# Influence of building geometry on the environmental impact of building structures

Pierre Navaro Auburtin, Myriam Saadé, Manuel Manthey, Mathilde Louërat, Jean-Luc Martin, Olivier Baverel

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Pierre Navaro Auburtin, Myriam Saadé, Manuel Manthey, Mathilde Louërat, Jean-Luc Martin, et al. Influence of building geometry on the environmental impact of building structures. CISBAT 2023, Sep 2023, Lausanne, Switzerland. pp.152001, 10.1088/1742-6596/2600/15/152001. hal-05223836

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# Influence of building geometry on the environmental impact of building structures

To cite this article: P Navaro Auburtin et al 2023 *J. Phys.: Conf. Ser.* **2600** 152001

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# Influence of building geometry on the environmental impact of building structures

P Navaro Auburtin<sup>1,2,3</sup>, M Saadé<sup>1</sup>, M Manthey<sup>2</sup>, M Louërat<sup>2</sup>, J-L Martin<sup>3</sup>, O Baverel<sup>1</sup>

E-mail: pierre.navaro-auburtin@enpc.fr

Keywords: structure, LCA, design, multicriteria, building

**Abstract.** In France, embodied carbon represents 35% of the total green house gases emissions from the construction sector. This proportion can reach up to 70% when considering recent buildings. In particular, structural elements for a large portion of the material quantities hence environmental impacts. There is a large panel of options for designing structure and the early stage choices have the biggest impact on the final product. In order to help designers, understanding the influence of structure parameters such as material, span, grid choices or the number of level is essential. To do so, the article uses a methodology coupling structural analysis and environmental assessment. Wood, steel and concrete structures are considered with various span based on a 3 meters by 3 meters grid. The life cycle analysis uses french based scenarios and indicators from the european norm EN15804+A2. Results show that there is no material which is better on every indicators. Also the increase of span or the decrease of the number of level leads to higher impacts no matter the indicator. Finally slabs and beams represent the largest part of the impacts.

## 1. Introduction

The International Panel for Climate Change is clear, a strong reduction of green house gases (GHG) emissions is needed if we want to keep the global warming between 1.5 and 2°C [1]. 65 to 85% of GHG of newly constructed building in France come from the materials and equipments used in the construction [2]. As structural elements represent a large part of the mass used for the construction and may use materials with high carbon intensity, they are also a great lever to reduce those emissions. GHG emissions of structure have been tackle through different studies [3][4]. They compare different structural systems with different materials but only focus on GHG emissions. Concerning the influence of height, the impact of lateral stability systems for high buildings has been studied in [5]. Many life cycle analysis (LCA) of buildings have been conducted ([6], [7]) however they lack a parametric approach concerning building structures. In this article we aim to fill this gap providing answers on the influence of building height, span and geometry on various environmental indicators for low rise cases.

For this purpose we design different structure geometries of typical buildings, get the material quantities and run an LCA based on EN15804+A2 for products and EN15978 for building level.

<figure><img src="image_4.png" /></figure>

<footnote n="1"><sup>1</sup> Laboratoire Navier, UMR 8205, ENPC, IFSTTAR, CNRS</footnote>

<footnote n="2"><sup>2</sup> Centre Scientifique et Technique du Bâtiment (CSTB), France</footnote>

<footnote n="3"><sup>3</sup> AREP, 16 Av. d'Ivry, F-75013 Paris Cedex 13. France</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Plan configurations where filled squares are columns, thin lines are beams and thick lines are walls</figcaption></figure>

## 2. Methodology

### 2.1. *Structure hypothesis*

The structural systems considered are composed of slabs, beams, columns and foundations which transmit and resist to the vertical loads. We also consider walls for lateral stability of the system. Every element is precalculated for specific spans, live and permanent loads. We consider a housing situation hence live loads of 1.5kN/m², a permanent load of 1.2kN/m² for the slabs that consist in equipment and we add the self weight of the structures. For slabs an acoustic insulation value of 53dB is considered. The structure fire resistance requirement is set at 60 minutes of fire following the ISO 834 curve. The functionnal unit considered is 1m² of habitable floor.

For wood structures, the beams and columns are made of glued laminated timber GL24h. They are sized considering the eurocode 5 NF EN 1995. The slab system considered is a cross laminated timber (CLT) slab. On top of this slab is set a concrete screed for acoustic purposes. This screed is necessary to compare the slabs which are made with different materials. For a cast in place slab, the concrete serves a structural and an acoustic purpose. Hence to compare the system with the same functionnal unit a screed is added on the CLT system. For steel structures, the beams and columns are made of S235 steel. They are sized considering the eurocode 3 NF EN 1993. The slab system considered is a composite concrete and steel deck.

