Comparative Life-Cycle Assessment of a High-Rise Mass Timber Building with an Equivalent Reinforced Concrete Alternative Using the Athena Impact Estimator for Buildings

Buildings consume large amounts of materials and energy, making them one of the highest environmental impactors. Quantifying the impact of building materials can be critical to developing an effective greenhouse gas mitigation strategy. Using Athena Impact Estimator for Buildings (IE4B), this paper compares cradle-to-grave life-cycle assessment (LCA) results for a 12-story building constructed from cross-laminated timber (CLT) and a functionally equivalent reinforced concrete (RC) building. Following EN 15978 framework, environmental impacts for stages A1–A5 (product to construction), B2, B4, and B6 (use), C1–C4 (end of life), and D (beyond the building life) were evaluated in detail along resource efficiency. For material resource efficiency, total mass of the CLT building was 33.2% less than the alternative RC building. For modules A to C and not considering operational energy use (B6), LCA results show a 20.6% reduction in embodied carbon achieved for the CLT building, compared to the RC building. For modules A to D and not considering B6, the embodied carbon assessment revealed that for the CLT building, 6.57 × 105 kg CO2 eq was emitted, whereas for the equivalent RC building, 2.16 × 106 kg CO2 eq was emitted, and emissions from CLT building was 70% lower than that from RC building. Additionally, 1.84 × 106 kg of CO2 eq was stored in the wood material used in the CLT building during its lifetime. Building material selection should be considered for the urgent need to reduce global climate change impacts.


Introduction
The building sector is one of the major industries producing a large amount of greenhouse gases [1,2]. It contributes about 36% of global energy use and 39% of energy-related carbon dioxide (CO 2 ) emissions [3]. Therefore, the building sector represents significant potential for reducing emissions and mitigating global climate change.
Two major approaches are currently being adopted to reduce environmental impacts from the building sector. One is selecting materials wisely, and the other is optimizing energy use during the service life of a building [4,5]. Hafner [1] calculated through life-cycle analysis that the operational phase accounts for 45% to 80% of total carbon emission, depending on the energy standard with which the buildings are built. That leaves 20% to 55% of total carbon emissions related to materials used in buildings and their embodied carbon. Therefore, the greenhouse gas (GHG) emissions of whole buildings can be significantly influenced by the selection of construction material. Considering the (WBLCA) focusing on US data. They are Tally and IE4B. Tally is an Autodesk Revit application to help architects and engineers to compare environmental impacts of different building designs. Tally extracts data from Revit models to calculate embodied impacts. We chose IE4B, because we do not have the Revit model, but only material take-off (Table A1) data from the architects. The embodied carbon for both buildings is analyzed and reported, to compare their environmental impacts. Embodied carbon refers to the GHG emissions from the life-cycle stages of the whole building except for operational energy emissions.

Building Design
The building studied here was based on the Framework building designed by LEVER Architecture, a 12-story mixed-use building with 8,360 m 2 floor area ( Figure 1). The Framework building was the first high-rise MT building designed in the United States to obtain approval for construction. It was designed to be built for mixed-use with combined retail and public exhibition on the ground floor, five levels of office, and 60 units of affordable housing. The framework's structural system was to be made from glulam columns and beams, and the floors and walls were from large amounts of CLT materials. The foundation, one of the heaviest parts of the building, would have been constructed of concrete and steel rebar.
Sustainability 2020, 12, x FOR PEER REVIEW 3 of 17 This paper aims to investigate the environmental impacts of a MT high-rise multi-functional building, compared with its more traditional RC counterpart, using Athena Impact Estimator for Buildings (IE4B). There are two commercially available software tools to perform Whole Building LCA (WBLCA) focusing on US data. They are Tally and IE4B. Tally is an Autodesk Revit application to help architects and engineers to compare environmental impacts of different building designs. Tally extracts data from Revit models to calculate embodied impacts. We chose IE4B, because we do not have the Revit model, but only material take-off (Table A1) data from the architects. The embodied carbon for both buildings is analyzed and reported, to compare their environmental impacts. Embodied carbon refers to the GHG emissions from the life-cycle stages of the whole building except for operational energy emissions.

