Does height matter? The Embodied impacts of tallness, slab thickness, building code and design tranches.

Does height matter? 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The Embodied impacts of tallness, slab thickness, building code and design tranches. Avery Hoffer, Evan Bentz, Shoshanna Saxe This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6001700/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Societies across the globe are simultaneously trying to build significantly more housing to meet the needs of a growing population and emit significantly less greenhouse gas emissions to meet the pressures of the climate crisis. This is precipitating debates about the nature of sustainable housing. ‘Conventional wisdom’ holds that tall buildings are bad for the environment coinciding with longstanding skepticism of such buildings, yet the research and data on this question is often anecdotal or incomplete and contradictory. In this paper we wade into the debate on how to build sustainable housing, particularly with regards to building design and height. This paper examines the effects of building height on embodied greenhouse gas emissions for 5-to-20 storey reinforced concrete buildings. We find that while height does minimally increase embodied emissions per rentable area, the impact is within the noise of other design choices, particularly slab thickness and design tranches – number of storeys with identical design. These results show that the debate around tall buildings and sustainability is too often focused on the wrong question and opportunities to design much better buildings are being overlooked. Civil Engineering Environmental Engineering Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION Worldwide, we are facing two diverging yet pressing issues. The first, a housing crisis, with a global demand for 96,000 new housing units every day 1 . The second, a climate crisis, with emissions needing to be reduced by 45% by 2030, and net zero by 2050 2 . These two crises are in direct conflict, as increasing housing supply requires material, energy and transport, all producing greenhouse gas emissions. The construction required to meet today’s housing demand alone is threating our ability to meet climate change commitments 3 – 7 . Consequently, finding solutions for how to build more housing with less greenhouse gas (GHG) emissions is a pressing challenge facing nearly all of humanity. While tall multi-unit residential buildings – apartments or condominiums - allow for more units with reduced land use and better infrastructure efficiency, tall buildings have been denounced since their first appearance in the 1850s 8 on social, economic and political levels 9 . Tall buildings were considered “Advertisements of ego”, “thieves of light and air and energy” and “vulgar, immodest and unnecessary” 10 , and are often portrayed negatively in media, through evil villains scheming over Gotham from the penthouse, or even as a literal villain in the novel “High-Rise” 11 . The suspicions over tall buildings have persisted into the sustainability and resilience debates 12 , 13 . As buildings became more efficient to operate and energy production less polluting, the primary resource use in construction and the associated GHG emissions increasingly dominate the sustainability credentials of buildings. Naturally, a debate about which types of buildings are the best for embodied GHG has followed 14 . The resulting literature produced a mix of findings. On one hand, studies found that increasing building height can be detrimental to embodied emissions 15 , 16 , and that buildings shouldn’t exceed 5 storeys 17 , 18 . On the other hand, different research studies found that increasing building height has relatively little impact on increasing embodied GHG emissions, and that other factors, such as floor type 19 , building lifetime 20 and density 21 play a much larger role in affecting the emissions output, while building taller has minimal impacts on other non-structural systems 22 . Sometimes adding height was found to decrease embodied emissions 19 , 23 . While the research is mixed the narrative of tall buildings being bad for the environment has become ‘common knowledge’ and is used to fight the construction of tall buildings 24 – 26 . In practice, while the studies that find height is bad describe existing cities (e.g. London, England, and Paris, France), many of the papers use small datasets, neglect foundations and make use of simplified structural systems 27 . As the need to build more with less pollution accelerates, evidence-based decision-making about what really drives and improves the environmental impacts of construction are needed. There is a real risk that we avoid tall buildings not because they are actually more polluting, but because thinking they are aligns with nearly 200 years of suspicion. Thus, this paper wades into the debate on height and sustainability by (i) calculating from first principles the effects of building height on embodied GHG emissions of reinforced concrete structures, (ii) identifying impactful parameters to minimize the embodied GHG effects of residential buildings (height or otherwise) and (iii) comparing the impact of height on embodied GHG to other aspects of the built environment. We focus on the structure and substructure design as these building elements contribute the largest share of embodied emissions and are the ones most impacted by height (e.g. cladding and insulation change much less over the height of a building). We take a bottom-up approach calculating the structural member sizes through an iterative design loop to find optimal dimensions for the column, beam, wall, substructure pads, piles and footings. The research is carried out using both the current (as of 2024) Canadian and American structural design codes 28 , 29 , which govern building design over much of the western hemisphere. The Canadian code, which takes an advanced approached to shear design and column width will be featured in the discussion of our findings and compared to the American code (detailed design results for both approaches can be found in the supplementary information). RESULTS Effect of height on structural members When considering the relationship between height and embodied greenhouse gas, there is a ‘double dependence’ 19 that causes an exponential relationship between material consumption and building height. The first dependence is that for each added floor there is the added material needed to build that floor. The second is that for each added floor, the weight of the building and other loads (e.g. wind loading) increase and the lower floors must be strengthened to resist the additional loading, some of which increase quadratically with height. This increased complexity in structural design is the foundation of the argument for taller buildings being more resource and embodied GHG intensive. However, in parallel there are the materials savings of not needing to build a second building with its own, for example, material intensive foundation and roof. Complicating the question further, buildings are not only designed for self-weight, they also must resist deflection and vibration; which design parameter controls the structural design changes as the building height increases (e.g. usually strength for short buildings and deflection or vibration in tall buildings) 30 . As such, in taller buildings there is often unused strength capacity that could support more floors without more material being required. For very tall buildings, acceleration from vibration governs structural design, though this is outside the scope of this paper as all studied buildings are below the vibration dominance threshold. Does height matter? Across 128 buildings ranging from 5 to 20 storeys we tested the impacts of varying building height (number of storeys), slab thickness, design tranches, and choice of design code (CSA versus ACI). In this paper the term “design tranches” is used to refer to the number of stories that have the same reinforcement and concrete design. While these groupings are common to simplify the construction process, they do not have a universally agreed on technical term. For this paper we are using the term design tranches which is used by some structural engineers in practice. To properly compare the results of different building heights, the total embodied GHG emissions are normalized by the net rentable area (NRA), defined as the area for which rent can be charged; the gross floor area minus the hallways and elevator core. Figure 1 illustrates how the emissions per NRA vary as height increases from 5 to 20 storeys. For a 200 mm thick slab (light blue dots in Fig. 1 ), GHG emissions increase by 3% from 5 to 10 storeys. Beyond this height, the embodied