For concrete structures, the beams and columns are made of reinforced concrete C20/25 composed of CEMII cement. They are sized considering the eurocode 2 NF EN 1992. The slab system considered is a cast in place deck.

Two foundations systems are considered : isolated footing under columns and continuous footing. Isolated footings are sized following [8] using a pressiometric method. Continuous footing are calculated as isolated footing receiving a 1m width column. A clay and silt soil with an effective pressure of 0.7 MPa and a pressiometric factor of 1.30 is considered. Walls are fixed to a width of 20cm and a steel reinforcing rate of 40kg/m³.

Figure 1 shows different plan considered to study the span of the structure. A central staircase is considered with walls. Then a square of 12 by 12 meters is considered at each side allowing a grid of 3x3. 9 configurations are tested : 3x3, 3x6, 6x3, 6x6, 3x12, 12x3, 6x12, 12x6, 12x12. The first slab, directly in contact with the ground, is always made with a concrete slab whatever the typology chosen.

### 2.2. *Life Cycle Analysis hypothesis*

Each material life cycle has been model with the software brightway2 and the database ecoinvent 3.7.1. For scenario modeling, we used the french based scenarios coming from specific environmental declaration product (EPD) of INIES Database used for french environmental
regulation for building (RE2020) (cf. Table 1).

<table><caption>**Table 1. Material scenarios**</caption><thead><tr><td>Material</td><td>INIES id</td></tr></thead><tbody><tr><td>Steel S235</td><td>32987</td></tr><tr><td>Glue laminated timber</td><td>26812</td></tr><tr><td>Concrete with CEMII cement</td><td>27741</td></tr><tr><td>Reinforcing steel</td><td>Diogen</td></tr><tr><td>metal sheet</td><td>32988</td></tr><tr><td>Cross laminated timber</td><td>27250</td></tr><tr><td>Concrete screed</td><td>12465</td></tr></tbody></table>

The main differences for the steels come from the origin of production. 33% of steel S235 come from converters and 67% from electric converters. For steel sheets, 80% come from converters and 20% from electric converters. For reinforcing steel, 2% come from converters and 98% come from electric converters. This has a direct impact on GWP as converters produce much more GHG emissions.

Concerning concrete with CEMII cement and concrete screed the main difference is the cement content. As a concrete screed needs less resistance it has lower cement content hence lower GHG emissions.

The set of indicators used come from the norm EN15804+A2. The LCA made is attributionnal and static. Following the terms of NF EN 15978, a reference study period of 50 years for the building structures and a full renewal of the elements are considered.

# 3. Results

## 3.1. Influence of span

Those first results are calculated with an eight storey building. Figure 2 shows the influence of span and material over different LCA indicators. The bigger the symbol the higher the span of structure considered. For concision purposes the set of indicator has been reduce showing different caracteristic impacts depending on the materials used. Each value is per habitable square meter.

Globally, the higher the span the higher the impact. As the span rises, there are less columns however the beams and columns need to be thicker. Quantitatively there is a rise in the material quantity with the rise of the span hence the impact is more important. Grids of 6x3, 12x3 and 12x6 have a lesser impact than grids of 3x6, 3x12 and 6x12 respectively as they have small span beams because of the central walls. Even if 3x6, 3x12 and 6x12 have less columns what matters quantitatively are the beams.

For acidification, wood and steel solutions are more impactful than concrete structures. They range from 0.37 $molH^+eq/m^2$ (concrete 3x3) to 0.78 $molH^+eq/m^2$ (wood 12x12).

For climate change, wood structures range from 80 $CO2eq/m^2$ to 140 $CO2eq/m^2$, steel structure from 136 $CO2eq/m^2$ to 217 $CO2eq/m^2$ and concrete structure from 132 $CO2eq/m^2$ to 232 $CO2eq/m^2$. For smaller span, concrete seems to be better than steel. This tendency is inverted when considering higher span. Wood solutions have lower emissions as these solutions contain less carbon intensive material. The proximity between the result of steel and concrete structures cna be explaining by the fact that steel structures contain lots of concrete and concrete structures contain lots of steel.