Building Design
The building studied here was based on the Framework building designed by LEVER Architecture, a 12-story mixed-use building with 8,360 m 2 floor area ( Figure 1). The Framework building was the first high-rise MT building designed in the United States to obtain approval for construction. It was designed to be built for mixed-use with combined retail and public exhibition on the ground floor, five levels of office, and 60 units of affordable housing. The framework's structural system was to be made from glulam columns and beams, and the floors and walls were from large amounts of CLT materials. The foundation, one of the heaviest parts of the building, would have been constructed of concrete and steel rebar.

Goal and Scope
The goal of this study was to conduct a whole building LCA using the IE4B, to estimate environmental impacts from the building materials of assemblies, construction, and the end of building life treatment during a 60-year period [29]. Comparison of the LCA was between the MT building and a

Goal and Scope
The goal of this study was to conduct a whole building LCA using the IE4B, to estimate environmental impacts from the building materials of assemblies, construction, and the end of building life treatment during a 60-year period [29]. Comparison of the LCA was between the MT building and a functionally-equivalent RC building specifically examining the effect of CLT use in high-rise residential buildings for global climate change mitigation potential.
The system boundary was defined as "cradle to grave," which includes the product stage (A1-A3), construction stage (A4-A5), use stage (B2, B4), and end of life stage (C1-C4) [30], as shown in Figure 2. System expansion was employed to take into account the net benefits related to reuse and recycling Sustainability 2020, 12, 4708 4 of 15 of the materials and products, as well as energy recovery from materials, such as wood incineration beyond the system boundary (D). functionally-equivalent RC building specifically examining the effect of CLT use in high-rise residential buildings for global climate change mitigation potential.
The system boundary was defined as "cradle to grave," which includes the product stage (A1-A3), construction stage (A4-A5), use stage (B2, B4), and end of life stage (C1-C4) [30], as shown in Figure 2. System expansion was employed to take into account the net benefits related to reuse and recycling of the materials and products, as well as energy recovery from materials, such as wood incineration beyond the system boundary (D). For this study, several stages within Module B were excluded from the comparative LCA, because of the assumption of functional equivalence for the two buildings in their use stage and lack of empirical data for repairing and refurbishing of MT buildings and lack of water consumption data. Thus, B1-Use, B3-Repair, B5-Refurbishment, and B7-Operational Water were excluded from the analysis. Although the global warming impact from the whole building LCA comes mainly from operational energy, such as electricity and natural gas used during the building's lifetime, the two comparison buildings were designed as functionally equivalent. Thus, no difference would be shown for the operational energy impacts (B6). This analysis mainly focuses on the impacts resulting from materials and other activities.

Software Tool
IE4B is an open free software tool that provides a cradle-to-grave life-cycle inventory (LCI), mostly developed for north American building industries, to estimate a whole building's environmental For this study, several stages within Module B were excluded from the comparative LCA, because of the assumption of functional equivalence for the two buildings in their use stage and lack of empirical data for repairing and refurbishing of MT buildings and lack of water consumption data. Thus, B1-Use, B3-Repair, B5-Refurbishment, and B7-Operational Water were excluded from the analysis. Although the global warming impact from the whole building LCA comes mainly from operational energy, such as electricity and natural gas used during the building's lifetime, the two comparison buildings were designed as functionally equivalent. Thus, no difference would be shown for the operational energy impacts (B6). This analysis mainly focuses on the impacts resulting from materials and other activities.