emissions increase at ~ 1.3% / storey through 20 storeys. Much of this increase comes from the double dependence impact on the columns, beams and walls mentioned earlier. Similarly, the impact of floor slab on material use and embodied GHG is examined between 175 mm and 250 mm in 25 mm increments, also shown in Fig. 1 . The results show that we find larger impacts from changing the slab thickness than the number of storeys. Importantly, 20-storey buildings with 175 mm slabs (Point A, Fig. 1 ) have less embodied GHG per m 2 than 5-storey buildings with 225 mm thick slabs (Point B, Fig. 1 ). Therefore, while height does affect embodied GHG emissions, a 50 mm increase in slab thickness has a larger impact than a 15 storey increase in height. On average, we find that decreasing the slab thickness by 25 mm (1”) has the same effect as an 8 storeys change in height. This suggests that optimizing slab thickness in buildings is often a more important target for engineering time than the overall height of the building in regard to embodied GHG. To meet code requirements, traditional structural design practice (which we have followed in this paper) requires that thinner slabs necessitate more columns (Table 9.2 in CSA 28 and 7.3.1.1 in ACI 29 ). While this study uses the traditional design practices where column spacing is linearly proportional to slab thickness, additional slab deflection checks can be considered to remove this dependance and keep consistent column spacing with thinner slab thicknesses. In this research we observe a change in columns per floor from 36 to 22 between thinnest and thickest slabs. In shorter buildings, this leads to thinner slabs having more embodied GHG in the columns than their thicker counterparts. This trend reverses after 10 to 14 storeys, when strength design begins to dominate over minimum column size requirements. By choosing to utilize a slab that is 25 mm thinner in short buildings, the additional columns contribute an extra 0.6% of embodied emissions, over 12x less than the GHG reduced from decreased slabs, showing the trade-off of decreasing slab thickness with increasing columns has a large net positive effect in terms of embodied emissions. While additional columns decrease the flexibility of the floor slab this is not a big concern for residential buildings studied here which always have a lot of internal walls separating units and rooms. The non-uniformity in vertical gaps between different slab thicknesses (for example Point C, Fig. 1 ) is due to the dependance of column layout on slab thickness, as there are 14 more columns per floor in buildings with 175 mm thick slabs than 250 mm slabs. The change in the number of columns per floor as slab thickness decreases is not necessarily constant, which is why the vertical gaps are not uniform between slab thicknesses (Point C, Fig. 1 ). Additionally, the buildings with the 2 lowest slab thicknesses (175 mm and 200 mm) have bilinear curves, changing at the 10-storey mark. This is due to what causes the foundation to fail, which is originally due to soil failure, and changes to shear failure (both one-way and punching). This change in failure type requires additional materials for thicker foundation footings, and therefore have higher embodied emissions. While this also occurs in the structures with the 2 largest slab thicknesses (225 mm and 250 mm), the additional material needed for the change in failure type is lower, and as a result does not show a bilinear curve. As shown in Fig. 2, while the column, beam and wall thicknesses (Left, Fig. 2) do change with height, on average the impact of slab thickness (Right, Fig. 2) is larger and drives overall material use and embodied GHG, accounting for 60–75% of the GHG emissions embodied in the structures. The foundations (Middle, Fig. 2) have a relatively small impact contributing 10–15% of the emissions in each building. Figure 2: Embodied GHG emissions in kg CO 2 e/m 2 for the Columns, Beams and Walls (Left), Foundations (Middle) and Slabs (Right), as storeys and slab thickness increase. Figure 3 illustrates the embodied GHG emissions in kgCO 2 e/m 2 as height and slab thickness change. Increasing height by 5-storey increments increases emissions by 5%, while 25 mm increase in slab thickness increases emissions by 9%. In these increments, slab thickness is roughly twice as critical for embodied GHG than height, and while less height means less housing - thinner slabs require more careful construction to achieve sufficient concrete coverage of reinforcing steel but minimal impact of the housing function of the building. Design Tranches and Embodied Emissions In addition to the height and slab thickness, we tested the customization of the design to the loadings at each floor. For simplicity in design and construction, it is common to use the same column and walls thicknesses for multiple floors of a building – meaning some are overdesigned as all are determined by the worst load case (usually on the lowest floor of the group or tranche). These groupings are common but do not have a universally agreed on technical term. For this paper we are using the term design tranches which is used by some structural engineers in practice. In current practice, buildings around 20 storeys tend to be designed roughly every 10 storeys, therefore having 2 design tranches in a building. Buildings in the 10–15 storey range often only have 1 design tranche throughout the structure. Decreasing the number of floors designed together (and thereby increasing the number of design tranches) allows the columns, beams and walls to be more closely tailored to the loads they experience. This requires more effort in design and construction but has no impact on the function of the building. For buildings in the 15–20 storey range maximizing the number of design tranches – designing each storey for its own loading - can save over 45% of the column, beam and wall concrete leading to 11% savings in total embodied carbon emissions of the structure. This is calculated as an upper bound potential impact as there would be non-trivial increases in design and construction time required to construct such a building, not to mention the increased potential for construction errors. For buildings under 10 storeys the potential for savings is lower due to minimum dimension limits set in design codes (e.g. a column cannot be smaller than 250 mm according to clause 21.4.2.2A of the Canadian code 28 ); For 10 storey buildings, there are 5% savings in total embodied emissions by increasing design tranches, while 5 storey buildings have almost no savings. These savings also trickle down to the foundation – as the superstructure uses less material and weighs less, the substructure has less to support, reducing embodied GHG emissions there as well. Building Code Building code requirements have the potential for a significant GHG impact on buildings. The results presented above use the Canadian Standards Association (CSA) building code. While the older 2014 American Concrete Institute (ACI-14) building code is quite similar in terms of embodied GHG emissions, the newest 2019 American code (ACI-19) differs on the shear capacity of floor slabs, with a reduction in capacity by as much as 40% from its previous version for lightly reinforced one-way slabs. To match the CSA’s simplified shear capacity, ACI necessitates a 30% increase in floor slab embodied emissions through additional slab rebar, or even higher emissions if the code change is accommodated through increased slab thickness. With floor slabs accounting for 60–75% of total building embodied emissions, building code location / version can play a far larger role than height in embodied emissions of tall buildings. In a CSA and ACI comparison, we found a 15–45% increase in slab rebar quantities due to ACI’s 2019 reduced one-way shear capacity. As the newer ACI-19 begins being implemented, this may cause a spike in housing embodied GHG emissions in the United States. In addition to the metrics discussed above we tested the impact of storey height on material use and embodied GHG. Common storey heights in modern buildings are between 2.4-3.0 metres (8 to 10 feet). In line with the minimal impact observed from slightly taller buildings, we find low impact from increased storey height. Reducing the 2.7 metre storey height 0.3 metres (1 foot) there is a maximum savings of 4% total embodied GHG emissions. This height reduction from 2.7 metres to 2.4 metres would provide the same benefit as a 10 mm slab thickness reduction in a 20 storey building. DISCUSSION There is a widespread belief that height is the primary factor driving upfront construction emissions. In contrast, we find that the focus on height is overlooking more powerful drivers of embodied GHG emissions, particularly