carbon footprint is optimised considering synergies between efficient wood utilisation for CLT panels, construction of CLT buildings with efficient connections, and plausible improved service-life risk management of CLT structures. The study connects structural engineering aspects of CLT with life cycle analysis in a particularly multi-disciplinary way, with the goal of reducing the life cycle carbon footprint.

2. Synergy between structural engineering design and carbon footprint of CLT

In this study, the synergy effect between structural engineering design solutions and carbon footprint of CLT in multi-storey building is explored based on the knowledge in a research project in Sweden (Linnaeus University 2020). It is a collaborative project between the academia and industry to address open research issues for competitive CLT-based building systems. The project addresses four aspects of CLT in building construction, with focus on the synergy between these aspects: optimised material utilisation, construction and connections, building risks during service-life and life cycle carbon footprint.

The aspect on optimised material utilisation concerns the efficiency of raw material use and the development of appropriate grading schemes for the production of strong and stiff CLT panels. Issues explored include mechanical material properties of the final CLT panels. The construction and connection aspect concerns the development of connectors with optimal material usage and efficiency in the assembly phase of CLT construction. This part of the project develops design models for dowel-type connectors and assesses connectors for the efficient disassembly of CLT elements at end-of-life buildings, among other things. The building’s risk part concerns the assessment and mitigation of plausible risks during the service life of CLT buildings. This conducts instrumentation of hygrothermal parameters and monitoring of long-term structural performance of CLT buildings by installed sensors. It identifies property changes related to , for example, moisture, and develops models in addition to measurement data for the training of neural networks, to detect potential issues or even damages related to the moisture and development of mould. The life cycle carbon footprint aspect explores strategies to optimise the climate impact of multi-storey CLT buildings, considering different technical solutions for the construction and synergies identified in the engineering aspects.

2.1. Optimised material utilisation

Sawn timber used for CLT exhibits large variations in strength, modulus of elasticity and density, which are properties correlated with each other. In CLT, lamella with high modulus of elasticity should preferably be placed in the outer, surface layers, while timber of lower quality should be placed in the centre of the panel. Today, however, many producers use the same grade (for example C24) for all layers and therefore miss the opportunity to optimise the panel. If lamellae of Norway spruce from Sweden and other Nordic countries are divided in two different grades, one with higher and one with lower modulus of elasticity, it is possible to increase the bending stiffness, or reduce the thickness of the CLT panel, compared to the case when a single grade is used in all layers. The potential improvement depends on several factors and this is currently investigated. An estimate is that appropriate grading of lamellae would enable a reduction of the thickness of the CLT panel by five to ten per cent, in cases where out-of-plane bending stiffness is crucial (Ström 2020). The use of lamellae with high modulus of elasticity in the surface layers of panels also means that the average wood density in the surface layers is higher. This may be an advantage when it comes to strength of connections with mechanical fasteners (this is studied in the construction and connection part) since higher density means higher embedment strength and withdrawal capacity of fasteners. Characteristic values of grade determining properties of C-classes, which are dominating today, are specified in EN 338 (2016). A study of stiffness and strength at in-plane loading of CLT-elements is presented in Danielsson et al. (2017).

2.2. Construction and connection

The design of connections is crucial for the global reliability of CLT structures and governs load transfer within the structure. A more efficient and targeted design of connections can thus lead to a more efficient load transfer and to an increased reliability of CLT structures. Efficiency can, moreover, be achieved by a design for prefabrication and fast assembly on-site during construction as well as, at the end-of-life, easy dissembling of the structural elements. A number of innovative connection systems, especially for CLT, have been developed during the last years, while traditional connection systems, such as hold-downs and angle brackets, are still most commonly used. A high number of laterally and axially loaded metal fasteners (most commonly screws) are used for the different types of connections in CLT construction systems. Innovative CLT connectors rely on the same types of load transfer mechanisms but aim at a reduced number of connectors and pre-fabrication for faster assembling.