Software Tool
IE4B is an open free software tool that provides a cradle-to-grave life-cycle inventory (LCI), mostly developed for north American building industries, to estimate a whole building's environmental impacts. The LCI results comprise flows from and to nature: energy and raw material inputs into the system and emissions out to air, water, and land.
Athena has developed a set of regional North American LCI datasets for major building products, such as concrete, steel, and wood products, covering 90% to 95% of the structural and envelope systems applicable to typical commercial, institutional, light industrial, and residential buildings. Additional databases related to regional electricity grids, fuel use, and transportation by various modes are taken from the U.S. LCI Database.
The IE4B tool conforms to EN 15978 standard, which includes impacts from production, transportation, construction, use, and demolition, as well as operational energy and water use. End of life (EoL) building disposal and transportation were also included in the assessment. The process of calculation for IE4B is shown in Figure 3. The IE4B tool used the "Tool for the Reduction and Assessment of Chemical and other environmental Impacts" (TRACI) environmental impact assessment method to aggregate the impacts from the LCI flows [31]. TRACI provides characterization factors for the life cycle impact assessment (LCIA). Characterization factors quantify the potential impacts that inputs and emissions have on specific impact categories in common equivalence units, such as global warming (GW) (kg CO 2 eq), acidification (AC) (kg SO 2 eq), eutrophication (EU) (kg N-eq), ozone depletion (OD) (kg chlorofluorocarbons-11-eq), smog formation (SF) (kg O 3 -eq), and human health (HH) (CTU). system and emissions out to air, water, and land.
Athena has developed a set of regional North American LCI datasets for major building products, such as concrete, steel, and wood products, covering 90% to 95% of the structural and envelope systems applicable to typical commercial, institutional, light industrial, and residential buildings. Additional databases related to regional electricity grids, fuel use, and transportation by various modes are taken from the U.S. LCI Database.
The IE4B tool conforms to EN 15978 standard, which includes impacts from production, transportation, construction, use, and demolition, as well as operational energy and water use. End of life (EoL) building disposal and transportation were also included in the assessment. The process of calculation for IE4B is shown in Figure 3. The IE4B tool used the "Tool for the Reduction and Assessment of Chemical and other environmental Impacts" (TRACI) environmental impact assessment method to aggregate the impacts from the LCI flows [31]. TRACI provides characterization factors for the life cycle impact assessment (LCIA). Characterization factors quantify the potential impacts that inputs and emissions have on specific impact categories in common equivalence units, such as global warming (GW) (kg CO2 eq), acidification (AC) (kg SO2 eq), eutrophication (EU) (kg N-eq), ozone depletion (OD) (kg chlorofluorocarbons-11-eq), smog formation (SF) (kg O3-eq), and human health (HH) (CTU).