the slab thickness and approach to design. Thinner slabs can facilitate 15 storey taller buildings and the dramatic increase in housing that would provide for the same embodied GHG. While height does increase emissions somewhat, the differences are small and often swamped by the increased infrastructure needs of more buildings. Assuming construction with 200 mm thick floor slabs and the same total net rentable area in each case, four 5-storey buildings have structural embodied GHG of 900,000 kgCO 2 e, two 10 storey buildings 930,000 kgCO 2 e and one 20-storey building 1,050,000 kgCO 2 e. This implies a savings of 120,000 kgCO 2 e if two 10 storey buildings are built instead of one 20-storey and 150,000 kgCO 2 e if four 5 storey buildings are built instead of one 20-storey. For comparison, a large (900 m 2 ) wood framed single-family home has an embodied GHG of 132,000 kgCO 2 e 31 , about the difference between our 3 scenarios. The emissions savings gap of choosing two 10-storey buildings (four 5-storey buildings) is equivalent to either 300 metres (380 metres) of 2 lane, 7 metre wide tertiary roads 32 , 1400 metres (1800 metres) of 300 mm diameter concrete water transmission pipes with concrete bedding 33 , 34 , or 5.5 (7) elevators (shown in S.I.). Therefore, these savings from decreasing building height, shown in Fig. 4 , are minimized or eliminated when accounting for other services needed for the additional buildings to house the units removed by this suspicion of height. Overall, we find that adding a storey between 5- and 20 storey buildings produces a 1% increase in embodied GHG emissions per storey increase, falling within the margin of error for this research, contradicting historical narratives and recent findings that height is a primary factor in increasing emissions. Conversely, 2 other parameters are shown to have a much larger effect, neither of which changed the dimensions, structural layout, function or appearance of the building 27 . Reinforced concrete floor slab thickness and increased design tranches for the lateral load resisting members (columns, beams and walls) had larger impacts on emissions than height. The former is shown to have an average of 9% savings per 25 mm slab thickness decrease, consistent with studies varying slab types 15 , and the latter savings of up to 14% of total emissions by designing members on a per-storey basis. These results tell us that while there can be increased emissions due to height, there are more critical parameters that can be optimized to decrease embodied GHG without sacrificing needed housing. We find that tall buildings in many cases have lower embodied GHG than short ones when careful design considerations are applied and that the ‘common knowledge’ is due for an update. METHODS Scope of Structural Design The studied buildings are designed using first principles, varying height, slab thickness and design tranches to calculate material volumes and mass and associated embodied GHG emissions. Each building is modelled with structural design software and the building element sizes and strengths are designed to meet either the Canadian Standards Association (CSA) or American Concrete Institute (ACI) design codes The tested number of storeys is 5 to 20. Twenty is selected as the upper bound given fewer than 5% of buildings in the US are taller than 20 storeys 35 . Five is selected as the lower bound as buildings less than 5 storeys usually have either smaller or much wider building footprints. Additionally, as buildings get smaller, there is less ability to adjust the structural design given minimum element sizes for constructability in codes. All the modelled buildings employ a column and slab structural system, a central elevator core with dimensions 9 m x 6 m and a 1.5 metre hallway wrapping around the core exterior. The ground conditions are held steady with an allowable bearing capacity of 300 kPa – typical of compact sands and/or very stiff clays. This structure is assumed to be in a non-seismic zone. The floor dimensions of the building are modelled after the 19-storey ‘Museum House’ in Toronto, Canada, and are 35 m by 15 m with an equal size on each floor. These are chosen as they are commercially reasonable dimensions for a 5-storey building through a 20-storey building. Software We use 4 main software programs to automate, create and quantify buildings in this research project. The first is the Python environment, used to automate model runs, calculate concrete and steel quantities, and add additional design checks wherever necessary. Second, Altair S-Frame 36 (V2024.1) is used to analyze the superstructure model and export forces, moments and displacements. Third, Altair S-Concrete 37 (V2024.1), to design and optimize the columns, beams and walls cross sections. Lastly, Altair S-Foundation 38 (V2024.0), is used to design the substructure. The following section describes how these 4 programs are used in conjunction design the entire building structure from first principles. System Flow To begin the process outlined in Fig. 5, inputs for slab thickness, storeys and design tranches are used to create the initial building file, where building loads and load combinations are calculated and applied. Placeholder dimensions are used for the columns (300 mm x 300 mm), beams (300 mm x 800 mm) and walls (300 mm thick) to get the model started. A linear analysis is then run in S-Frame, where the demand for each structural member is calculated. In S-Concrete, this demand is evaluated against each member’s capacity, and any unsafe members are strengthened through increased dimensions and/or reinforcing steel. Members with reserve capacity are made smaller when possible. The model with the now updated members is rerun, and the cycle continues until the design of the columns, beams and walls has converged, at which point the quantities of concrete and rebar are calculated. Specific details and results can be found in the supplementary information. Figure 5: Project workflow consisting of 4 software After the column, beam and wall design is finalized, the slabs and foundations are designed and quantified. In S-Frame, slabs are split into one-dimensional strips and forces are exported. Reinforcement is then designed for each strip to withstand its respective forces using design code equations. Lastly, the foundations are designed manually to optimize the substructure to high design variability. Support reactions are exported from the S-Frame model and used as S-Foundation loads. Foundation members like pedestals, piles and footings are designed in terms of both dimensions and reinforcement to withstand the applied loading conditions. After calculating the material quantities for all the building members, the embodied GHG emissions are calculated as follows: $$\:\varvec{C}{\varvec{O}}_{2}\varvec{e}=\:\frac{{{\varvec{G}\varvec{W}\varvec{P}}_{\varvec{C}\varvec{o}\varvec{n}\varvec{c}\varvec{r}\varvec{e}\varvec{t}\varvec{e}}\times\:(\varvec{C}}_{\varvec{C}\varvec{B}\varvec{W}}+{\varvec{C}}_{\varvec{S}}+{\varvec{C}}_{\varvec{F}})+\:{{\varvec{G}\varvec{W}\varvec{P}}_{\varvec{S}\varvec{t}\varvec{e}\varvec{e}\varvec{l}}\times\:(\varvec{R}}_{\varvec{C}\varvec{B}\varvec{W}}+{\varvec{R}}_{\varvec{S}}+{\varvec{R}}_{\varvec{F}})}{\varvec{N}\varvec{R}\varvec{A}}$$ Where the subscripts CWB, S and F represent the Column, Beam & Wall, Slab and Foundation aspects respectively for concrete and rebar, denoted as “C” and “R”. These quantities, in kg, are multiplied by their respective global warming potentials (GWPs) 39 taken from their Environmental Product Declarations (EPDs), in units of kgCO 2 e/kg. This summation is then divided by the net rentable area (NRA) in m 2 . Net rentable area is defined as the area for which rent can be charged, calculated as the gross floor area less the hallways and vertical penetrations such as elevator shafts and stairwells. Material Usage By mass, concrete and steel accounted for 96% and 4% of the materials respectively. However, when converting the materials to emissions using EPDs, there is a range of GWP values for concrete and steel 40 depending on the manufacturer. For this report, the average industry EPD values are taken, as the focus is on structural design rather than material selection. For concrete with a 20 MPa cylinder strength, this is taken as 0.092 and 1.016 kgCO 2 e/kg 41,42 for Canada and the United States respectively, and 0.85 kgCO 2 e/kg 43 for steel for both countries, resulting in rebar accounting for roughly 15% of total embodied emissions. Mesh and Slab Strip Sensitivity Studies As previously mentioned, additional studies aside from the varying of slab thickness, building height and design tranches are conducted to not just determine the effects of these parameters on embodied emissions, but rather the