Keywords

1 Introduction

Cold-formed steel constructions have started to gain popularity due to their efficient use of materials and ease of fabrication. Cross-section shape flexibility represents a particularity for cold-formed steel profiles and makes them an economic alternative for secondary load bearing structures such as racking or high bay warehouses, but most recently also for small to medium rise buildings. The lightweight nature of these elements coupled with precise manufacturing tolerances allows for quick construction, typically without the need for heavy equipment. Cold-formed elements are typically manufactured from galvanised coils with thicknesses ranging from 0.5 mm to 3 mm and steel grades with strengths greater than 300 MPa. The profiles are commonly connected using bolts and screws, often avoiding welded elements, making them easily stackable for transportation and storage on-site.

An ever-growing concern regarding climate change is driving governmental bodies to promote and enforce sustainable practices for the built environment. The construction industry is recognised as one of the primary contributors to carbon emission. According to the Global Status Report for Buildings and Constructions [1], in 2021 the production of construction materials was estimated to contribute 37% to energy-related CO2 emissions. The focus in recent decades has been to reduce the operational footprint of the building by improving energy efficiency. This focus was justified by the fact that operational carbon amounts to 28% while embodied carbon accounts for 11% according to a report by the World Green Building Council [2]. However, as the built environment evolves through continuous investment and innovation, it is expected that the embodied carbon footprint will become the main contributor to the overall footprint of the building. The need for decarbonisation is accelerated by the net-zero greenhouse gas emission target set for 2050 by the European Green Deal.

To evaluate the carbon emissions of buildings, a Life Cycle Assessment (LCA) can be used. LCA is a science-based and standardized methodology [3, 4] designed to quantify and report environmental impacts. The LCA serves to measure and guide the reduction of carbon emissions related to buildings and their components across their entire life cycles. This involves examining stages such as pre-use, during use, and at the End-Of-Life (EOL) of the building.

The purpose of this paper is to assess the embodied carbon footprint of a typical industrial hall roofing system. According to previous research [5], sigma cold-formed steel purlins have a lower global warming potential compared to prestressed prefabricated concrete or glulam timber. The focus of this research is to quantify the environmental impacts of a roof system consisting of cold-formed steel Z-shaped purlins and trapezoidal sheeting.

2 Methodology

Life Cycle Assessment (LCA) is a scientific and quantitative method designed to determine and evaluate environmentally relevant processes. Initially developed for evaluating products, it is now also applied to evaluate constructions.

The standard [3] outlines the steps that must be followed to complete a Life Cycle Assessment of a building: (1) Purpose and object of the assessment; (2) Boundaries of the analysis; (3) Life cycle inventory (LCI); (4) Calculation of the environmental indicators; (5) Interpretation of results; (6) Conclusions.

2.1 Purpose and Object of Assessment

The purpose of this LCA is to quantify the environmental performance of equivalent purlin roof configurations, composed of cold-formed steel elements and different steel grades. This comparative LCA can support the different players in the construction chain (e.g. engineers, architects etc.) in the decision-making process by providing comparisons of the global warming potential (GWP) expressed in kgCO2eq as defined in EN 15804 + A2 [6] for different design options and by indicating the potential for GWP reduction. The declared unit of the LCA to be used for the assessment and comparison between the different design variants is the square metre (m2) of the purlin roof system.

2.2 Design Variants

The trapezoidal sheeting that represents the top outer layer of the industrial hall is supported by Z-shaped cold-formed purlins that span over the rafters. The continuity of the bending moment over the rafters was ensured in two options: continuous and overlapped purlin. Previous research [7] shows that overlapped purlins are superior from a mechanical behaviour point of view compared to sleeved connections. The use of high-grade steel has been shown to increase material efficiency by previous work of the authors [8], therefore three steel grades have been investigated S350GD + Z and S550GD + Z. ArcelorMittal’s HyPer® high-strength steel grade S550GD + ZM satisfies additional requirements compared to the EN 10346 standard [9] by ensuring that ductility requirements are respected as defined in EN 1993-1-1, EN 1993-1-3 and EN 1993-1-12 [10,11,12]. Zinc-magnesium (ZM) coating is a brand product of ArcelorMittal called Magnelis®. In the framework of the EN 10346 standard [9], Magnelis® shall be classified as a ZM coating [13]. The coating is produced on a classic hot dip galvanising line in which the molten bath has a unique chemical composition that includes zinc, 3.5% aluminium and 3% magnesium.

The purlins were designed using a validated finite element model, as shown in Fig. 1, developed in the previous work of the authors [8]. The model consists of two equal spans of 4 m and 6 m.

