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Embodied emissions in structures.   Impact and mitigation

Life cycle stages of constructions

Greenhouse gas emissions due to human activities are mainly associated with the pro- duction and consumption of energy. The energy demand of the built environment can be divided into two principal contributions, respectively, operational and embodied energy (EE). The EE refers to the energy required for the product stage (A1–A3), maintenance, repair/replacement/refurbishment processed during the use stage (B2–B5) and the end of life management of structures including demolition, waste processing and final disposal (C1–C4), see Figure 1. A life cycle assessment of the EE and the associated greenhouse gas emissions (ECO2e), may also account for the potentially large benefits related to recycling of materials or reuse of structural components or entire systems (stage D in Figure 1).

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Fig. 1: Life cycle stages of constructions according to EN15978:2011

A more than relevant contribution

In buildings, only a certain portion of the total EE and associated ECO2e is associated with the load-bearing structure (beams, columns, slabs, foundations, etc.), whereas the rest is attributable to non-structural components (e.g. suspended ceiling, lining, cladding, etc.). However, a review of different studies, revealed that the contribution of structures to the total EE and ECO2e of buildings ranges between 30 and 80%. An illustrating example for this given in Figure 2 for 30 office buildings investigated by Davis Langdon, published by Clark (2013). The breakdown of their total ECO2e into the contribution of the structure, the facade, internal non- load-bearing components and installations (M&E), reveals that the (mainly steel-framed) structures accounts on average for around 60%. Generally, however, the share of structure-embodied CO2e in buildings is strongly case-dependent. Find out more on the underlying influences in section 2.2 of the paper you find under this link.  

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Fig. 2: Percentage contribution of different components to total ECO2e in 30 office buildings (S = steel framed, C = concrete framed, M = mixed; numbers = no. of storeys). Figure reprinted from "What colour is your building?", with permission from D. Clark.

So what does this mean at a national or global scale? Here a rough estimation for Germany, where the 10 year (2012–2021) average annual useful floor area in new buildings (residential and non-residential) amounts to around 57 million m2 (Bundesamt 2021). Considering a central estimate for cradle-to-gate (LC stages A1-A3) embodied CO2e in building structures of the order of 300-400 kgCO2e/m2 of floor area (see e.g. De Wolf et al 2016), this implies an annual amount of approximately 20 million tons of ECO2e in structural materials for buildings. Adding a 20–30% average contribution of life cycle stages beyond the production stage (see e.g. Hart et al. 2021) and up-scaling the result from the useful (UFA) to the total building floor area (TFA) assuming a corresponding average ratio of UFA/TFA=0.7, leads to a roughly estimated 38 million tons of annually embodied CO2e in German building structures during the last decade. In comparison to the total German annual average emissions (across all sectors) in this decade (see here), this corresponds to a significant share of 4 to 5%. In developing countries with comparatively higher construction rates, this share can be expected to be significantly higher.

Mitigation opportunities

Structural performance, e.g. in terms of load-bearing capacity, is, generally speaking, a function of the demands on a structure, the properties of its constitutive materials and its geometrical characteristics. Both material and geometry have also a relevant impact on the environmental performance of structures and corresponding decisions should be adopted carefully. Depending on the particular circumstances of a structural project, designers can influence such decisions to certain extent by good engineering practices. The opportunities for this are highest at the conceptual structural design stage, which mainly consists in selecting the type, layout and main dimensions of the load-bearing system, its main constitutive members and most prominent details, as well as the appropriate materials. See section 2.2.1 and 2.2.2 under this link.

In comparison to the conceptual design stage, the subsequent detailed member design (e.g. dimensioning of cross-sections) offers only limited opportunities to designers for savings of materials and EE and ECO2e. Detailed member design is subject to the constraints imposed by the compulsory decision rules defined in structural codes and standards, for which compliance must be demonstrated. Potential research-based modifications to these rules, with the aim to improve the sustainability performance of structures, are discussed in Sections 3–5 go the paper you can find here.

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