A Number and a Story  

Photo courtesy of LaGuardia Gateway Partners/Jeff Goldberg

LaGuardia Airport’s renovated Terminal B (2022), designed by HOK with WSP, HOK, and Thornton Tomasetti as structural engineers.

As most practicing architects now recognize, decarbonization is an imperative for the profession, the nation, and the planet. Yet decarbonization until recently has been understood as essentially synonymous with reductions in operational carbon emissions, which are only part of the story. That story is now changing as more architects, engineers, clients, and citizens recognize the importance of reducing embodied carbon (EC), and as instruments for estimating it, measuring it, and controlling it become increasingly accessible. EC includes the amount of greenhouse-gas (GHG) emissions associated with the extraction, production, transportation, installation, and end-of-life stage of any product in a building, or of the building as a whole. On the global scale, the manufacture of construction materials and products accounts for 11 percent of annual GHG emissions (EPA 2024), implying significant atmospheric gains from improved performance in this area. Whether carbon accounting is driven by clients’ or other organizations’ sustainability goals, regulatory imperatives, business considerations, or any other type of incentive, reporting and reducing EC is an essential aspect of contemporary design and construction. Decarbonization efforts also involve a proliferation of benchmarks, complicating communication and collaboration among architects, engineers, contractors, and clients.

Images courtesy of author’s screen captures, June 1, 2024

Figure 1A (left): Google Books Ngram Viewer data since 1900. Figure 1B (right): Google Trends data since 2004 on the frequency of the phrase “embodied carbon” in books published in English and in online searches.

Awareness of EC as well as operational carbon, says Max Driscoll, AIA, LEED-AP, vice president for sustainability at AECOM Tishman in New York, has taken the shape of “a hockey-stick curve. It was basically zero five years ago, and it’s really going up and up.” (Measurements of the use of the phrase “embodied carbon” using Google Books Ngram Viewer over the past century and Google Trends for online searches over the past 20 years bear out Driscoll’s assessments; see Figures 1A and 1B.) Accounts of attention to EC, like the familiar hockey-stick curve of geophysicist Michael Mann and colleagues showing global temperature rising sharply beginning with the 20th century (Mann et al. 1999), involve nuances and margins of error, yet Driscoll and other commentators describe EC as a critical component of contemporary practice, an area where recognized strategies can combine with ample low-hanging fruit to yield substantial environmental benefits.

Luke Johnson, sustainability specialist at the steelmaker Nucor, notes that operational carbon is intuitively easier to grasp, while EC presents more of a learning curve. “I think it’s easy to understand heating and cooling costs or electrical costs and riding a bike to work versus driving your car; those are things that you can relate to any building. Now, when you talk about a steel building or a concrete building or a wood building, the supply chains there are dramatically different, and therefore the embodied carbon in that supply chain is harder to understand, because it’s not an apples-to-apples comparison between materials.... A full life-cycle assessment is what you really have to do to fully understand the embodied carbon, and that’s another added kind of time and cost.”

The building and construction industries in general, according to Carbon Leadership Forum (CLF) figures taken from the United Nations Environment Programme and International Energy Agency’s joint 2017 report, account for nearly 40 percent of global energy-related carbon dioxide emissions in constructing and operating buildings, including the impacts of upstream power generation (CLF “Climate”). Ironmaking and steelmaking account for 7-8 percent of annual global carbon emissions (IEA), and about half of all steel produced is for the construction sector, placing direct influence on about 4 percent of global carbon emissions in the hands of structural engineers using steel. Since U.S. building codes mainly address operational carbon, EC reporting and management are a professional responsibility.