Stiffness and strength of fasteners are, in addition to the fastener properties itself, governed by the local material behaviour of the CLT. In engineering design, CLT is treated as a quasi-homogeneous product (see e.g. Ringhofer et al. 2018). One objective of the current research project is to investigate the potential of considering CLT layers with different qualities or strength classes and the layered structure of the building product for a more reliable and efficient design of connections. An analytical approach for the strength of connections, considering the layered structure, was presented in Blaß and Uibel (2007). Kinematically compatible behaviour of connections can be predicted by using a so-called Beam-on-Foundation (BOF) modelling approach (Lemaitre et al. 2018). Preliminary results with the latter approach show that the strength of connections can be increased when higher strength classes are used in the outer layers, when loading occurs parallel to the fibre direction of the outer layer.

Connection design approaches rely on single fastener properties, this procedure is quite conservative and may lead to inefficient designs, since it neither exploits the full potential of connections in CLT, nor does it provide a link between the connector and the prefabricated elements or the entire structures. The latter is particularly important for a reliable, robust and efficient design. Laterally loaded connections in CLT exhibit a very ductile behaviour due to the crosswise layup and an internal reinforcement effect. This allows for a ductile design of connections and load redistribution in structures (Rosenberg and Henriksson 2019). Thus, not the design for the characteristic strength of a single fastener will govern overall failure of the structure, but the higher characteristic strength of several fasteners or connections. Such a system effect is considered in the CLT product itself, where the characteristic value of a single board is multiplied by a factor of up to 1.15 for considering a homogenisation effect (Gustafsson 2019). A similar approach is also applied for the design of connection in timber frame shear walls, where the fastener capacity is multiplied by a factor of 1.2 to account for homogenisation effects due to a high number of ductile fasteners (Eurocode 5: EN 1995-1-1:2004+AC:2006+A1 2008). Exploiting this beneficial characteristic could lead to a reduced number of fasteners by considering a system factor, depending on the number of fasteners or connections.

As an alternative to metal fasteners, the use of wooden, carpentry-type connections is expected to have a great benefit in a holistic perspective that includes the impact on the environment. Indeed, such connection systems are available for certain types of connections, e.g. for connections between adjacent CLT floor elements. Due to the automatic production using CNC machining, and a trend towards integration of automated manufacturing with the design, e.g. for wall-to-floor connections or panel-to-panel connections, such connection systems are expected to become more effective and competitive in the near future.

Labour and installation time have a measurable impact on the direct cost and on carbon cost of construction. Both depend largely on the type of connections and the number of connectors used. A pilot study showed that the time for assembly can be reduced by more than 30% if screws are inserted into predrilled holes (Finnhult and Petersson 2020). In addition to reduced assembly time, the quality of the position of the screws considerably improved. This is only one example that highlights the potential for a more efficient assembly through pre-fabrication.

2.3. Service life risk management

Moisture content is a significant parameter for the performance of timber elements, affecting mechanical properties, such as strength and stiffness. Excessive moisture levels, coupled with conducive temperature conditions and suitable oxygen supply, can potentially lead to fungal decay in timber structures (Schmidt et al. 2019). This can have influence on the durability and expected service life of CLT-based structural elements, as other timber-based structures. Studies (e.g. Serrano et al. 2014, Dorn et al. 2019) have been done on both timber elements and large-scale structures to investigate their performance with time and the influence of different parameters which affect the structural health of timber buildings.

Service life risk management is undertaken in the research project to establish parameters which potentially affect the durability and service life of timber structures, particularly CLT elements. Monitoring systems are being deployed to assess certain properties of CLT building elements and structures during the service life. In addition to moisture and temperature in the CLT elements, other parameters, such as displacements and vibrations, are also monitored (Serrano et al. 2014, Dorn et al. 2019). Since monitoring over a long-time span is required, reliability is of great importance. Automated data collection, storage and processing routines are deployed as well as evaluation routines that allow for automated interpretation of data, e.g. with hindsight towards abnormal behaviour or the potential for biological decay. With these, service life prediction models would be developed to facilitate proper building risk management focusing on preventive maintenance, which minimises repairs, replacements and damages of CLT elements or structures.