Fig. 1.
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Overlapped Z-purlin [8]

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To capture the sensitivity to buckling of the thin-walled members, the geometrically and materially nonlinear analysis with imperfections included (GMNIA) method of analysis was used.

The roofing components were designed according to the Eurocode framework. The loads considered for the ultimate and serviceability limit states were equal to 3.15 kN/m2 respectively 2.2 kN/m2. The material factor was assumed to be equal to 1.00 according to EN 1993-1-3 [11]. The individual vertical deformation of each component was limited to L/200 where L is the span. The sheeting was assumed to span continuously over three spans. The design variants used in the LCA are detailed in Table 1.

Table 1. Design variants parameters.
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Figures 2 and 3 present the weight of each of the different variants per area of the roof. The efficiency of using an overlapped connection over the rafters is demonstrated through the reduction in steel use by up to 40%.

Fig. 2.
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Steel consumption for 4 m span variants.

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Furthermore, increasing the yield strength from 350 N/mm2 to 550 N/mm2 reduced steel consumption by an additional 27%. The overlapped system designed using high-grade steels leads to an increase in purlin spacing by a factor of 2.5 for 4 m span beams and 2.2 for 6 m spans.

Fig. 3.
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Steel consumption for 6 m span variants.

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2.3 Boundaries of the Analysis

The European Standard EN 15804+A2 [6] establishes a common life cycle model for materials and products applied to building and construction works. The life cycle model, shown in Fig. 4, includes modular definitions for the life cycle stages, allowing each stage to be compared in isolation with other stages.

Fig. 4.
figure 4

The life-cycle model [6]

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This current LCA specifically focusses on Product Stage A1–A3. Within this life cycle stage, emissions associated with resource extraction (A1), emissions related to resource transportation (A2), and emissions related to the manufacturing process, including the completion of finished products at the factory gate (A3), were considered.

2.4 Life-Cycle Inventory (LCI)

Environmental product declarations (EPDs) provide quantified information on the environmental impacts and aspects of products and services that are used in an LCA. The main EPDs and environmental data used in the current LCA are presented in Table 2 together with their impact at the Product Stage (A1–A3) in terms of their declared functional unit (FU).

Table 2. EPDs and Product Stage GWP emission factors.
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ArcelorMittal’s XCarb® Recycled and Renewably Produced (RRP) is applied to steels made in an electric arc furnace (EAF) using high levels of scrap (high recycled content) and 100% renewable electricity. Therefore, a great reduction in GWP related to metallic coated steel profiles can be achieved as presented in Table 2 when comparing, for example, the XCarb® Recycled and Renewable hot dip galvanised steel with Magnelis® coating versus standard steel produced in blast furnaces/blast oxygen furnaces (BF/BOF) as represented by hot-dip galvanised steel sheet (OEKOBAU.DAT).

2.5 Calculation of the Environmental Indicator

The LCA analysis focusses on the GWP to describe the environmental impacts. The value of the GWP indicator is calculated for the product life cycle stages (A1–A3) based on a matrix calculation routine, as illustrated in Fig. 5.

For i = to the assessed life cycle stages [A1–A3].

The basic principle of this matrix calculation routine consisted of multiplying each product/material quantified in a module of the building life cycle with its respective value for any environmental indicator. Equation 1 exemplifies the resulting calculation routine for the quantification of the GWP of stage i:

$$ GWP_{i} = a_{1,i} \times GWP_{a1,i} + \, a_{2,i} \times \, GWP_{a2,i} + a_{3,i} \times \, GWP_{a3,i} + \, \ldots \, + \, a_{N,i} \times \, GWP_{aN,I} $$
(1)

  • where:

  • GWPi is the global warming potential quantified for module i of the purlin roof;

  • an,i is the quantity of product/material used in the module i of the building (n = 1, 2, 3, …, N);

  • GWPan,i is the global warming potential of the product/material used in the module i of the building (n = 1, 2, 3, …, N).

Fig. 5.
figure 5

The life-cycle model [6].

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3 Results and Discussion

For the purlin design Variant 3 and Variant 6, two additional GWP assessments were made. The additional assessments referred as Variant 3 XCarb® and Variant 6 XCarb® highlight the benefits of the EAF process using recycled steel (scrap) and 100% renewable electricity during the steelmaking process. The results are presented in terms of the area of the purlin roof system. Figure 6 and Fig. 7 show the GWP result, highlighting the contribution of the different components to the GWP for two span configurations: 4 m and 6 m.

In general, regardless of the variant, the roof component with the most significant impact on the GWP is the cold-formed sheet decking, followed by the cold-formed Z profile. This outcome is in line with expectations, since both components account for the largest share of materials in the overall roof system. Specifically, cold-formed steel decking can contribute to overall GWP emissions by up to 71% (Variant 6). Similarly, cold-formed sigma purlins can influence the overall GWP by up to 45% (Variant 4).