Efforts to reduce EC “gained a lot more traction earlier on the West Coast,” says Michael Cropper, P.E., LEED AP, associate principal at Thornton Tomasetti, DC Structures Practice Leader in Washington, DC, and co-leader of the firm’s Embodied Carbon Community of Practice. Interest in EC, Cropper says, is particularly high in larger markets, among institutional clients with “strong ESG [environmental, social, and governance] and sustainability goals,” and at larger architecture and engineering firms with the resources to perform the necessary research. There are costs associated with managing information about EC, and “because it’s a new topic, there’s a sense of inherent risk,” Cropper continues, yet “what we have seen is a lot more acceptance.... Once in a market, you start to see a successful project implement these kinds of reductions and gain traction from leasing or good press.” The LEED system, currently in version 4.1, addresses EC in its Materials and Resources credits and will give up to four LEED points for EC assessment and reduction in the forthcoming version 5 of the program, expected to appear in 2025. State and federal Buy Clean initiatives create similar incentives to keep EC below designated thresholds in publicly supported projects.

Allan M. Paull, senior vice president for civil and structural engineering at AECOM Tishman, emphasizes three criteria for evaluating measures to reduce EC. “Whatever solutions you have have to work, have to be sustainable from a building standpoint, and have to be cost-effective.” Clients’ commitment to EC reduction, he notes, is also quantifiable: “Maybe they’re willing to pay 2 or 3 percent more; no one’s willing to pay 50 percent more to be environmentally sensitive.” Paull cites environmental/energy scientist Vaclav Smil’s 2022 book How the World Really Works for a sobering, factually rigorous explanation of processes involved in the transition to cleaner energy, which Smil says is contingent on deliberate human choices (rejecting determinism in the direction of either “catastrophism” or “techno-optimism” [Smil 9]) and unlikely to proceed as swiftly as many would prefer. The four key material ingredients of civilization that have allowed the world population to grow to 8 billion people, Smil asserts, are steel, concrete, ammonia, and plastics. “Steel and concrete,” Paull says, are thus “two of the four foundations of society that allow the planet to support way in excess of what it would support without that.”

The steel industry, at least in the United States, has moved decisively toward circular manufacturing processes over recent decades (see “Structural Steel for Low-Carbon-Emission Lightweight Frames,” Architectural Record, March 2024); the concrete industry has further to go in this regard, commentators suggest, and may offer opportunities for considerable improvement if new decarbonization processes prove scalable. Since “56 percent of our carbon footprint is tied up in those two trades,” as Paull’s colleague Driscoll points out, design teams and structural engineers working with those two materials are in a position to make substantial progress through detailed attention to the EC associated with structural components.

PROLIFERATING TOOLS AND THEIR BEST USE

Multiple organizations and programs have offered guidance to professionals seeking to understand and control EC, including the CLF, the American Institute of Architects’ AIA 2030 Commitment (and its Design Data Exchange), the Structural Engineering Institute (SEI)’s SE 2050 Commitment, the Associated General Contractors of America (AGC), the Aluminum Extruders Council, the Urban Land Institute, the Rocky Mountain Institute, the U.S. Environmental Protection Agency (EPA), and others. The World Resources Institute’s GHG Protocol sets standards for direct (Scope 1) and indirect (Scopes 2 and 3, defined by positions in the value chain) emissions and provides guidance on reduction strategies tailored to different economic sectors. The CLF maintains a useful list of tools for whole-building life-cycle assessments (LCAs) and calculators for individual materials or assemblies, from the Embodied Carbon in Construction Calculator (EC3), a widely used free database of Environmental Product Declarations (EPDs), to more product-specific tools and proprietary software programs, including OneClick, Tally, Athena, the Buildings and Habitat Object Model (BHoM), Build Carbon Neutral, Early Phase Integrated Carbon (EPIC), and many more.

The AGC’s Decarbonization and Carbon Reporting Playbook provides contractors with step-by-step recommendations to be used alongside online emissions calculators, eight of which are linked at the AGC website; the Playbook is free, not hidden behind a paywall (an advantage for small to medium-sized firms, Driscoll comments). CarbonCare, created by a not-for-profit logistics organization based in Engelberg, Switzerland, allows input of shipment methods and routes, calculating the EC associated with multimodal transportation options and informing decisions about the relative GWP of local or global material sources. SE 2050 includes Embodied Carbon Action Plans that focus on four areas, extending beyond single project-level concerns: education, reporting, advocacy, and reduction. Thornton Tomasetti, Cropper notes, was a co-founder of the original SE 2050 Challenge and a founding signatory of its successor the SE 2050 Commitment; as of this writing, the firm has submitted 65 projects to the SE 2050 database, and its R&D studio CORE has developed its own Revit plugin for EC takeoffs and Global Warming Potential (GWP) calculations, Beacon, an open-source program free for anyone’s use.

