Assessing Structural Embodied Carbon

To view the figures and tables associated with this article, please refer to the flipbook above.

In 2019, the Carbon Leadership Forum issued the SE 2050 Challenge. In response, the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) has responded with the SE 2050 Commitment Program with the goal of eliminating embodied carbon in building structures by 2050. Over 140 firms are currently committed to SE 2050 and actively measuring and reducing the embodied carbon of their projects. As this SE 2050 Commitment Program gained traction, participating firms, engineers, and designers learned the approaches to measuring embodied carbon can vary greatly among engineers, design stages, projects, and firms. To simplify the measurement approaches and provide consistency, the members of the SEI Sustainability Committee developed and published an ASCE/SEI Prestandard. Like other Prestandards, it is peer-reviewed and published with the goal of standardizing practices and reducing inconsistencies. Note that Prestandards create the basis for future Standards but are not typically adopted by an Authority Having Jurisdiction.

The “ASCE/SEI Prestandard for Assessing the Embodied Carbon of Structural Systems for Buildings” recommends scope and a calculation methodology for assessing the embodied carbon emissions of structural systems. It breaks the assessment process into a manageable framework for the user. The Prestandard assumes the user has basic familiarity with structural engineering. For users without a basic familiarity of embodied carbon concepts and/or life cycle assessment (LCA), the Prestandard directs users in the assessment process and provides additional resources to start measuring and reporting embodied carbon.

Embodied carbon assessments are performed for a variety of reasons. By defining Assessment Tiers, the Prestandard provides structural engineers with flexibility in meeting the needs of the embodied carbon assessment with the available data, ultimately helping structural engineers and the wider design team make informed carbon decisions.

Embodied carbon assessments require a clear goal and scope for a successful outcome. The Prestandard requires the user to set a goal for the embodied carbon assessment, including the reason for the assessment, the intended audience, and how the results will be used. After defining the goal, the user is able to determine the Assessment Tier to establish minimum requirements for all further stages of the carbon assessment, creating a consistent set of scopes and methodologies for evaluating embodied carbon of structural systems.

Accurate material quantities are critical to a meaningful embodied carbon assessment. The Prestandard outlines the building components scope (inclusions and exclusions) specific to each Assessment Tier. Four material categories are defined alongside definitions and commentary for categorizing elements of a structural system: “Excluded,” “Primary,” “Accessory,” and “Ancillary.” For example, “Primary” structural components include all major building components that participate in the primary gravity and lateral load path (e.g. beams, columns, braces), while their connections are “Accessory” components. “Ancillary” components do not participate in primary gravity and lateral load paths, for example finish and facade support. The higher the Assessment Tier, the more robust the material quantities need to be included (See Table 3-1 from the Prestandard).

The Prestandard outlines three acceptable data sources for material quantities, i.e., how the user should obtain material quantities and minimum acceptable data sources based on the Assessment Tier (See Table 3-2 of the Prestandard). The Prestandard defines the data sources in detail and explains how they are to be applied to a structural carbon assessment dependent on the Tier.

Material quantity takeoff provided by contractor procurement records or submittals are the most accurate and typically reflect the as-built condition more accurately than the design documents and, in some cases, the material quantities may even account for material wastage. These quantity takeoffs are allowed for Tier 1, Tier 2, and Tier 3 assessments.

Quantity takeoff from design documentation can include project drawings, sketches, narratives, or a structural building information model (BIM). Models that are sufficiently detailed can provide highly accurate and accessible sources of material quantities. However, the user must exercise judgement especially when obtaining material takeoff from a BIM as modeling practices vary widely, and it is common to have overlapping, duplicated, or missing elements/materials (e.g., rebar not modeled in concrete elements or kickers not modeled at a perimeter wall condition). Quantity takeoff from design documentation is allowed in Tier 1 and Tier 2 assessments. In limited instances, it is permitted in Tier 3.

Quantity takeoff from manual approximation includes material quantities estimated using techniques such as rule-of-thumb estimation, use of design tables, or extrapolating results from design of a single bay over the entire building. This type of quantity takeoff is intended for early stages of a project and allowed for Tier 1 assessments. In limited instances, manual approximation is permitted for Tier 2 assessments, specifically for accessory or ancillary components where information is not explicitly included in the design documentation (e.g., primary framing connection material).