2.4. Life cycle analysis

In a building’s life cycle, CO₂ may be emitted from non-energy-related sources, including industrial process reactions and land-use practices, and from the combustion of fossil fuels linked to material production, transportation, construction, service life, maintenance and demolition activities. Accurate analysis and optimisation of climate impacts of CLT buildings need to consider these dynamics and their potential synergies, to design CLT buildings that minimise carbon emissions. Figure 1 is illustrative and shows examples of synergies effect between the described structural engineering aspects and carbon footprint of CLT in multi-storey buildings. As an example, optimised material utilisation and improved construction and connections have implications for the carbon footprint at the production, construction, service-life and the demolition stages of a CLT building, while building risk information has implications for maintenance regimes and the service life period of the building. Hence knowledge from these engineering aspects is incorporated in the present life cycle analysis, to optimise the carbon footprint of the studied CLT building.

Figure 1. Illustration of the explored synergies between engineering design and carbon footprint of CLT building.

3. Methods and assumptions

This study used a process-based life cycle analysis approach to quantify and optimise the climate impact of a case-study CLT multi-storey building. The analysis incorporates principles of normative standards (e.g. EN 15978 2011, ISO/TS 14067 2013) for the assessment of buildings environmental impacts.

3.1. Case study CLT building

An eight-story CLT-based multi-family building in Växjö, Sweden is used as the object for this study. The building has a total heated floor area of 3374 m² and contains 33 apartments. Figure 2 shows a drawing and a floor plan of the building. The foundation and ground floor are constructed of reinforced concrete, while the second to eighth floors are constructed of CLT panel elements. Three CLT wall types are used for the construction of the load-bearing frame in the building. Schematic representation of the wall and floor constructions is given in Figure 3, from Serrano (2009). A detailed documentation of the construction characteristics of the building is given in Jarnerö (2008) and Serrano (2009).

Figure 2. Illustrations of the west facade and a floor plan of the studied CLT multi-storey building.

Figure 3. Schematic representation of the different CLT walls [(a) exterior wall, (b) apartment separating wall, (c) other interior load bearing wall (within apartment)] and floor constructions of the studied building.

The total mass of the finished building is 2670 air-dried metric tons, of which CLT is 369.9 air-dried metric tons (Table 1). In terms of the total floor area of the building, this corresponds to 109.6 kg of CLT per unit area (m²). The floors used 49% of the total amount of the CLT, while the interior and exterior walls and the balconies used 37% and 14%, respectively, of the CLT. The mass of all materials in the finished building is documented in Serrano (2009) and Dodoo and Muszyński (2021).

Table 1. Mass of CLT in different parts of the studied CLT multi-storey building.

Structural component	Amount of CLT (air-dried metric tons)
Floors	180.8
Exterior and interior walls	135.1
Balconies	51.6
Stairwells	2.4
Total	369.9

3.2. Scope, system boundary and data

The carbon footprint over the life cycle of the building is modelled as-built, as a reference. In addition, the identified synergies in structural engineering are used to develop plausible assumptions to model further life cycle carbon footprint scenarios, for insight on how these influence the climate impact of a CLT building. The entire building is taken as the functional unit in this analysis and the carbon footprint is calculated in kgCO_2eqv/m² (heated floor area), considering GHG flows linked to building material production, construction, service life and end-of-life stages, including associated potential benefits and burdens. In this study, 100-year time horizon global warming potential factors of different GHGs are used. The system boundary for the analysis is illustrated in Figure 4, following the modularity of activities in EN 15978 (2011). Due to the study’s goal, the modules regarding the use, refurbishment, operational energy and water uses are not included in the analysis. The modules considered in this study are marked with dash borders in Figure 4.

Figure 4. System boundary of activities in the study. The dash borders show what is covered in the analysis.

The carbon footprint for material production (A1–3) is calculated based on the material mass in the finished building (Table A1), typical material wastage factors during construction in Björklund and Tillman (1997), and life cycle inventory data from the most recent version (3.7.1) of the Ecoinvent database (Wernet et al. 2016, Ecoinvent 2020). The Ecoinvent database accounts for energy and environmental flows linked to the extraction, processing and transportation of a wide range of materials in different sectors, and it is recognised as one of the most comprehensive life cycle inventory databases (Cobut et al. 2015, Takano et al. 2015). The Appendix (Table A1) gives a summary of the building’s materials, retrieved from Serrano (2009) and, Gustavsson et al. (2010) as well as the building’s architectural/construction drawings, and the list of the Ecoinvent processes for the life cycle inventory and impact calculations. The data used are mostly representative values of manufacturers in Europe (RER). For cases where European or Swedish representative values are not available, data for Switzerland (CH) in Ecoinvent are used. The IPCC 2013 (IPCC 2013) global warming potential (100a) values and the cut-off system model in Ecoinvent are applied in this study. All the wood-products, including the CLT, are assumed to come from a sustainably managed forest, as in Sweden (Dodoo et al. 2014, Swedish Wood 2020). Fossil-based GHGs, from the use of fossil fuels, and biogenic GHGs, from the use of forest-based fuels, are distinguished in this study. In the present calculations, fossil-based GHG emissions are considered and biogenic GHG emissions are excluded, following the assumptions of Piccardo et al. (2020).