The energetic response by multiple organizations indicates that EC management is not a transient trend but an enduring aspect of design and construction practice. It also suggests that aligning different measurement and reporting systems, assumptions, and responses can create confusion. No single industry standard for EPDs or LCAs has emerged, though the CLF and other professional organizations (including the U.S. Green Building Foundation, AIA, SEI, and Architecture 2030) have formed an Embodied Carbon Harmonization and Optimization (ECHO) Project aimed at clarifying carbon reporting standards and practices. Cropper notes that it is too early for “a consensus yet on what the right tools are for the right project stages, and for the right projects, and for the right design professionals,” though EC3 has enough adherents to be approaching the status of a standard. Though preferences among these instruments vary widely, “the math is very simple,” says Driscoll: “quantity of material times the GWP of the product equals carbon footprint.... It’s actually simpler and more manageable than people think it is.”

As an aphorism attributed to the Nobel Prize-winning cognitive scientist and economist Daniel Kahneman holds, “No one ever made a decision because of a number. They need a story” (Lewis 250). While LCAs provide essential quantitative information, placing it in the context of a persuasive narrative about what a building can accomplish is up to architects and engineers. Though some clients may cite immediate economic justifications for declining to support the work involved in carbon accounting and reduction, the larger story of a built environment transformed from linear and wasteful to circular and sustainable may ultimately be more persuasive. Cropper points out that the multipart mission of SE 2050 calls for not only EC reporting and reduction in projects but communication with architects and “internally educating our engineers and disseminating strategies and knowledge about how to do this, like with our Communities of Practice.”

“The EC3 is basically a database of EPDs,” notes Johnson. “It’s a great idea to help harmonize where information is housed and how it’s collected. In general, though, you have to be aware of what data you’re using: within that tool, you have to have gone through some sort of design. If you just go in there and you want to compare, say, steel to concrete, just looking at the numbers within a tool, you don’t really have a sense of which one could create a lower-embodied-carbon structure, because you haven’t taken it through a design; you’re just comparing EPD values.”

“We always have to be careful to compare apples to apples,” Johnson continues. “For us that means comparing a building with a building, or a system with a system.” Specialists he has worked with find that “all the data points within a building are very difficult. I heard one person say once that there might be up to 2,800 components that could go into a building, and that’s everything: electrical, carpeting, paints, structure, all that sort of stuff. For an architect [who] has to go through and collect all these data points, that’s a really daunting task.... Even within steel, if you just take a wide-flange section and a rebar, you can’t really just compare those EPD values, because the way that those items function within a building is not the same, and the same quantity of material is not used to make it a functional piece of the structure.”

As an example of a comparability challenge, Johnson describes a design that includes a 15-foot column, either a wide-flange section or a hollow structural section (HSS). “Both have equivalent axial capacities; I think it’s around 370 kips. If you multiply through the embodied carbon for the wide-flange column, it comes out to be 440, and if you multiply it out for the HSS section, it’s like 434, very close. But if you look at the EPD values, it’s 1100 versus 1550. So if you just looked at the HSS one, you’d say, ‘Well, this one’s 40 percent higher,’ but if you carry it through a design and get a quantity and a takeoff, they’re pretty much comparable materials at that point.” Comparisons outside the steel domain can become incoherent, he adds: “Steel reports usually in tons within an EPD; wood reports in cubic meters of material. It’s a volume versus a weight, so comparing those is just impossible to do.”