validity and accuracy of the structural models used. Primarily, hand calculations were completed to ensure the results calculated through S-Frame are accurate, based on first principles and tributary area, under both point loading and area loading. Once confirmed, a mesh sensitivity study was conducted to determine the optimal size of the finite element mesh. A coarse mesh with too few elements creates results that are inaccurate, while a too fine mesh is computationally intensive, and can be unnecessary. The number of square finite elements within a 1 metre block was increased from 4 (2 x 2 grid) elements to 400 (20 x 20 grid), and once the results began to converge, occurring at 64 elements (8 x 8 grid), this was taken as the value used in these models. Mentioned previously, the 1-dimensional slab strip study was conducted as well, where the tributary width of the slabs decreased from 8 metres to 1 metre. Like the finite element mesh study, the results began to converge, occurring at 2 metre width. As the widths decreased, the results additionally began to become unrealistic, tending towards infinite shear at the edges of triangular mesh due to limitations of the element types used. Declarations DATA AVAILABILITY All data that supports the findings of this study are included within the article (and any supplementary information files). CODE AVAILABILITY The underlying code for this study is available through GitHub in AveryHoffer/PaperCode and can be accessed via https://github.com/AveryHoffer/PaperCode.git. ACKNOWLEDGEMENTS This research is funded by the Centre for the Sustainable Built Environment (CSBE) at the University of Toronto and the Canada Research Chair in Sustainable Infrastructure, Grant/Award Number: 232970. CSBE in turn is funded by an NSERC Alliance Grant (ALLRP 582941–23), the Climate Positive Energy Initiative and the School of Cities both at the University of Toronto, and 12 industry partners (Colliers; the Cement Association of Canada; Chandos Construction; Mattamy Homes; Northcrest; Pomerleau; Purpose Building, Inc.; ZGF Architects; Arup; SvN Architects + Planners; Entuitive; and KPMB Architects). We thank Matthew Sauer and Altair for their assistance in this research project. 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ASTM International. Environmental Product Declaration. United States Census Bureau. American Housing Survey (AHS). Census.gov https://www.census.gov/AHS. Altair. S-FRAME 2021.1 Quick-Start Guide. https://help.altair.com/2021.1/sframe/S-FRAME-2021-Quick-Start_Guide.pdf (2024). Altair. S-CONCRETE 2021.1 Reference Manual. https://help.altair.com/2021.1/sframe/S-CONCRETE-Reference-Manual.pdf (2024). Altair. S-FOUNDATION 2021.1 Quick-Start Guide. https://help.altair.com/2021.1/sframe/S-FOUNDATION-Quick-Start_Guide.pdf (2024). US Environmental Protection Agency (EPA). Understanding Global Warming Potentials. https://www.epa.gov/ghgemissions/understanding-global-warming-potentials (2016). EC3 - Find & Compare Materials. https://buildingtransparency.org/ec3/material-search. National Ready Mixed Concrete Association (NRMCA). Environmental Product Declaration. (2019). Concrete Ontario. Environmental Product Declaration. (2022). Concrete Reinforcing Steel Institute (CRSI). Environmental Product Declaration. (2022). Additional Declarations The authors declare no competing interests. Supplementary Files SupplementaryInformation.xlsx Supplementary Information - Excel SupplementaryInformationCodeOutput.ipynb Supplementary Information - Python Code Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6001700","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":413840830,"identity":"e0d831a9-21a5-4c36-9d2f-1107ce21cae4","order_by":0,"name":"Avery Hoffer","email":"data:image/png;base64,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","orcid":"https://orcid.org/0009-0003-6228-7078","institution":"University of Toronto","correspondingAuthor":true,"prefix":"","firstName":"Avery","middleName":"","lastName":"Hoffer","suffix":""},{"id":413840831,"identity":"7c2772a5-946c-4dae-8fef-c8cf1badbcb9","order_by":1,"name":"Evan Bentz","email":"","orcid":"https://orcid.org/0000-0001-9368-8681","institution":"University of Toronto","correspondingAuthor":false,"prefix":"","firstName":"Evan","middleName":"","lastName":"Bentz","suffix":""},{"id":413840832,"identity":"af86654c-411d-4186-955d-2feb883c7e03","order_by":2,"name":"Shoshanna Saxe","email":"","orcid":"https://orcid.org/0000-0002-4665-8890","institution":"University of Toronto","correspondingAuthor":false,"prefix":"","firstName":"Shoshanna","middleName":"","lastName":"Saxe","suffix":""}],"badges":[],"createdAt":"2025-02-10 19:15:01","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-6001700/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6001700/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":76107787,"identity":"9f5cea57-3db7-420c-a331-9c748df73df4","added_by":"auto","created_at":"2025-02-12 11:27:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":73609,"visible":true,"origin":"","legend":"\u003cp\u003eGlobal Warming Potential in CO\u003csub\u003e2\u003c/sub\u003e equivalent as the number of storeys increases for 4 different slab thicknesses.\u003c/p\u003e","description":"","filename":"Fig1Final.png","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/5e3af70af6aa30faa8a57ce9.png"},{"id":76107789,"identity":"cd6665b7-fc72-4e78-945c-de11a88b855d","added_by":"auto","created_at":"2025-02-12 11:27:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":299860,"visible":true,"origin":"","legend":"\u003cp\u003eEmbodied GHG emissions in kg CO\u003csub\u003e2\u003c/sub\u003ee/m\u003csup\u003e2\u003c/sup\u003e for the Columns, Beams and Walls (Left), Foundations (Middle) and Slabs (Right), as storeys and slab thickness increase.\u003c/p\u003e","description":"","filename":"Fig2Final.png","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/3a6059364d2b680c4ea3766a.png"},{"id":76107791,"identity":"fdb63ed3-ec49-4b76-8b9d-73af8c044564","added_by":"auto","created_at":"2025-02-12 11:27:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":53723,"visible":true,"origin":"","legend":"\u003cp\u003eHeatmap of CO\u003csub\u003e2\u003c/sub\u003ee emissions in units of kg CO\u003csub\u003e2\u003c/sub\u003ee/m\u003csup\u003e2\u003c/sup\u003e as both the number of storeys and slab thickness increase.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/d26af1fe214fb410415e3ae6.png"},{"id":76107796,"identity":"13929333-233d-4c6a-8d04-fdd31bf12ecc","added_by":"auto","created_at":"2025-02-12 11:27:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":185720,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of reducing height from one 20-Storey building to two 10-Storey and four 5-Storey buildings in terms of required services. Embodied GHG emission savings from decreasing height are negated by non-structural infrastructure.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/c6ce334cff0cf7926d237b8f.png"},{"id":76108308,"identity":"400aedea-f34e-43ed-ab30-c3e4452ad1f5","added_by":"auto","created_at":"2025-02-12 11:35:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":257228,"visible":true,"origin":"","legend":"\u003cp\u003eProject workflow consisting of 4 software\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/4a92b5220f858d08b1e0ced1.png"},{"id":76111139,"identity":"f3c5ade3-4521-4e0d-8b3c-a21decfe419b","added_by":"auto","created_at":"2025-02-12 11:59:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1237083,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/acd2620c-2d90-4a56-844f-37ea35e339f6.pdf"},{"id":76108307,"identity":"39af7bec-4b90-4b91-86db-80aaf6e284e8","added_by":"auto","created_at":"2025-02-12 11:35:29","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":96742,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information - Excel\u003c/p\u003e","description":"","filename":"SupplementaryInformation.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/49773c13d2c3dc885b99fbfb.xlsx"},{"id":76107797,"identity":"c70074e4-85cd-42bd-bca6-fcd2b16ff848","added_by":"auto","created_at":"2025-02-12 11:27:29","extension":"ipynb","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":349411,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information - Python Code\u003c/p\u003e","description":"","filename":"SupplementaryInformationCodeOutput.ipynb","url":"https://assets-eu.researchsquare.com/files/rs-6001700/v1/cf609482a0aea8710b4e0bc9.ipynb"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eDoes height matter? The Embodied impacts of tallness, slab thickness, building code and design tranches.