Building Material Comparison
Material inputs for the building LCA were taken from the building's design blueprints provided by LEVER Architecture, the Framework designers. The MT and RC buildings were designed to be functional equivalent for a fair comparison. Building materials identified by the architectural blueprints are summarized in Appendix A. The total mass of the major (structural) materials for the MT Building was 5.30 × 10 6 kg, whereas the RC building was about 50% heavier at 7.93 × 10 6 kg.
The materials of the MT and RC buildings were grouped into different building assemblies, such as ceilings/roof, floors, foundations, columns and beams, and walls (Appendix A). Figure 4 shows the materials classified into the steel, wood, concrete, gypsum board, mortar and the rest in others based on the summary of bill of materials (BOM) used in both buildings. The most material used by weight in both buildings was concrete, which was followed by gypsum board and steel in the RC building, and wood and gypsum board in the MT building. Figure 4 shows that 1.12 × 10 6 kg more wood was used in the MT building than the RC building, and 3.8 × 10 6 and 0.22 × 10 6 kg more concrete and steel used, respectively, in the RC building. Additionally, 0.23 × 10 6 kg of gypsum board was added to the MT building to meet fire codes, which added more environmental burdens to LCA results for the MT building. Considering carbon storage in wood products and assuming wood moisture content at 12%, a total of 1.14 × 10 6 kg of wood material used in the MT building translates to about 1.84 × 10 6 kg of CO 2 eq stored in the building during its service life. In addition, using the mass of wood building materials and the carbon displacement factor [32,33] to estimate avoided product emissions, results showed that the MT building saved 3.51 × 10 6 kg CO 2 eq of GHG emissions. The displacement factor Sustainability 2020, 12, 4708 6 of 15 computes the sum of GHG emission reduction attained per unit of wood use. In essence, building with wood avoids the consumption of about three times as much fossil fuel as a wood product needs, a substantial GHG benefit even when considering biogenic CO 2 emissions. Figure 5 demonstrates the material quantity and percentage for different building assemblies in both buildings. Floors made up the largest share (by mass) of materials among all assemblies, followed by walls. and gypsum board in the MT building. Figure 4 shows that 1.12 × 10 6 kg more wood was used in the MT building than the RC building, and 3.8 × 10 6 and 0.22 × 10 6 kg more concrete and steel used, respectively, in the RC building. Additionally, 0.23 × 10 6 kg of gypsum board was added to the MT building to meet fire codes, which added more environmental burdens to LCA results for the MT building. Considering carbon storage in wood products and assuming wood moisture content at 12%, a total of 1.14 × 10 6 kg of wood material used in the MT building translates to about 1.84 × 10 6 kg of CO2 eq stored in the building during its service life. In addition, using the mass of wood building materials and the carbon displacement factor [32,33] to estimate avoided product emissions, results showed that the MT building saved 3.51 × 10 6 kg CO2 eq of GHG emissions. The displacement factor computes the sum of GHG emission reduction attained per unit of wood use. In essence, building with wood avoids the consumption of about three times as much fossil fuel as a wood product needs, a substantial GHG benefit even when considering biogenic CO2 emissions. Figure 5 demonstrates the material quantity and percentage for different building assemblies in both buildings. Floors made up the largest share (by mass) of materials among all assemblies, followed by walls.   Table 1 lists specific breakdowns of material usage for the two buildings. A thin sheet of concrete is used on top of CLT floors for sound-deadening purposes. About 0.61 × 10 6 kg of CLT was in the floor assembly for the CLT building; this reduced concrete use in the floor assembly from 4.69 × 10 6 kg for the RC building to 2.33 × 10 6 kg for the CLT building. The total mass of floor materials is 3.02 × 10 6 kg in the CLT building and 4.88 × 10 6 kg in the RC building ( Figure 5 and Table 1). Similarly, for walls, the total mass was 1.37 × 10 6 kg in the CLT building land 2.07 × 10 6 kg in the RC building. This was due to the use of 0.24 × 10 6 kg CLT to replace the use of 1.0 × 10 6 kg concrete and 0.11 × 10 6 kg steel, although there is 0.23 × 10 6 kg more gypsum board used in the CLT building. Additional gypsum board was required for high-rise MT buildings in the roof and walls to meet building code requirements. Table 1 was extracted from the BOM generated in the IE4B. The BOM is an ingredients list for constructing the buildings. The materials were classified into five groups: steel, wood, concrete, mortar, and gypsum board. CLT and glulam were classified as wood. Only the top five weighed materials are listed in Table 1, which accounted for 97.26% and 98.71% of the total mass for the MT building and RC buildings, respectively. Floors and walls make up most of the total mass in the building, so the designer may consider selecting less fossil-fuel intensive materials in building designs to reduce environmental impacts, while still meeting building code and other construction requirements.   Table 1 lists specific breakdowns of material usage for the two buildings. A thin sheet of concrete is used on top of CLT floors for sound-deadening purposes. About 0.61 × 10 6 kg of CLT was in the floor assembly for the CLT building; this reduced concrete use in the floor assembly from 4.69 × 10 6 kg for the RC building to 2.33 × 10 6 kg for the CLT building. The total mass of floor materials is 3.02 × 10 6 kg in the CLT building and 4.88 × 10 6 kg in the RC building ( Figure 5 and Table 1). Similarly, for walls, the total mass was 1.37 × 10 6 kg in the CLT building land 2.07 × 10 6 kg in the RC building. This was due to the use of 0.24 × 10 6 kg CLT to replace the use of 1.0 × 10 6 kg concrete and 0.11 × 10 6 kg steel, although there is 0.23 × 10 6 kg more gypsum board used in the CLT building. Additional gypsum board was required for high-rise MT buildings in the roof and walls to meet building code requirements. Table 1 was extracted from the BOM generated in the IE4B. The BOM is an ingredients list for constructing the buildings.
The materials were classified into five groups: steel, wood, concrete, mortar, and gypsum board. CLT and glulam were classified as wood. Only the top five weighed materials are listed in Table 1, which accounted for 97.26% and 98.71% of the total mass for the MT building and RC buildings, respectively. Floors and walls make up most of the total mass in the building, so the designer may consider selecting less fossil-fuel intensive materials in building designs to reduce environmental impacts, while still meeting building code and other construction requirements.  Figure 6 shows the whole building life cycle (from Module A to C, excluding maintenance, replacement, operational energy, and water use in Module B) environmental impacts of the CLT MT building and its equivalent concrete alternative. The results illustrate that the MT building had a lower environmental impact than the RC building in three of six categories: GW, OD, and EU. The MT building demonstrated 8%, 29%, and 21% reductions in OD, EU, and GW, respectively, over its RC counterpart. The MT building had higher impacts in categories of HH, AC, and Smog (SF). Figure 7 displays the environmental impacts of each stage (excluding the operational energy emission) of the whole building LCA results. Over 75% of the total environmental loads are from the production and construction stages for both the MT and RC buildings, except HH. Future research should be focused on ways to reduce impacts of the production and construction stages.  Figure 6 shows the whole building life cycle (from Module A to C, excluding maintenance, replacement, operational energy, and water use in Module B) environmental impacts of the CLT MT building and its equivalent concrete alternative. The results illustrate that the MT building had a lower environmental impact than the RC building in three of six categories: GW, OD, and EU. The MT building demonstrated 8%, 29%, and 21% reductions in OD, EU, and GW, respectively, over its RC counterpart. The MT building had higher impacts in categories of HH, AC, and Smog (SF). Figure 7 displays the environmental impacts of each stage (excluding the operational energy emission) of the whole building LCA results. Over 75% of the total environmental loads are from the production and construction stages for both the MT and RC buildings, except HH. Future research should be focused on ways to reduce impacts of the production and construction stages.