Driscoll cautions that when a design team performs an LCA, gets carbon-reduction figures for specified components, and obtains a LEED credit, that step is not the end of the story. The USGBC does not require firms to demonstrate how much of the materials mentioned in an LCA are actually procured and used onsite, he says, or how designs evolve. “Our role is important: to take the baton and deliver on what that LCA said the project was going to deliver—if not do better, because the LCA doesn’t often get the procurement nailed to the level of accuracy that we’re going to have, because we’re actually buying the material.”

Francesca Meola, principal and senior project engineer at HOK, and her colleague Mark Hendel, engineering practice leader, both emphasize the value of early coordination among members of the full team of design professionals, engineers, sustainability consultants, and contractors. Having worked with steel, concrete, timber, and hybrid structures, they note that the different materials and associated trades have different requirements and that estimates of GWP can inform design decisions at each stage of the process, from initial conceptualization and material choices through schematic design (SD) and detailed design (DD), when architects and engineers still have the bandwidth to make changes to the design ensuring they can achieve their EC goals. “There is a benefit with doing LCA at the very beginning to understand if it would be better to go with a timber construction versus steel versus concrete,” Meola says. “Of course, you don’t have final numbers, but you have an order of magnitude, and you know what would be the best solution in general.” LCA information remains important after a structural material is selected, she adds, informing the details of bay spacing, foundations, floor-to-floor heights, curtain-wall dimensions, and other components. Meola and Hendel allocate EC-reducing decisions to three categories, ranging from low-hanging fruit with no added cost to ambitious maneuvers that can bring dramatic improvements in overall GWP if clients are receptive. “Basket One,” as Hendel terms it, “is stuff that we don’t even want to talk about; we’re doing it, and it’s not going to cost you anything, and you’re going to be happy for it.... It doesn’t even come up; we write them into our spec. The client has no issue with it. The contractor follows the spec. Done.” Examples from Basket One might be choosing steel manufactured with an electric arc furnace (EAF) rather than a blast furnace, or concrete masonry units using recycled glass pozzolan rather than coal fly ash as a cement binder (Meola, Cropper, and Paull all cite Pozzotive, a ground-glass product by Urban Mining Industries of New Rochelle, NY, as an example of an affordable material whose benefits include EC reduction). “You can take a big leap forward just by paying attention,” Hendel says, “just by looking at it and saying, ‘Okay, let’s make sure we’re at least doing the easy ones.’” “When we have our drawings and the specs,” Meola adds, “we do specify the GWP, the content of your target for concrete, for foundations, for steel, and so on. And so, by putting numbers on our drawings and our specs, we are basically saying we want to achieve these GWP contents. And implicitly, you’re saying who can do it and who cannot do it, but that doesn’t have any impact on cost.”

Basket Two comprises green choices with a slight cost premium that might be offset by sacrificing finishes or other details deemed noncritical. On the structural side, material replacements or column-grid adjustments can constitute Basket Two items; “if you want to do really massive spans, then that is going to have an embodied-carbon implication and a tonnage implication,” Hendel says. Basket Three includes “the pie-in-the-sky ideas, and I think the clients are very receptive to those. It’s on us to give them the information to guide them through that decision-making process, and we might be surprised just how far they’re willing to go, especially if it’s a major client.” Major design changes—not revised grid spacing or floor-to-floor heights, but reductions in a tower’s volume or in basement levels—make Basket Three a domain where large pendulum swings of reduced or added cost command high-level attention.

Incorporating EC data into their practice, Meola says, has been straightforward. HOK has chosen OneClick as the firm’s LCA tool; “when we have detailed information,” she says, “we can extract information from our BIM model from Revit and then push it directly into OneClick with minor adjustment. And that’s really helpful to get instantaneous feedback on the quantities and on the GWP for the project, but we use it also without going through Revit, and we have our spreadsheets with our takeoffs, you know, when we’re still in concept.” The output format (including pie charts and other visualizations) is convenient, she says, both at that early stage and when the team has developed a model. Development of an internal HOK system is also in the works, since “we have all these disciplines in-house [and] we are in a good position to get information from the facade team, the architectural team, mechanical, and us... You want to have a tool that clearly shows what are the aspects of the designs that can be changed and have a large impact on the final number.”