\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eWorldwide, we are facing two diverging yet pressing issues. The first, a housing crisis, with a global demand for 96,000 new housing units every day\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. The second, a climate crisis, with emissions needing to be reduced by 45% by 2030, and net zero by 2050\u003csup\u003e2\u003c/sup\u003e. These two crises are in direct conflict, as increasing housing supply requires material, energy and transport, all producing greenhouse gas emissions. The construction required to meet today\u0026rsquo;s housing demand alone is threating our ability to meet climate change commitments\u003csup\u003e\u003cspan additionalcitationids=\"CR4 CR5 CR6\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Consequently, finding solutions for how to build more housing with less greenhouse gas (GHG) emissions is a pressing challenge facing nearly all of humanity.\u003c/p\u003e \u003cp\u003eWhile tall multi-unit residential buildings \u0026ndash; apartments or condominiums - allow for more units with reduced land use and better infrastructure efficiency, tall buildings have been denounced since their first appearance in the 1850s\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e on social, economic and political levels\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Tall buildings were considered \u0026ldquo;Advertisements of ego\u0026rdquo;, \u0026ldquo;thieves of light and air and energy\u0026rdquo; and \u0026ldquo;vulgar, immodest and unnecessary\u0026rdquo;\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, and are often portrayed negatively in media, through evil villains scheming over Gotham from the penthouse, or even as a literal villain in the novel \u0026ldquo;High-Rise\u0026rdquo;\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. The suspicions over tall buildings have persisted into the sustainability and resilience debates\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. As buildings became more efficient to operate and energy production less polluting, the primary resource use in construction and the associated GHG emissions increasingly dominate the sustainability credentials of buildings. Naturally, a debate about which types of buildings are the best for embodied GHG has followed\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. The resulting literature produced a mix of findings. On one hand, studies found that increasing building height can be detrimental to embodied emissions\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, and that buildings shouldn\u0026rsquo;t exceed 5 storeys\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. On the other hand, different research studies found that increasing building height has relatively little impact on increasing embodied GHG emissions, and that other factors, such as floor type\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, building lifetime\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e and density\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e play a much larger role in affecting the emissions output, while building taller has minimal impacts on other non-structural systems\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Sometimes adding height was found to decrease embodied emissions\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. While the research is mixed the narrative of tall buildings being bad for the environment has become \u0026lsquo;common knowledge\u0026rsquo; and is used to fight the construction of tall buildings\u003csup\u003e\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. In practice, while the studies that find height is bad describe existing cities (e.g. London, England, and Paris, France), many of the papers use small datasets, neglect foundations and make use of simplified structural systems\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. As the need to build more with less pollution accelerates, evidence-based decision-making about what really drives and improves the environmental impacts of construction are needed. There is a real risk that we avoid tall buildings not because they are actually more polluting, but because thinking they are aligns with nearly 200 years of suspicion.\u003c/p\u003e \u003cp\u003eThus, this paper wades into the debate on height and sustainability by (i) calculating from first principles the effects of building height on embodied GHG emissions of reinforced concrete structures, (ii) identifying impactful parameters to minimize the embodied GHG effects of residential buildings (height or otherwise) and (iii) comparing the impact of height on embodied GHG to other aspects of the built environment. We focus on the structure and substructure design as these building elements contribute the largest share of embodied emissions and are the ones most impacted by height (e.g. cladding and insulation change much less over the height of a building). We take a bottom-up approach calculating the structural member sizes through an iterative design loop to find optimal dimensions for the column, beam, wall, substructure pads, piles and footings. The research is carried out using both the current (as of 2024) Canadian and American structural design codes\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, which govern building design over much of the western hemisphere. The Canadian code, which takes an advanced approached to shear design and column width will be featured in the discussion of our findings and compared to the American code (detailed design results for both approaches can be found in the supplementary information).\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eEffect of height on structural members\u003c/h2\u003e \u003cp\u003eWhen considering the relationship between height and embodied greenhouse gas, there is a \u0026lsquo;double dependence\u0026rsquo;\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e that causes an exponential relationship between material consumption and building height. The first dependence is that for each added floor there is the added material needed to build that floor. The second is that for each added floor, the weight of the building and other loads (e.g. wind loading) increase and the lower floors must be strengthened to resist the additional loading, some of which increase quadratically with height. This increased complexity in structural design is the foundation of the argument for taller buildings being more resource and embodied GHG intensive. However, in parallel there are the materials savings of not needing to build a second building with its own, for example, material intensive foundation and roof. Complicating the question further, buildings are not only designed for self-weight, they also must resist deflection and vibration; which design parameter controls the structural design changes as the building height increases (e.g. usually strength for short buildings and deflection or vibration in tall buildings)\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. As such, in taller buildings there is often unused strength capacity that could support more floors without more material being required. For very tall buildings, acceleration from vibration governs structural design, though this is outside the scope of this paper as all studied buildings are below the vibration dominance threshold.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eDoes height matter?\u003c/h3\u003e\n\u003cp\u003eAcross 128 buildings ranging from 5 to 20 storeys we tested the impacts of varying building height (number of storeys), slab thickness, design tranches, and choice of design code (CSA versus ACI). In this paper the term \u0026ldquo;design tranches\u0026rdquo; is used to refer to the number of stories that have the same reinforcement and concrete design. While these groupings are common to simplify the construction process, they do not have a universally agreed on technical term. For this paper we are using the term design tranches which is used by some structural engineers in practice. To properly compare the results of different building heights, the total embodied GHG emissions are normalized by the net rentable area (NRA), defined as the area for which rent can be charged; the gross floor area minus the hallways and elevator core.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates how the emissions per NRA vary as height increases from 5 to 20 storeys. For a 200 mm thick slab (light blue dots in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), GHG emissions increase by 3% from 5 to 10 storeys. Beyond this height, the embodied emissions increase at ~\u0026thinsp;1.3% / storey through 20 storeys. Much of this increase comes from the double dependence impact on the columns, beams and walls mentioned earlier.