Comparison of Global Warming Impacts
Global warming (GW) impact is a term used to describe the contribution of a product or service to potential warming of the atmosphere, which could contribute to climate change.

Global Warming Summary
The GW impact for each life-cycle stage of each building type is shown in Table 2 and plotted in Figure 8 (for stage B, only including modules B2 and B4). In Table 2, a negative rate of change indicates that the RC building performs better than the MT building for the specific stage. The operational energy usage (B6) is the same for both buildings, according to the data provided by the architects who designed the two buildings, so the focus can be on structural elements (i.e., embodied carbon). Not including B6, total carbon dioxide emissions from modules A through C was about 2.15 × 10 7 kg CO 2 eq for the RC building and 1.70 × 10 7 kg CO 2 eq for the CLT building. When considering module D, total carbon dioxide emissions for the RC building increased to 2.16 × 10 7 kg CO 2 eq, but it decreased to 6.57 × 10 6 kg CO 2 eq for the CLT building. Stage D involves reuse, recycling, and recovery of building materials. The increase of GHG emissions for the RC building is due to the carbon dioxide emissions during transportation or reuse activities. Due to the stored carbon in wood, the MT building exhibits substantially lower global warming impacts over its RC counterpart in module D.

Comparison of Global Warming Impacts
Global warming (GW) impact is a term used to describe the contribution of a product or service to potential warming of the atmosphere, which could contribute to climate change.

Global Warming Summary
The GW impact for each life-cycle stage of each building type is shown in Table 2 and plotted in Figure 8 (for stage B, only including modules B2 and B4). In Table 2, a negative rate of change indicates that the RC building performs better than the MT building for the specific stage. The operational energy usage (B6) is the same for both buildings, according to the data provided by the architects who designed the two buildings, so the focus can be on structural elements (i.e., embodied carbon). Not including B6, total carbon dioxide emissions from modules A through C was about 2.15 × 10 7 kg CO2 eq for the RC building and 1.70 × 10 7 kg CO2 eq for the CLT building. When considering module D, total carbon dioxide emissions for the RC building increased to 2.16 × 10 7 kg CO2 eq, but it decreased to 6.57 × 10 6 kg CO2 eq for the CLT building. Stage D involves reuse, recycling, and recovery of building materials. The increase of GHG emissions for the RC building is due to the carbon dioxide emissions during transportation or reuse activities. Due to the stored carbon in wood, the MT building exhibits substantially lower global warming impacts over its RC counterpart in module D.