For particulate matter formation, wood structure have higher emissions. This is due to the sawing of wood to make cross laminated timber and glued laminated timber.
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Impact of building structure following material and span; AE: acidification; CCtot: climate change total; PMF: particulate matter formation; HT: human toxicity; LU: land use; ERnr: energy resources fossils; WAT: water use</figcaption></figure>

Concerning human toxicity, the highest impact comes from the steel structures whereas for land use wood structures have the greatest impact. This last result comes from the fact that forest exploitation necessitates a lot of land to grow trees. Even if we can argue that those exploitations are non artificialised soil, they still are industrialized places where there is low biodiversity compared to non transformed land. The consumption of energy ressources and more precisely fossil fuels is the highest for steel structures followed by concrete structures. The production of both this materials is highly energy intensive as the steel needs to be melted and the clinker for concrete necessitates high temperatures. Steel structures use more water than concrete structures. The consumption range from 29 $m^3/m^2$ (wood 3x3) to 81 $m^3/m^2$.

Figure 3 focuses on the repartition climate change indicator and shows the repartition of impact following the structural elements. Beams and slabs (taking into account roof and bottom slab) represent from 73% (wood 3x3) to 88% (concrete 12x12) of climate change impact. As such, they represent the largest lever for building structures to reduce CO2eq emissions. On the other side, foudations account only from 3% (wood 3x3) to 10% (concrete 10x10). This can be discussed as we only considered the necessary material to transmit the loads to the floor. However generally a large volume of concrete is added to attain the strong soil. Columns account for 3% (wood 12x12) to 10% (wood 3x3), walls for 4% (concrete 12x12) to 11% (wood 3x3). As the span increases, the emissions linked to columns decrease. Even if thicker columns are needed, the fact that there are less columns when increasing the span is the most significative
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Repartition on the elements of CCtot with variable span and 8 levels</figcaption></figure>

factor.

As the span increases the beams and slab system increases drastically. For the steel structure, in the 3x3 configuration beam and slab emit 75 kgC02eq/m² where the 12x12 configuration they emit 150 kgC02eq/m². Beam value increases more than slab value as the chosen systems tend to add secondary beams increasing the span of the slab elements.

## 3.2. Influence of the number of floors

Figure 4 focuses on the climate change indicator and shows the repartition of impact following the elements of structure. As the building number of level decreases, the CO2eq emitted increases. To be more precise, as with the decrease of the number of level, the roof structure part and the bottom slab part increase as there are less intermediate levels. Specifically for the roof, this means that there are more built elements per habitable meter square hence more CO2eq emitted per meter square. However this gain with height reduces quickly.

For wood structures the values range from 80 kgC02eq/m² to 296 kgC02eq/m². For steel from 136 kgC02eq/m² to 368 kgC02eq/m². For concrete, from 132 kgC02eq/m² to 381 kgC02eq/m².

The increase of CO2eq emissions is higher for wood when reducing the number of floors due to the fact that this reduction increases the part of bottom slab per meter square which is in concrete.

As the number of floor rises, the columns have an higher impact. As the building gets higher, the columns take higher loads and need to be thicker. The same phenomenon occurs for foundations.

## 4. Discussion

The article doesn't present the impact of the number of levels on architectural elements nor the impact on the energetic exploitation part of the building. As mentioned in the results, as the number of levels increases, the CO2eq linked to the structure elements decreases, however, the necessity to add elevators or other technical elements for height might lead to higher emissions.

A sensitivity analysis should also be lead with different materials life cycle hypothesis. For instance, considering other types of concrete, evaluating the sensitivity of steel solutions to the production mode or trying different end of life scenarios for wood are good leads.

A detailed analysis of abiotic resources and of the impact on biodiversity should be discussed. Building eight storey towers or one storey doesn't have the same impact on land occupation and urbanistic parameters. This study consists in a precise view of the structure of building but needs to be contrasted to an overview of the construction sector.
<figure><img src="image_8.png" /><figcaption>**Figure 4.** Repartition on the elements of climate change total impact with number of floor</figcaption></figure>

**5. Conclusion**

Results show that material, span and number of floor have a great influence on the environmental impact of building structures. Shorter spans are less impactful than long ones, also the higher the number of floors, the lower the impact of the building structure per square meter. However the quality of the architectural space and reversibility of the building might not be as good. Wood structures are less carbon intensive at fixed parameters but have higher particulate matter formation and use more land. As no material behave better on every indicator, designers should be careful when reducing carbon emissions to not transfer pollution on other impacts. Also efforts should focus on the structural elements that matter i.e slab and beams.

**Acknowledgements**

This work was supported the National Association for Research and Technology (ANRT) within the framework of an industrial agreement for training through research.

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