The carbon footprint, regarding construction activities (A4–5), encompasses GHG emissions for material transportation to the construction site and the equipment used to assemble the materials into the ready building. This is calculated based on the data presented by Dodoo and Muszyński (2021) and assumptions in Gustavsson et al. (2010) and Dodoo and Gustavsson (2012), which assume that half of the energy for the construction activities is diesel oil and the remainder is electricity.

Currently, none of the buildings constructed in CLT has reached mid-service life and hence reliable empirical data to support rigorous analysis of this stage of CLT buildings are lacking. The original balcony structure of the studied CLT building was replaced due to moisture-related issues in 2020 (Dodoo and Muszyński 2021), about a decade after construction. This served as the basis for a simplified analysis of the service life in this study. Besides replacement of the balcony structure, the building is assumed to be painted every decade (Takano et al. 2015) during the service life, taken to be 50 years in this study. Material-related life cycle GHG emission data from the Ecoinvent database are used to model the carbon footprint for the service life activities (B2–4).

The activities analysed at the end-of-life stage are deconstruction and transportation of the building materials to treatment sites for reprocessing (C1–3). The potential benefits and burdens (D) associated with recovery of the post-use CLT and reinforced concrete, the dominant material by mass in the building are analysed. Empirical data on end-of-life management of CLT buildings are currently lacking. Dodoo and Muszyński (2021) explored the implications of options for end-of-life management of CLT buildings for optimised material recovery and environmental benefits, and data from the study are used here. Following this, the post-use CLT is assumed to be cascaded via reprocessing into particleboards for building applications and subsequently used as bioenergy to replace fossil fuels. Production process and data for particleboards production based on recycled wood do not exist in Ecoinvent v3.7.1. Hence the benefits and burdens associated with recycling the post-use wood (CLT) into particleboards are estimated based on data from Sathre and Gustavsson (2006) and Merrild and Christensen (2009). The benefits and burdens of the subsequent energy recovery after the particleboard’s useful life are estimated using data from Dodoo et al. (2008, 2009). The accounting of the climate impact of post-use CLT cascading considers both the burdens from the end-of-life management and the benefits due to the displaced processes and activities. The burdens comprise emissions linked to dismantling of the post-use CLT panels, transportation to end-of-life processing or disposal sites at assumed distance of 50 km, and crushing the CLT panels into smaller pieces and reprocessing these into particleboard. The accounted benefits encompass the displaced forest processes and activities associated with acquisition of fresh wood and wood processing into particleboard, and the avoided burning of fossil fuel due to the availability of bioenergy. In the analysis here, 70% of the post-use CLT is assumed to be recovered and cascaded, based on data presented in Passarelli (2018).

The reinforced concrete is assumed to be recycled as scrap for the secondary steel and sub-base filling material, based on data from Dodoo et al. (2014). The concrete is assumed to be crushed into aggregate to displace natural aggregate-filling materials. The reinforcing steel in the concrete is assumed to be sorted out for recycling during the concrete crushing, for shredding and melting to replace ore-based steel to produce new reinforcing steel. This analysis use a simplified approach, allocating the burdens associated with post-use management to the final product made from the end-of-life materials and accrediting the avoided impacts to the material end-of-life options.

3.3. Modelling the identified synergy effects

The emerging knowledge, described in section 2.1–2.3, is considered as the basis to develop plausible scenarios to model the implications of the synergy effects between the structural engineering design aspects and carbon footprint for CLT building. Table 2 summarises how the described structural engineering aspects are linked to the life cycle carbon footprint modelling.