It is essential, Meola emphasizes, for everyone on a project to work with the same information. Variability in the quality of EPDs presents complications, since industry-average GWP data and data from individual manufacturers may differ. EPDs for different materials may also use different operational units, and the dates and accuracy of EPD data are not always comparable. (Direct confirmation with manufacturers that figures are current and products are available in the desired quantities, several commentators note, is usually advisable.) “By using OneClick we have access to the industry-average numbers,” she says, “and so when we are then contacted by manufacturers that can provide a lower GWP, we don’t just take it for granted that they can lower the GWP numbers by 50 percent or something like that. But for us, it’s more getting an understanding of what can be achieved out there, instead of using the industry-average numbers, and just knowing that those numbers can be lowered by going with specific manufacturers.”

Distinctions among types of product-specific EPDs are essential. The International Organization for Standardization (ISO) recognizes three types of environmental documentation: Type I (ISO 14024) claims label products that have met requirements set by governmental or professional bodies (e.g., EnergyStar or the Forest Stewardship Council); Type II (ISO 14021) are self-declared by businesses and do not require LCA studies; and Type III (ISO 14025) have been externally verified by a third party performing an in-depth LCA. The exacting Type III EPDs are most accurate and informative.

“We’ve gotten to a point where almost every manufacturer has an EPD, and that gives us the ability to do things that we didn’t have the ability to do 10 years ago,” says Joseph Dardis, P.E., head of presales for construction projects at ArcelorMittal International North America (a Luxembourg-based steel manufacturer). “Seven or eight years ago, we had some industry-wide EPDs; that was a step in the right direction, but that still didn’t really give you the ability to track individual emissions on your specific project.... Now we’ve gotten to the point where almost every manufacturer has their own, and they have it on a facility-specific basis, so you can get much more granular in tracking those emissions. The more specific you can get with a manufacturer and a facility-specific EPD, the better the accuracy of that LCA. And generally when they are a Type III, externally verified EPD, you can have a greater sense of accuracy.”

The system is not perfect, Dardis notes: “There are still dozens of different EPD standards out there, and you can do an EPD accurately according to other standards, and you can have two different results.” Although ArcelorMittal makes its structural steel in Europe, he says, the firm considered it important to publish EPDs to a North American standard for the sake of transparency and comparability to U.S. producers. Databases like EC3 can aggregate data from the wide range of EPDs and inform comparisons; using EC3, he says, “I can search globally for structural steel sections, and I can see 10 different manufacturers and 30 different production facilities, and the embodied carbon of the steel coming from each one of these places. That is a level of granularity that gives me the ability, if I’m a designer, to make very impactful decisions on a project.”

As the field has progressed from industry-wide EPDs to manufacturer-specific EPDs to facility-specific EPDs, Dardis finds that the SE 2050 program is filling in information gaps about emissions for materials in the contexts of project types. “What is the standard,” he asks, “for a healthcare facility, for an office project, for a school? What is the baseline embodied carbon per square foot? If these design firms aren’t compiling this information, we don’t have it, and we don’t know what ‘good’ is and what ‘bad’ is. So I think the SE 2050 program is enabling this to happen.... We manage what we can measure, so if we don’t know what is an acceptable GWP per square foot for an office building, how do we know if we’re better or worse?” It is increasingly possible to say, for example, “Here’s the baseline for a 30-story office building; we did 30 percent better because we have all these tools available to us, and we can show that by procuring specific materials in a certain way, we can address this.”

MATERIAL CHOICES, AFTER AND BEFORE REVOLUTIONS

Reducing EC operates on two basic levels: choosing greener materials and designing structures to use less of them. The material-choice EC reflects the energy used in manufacture, shipping, and construction, with some newer materials offering measurable advantages (e.g., high-strength steels enabling the use of less volume of steel to support equivalent loads). Design strategies, such as optimization to reduce the amount of steel plate, bolts, and welds used, can thus synergize with material choices.