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSimilarly, the impact of floor slab on material use and embodied GHG is examined between 175 mm and 250 mm in 25 mm increments, also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The results show that we find larger impacts from changing the slab thickness than the number of storeys. Importantly, 20-storey buildings with 175 mm slabs (Point A, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) have less embodied GHG per m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e than 5-storey buildings with 225 mm thick slabs (Point B, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Therefore, while height does affect embodied GHG emissions, a 50 mm increase in slab thickness has a larger impact than a 15 storey increase in height. On average, we find that decreasing the slab thickness by 25 mm (1\u0026rdquo;) has the same effect as an 8 storeys change in height. This suggests that optimizing slab thickness in buildings is often a more important target for engineering time than the overall height of the building in regard to embodied GHG.\u003c/p\u003e \u003cp\u003eTo meet code requirements, traditional structural design practice (which we have followed in this paper) requires that thinner slabs necessitate more columns (Table\u0026nbsp;9.2 in CSA\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and 7.3.1.1 in ACI\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e). While this study uses the traditional design practices where column spacing is linearly proportional to slab thickness, additional slab deflection checks can be considered to remove this dependance and keep consistent column spacing with thinner slab thicknesses. In this research we observe a change in columns per floor from 36 to 22 between thinnest and thickest slabs. In shorter buildings, this leads to thinner slabs having more embodied GHG in the columns than their thicker counterparts. This trend reverses after 10 to 14 storeys, when strength design begins to dominate over minimum column size requirements. By choosing to utilize a slab that is 25 mm thinner in short buildings, the additional columns contribute an extra 0.6% of embodied emissions, over 12x less than the GHG reduced from decreased slabs, showing the trade-off of decreasing slab thickness with increasing columns has a large net positive effect in terms of embodied emissions. While additional columns decrease the flexibility of the floor slab this is not a big concern for residential buildings studied here which always have a lot of internal walls separating units and rooms.\u003c/p\u003e \u003cp\u003eThe non-uniformity in vertical gaps between different slab thicknesses (for example Point C, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) is due to the dependance of column layout on slab thickness, as there are 14 more columns per floor in buildings with 175 mm thick slabs than 250 mm slabs. The change in the number of columns per floor as slab thickness decreases is not necessarily constant, which is why the vertical gaps are not uniform between slab thicknesses (Point C, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAdditionally, the buildings with the 2 lowest slab thicknesses (175 mm and 200 mm) have bilinear curves, changing at the 10-storey mark. This is due to what causes the foundation to fail, which is originally due to soil failure, and changes to shear failure (both one-way and punching). This change in failure type requires additional materials for thicker foundation footings, and therefore have higher embodied emissions. While this also occurs in the structures with the 2 largest slab thicknesses (225 mm and 250 mm), the additional material needed for the change in failure type is lower, and as a result does not show a bilinear curve.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;2, while the column, beam and wall thicknesses (Left, Fig.\u0026nbsp;2) do change with height, on average the impact of slab thickness (Right, Fig.\u0026nbsp;2) is larger and drives overall material use and embodied GHG, accounting for 60\u0026ndash;75% of the GHG emissions embodied in the structures. The foundations (Middle, Fig.\u0026nbsp;2) have a relatively small impact contributing 10\u0026ndash;15% of the emissions in each building.\u003c/p\u003e \u003cp\u003e \u003cem\u003eFigure 2: Embodied GHG emissions in kg CO\u003c/em\u003e \u003csub\u003e \u003cem\u003e2\u003c/em\u003e \u003c/sub\u003e \u003cem\u003ee/m\u003c/em\u003e \u003csup\u003e \u003cem\u003e2\u003c/em\u003e \u003c/sup\u003e \u003cem\u003efor the Columns, Beams and Walls (Left), Foundations (Middle) and Slabs (Right), as storeys and slab thickness increase.\u003c/em\u003e\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e illustrates the embodied GHG emissions in kgCO\u003csub\u003e2\u003c/sub\u003ee/m\u003csup\u003e2\u003c/sup\u003e as height and slab thickness change. Increasing height by 5-storey increments increases emissions by 5%, while 25 mm increase in slab thickness increases emissions by 9%. In these increments, slab thickness is roughly twice as critical for embodied GHG than height, and while less height means less housing - thinner slabs require more careful construction to achieve sufficient concrete coverage of reinforcing steel but minimal impact of the housing function of the building.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eDesign Tranches and Embodied Emissions\u003c/h3\u003e\n\u003cp\u003eIn addition to the height and slab thickness, we tested the customization of the design to the loadings at each floor. For simplicity in design and construction, it is common to use the same column and walls thicknesses for multiple floors of a building \u0026ndash; meaning some are overdesigned as all are determined by the worst load case (usually on the lowest floor of the group or tranche). These groupings are common but do not have a universally agreed on technical term. For this paper we are using the term design tranches which is used by some structural engineers in practice. In current practice, buildings around 20 storeys tend to be designed roughly every 10 storeys, therefore having 2 design tranches in a building. Buildings in the 10\u0026ndash;15 storey range often only have 1 design tranche throughout the structure. Decreasing the number of floors designed together (and thereby increasing the number of design tranches) allows the columns, beams and walls to be more closely tailored to the loads they experience. This requires more effort in design and construction but has no impact on the function of the building. For buildings in the 15\u0026ndash;20 storey range maximizing the number of design tranches \u0026ndash; designing each storey for its own loading - can save over 45% of the column, beam and wall concrete leading to 11% savings in total embodied carbon emissions of the structure. This is calculated as an upper bound potential impact as there would be non-trivial increases in design and construction time required to construct such a building, not to mention the increased potential for construction errors. For buildings under 10 storeys the potential for savings is lower due to minimum dimension limits set in design codes (e.g. a column cannot be smaller than 250 mm according to clause 21.4.2.2A of the Canadian code\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e); For 10 storey buildings, there are 5% savings in total embodied emissions by increasing design tranches, while 5 storey buildings have almost no savings. These savings also trickle down to the foundation \u0026ndash; as the superstructure uses less material and weighs less, the substructure has less to support, reducing embodied GHG emissions there as well.\u003c/p\u003e\n\u003ch3\u003eBuilding Code\u003c/h3\u003e\n\u003cp\u003eBuilding code requirements have the potential for a significant GHG impact on buildings. The results presented above use the Canadian Standards Association (CSA) building code. While the older 2014 American Concrete Institute (ACI-14) building code is quite similar in terms of embodied GHG emissions, the newest 2019 American code (ACI-19) differs on the shear capacity of floor slabs, with a reduction in capacity by as much as 40% from its previous version for lightly reinforced one-way slabs. To match the CSA\u0026rsquo;s simplified shear capacity, ACI necessitates a 30% increase in floor slab embodied emissions through additional slab rebar, or even higher emissions if the code change is accommodated through increased slab thickness. With floor slabs accounting for 60\u0026ndash;75% of total building embodied emissions, building code location / version can play a far larger role than height in embodied emissions of tall buildings. In a CSA and ACI comparison, we found a 15\u0026ndash;45% increase in slab rebar quantities due to ACI\u0026rsquo;s 2019 reduced one-way shear capacity. As the newer ACI-19 begins being implemented, this may cause a spike in housing embodied GHG emissions in the United States.