Table 2. Structural engineering aspects and their implications for the life cycle carbon footprint modelling.

Engineering aspect	Life cycle phase influenced	Description of influence
Optimised material utilisation	Production, end-of-life stage and post-use benefits	Amount of CLT used and recovered for post-use management
Efficient connection and construction	Production, construction, end-of-life stage and post-use benefits	Amount of fasteners used, assemble and disassemble of CLT, amount of CLT recovered for reuse
Service life risk management	Service life	Early identification of repairs and maintenance needs, thereby avoiding major structural replacements

Material utilisation optimisation and connections efficiency of CLT panels have implications for the carbon footprint at the production and construction stages, as well as at the end-of-life and post-use material management stages. The thickness of CLT panel may be reduced by 5–10% while fulfilling the same structural function (section 2.1). Consequently, the implications of a 7.5% average reduction of the used amount of CLT in the studied building are explored in the life cycle carbon footprint modelling.

There is a potential reduction of the amount of fasteners when using efficient and reliable connection systems coupled with the synergy with optimised CLT panel thickness (section 2.2). Following this, the amount of steel-made CLT connection is assumed to be reduced by 25%. Efficient connections additionally present greater possibilities for effective post-use CLT recovery e.g. reuse and cascading. In a recent demonstration project, where CLT panels were deconstructed and subsequently used for a new construction, Passarelli (2018) reported that the usable areas of the recovered CLT panels significantly decreased due to the previously used connections, and about 30% of the recovered CLT panels were eventually lost as waste during the reprocessing for the new construction. This served as the basis for the modelling of the potential recoverable post-use CLT in the reference case. Informed by this, this analysis assumes that potential recoverable post-use CLT for further use improves from 70% to 90% with the efficient connection systems, which facilities deconstruction and material recovery.

Replacement of the balconies during the service-life is modelled in the reference case of the building. Crucial long-term data to substantiate modelling of service life activities, such as maintenance, repairs and replacements regimes in CLT buildings, are generated in the research project on which this study is based. The knowledge generated, among other things, will facilitate prediction of potential damages and hence timely preventive actions in CLT buildings. Given the lack of relevant long-term data, a simplified assumption is made and here the damage-related replacement in the reference case is halved for the scenario of effective building risk management. Through the instrumentation, potential issues, which might adversely affect the service life performance, may be identified and hence timely repaired or maintained.

4. Results

The material production carbon footprint of the building is presented in Figure 5, and in Figure 6 the relative contributions of the materials to the building’s total mass and material production carbon footprint are illustrated. CLT constitutes 14% of the building’s total material mass and accounts for 18% of the material production carbon footprint. Steel and concrete together represent 38% of the material production carbon footprint, and these account for almost 60% of the total mass of the building. Rock wool insulation constitutes 11% and 3% of the material production carbon footprint and the building’s total material mass, respectively.

Figure 5. Material production (A1–3) carbon footprint for the reference building.

Figure 6. Relative distributions of materials of the reference building in terms of mass and carbon footprint.

In Table 3 the material production carbon footprint is broken into those connected to the CLT and non-CLT structural components in the building. The CLT structural component comprises other materials besides the CLT. For example, the CLT floor structure in the studied building contains insulation, steel fasteners, plasterboards, plywood among others. The CLT structural components represent 53 and 47%, respectively, of the building’s total material production carbon footprint. The exterior and interior walls represent 51%, while the floor represents 38% of the carbon footprint (A1–3) linked to the CLT structural components. The foundation and ground floor account for 67% of the carbon footprint for the non-CLT structural components. Optimised material utilisation and efficient connection systems give 2.7% and 1.1% reductions, respectively, in the material production carbon footprint for the CLT structural components. The corresponding carbon footprint reductions when considering both the CLT and non-CLT structural components are 1.5% and 0.7%, respectively. These values are presented in Table 4, which illustrates how the modelled structural engineering aspects influence the carbon footprint in the different building life cycle modules. The modelled risk management scenario suggests about 25.6% reduction in the carbon footprint during the service life. The post-use benefits of CLT are increased with efficient connection systems as the recoverable post-use CLT for cascading is greater in this case compared to the reference building. For the case of optimised material utilisation, the recoverable post-use CLT and its associated benefits are reduced compared to the reference building. This is due to the drop (7.5%) in the total mass of CLT in the building as the thicknesses of the panels are reduced.