The two major structural materials, steel and concrete, differ drastically in their current and future EC footprints and the questions they pose for design teams at various stages of a project. “Steel is much more mature than concrete when it comes to recycling and CO2.” comments Paull. Concrete remains more environmentally troublesome, he finds, though he speculates that innovators in that industry are “just on the verge of coming out with some pretty dramatic changes.” Driscoll describes the current condition as asymmetrical: “Steel has been at it for longer, so they’re a little more mature. But there’s more potential right now to decarbonize concrete than there is to decarbonize steel. Concrete is harder to tackle because it’s more regional; steel tends to be more national. For a project in LA, I might get steel from Arkansas. For that same project in LA, I’m not getting concrete from Arkansas.”

Changes in the steel industry over recent decades, Paull comments, are both a case study in technological disruption and an indication of how a sector can progress, though not without costs and casualties. “Up until a couple years ago, nobody was really much concerned about what was going on in embodied material,” he says, but after the transition from basic oxygen furnaces to electric arc furnaces (EAFs) in the 1980s and 1990s, “the two dominant U.S. steel mills lost market share because they didn’t convert fast enough. U.S. Steel is still in business, but with diminished capacity, and Bethlehem Steel no longer exists.” The contemporary industry resulting from this shakeup is a model of circularity by several important metrics: today’s American hot-rolled structural steel is 100 percent recyclable and 93 percent recycled, according to the American Institute of Steel Construction’s figures.

With about 70 percent of all steel produced in the U.S. coming from scrap-fed EAFs, a key determinant of its EC content is the indirect emissions component (Scope 2 in the GHG Protocol’s system), the footprint of the electricity used in the process, which varies widely by region. Where power comes from renewables (wind, solar, and hydroelectric; nuclear is very low in GHG emissions but not considered renewable, since its fuels are mined), the steel’s EC is competitively low. Where power depends on fossil fuels, its EC is correspondingly higher. Steel mills’ locations relative to most job sites make transportation a larger component of the material’s overall EC calculations, though generally not large enough to be the decisive variable. ArcelorMittal’s Dardis notes that “the argument we always get is that as a European manufacturer, what is the effect of bringing structural steel from Europe versus procuring it domestically? What’s the penalty; what’s the tradeoff there? Steel needs to be thought of a little bit differently than regionality. Process and electrical grids are more important. Of course, transportation does matter, but typically, at the end of the day, it’s a pretty small percentage.”

Nucor’s Johnson estimates that transportation costs might typically account for 5 to 10 percent of the steel EC on a project. For practitioners interested in the power sources for particular steel mills, he points to a resource maintained by the EPA, the Emissions and Generation Resource Integrated Database (eGRID) website, which gives data on the power-grid mix for regions of the U.S.: “how many coal-fired plants and renewable sources (being hydro, wind, and solar) that fall within that grid. And then you can utilize that information within that specific mill’s EPD to assess your Scope 2 impacts for the utilization of electricity within that region.” The eGRID data, he says, can lag roughly 18 months behind the present, though regions’ power profiles rarely change dramatically year to year unless major assets like a large wind farm come online or are decommissioned. Examination of power-grid profiles for specific steelmakers adds valuable information to the material’s EPD.

In contrast, “concrete clearly produces a lot of CO2,” Paull says, analyzing the component processes: “You have the CO2 in mining the limestone; you have to heat the limestone and the clay and the sand to 2,700°F, which not only releases carbon dioxide from the limestone but requires a lot of energy from hydrocarbon fuels such as natural gas. And then you grind up the resultant product called clinker, which requires more energy, and mix it as a binder in concrete. Cement by itself can only produce around 6,000 psi concrete. So about the early part of this century, the buildings got bigger, the demand on the concrete got higher, and we needed to get the strengths 8, 10, 12, 14,000 psi, and there’s no way to do that with Portland cement, so we started replacing Portland cement with slag and fly ash, slag being the byproduct of the steelmaking industry, and fly ash being the byproduct of coal-burning power plants. In simple terms, what those two products do is use the byproducts that aren’t gluing the concrete together and make more cement, and that allows us to get to a higher strength; it also reduces the temperature gain, which limits thermal cracking issues, which can weaken concrete. But no one really was concerned about CO2; we were concerned about the performance of the concrete. But inadvertently, we reduced the CO2 content of Portland cement. My opinion is, we’ve probably gone as far as we can with Portland cement” in improving its carbon footprint.