\u003c/p\u003e \u003cp\u003eIn addition to the metrics discussed above we tested the impact of storey height on material use and embodied GHG. Common storey heights in modern buildings are between 2.4-3.0 metres (8 to 10 feet). In line with the minimal impact observed from slightly taller buildings, we find low impact from increased storey height. Reducing the 2.7 metre storey height 0.3 metres (1 foot) there is a maximum savings of 4% total embodied GHG emissions. This height reduction from 2.7 metres to 2.4 metres would provide the same benefit as a 10 mm slab thickness reduction in a 20 storey building.\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThere is a widespread belief that height is the primary factor driving upfront construction emissions. In contrast, we find that the focus on height is overlooking more powerful drivers of embodied GHG emissions, particularly the slab thickness and approach to design. Thinner slabs can facilitate 15 storey taller buildings and the dramatic increase in housing that would provide for the same embodied GHG. While height does increase emissions somewhat, the differences are small and often swamped by the increased infrastructure needs of more buildings. Assuming construction with 200 mm thick floor slabs and the same total net rentable area in each case, four 5-storey buildings have structural embodied GHG of 900,000 kgCO\u003csub\u003e2\u003c/sub\u003ee, two 10 storey buildings 930,000 kgCO\u003csub\u003e2\u003c/sub\u003ee and one 20-storey building 1,050,000 kgCO\u003csub\u003e2\u003c/sub\u003ee. This implies a savings of 120,000 kgCO\u003csub\u003e2\u003c/sub\u003ee if two 10 storey buildings are built instead of one 20-storey and 150,000 kgCO\u003csub\u003e2\u003c/sub\u003ee if four 5 storey buildings are built instead of one 20-storey. For comparison, a large (900 m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) wood framed single-family home has an embodied GHG of 132,000 kgCO\u003csub\u003e2\u003c/sub\u003ee\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, about the difference between our 3 scenarios.\u003c/p\u003e \u003cp\u003eThe emissions savings gap of choosing two 10-storey buildings (four 5-storey buildings) is equivalent to either 300 metres (380 metres) of 2 lane, 7 metre wide tertiary roads\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, 1400 metres (1800 metres) of 300 mm diameter concrete water transmission pipes with concrete bedding\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, or 5.5 (7) elevators (shown in S.I.). Therefore, these savings from decreasing building height, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, are minimized or eliminated when accounting for other services needed for the additional buildings to house the units removed by this suspicion of height. Overall, we find that adding a storey between 5- and 20 storey buildings produces a 1% increase in embodied GHG emissions per storey increase, falling within the margin of error for this research, contradicting historical narratives and recent findings that height is a primary factor in increasing emissions. Conversely, 2 other parameters are shown to have a much larger effect, neither of which changed the dimensions, structural layout, function or appearance of the building\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Reinforced concrete floor slab thickness and increased design tranches for the lateral load resisting members (columns, beams and walls) had larger impacts on emissions than height. The former is shown to have an average of 9% savings per 25 mm slab thickness decrease, consistent with studies varying slab types\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, and the latter savings of up to 14% of total emissions by designing members on a per-storey basis. These results tell us that while there can be increased emissions due to height, there are more critical parameters that can be optimized to decrease embodied GHG without sacrificing needed housing. We find that tall buildings in many cases have lower embodied GHG than short ones when careful design considerations are applied and that the ‘common knowledge’ is due for an update.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"METHODS","content":"\u003ch2\u003eScope of Structural Design\u003c/h2\u003e\n\u003cp\u003eThe studied buildings are designed using first principles, varying height, slab thickness and design tranches to calculate material volumes and mass and associated embodied GHG emissions. Each building is modelled with structural design software and the building element sizes and strengths are designed to meet either the Canadian Standards Association (CSA) or American Concrete Institute (ACI) design codes\u003c/p\u003e\n\u003cp\u003eThe tested number of storeys is 5 to 20. Twenty is selected as the upper bound given fewer than 5% of buildings in the US are taller than 20 storeys\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Five is selected as the lower bound as buildings less than 5 storeys usually have either smaller or much wider building footprints. Additionally, as buildings get smaller, there is less ability to adjust the structural design given minimum element sizes for constructability in codes. All the modelled buildings employ a column and slab structural system, a central elevator core with dimensions 9 m x 6 m and a 1.5 metre hallway wrapping around the core exterior. The ground conditions are held steady with an allowable bearing capacity of 300 kPa \u0026ndash; typical of compact sands and/or very stiff clays. This structure is assumed to be in a non-seismic zone.\u003c/p\u003e\n\u003cp\u003eThe floor dimensions of the building are modelled after the 19-storey \u0026lsquo;Museum House\u0026rsquo; in Toronto, Canada, and are 35 m by 15 m with an equal size on each floor. These are chosen as they are commercially reasonable dimensions for a 5-storey building through a 20-storey building.\u003c/p\u003e\n\u003ch3\u003eSoftware\u003c/h3\u003e\n\u003cp\u003eWe use 4 main software programs to automate, create and quantify buildings in this research project. The first is the Python environment, used to automate model runs, calculate concrete and steel quantities, and add additional design checks wherever necessary. Second, Altair S-Frame\u003csup\u003e36\u003c/sup\u003e (V2024.1) is used to analyze the superstructure model and export forces, moments and displacements. Third, Altair S-Concrete\u003csup\u003e37\u003c/sup\u003e (V2024.1), to design and optimize the columns, beams and walls cross sections. Lastly, Altair S-Foundation\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e (V2024.0), is used to design the substructure. The following section describes how these 4 programs are used in conjunction design the entire building structure from first principles.\u003c/p\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003eSystem Flow\u003c/h2\u003e\n \u003cp\u003eTo begin the process outlined in Fig.\u0026nbsp;5, inputs for slab thickness, storeys and design tranches are used to create the initial building file, where building loads and load combinations are calculated and applied. Placeholder dimensions are used for the columns (300 mm x 300 mm), beams (300 mm x 800 mm) and walls (300 mm thick) to get the model started. A linear analysis is then run in S-Frame, where the demand for each structural member is calculated. In S-Concrete, this demand is evaluated against each member\u0026rsquo;s capacity, and any unsafe members are strengthened through increased dimensions and/or reinforcing steel. Members with reserve capacity are made smaller when possible. The model with the now updated members is rerun, and the cycle continues until the design of the columns, beams and walls has converged, at which point the quantities of concrete and rebar are calculated. Specific details and results can be found in the supplementary information.\u003c/p\u003e\n \u003cp\u003e\u003cem\u003eFigure 5: Project workflow consisting of 4 software\u003c/em\u003e\u003c/p\u003e\n \u003cp\u003eAfter the column, beam and wall design is finalized, the slabs and foundations are designed and quantified. In S-Frame, slabs are split into one-dimensional strips and forces are exported. Reinforcement is then designed for each strip to withstand its respective forces using design code equations.\u003c/p\u003e\n \u003cp\u003eLastly, the foundations are designed manually to optimize the substructure to high design variability. Support reactions are exported from the S-Frame model and used as S-Foundation loads. Foundation members like pedestals, piles and footings are designed in terms of both dimensions and reinforcement to withstand the applied loading conditions.