Table 3. Material production carbon footprint (kgCO_2eqv/m²) broken into CLT and non-CLT structural components.

Structural component	Reference	Optimised material utilisation	Efficient connection and construction
CLT structures:	107.5	104.5	106.3
Floors	41.3	40.0	41.3
Exterior and interior walls	55.2	54.0	54.1
Stairwells	0.6	0.6	0.6
Balconies	10.3	9.9	10.2
Non-CLT structures:	95.9	95.9	95.7
Foundation and ground floor	64.0	64.0	64.0
Windows and doors	10.1	10.1	10.1
Interior finishes	16.9	16.9	16.9
Roof	4.9	4.9	4.7
CLT + non CLT structures	203.4	200.4	201.9

Table 4. Reductions of carbon footprint for the structural engineering aspects compared to the building in the reference case. Negative values denote increased benefit, while positive values denote reduced benefit.

Building module and stage	Optimised material utilisation	Efficient connection and construction	Service life risk management
Production (A1–3)	−1.5%	−0.7%	-
Construction (A4–5)	–	–	–
Service life (B2–4)	–	–	−25.8%
End-of-life (C1–3)	–	–	–
Post-use benefits of CLT (D)	7.5%	−27.9%	–

Table 5 shows the carbon footprint due to the synergy effect, when the interactions and trade-offs between the different structural engineering aspects are combined. The carbon footprints are given for different life cycle modules and are compared to that of the building in the reference case. Compared to the reference case, the synergy effect gives about 43.3% reduction in life cycle GHG balance of the building. The biggest GHG reduction is linked to improved post-use CLT recovery due to efficient connection systems. Still the carbon footprint reduction achievable in the material production stage is noteworthy given that this comes from only the CLT and connections. When excluding the benefits associated with post-use CLT recovery, the synergy effect results in 4.4% reduction in life cycle carbon footprint compared to the reference case. Figure 7 illustrates the relative carbon footprints of the synergy effect and reference case for the different life cycle modules. This demonstrates the significance of a synergy approach between engineering aspects of CLT in building for carbon footprint reduction.

Figure 7. Relative distribution and net balance of the carbon footprint for the reference building and the synergy effect of structural engineering design.

Table 5. Carbon footprint and life cycle GHG balances for the reference building and the synergy effect of structural engineering design.

Building module and stage	Carbon flow (kgCO2eqv/m^2)		GHG savings
	Reference	Synergy	kgCO2eqv/m^2	%
Production and construction stage:
Module A1–3	203.4	199.2	4.1	2.0
Module A4–5	40.4	40.4	–	–
Service life stage:
Module B2–4	32.2	23.9	8.3	25.8
End-of-life stage:
Module C1–3	4.5	4.5	–	–
Potential benefits and burden:
Module D			-
CLT/wood recovery of energy	−152.4	−181.3	28.9	19.0
Concrete & steel recycling	−35.7	−34.3	−1.4	−3.9
Life cycle balance (With Module D)	92.4	52.4	39.9	43.3
Life cycle balance (Without Module D)	280.5	268.0	12.4	4.4

5. Discussion

The focus of this study is to better understand the synergy between structural engineering design solutions and life cycle carbon footprint of CLT multi-storey buildings. The analysis is based on results in an on-going multi-disciplinary research project, exploring strategies to improve the competitiveness of CLT-based building systems through improved engineering design and reduced climate impact. It identifies opportunities to reduce climate impact of CLT-based buildings, through a synergy approach integrating knowledge from wood material utilisation, wood construction and connections, building risk management and life cycle analysis. The findings suggest that the synergy effect between the explored aspects can result in a significant reduction in life cycle carbon footprint of CLT-based buildings.