For the 4 m span configuration the reduction of GWP between Variant 1, Variant 2 and Variant 3 is related to the reduction of the material consumption, since equivalent GWP emission factors were used to estimate the CO2eq emissions of the different steel components. The reduction was achieved because using an overlapped purlin and increasing the yield strength of the profile. Overall, the GWP can be reduced by up to 29% when Variant 1 is compared with Variant 3. Similarly, for the 6m span configuration the reduction of GWP from Variant 4, Variant 5 and Variant 6 is also related to the optimisation of weight. Overall, the GWP can be reduced up to 13% when Variant 4 is compared with Variant 6.

Fig. 6.
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GWP comparison in terms of components (4 m span).

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Fig. 7.
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GWP comparison in terms of components (6 m span).

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In contrast to Variants 1 to 6, where the average European virgin steel production (BOF) is assumed for cold-formed steel elements, two additional variants, Variant 3 XCarb® and Variant 6 XCarb®, were considered. These variants integrate low-emission steel XCarb® for elements such as the roof deck, Z purlins, and cleats. These adjustments resulted in a substantial 77% reduction in the overall Global Warming Potential (GWP) when comparing Variant 1 and Variant 3, and a 72% reduction when comparing Variant 4 and Variant 6. These CO2eq reductions are related not only to the weight optimisation of Variant 3 and Variant 6 compared to their counterparts, Variant 1 and Variant 4, but also to the use of low-emission XCarb® steel. XCarb® is produced from an Electric Arc Furnace (EAF) with high recycled material content (steel scrap) and renewable electricity, leading to reductions in embodied carbon related to product stages A1–A3. The impact of XCarb® on the reduction of the GWP of global warming is evaluated by comparing Variants 3 and 6 with their respective XCarb® variants. XCarb® results in a GWP reduction of up to 68% when comparing Variant 6 to Variant 6 XCarb®.

4 Conclusions

The purpose of this LCA is to quantify the environmental performance of equivalent purlin roof configurations, composed of cold-formed steel elements and different steel grades and support the different construction chain players (e.g. engineers, architects etc.) in the decision-making process by providing comparisons of the global warming potential. In this research different purlin solutions were adopted. Two span configurations of 4 m and 6 m were investigated. A high-grade steel (S550) was used as an alternative to the more traditionally used S350. The connection over rafters was investigated in two options: continuous profile or overlapped purlin.

A Life Cycle Assessment from cradle-to-factory gate (product stages A1–A3) was conducted, and the Global Warming Potential for the six proposed variants was calculated. In the overall analysis, it was found that the roof deck and the Z-purlins account for the majority of CO2eq emissions, reaching up to 71% for the roof deck in Variant 6 and up to 45% for the Z-purlin in Variant 4. These findings underscore the importance of optimising the design of purlins and roof decks in terms of weight by using high-strength steels, emphasising the need for using low-emission materials with high recycled content for these specific roof components.

In the 4 m span configuration, Variant 3 achieved a 29% reduction in Global Warming Potential (GWP) compared to Variant 1. This GWP reduction is associated with a decrease in material consumption through weight optimisation. This was facilitated by using a higher-grade steel together with an overlapped connection, instead of a continuous profile made of S350. Similarly, in the 6 m span configuration, Variant 6 yielded a 13% reduction in GWP compared to Variant 4. This reduction in GWP is similarly attributed to optimised material consumption. For the 4 m span configuration, Variant 3 reduced the GWP by 29% compared to Variant 1. This GWP reduction is related to a reduction in material consumption. Similarly, for the 6 m span configuration, Variant 6 reduced the GWP by 13% when compared to Variant 4.

Finally, two additional variants were examined, Variant 3 XCarb® and Variant 6 XCarb®, diverging from the standard use of average European virgin steel production (BOF) in Variants 1 to 6. These new variants, which incorporate low-emission steel, resulted in a 77% reduction in the overall Global Warming Potential when comparing Variant 1 and Variant 3 XCarb®, and a 72% reduction when comparing Variant 4 and Variant 6 XCarb®. These reductions are attributed not only to the weight optimisation of Variant 3 and Variant 6 compared to their counterparts, Variant 1 and Variant 4, but also to the use of XCarb® steel. XCarb® is produced from an Electric Arc Furnace with high recycled material content and renewable electricity, leading to substantial reductions in embodied carbon in the product stages A1–A3. The impact of XCarb® on the reduction in GWP was specifically assessed, revealing a potential 68% decrease when comparing Variant 6 to Variant 6 XCarb®.