Laura Micheli, PhD, Bruce Brothersen, SE, PE, PEng, Scott Russell, SE, PE. Peng

In the last decade or so, sustainability and environmental impacts have gained significant relevance in the structural design and  construction fields.

As a result, terms such as Life-Cycle Assessment (LCA) and Environmental Product Declaration (EPD) have become part of the common Architect, Engineering, and Construction (AEC) vocabulary. When it comes to designing and building steel buildings, it’s important to recognise the meaning of these terms—and other related terminology—and how they can help provide a better understanding of the environmental impacts of steel and how it compares to other structural options. And for those of you who are already familiar with these terms, a refresher never hurts.

Life-Cycle Assessments
Let’s start with LCA, which is a standardised method to evaluate the environmental impact of consumer products throughout their lifetimes, as defined by the International Organization for Standardisation (ISO). While still gaining traction in the AEC industry, LCAs are widely used in the consumer product manufacturing world to quantify the carbon emissions associated with different stages of a product’s life, ranging from raw material extraction to end-of-life. The environmental impact is typically estimated based on the energy inputs and greenhouse gas (GHG) emissions at each stage of the product’s production, construction, use, and end-of-life.

Typical life-cycle stages are depicted in Figure 1, including production (A1-A3), construction (A4-A5), use (B1-B5), and end-of-life (C1-C4) stages. An LCA can include all or only some of the life-cycle stages, depending on the scope and intended use of the assessment. When the LCA comprises only the production stage, the term “cradle-to-gate” is usually employed to designate the boundaries of the LCA, from resource extraction (cradle) to leaving the manufacturing facility (gate). The gate is typically the steel fabricator in the case of buildings and structures. If all four life-cycle stages are included, the LCA is referred to as “cradle-to-grave.” (Note that steel can be thought of as a “cradle-to-cradle” material, given that it is infinitely recyclable.)

The results of the LCA are presented in a tabular format, which includes six impact categories, namely global warming potential (GWP), ozone depletion potential, acidification potential (AP), eutrophication potential (think oxygen-hungry algae blooms in bodies of water), smog formation potential, and abiotic depletion potential (the usage of nonrenewable resources for energy production). The most well-known impact category indicator is the GWP, which is measured in kilograms of carbon dioxide equivalent (kg CO2 eq.) and represents the amount of energy/heat the emissions of one ton of a given gas will absorb over a given period of time, relative to the emissions of one ton of carbon dioxide. The larger the GWP value, the more a given gas warms the earth compared to carbon dioxide over a period of time, usually taken as 100 years.

Environmental Product Declarations
Another important sustainability term is an EPD, which is a report that summarises the LCA results of a given product, communicating its carbon footprint in a transparent and comprehensive way. For construction materials, EPDs are regulated by ISO 14025, ISO 21930, and EN 15804; in addition, the EPD must follow the guidelines and requirements of the appropriate product category rule (PCR); the PCR governing EPDs for structural steel is the “Product Category Rule (PCR) Guidance for Building-Related Products and Services.” While all the stages reported in Figure 1 could be included in the background LCA, EPDs typically include life stages A1 through A3 (cradle-to-gate). Note that the beyond end-of-life stage (D1-D4) is not considered a life-cycle stage by ISO 21930, but it could be included in the LCA as additional information.

An EPD must contain a description of the product and the life-cycle stages considered in the analysis, referred to as system boundaries. The LCA results are expressed in terms of environmental impact indicators, calculated based on a declared unit, such as one ton of product, as is the case of steel products EPDs. To ensure a transparent process, the EPD study commissioner must rely on a third-party company (the LCA practitioner) to perform the LCA study, which feeds data into an EPD, as well as an additional third-party company (the program operator) to review and verify the EPD. On the other hand, industry-average EPDs report the weighted industry average production for a number of companies manufacturing the same product. As an example, Table 1 summarises the industry average EPD of fabricated hot-rolled structural sections. Industry average EPDs of steel products can be found on the following websites:

It should be noted that EPDs of different construction materials (e.g., timber, steel, and concrete) are based on different PCRs and declared units. Therefore, a direct comparison between the data reported in their EPDs may lead to inaccurate results. Furthermore, choosing the material with the lowest GWP in the EPD doesn’t necessarily imply selecting the product that will yield the lowest overall carbon emissions since the entire life cycle of a building needs to be considered in the analysis. An accurate comparison of different construction materials can be achieved by accounting for the difference in declared units and considering all the life stages of the structure, from raw material supply to end-of-life. Using manufacturer-specific EPDs in lieu of industry-average values can also lead to more accurate estimates of embodied carbon. An example of this in the steel industry is the EPD difference between an electric arc furnace (EAF) recycling steel from scrap and a blast furnace making steel from ore. Another distinction is the country in which the steel is manufactured. In most cases, domestic steel production has less of a carbon footprint than imported steel.

Whole Building Life-Cycle Analysis
Taking the concept of a product LCA to a different level, the whole building life-cycle analysis (WBLCA) has emerged as a tool to estimate carbon emissions and energy consumption for an entire building. WBLCAs employ the same principles outlined above for LCAs and enable engineers  and other stakeholders to compare the environmental impact of different design solutions by providing information on embodied carbon and operational energy. In addition to stages A, B, and C, WBLCAs can also include stage D, which considers the carbon emissions related to recycling or reusing construction materials at the building’s end of life. Lastly, stages B6 and B7 (Figure 1) can be added to a WBLCA to account for operational energy, such as energy and water consumption.

WBLCA is usually performed by inputting the bill of materials for a given design into specialised software. The software output will be a summary of the six above-mentioned environmental impact indicators. Commonly used software packages are Athena, Tally (Revit), and One-Click LCA. In addition, the SEI Sustainability Committee has developed ECOM, a web-based platform that allows users to approximate the embodied carbon for construction materials and structural frames. The carbon footprint of various design scenarios can be compared by performing the WBLCA of different design solutions.

It is essential to understand that uncertainties inherent in the WBLCA results exist, as each software has its own database, inputs, bias, and assumptions. It is advisable to use multiple software programs and compare the results. Analysing the same building configuration with different software could lead to a different carbon footprint, and failure to include relevant life-cycle stages or processes could yield incomplete results. For instance, when biogenic carbon is included in the LCA of timber structures, it is  appropriate to extend the analysis to stage D to avoid considering full carbon sequestration without accounting for the possible CO2 release in the beyond life-cycle stage. As EPDs, LCA processes, and related software evolve over time, the GWP values they produce may change for a particular building or structure.