The carbon footprint analysis in this study follows recommendations of normative standards and a life cycle perspective approach, including the impacts of material production, construction, service life, and end-of-life management and potential benefits associated with post-use materials. GHG flows in the life cycle of a CLT multi-storey building are modelled to explore the implications of the synergy effect of the structural engineering aspects. In the studied building, CLT-structural components represent almost 60% of the total material production carbon footprint. However, emerging knowledge in this research project showed that there is potential to reduce CLT panel thickness by 5–10% (Ström 2020). On an average CLT panel thickness reduction of 7.5%, this analysis suggests that optimised material utilisation and efficient connection give about 3 and 1% reduction, respectively, in the material production carbon footprint for the CLT structural components. As a result, optimised material utilisation and efficient connection, respectively, give 1.5 and 0.7% reductions in the building’s total material production carbon footprint. Besides, these aspects have implications for the CLT available for post-use recovery. Furthermore, efficient connection systems facilitate the design and construction of CLT structures for disassembly. This enables realisation of the potential benefits offered by effective end-of-life management of CLT panels, including material recovery for reuse and cascading (Dodoo and Muszyński 2021). This can play a role in the current drive for transition to a bio-circular economy. The carbon footprint for the service life of a CLT building can be optimised by including a proper building risk management strategy to involve preventive maintenance to minimise the repair and replacement of building components.

In this study, a 43% reduction in life cycle GHG balance of the studied CLT building is noted through the synergy between structural engineering design and carbon footprint. This reduction is dominated by the benefits linked to improved post-use CLT recovery, which is connected to efficient connection and construction systems. When the benefits of post-use CLT recovery are not accounted for, about 4% reduction in the life cycle carbon footprint is noted for the synergy effect. Besides the carbon footprint reductions, the synergies in optimised material utilisation, efficient connection and reduced building service-life risk may result in cost savings. The issue of cost savings is not considered here and may be explored in further studies to further improve the competitiveness of CLT construction.

Bioenergy systems may influence the cycling of biogenic carbon in the biosphere and have positive, neutral or negative effects on the total carbon stock, depending on different factors including land-use management of the locations of the systems (Berndes et al. 2016). In this study, the inventory analysis did not account for the biogenic carbon sequestered or released from the wood-based materials as the wood is assumed to come from a well-managed sustainable forest, where carbon flows out of the forest are balanced by carbon uptake by growing trees at the landscape level (Dodoo et al. 2014). All biogenic carbon flows in buildings may be considered in the analysis of trade-offs and benefits of different strategies for climate change mitigation. A comprehensive analysis of biogenic carbon flows requires dynamic time-dependent accounting of carbon flows and may be complex to quantify, due partly to the high uncertainties in assumptions and lack of consensus on key methodological choices in such analysis (Werner et al. 2010, Berndes et al. 2016, Breton et al. 2018, Xu et al. 2018).

Data quality is crucial for the validity and reliability of a life cycle carbon footprint analysis. The European standard EN 15978 (2011) suggests that dataset for life cycle inventory may be from relevant and appropriate sources where data from environmental product declarations (EPDs) are not available for a building being analysed. This study used the most recent representative datasets from the Ecoinvent database, due to the unavailability of specific EPDs data for the materials in the building analysed. Life cycle inventory models and also EPDs may give significantly different carbon footprint values for the same type of product. For example, recent EPDs data for CLT suggest production (A1–3) carbon footprint values ranging from 34.0 to 53.8 kg CO_2eqv /m³ (Södra 2020, Stora Enso 2020). Such significant variations may be linked to various factors, including differences in fuels used for manufacture plants, transportation distances, production technologies, processing efficiencies and natural differences in physical properties of raw materials. Future research may explore the implications of data variabilities for the performance of the explored synergies in CLT building design and construction.

A growing discussion in our society today regards strategies to reduce GHG emission and thereby achieve carbon-neutral society (Government Offices of Sweden and Ministry of the Environment and Energy 2017, European Commission 2018). Achieving this ambitious target requires the deployment of various solutions, including low-carbon buildings and construction technologies. The growing body of knowledge emphasises the climate benefits of wood-based building systems in contrast to non-wood alternatives (IPCC 2007, Dodoo et al. 2009, Sathre and O’Connor 2010, Lippke et al. 2011, IPCC 2014, Tettey et al. 2019). A significant share of the material production GHG emission for wood-based materials is biogenic, arising from the use of bioenergy. Besides, biomass residues from the wood product chain are typically used as processing energy for wood materials and to replace other fuels (Gustavsson et al. 2006). The storage of carbon is dynamic which affects climate impacts of wood-based construction (Sathre and O’Connor 2010). This provides carbon sequestration from the atmosphere, howbeit typically temporary (Schlamadinger and Marland 1996). Notwithstanding these benefits, the rate of adoption of wood-based construction systems for multi-storey buildings remains slow (Mahapatra et al. 2012, Hemström et al. 2017, Hildebrandt et al. 2017). Hence studies to increase knowledge and competitiveness of wood-based multi-storey buildings are essential.