Calculation of environmental impact is a discussion worthy of its own article. The calculation pulls together all the threads of previous steps to tell a cohesive story about carbon impact. The Prestandard outlines major inputs and presents a consistent methodology for users.
A reference study period (RSP) of 60 years is required. This is generally consistent with green building standards and rating systems, such as LEED, with some exceptions. Choosing an appropriate RSP may at times contradict the requirements of these standards and rating systems; however, the RSP should be selected to support the intended service life of the building.

The Prestandard adopts life cycle stages and modules defined by EN 15643, a European framework for assessment of buildings. The assessment tier will dictate exact life cycle modules that are included, optional, and excluded from the structural carbon assessment. This same logic applies to acceptable sources for carbon data which quantify environmental impact. This data can be broken into two major buckets—primary and secondary data.

Primary data, reported in an Environmental Product Declaration (EPD), is carbon data specific to a given process or product and its manufacturing process. EPDs can be industry-average, region-specific, or product-specific, in order of increasing specificity.

Secondary data quantifies environmental impact with either global or regional carbon data sets. These contain generic emissions factors for processes and products and may not be representative of a given manufacturing process.

Both primary or secondary data are acceptable for a Tier 1 assessment although EPDs are preferred in all Tiers. (See Table 4-2 of The Prestandard).

The Prestandard provides a methodology for how to consider both biogenic carbon that provides structural engineers with a more nuanced understanding of the impact that biogenic carbon has on the climate change impacts of their system. However, this topic is often one of the most confusing for structural engineers as to when and how they should be considered. Here’s the crash course.

Biogenic carbon is considered in two ways within the Prestandard: biogenic carbon content and biogenic carbon flows. Biogenic carbon content is a metric that measures the total amount of carbon dioxide equivalent that is stored in the structural system. Whereas biogenic carbon flow considers when biogenic carbon is sequestered and emitted across the structural system’s lifespan.

Concrete carbonation, another confusing topic in LCA, is discussed in the Prestandard.

Comparison of different carbon assessments is particularly important if the user wishes to draw conclusions about design decisions or demonstrate reductions in our structural embodied carbon—a key focus of the SE 2050 Commitment and green building rating systems.

The Prestandard outlines requirements for reporting results of carbon assessments and methods of comparison for different carbon assessments.

There are two distinct approaches when performing a carbon assessment to compare different systems or buildings.

The first comparison approach is utilizing a carbon assessment to make design decisions. In this case, the intent of the requirements is not to provide a strict functional equivalence between comparisons, but to provide a basic level of equivalence. General items that must be held constant include site location, use-type, and code compliance in addition to structural design criteria such as risk category and environmental loads (snow, wind, earthquake, etc.).

In general, most structural decisions made throughout a project could be viewed through a carbon assessment lens.

This type of comparison is typical of situations early in the design process when considering different structural systems and evaluating general framing schemes. As with comparing design options in terms of cost, schedule, future plan flexibility, or other criteria, carbon assessment can be used as another weighting factor in these decisions. Some common strategies design decisions that would benefit from carbon assessment are:

The second comparison approach is utilizing a carbon assessment to demonstrate reductions from a reference structural system. For this type of comparison, the functional equivalency must be more stringent between the reference assessment and the proposed. This includes performance criteria such as deflections, vibration, durability, superimposed dead and live loads, and environmental loading among others. Both structural systems must be geometrically similar including number of stories, structural clear height, gross floor area, etc.

The referenced structural system is usually intended to represent “typical” construction practices for a building of the same type as the proposed system. For example, if a user intends to demonstrate reductions using a proposed wood-framed system in lieu of a reference steel-framed system, the two systems must be functionally equivalent in nearly every way. While spans may necessarily be shorter in the wood-framed system or the overall structural depth may vary between the two systems, each story’s clear height must be identical. This type of comparison will generally be required by Green Building Rating Systems or local, state, or federal policies.

In conclusion, the Prestandard presents a clear, consistent methodology for carbon assessment that is generally in alignment with existing references, rating systems, and available data. It breaks down a new and complicated concept into a manageable framework that allows for completeness in the assessment and comparison between design options and reference systems. Whether you are performing embodied carbon assessments weekly or are brand new to measuring embodied carbon, this Prestandard provides a standardized path for carbon assessments. It is free to download from the ASCE bookstore thanks to the SEI Futures Fund. ■

About the Authors

Ethan Fogle is a structural engineer with HGA in their Washington, DC office. He is HGA’s SE 2050 Embodied Carbon Champion, a member of their Sustainability Steering Committee, and a committee member of the ASCE Structural Engineering Institute’s (SEI) Sustainability Committee.

Jay Arehart, Ph.D, is faculty director of architectural engineering at the University of Colorado Boulder.