\u003c/p\u003e\n \u003cp\u003eAfter calculating the material quantities for all the building members, the embodied GHG emissions are calculated as follows:\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:\\varvec{C}{\\varvec{O}}_{2}\\varvec{e}=\\:\\frac{{{\\varvec{G}\\varvec{W}\\varvec{P}}_{\\varvec{C}\\varvec{o}\\varvec{n}\\varvec{c}\\varvec{r}\\varvec{e}\\varvec{t}\\varvec{e}}\\times\\:(\\varvec{C}}_{\\varvec{C}\\varvec{B}\\varvec{W}}+{\\varvec{C}}_{\\varvec{S}}+{\\varvec{C}}_{\\varvec{F}})+\\:{{\\varvec{G}\\varvec{W}\\varvec{P}}_{\\varvec{S}\\varvec{t}\\varvec{e}\\varvec{e}\\varvec{l}}\\times\\:(\\varvec{R}}_{\\varvec{C}\\varvec{B}\\varvec{W}}+{\\varvec{R}}_{\\varvec{S}}+{\\varvec{R}}_{\\varvec{F}})}{\\varvec{N}\\varvec{R}\\varvec{A}}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere the subscripts CWB, S and F represent the Column, Beam \u0026amp; Wall, Slab and Foundation aspects respectively for concrete and rebar, denoted as \u0026ldquo;C\u0026rdquo; and \u0026ldquo;R\u0026rdquo;. These quantities, in kg, are multiplied by their respective global warming potentials (GWPs)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e taken from their Environmental Product Declarations (EPDs), in units of kgCO\u003csub\u003e2\u003c/sub\u003ee/kg. This summation is then divided by the net rentable area (NRA) in m\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Net rentable area is defined as the area for which rent can be charged, calculated as the gross floor area less the hallways and vertical penetrations such as elevator shafts and stairwells.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003eMaterial Usage\u003c/h2\u003e\n \u003cp\u003eBy mass, concrete and steel accounted for 96% and 4% of the materials respectively. However, when converting the materials to emissions using EPDs, there is a range of GWP values for concrete and steel\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e depending on the manufacturer. For this report, the average industry EPD values are taken, as the focus is on structural design rather than material selection. For concrete with a 20 MPa cylinder strength, this is taken as 0.092 and 1.016 kgCO\u003csub\u003e2\u003c/sub\u003ee/kg\u003csup\u003e41,42\u003c/sup\u003e for Canada and the United States respectively, and 0.85 kgCO\u003csub\u003e2\u003c/sub\u003ee/kg\u003csup\u003e43\u003c/sup\u003e for steel for both countries, resulting in rebar accounting for roughly 15% of total embodied emissions.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003eMesh and Slab Strip Sensitivity Studies\u003c/h2\u003e\n \u003cp\u003eAs previously mentioned, additional studies aside from the varying of slab thickness, building height and design tranches are conducted to not just determine the effects of these parameters on embodied emissions, but rather the validity and accuracy of the structural models used. Primarily, hand calculations were completed to ensure the results calculated through S-Frame are accurate, based on first principles and tributary area, under both point loading and area loading. Once confirmed, a mesh sensitivity study was conducted to determine the optimal size of the finite element mesh. A coarse mesh with too few elements creates results that are inaccurate, while a too fine mesh is computationally intensive, and can be unnecessary. The number of square finite elements within a 1 metre block was increased from 4 (2 x 2 grid) elements to 400 (20 x 20 grid), and once the results began to converge, occurring at 64 elements (8 x 8 grid), this was taken as the value used in these models. Mentioned previously, the 1-dimensional slab strip study was conducted as well, where the tributary width of the slabs decreased from 8 metres to 1 metre. Like the finite element mesh study, the results began to converge, occurring at 2 metre width. As the widths decreased, the results additionally began to become unrealistic, tending towards infinite shear at the edges of triangular mesh due to limitations of the element types used.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data that supports the findings of this study are included within the article (and any supplementary information files).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCODE AVAILABILITY\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe underlying code for this study is available through GitHub in AveryHoffer/PaperCode and can be accessed via https://github.com/AveryHoffer/PaperCode.git.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research is funded by the Centre for the Sustainable Built Environment (CSBE) at the University of Toronto and the Canada Research Chair in Sustainable Infrastructure, Grant/Award Number: 232970. CSBE in turn is funded by an NSERC Alliance Grant (ALLRP 582941\u0026ndash;23), the Climate Positive Energy Initiative and the School of Cities both at the University of Toronto, and 12 industry partners (Colliers; the Cement Association of Canada; Chandos Construction; Mattamy Homes; Northcrest; Pomerleau; Purpose Building, Inc.; ZGF Architects; Arup; SvN Architects + Planners; Entuitive; and KPMB Architects).\u003c/p\u003e\n\u003cp\u003eWe thank Matthew Sauer and Altair for their assistance in this research project. Their expertise and guidance in facilitating understanding of the software, support in addressing complex problems, and generosity in providing access to the software were essential to the success of this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR CONTRIBUTIONS\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAH: Model Analysis, Code Creation, Calculations, Writing \u0026ndash; Original Draft, Visualization\u003c/p\u003e\n\u003cp\u003eEB: Conceptualization, Result Validation, Writing \u0026ndash; Review and Editing, Supervision\u003c/p\u003e\n\u003cp\u003eSS: Conceptualization, Result Validation, Writing \u0026ndash; Review and Editing, Funding Acquisition, Supervision\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCOMPETING INTERESTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eUnited Nations (UN). 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(2022).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[{"identity":"598cffff-0cd8-4574-af0c-af81938a0d83","identifier":"10.13039/501100000038","name":"Natural Sciences and Engineering Research Council of Canada","awardNumber":"582941–23","order_by":0},{"identity":"99c22eff-aaa1-4bff-8cb5-62014078d841","identifier":"10.13039/501100001804","name":"Canada Research Chairs","awardNumber":"232970","order_by":1}],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"University of Toronto","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6001700/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6001700/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSocieties across the globe are simultaneously trying to build significantly more housing to meet the needs of a growing population and emit significantly less greenhouse gas emissions to meet the pressures of the climate crisis. This is precipitating debates about the nature of sustainable housing. ‘Conventional wisdom’ holds that tall buildings are bad for the environment coinciding with longstanding skepticism of such buildings, yet the research and data on this question is often anecdotal or incomplete and contradictory. In this paper we wade into the debate on how to build sustainable housing, particularly with regards to building design and height. This paper examines the effects of building height on embodied greenhouse gas emissions for 5-to-20 storey reinforced concrete buildings. We find that while height does minimally increase embodied emissions per rentable area, the impact is within the noise of other design choices, particularly slab thickness and design tranches – number of storeys with identical design. These results show that the debate around tall buildings and sustainability is too often focused on the wrong question and opportunities to design much better buildings are being overlooked.\u003c/p\u003e","manuscriptTitle":"Does height matter? The Embodied impacts of tallness, slab thickness, building code and design tranches.","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-12 11:27:24","doi":"10.21203/rs.3.rs-6001700/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0a2a880d-c655-4096-87aa-1c45caca73ab","owner":[],"postedDate":"February 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":44136756,"name":"Civil Engineering"},{"id":44136757,"name":"Environmental Engineering"}],"tags":[],"updatedAt":"2025-02-12T11:27:25+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-12 11:27:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6001700","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6001700","identity":"rs-6001700","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}