Increasingly, studies (e.g. Pierobon et al. 2019, Tettey et al. 2019) show CLT as an example of a low-carbon alternative to steel or concrete in mid-rise multi-storey structures. This research has endeavoured to explore strategies to further improve the carbon footprint of CLT-based building systems through a synergy approach. It shows the importance of taking a life cycle perspective and integrating knowledge in different aspects of structural engineering to improve the carbon footprint of CLT multi-storey buildings.

6. Conclusions

This paper demonstrates the extent to which the synergy between structural engineering design solutions and life cycle carbon footprint analysis can contribute to improving the climate performance of CLT buildings. The analysis shows that the material production carbon footprint of the studied CLT building is 203.4 kgCO_2eqv/m², with 53% of the carbon footprint connected to the CLT structural components of the building. The application of the synergy approach yields a 4% reduction in the carbon footprint for material production of the building’s CLT structural components. This corresponds to a 2% reduction in the material production carbon footprint for both the CLT and non-CLT structural building components. The synergy approach yields additional 26% reduction in the carbon footprint during the service life of the building. Aggregated together, these synergetic benefits result in a 4% reduction of the life cycle carbon footprint of the building. When the flows associated with the end-of-life stage, including the recovery and cascading of post-use CLT, are considered, the synergy approach results in a 43% reduction of the overall life cycle carbon footprint of the building. This study shows that the synergy approach holds promise to further improve the climate performance of CLT buildings.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research work was funded by the Knowledge Foundation through the project ‘Improving the competitive advantage of CLT-based building systems through engineering design and reduced carbon footprint’ [20190026].

Appendix

Table A1. Quantities of inventoried materials (air-dry) contained in the reference building and data sources for the life cycle inventories analysis. RER and CH are short names for Europe and Switzerland in Ecoinvent.

Material	Mass (tonne)	Names of used dataset in the Ecoinvent database (v3.7.1)
CLT	369.9	Cross-laminated timber//[RER] cross-laminated timber production
Sawn timber	116.0	Structural timber//[RER] structural timber production
Plywood	1.2	Plywood//[RER] plywood production
Particleboard	12.6	Particleboard, uncoated//[RER] particleboard production, uncoated, average glue mix
Plasterboard	231.7	Gypsum plasterboard//[CH] gypsum plasterboard production
Rock wool	69.9	Stone wool, packed//[CH] stone wool production, packed
Concrete	1518.4	Concrete, high exacting requirements//[CH] concrete production, for building construction, with cement CEM II/A
Crushed stone	145.8	Gravel, crushed//[CH] gravel production, crushed
Rebars	49.6	Reinforcing steel//[Europe without Austria] reinforcing steel production
Steel sheets	21.6	Metal working, average for metal product manufacturing//[RER] metal working, average for metal product manufacturing
Aluminium	1.4	Aluminium alloy, AlMg3//[RER] aluminium alloy production, AlMg3
Plastics	5.7	Polystyrene, extruded//[RER] polystyrene production, extruded (average for CO2 blown, HFC-134a blown & HFC-152a blown)
Paints	4.7	Alkyd paint, white, without water, in 60% solution state//[RER] alkyd paint production, white, water-based, product in 60% solution state
Mortars	56.4	Cement mortar//[CH] cement mortar production
Sealants	9.5	Light mortar//[CH] light mortar production
Glue	0.4	Adhesive, for metal//[RER] market for adhesive, for metal
Asphalt	0.1	Synthetic rubber//[RER] synthetic rubber production
Roofing felt	2.6	Bitumen seal//[RER] bitumen seal production
Rubber	0.1	Latex//[RER] latex production
Glass	29.3	Flat glass, coated//[RER] flat glass production, coated
Ceramics	22.2	Ceramic tile//[CH] ceramic tile production
Porcelain	1.4	Sanitary ceramics//[CH] sanitary ceramics production