Product Stage Analysis
Product stages A1 through A3 include raw material extraction from nature, transportation to manufacturers, and product manufacturing. Both the RC and the MT buildings produced considerable amounts of GHG emissions. According to Sjunnesson [34], the production of concrete generates the most GHG emissions in the concrete building's life cycle, which constitutes approximately 85% of its total GW impact. For the RC building, 92% of the material mass is concrete (Figure 4), and this material accounted for 85% of total GHG emissions shown in Table 2 (emissions from A1-A3 divided by embodied emissions A-C). The MT building uses more wood and less concrete, so GHG emissions were 5.18 × 10 5 kg CO 2 eq (Table 2) less than the RC building, or 28.2% less than the RC building.

Construction Stage Analysis
The construction stage includes transportation of materials from factory to construction site by truck or rail (A4) and construction work, such as product installation and groundwork (A5). Table 2 shows that 1.55 × 10 5 kg CO 2 eq and 5.09 × 10 4 kg CO 2 eq emissions resulted from transportation and construction, respectively, for the MT building; these values for the RC building were 7.95 × 10 4 kg CO 2 eq and 7.47 × 10 4 kg CO 2 eq, respectively. GHG emissions from this stage were lower than from the product stage. This is because the Framework building has not been built, so the practical construction onsite energy use is not available, but the IE4B default fossil fuel energy use calculation for crane operation was included only in this stage of environmental impacts. For the RC building, the construction stage GHG emissions are only 7.1% (emissions from A4 to A5 divided by embodied emissions from A to C) of its total embodied carbon, whereas for the MT building it is about 11.7%. The reason for there being more carbon emissions from the MT building than from the RC building is the greater distance between the CLT manufacturer and the construction site. For concrete, given its weight and extensive use in urban areas, providers are normally within 25 miles of the building site, so the transportation of concrete tends to consume less fossil fuel than transportation of MT materials. There are limited CLT manufacturers in the United States and Canada, which leads to longer transportation distances and results in higher impacts for the MT building for A4. As additional CLT and other MT product manufacturers arise, these transportation distances will tend to shorten, resulting in lower A4 impact for MT buildings. Furthermore, the lighter weight of CLT building materials leads to lower GHG emissions during erection of the building structure compared to the RC building.

Use Stage Analysis
The use stage includes only maintenance (Module B2), replacement (Module B4), and Operational Energy (Module B6). Modules B1 (Use), B3 (Repair), B5 (Refurbishment), and B7 (Operational Water) are all excluded, due to the assumption of functional equivalency for the two buildings and a lack of empirical data for B3-Repair and B5-Refurbishing for MT buildings. Repair and Refurbishing of a building in the use stage can make an impact in GWP if the waste stream from repair and refurbishing is significant. However, for the current paper, due to the lack of such data from this new building sector, they are excluded, but only including the maintenance components (B2) and replacing building materials (B4). Both the MT and RC buildings produced very small amounts of GHG emissions for modules B2 and B4 (Table 2), about 0.1 % and 0.2% of their total embodied carbon from cradle to grave (A to C). Carbon emissions in this stage from the MT building were a little higher than those from the RC building, because the wood requires more maintenance to meet fire codes and prevent moisture intrusion into the building envelope.

End of Life Stage Analysis
The end of life stage includes energy use of the demolition equipment (C1), materials transportation from site to landfill (C2), waste sorting and processing (C3), and disposal equipment energy use (C4). Carbon emissions in this stage from the MT building and the RC building are only slightly different, 1.21 × 10 5 kg CO 2 eq and 1.23 × 10 5 kg CO 2 eq, respectively (Table 2). This means that, although the mass of materials used in the MT building was less than that in the RC equivalent, the two buildings consumed a similar amount of energy at the end of building life stage.

Beyond Building Life Stage
Stage D quantifies the net environmental benefits or loads due to reuse, recycling, and energy recovery resulting from the net flows of materials and energy exported beyond the system boundary. A biogenic carbon accounting methodology is adopted in IE4B, in accordance with the internationally accepted carbon footprint standards: PAS 2050 [35], ISO 14067 [36], and WRI GHG Protocol [37] for Products. According to the principle of these three standards, if the forest will be renewed after logging, forest growth leads to the reduction of carbon dioxide in the atmosphere, which is considered negative carbon emission [38]. Figure 9 demonstrates the negative carbon emissions derived in stage D for the large quantity of CLT used in the MT building. This carbon benefit has also been demonstrated in another paper [8]. Overall, the CLT wood product has a positive GW impact when considering carbon storage in products' life. For every cubic meter of CLT (380 oven dry kg wood) used in the MT building, about 190 kg of carbon are stored, which is equivalent to 700 kg CO 2 . Table 2 shows the embodied GHG emissions in relation to module D (beyond building life). Including D, it shows that the MT building produced 69.5% less GHG emissions than the RC building compared to 20.6% from stages A through C. When considering building operating energy, these numbers drop to 5.9% and 1.6%, respectively. This indicates the importance of building operating energy along with masking the importance of structural element material. For module D, the wood products used in the building exhibited an environmental carbon savings of 1.05 × 10 6 kg CO 2 eq from the carbon stored ( Figure 9).  Table 2 shows the embodied GHG emissions in relation to module D (beyond building life). Including D, it shows that the MT building produced 69.5% less GHG emissions than the RC building compared to 20.6% from stages A through C. When considering building operating energy, these numbers drop to 5.9% and 1.6%, respectively. This indicates the importance of building operating energy along with masking the importance of structural element material. For module D, the wood products used in the building exhibited an environmental carbon savings of 1.05 × 10 6 kg CO2 eq from the carbon stored ( Figure 9). Figure 10 shows the GHG emissions by assembly groups for the two building types from Stage A to D. GHG emissions were mainly from wall and floor for the RC building, in which floors accounted for almost 50% and wall accounted for 36% of its total emissions. For the MT building, floors accounted for only 24%, and walls accounted for 58% of its total carbon emissions. The value of GHG emissions from MT building walls and floors were dramatically reduced by 50% and 80% from the RC building assemblies, respectively. This is due to the large amount of CLT substituting concrete and steel, as shown in the materials summary of Figure 5 and Table 1. Therefore, in building construction, replacing traditional concrete and steel materials with CLT mass timber materials as structural members results in significant environmental benefits.  Figure 10 shows the GHG emissions by assembly groups for the two building types from Stage A to D. GHG emissions were mainly from wall and floor for the RC building, in which floors accounted for almost 50% and wall accounted for 36% of its total emissions. For the MT building, floors accounted for only 24%, and walls accounted for 58% of its total carbon emissions. The value of GHG emissions from MT building walls and floors were dramatically reduced by 50% and 80% from the RC building assemblies, respectively. This is due to the large amount of CLT substituting concrete and steel, as shown in the materials summary of Figure 5 and Table 1. Therefore, in building construction, replacing traditional concrete and steel materials with CLT mass timber materials as structural members results in significant environmental benefits.

Conclusions
This study conducted a comparative whole-building LCA of two functionally equivalent high-rise buildings using the Athena IE4B. The designs were based on the Framework building in Portland, Oregon, USA. For material resource efficiency, the MT building weighed about 67% of its RC equivalent, a substantial advantage for mass timber building designs. GW, AC, EU, and other impact indicators were reported from the whole-building LCA from cradle-to-grave analysis. Results showed that substituting wood for concrete and steel in these buildings would bring significant GHG emission reductions. From

Conclusions
This study conducted a comparative whole-building LCA of two functionally equivalent high-rise buildings using the Athena IE4B. The designs were based on the Framework building in Portland, Oregon, USA. For material resource efficiency, the MT building weighed about 67% of its RC equivalent, a substantial advantage for mass timber building designs. GW, AC, EU, and other impact indicators were reported from the whole-building LCA from cradle-to-grave analysis. Results showed that substituting wood for concrete and steel in these buildings would bring significant GHG emission reductions. From Stages A through C, the embodied carbon was 21% lower for the MT building due to these substitutions. If considering carbon stored by the CLT material in the MT building, then the GHG emissions reduction would increase to 69.5%. This comparative study demonstrated CLT as a smart choice for structural frames, such as walls and floors, compared to traditional concrete and steel building materials.
Research focusing on reducing emissions in the building sector has shifted from improving operational energy use and its associated emissions through efficiency measures to the selection of low-carbon-footprint materials for better environmental influence. Buildings constructed with wood products are considered to have low environmental loads, especially if carbon stored effects are considered. The results reported in this paper can support decision makers seeking better choices of material for high-rise buildings, in order to minimize environmental burdens and mitigate climate change.