However, Paull mentions some companies experimenting with low-EC alternatives to Portland cement, using electrolysis processes or chemical admixtures (including captured CO2) rather than high heat, that may eventually revolutionize the concrete industry as EAFs have revolutionized steel. As with many novel products, challenges include availability of both the materials and the expertise in using them, Hendel notes. “Contractors, when faced with it at the start of construction, are understandably going to say, ‘Well, if it limits the people who can bid on this project to one, we’re not going to do it.’” If a contractor is on board with using new materials, he recommends, “ask them who their subs are going to be, who their likely bidders are going to be, and if there are producers locally who have this technology.”

Driscoll notes that AECOM, as “the first construction management firm to have SBTi [Science Based Targets Initiative]-approved targets... the most aggressive carbon targets of any large contractor in the U.S.,” is in a position to pull the market to some degree. But not all firms can persuade clients to expand their comfort zone with new materials. “You’ve seen it a million times: great sustainability strategies in design come down to value engineering, and then half of them don’t make it to the finish line. And so we’re trying to de-risk that by understanding where the premium is. And we do it at a point where we have leverage, because those ready-mix suppliers have not yet won the job.”

“We really need the concrete industry to mimic what the steel industry is doing,” Meola summarizes. She estimates that with substitution of low-EC concrete and related processes, “hopefully in the next ten years we’ll see improvement.” At present, the experimental or niche status of advanced concrete stands in contrast to the EC-reduction gains already established by steel.

THE MATERIAL COSTS OF SIGHTLINES AND ATRIA

Clients present a range of levels of receptivity to the information-intensive activity of EC reporting, balancing it with other imperatives. Paull points out that business metrics, material metrics, and other aims can diverge. “We like to optimize weight, but our main goal is to optimize cost,” he says. “And optimizing cost doesn’t necessarily mean we’re optimizing weight. So there are strategies to take more steel out of a building, but it would add a lot of cost.” Designs intended to increase rentability or “to build a statement building” are also frequently profligate with materials and thus inefficient from the EC perspective, he adds: “We want to minimize material. But having column transfers, over-height floors, massive lobbies, that all takes a lot of material to do that. [At] the top of buildings, once upon a time, the building would go up, and we’d just square off the top, and that’s it. You don’t see that anymore; you see massive concrete or steel structures sitting at the top of the building that aesthetically are very pleasing but really don’t do anything.... If you’re putting a big hat on top of a building, that’s going to increase your carbon footprint.”

Structural design choices, Cropper points out, are powerful determinants of material volume and thus of EC—for better or worse, pitting tenants’ desire for column-free space (and thus a building’s leasability in a difficult commercial market) against EC metrics. “The further we space columns apart, the harder our structure is going to work,” he says, and “the more material we’re going to need. And what we tend to see is that the floor slabs and floor framing are big drivers of the embodied carbon in our structures. Vertical elements are less so, typically, especially if you’re talking about concrete buildings. Steel is the same way: a driver of tonnage is the column-grid spacing.”

On one project, Cropper recalls studying EC savings comparing a 30-by-45-foot office grid against a 30-by-30’-grid, “and it was [roughly] 30 percent right off the bat. I tell architects, ‘I’m not here to tell you that we need to go to 30-by-30 grids everywhere,’ but at the same time, maybe we need to think about that.... Every pound per square foot we can take off our building, we can reduce the material that we’re using in the building structure, and that reduces the embodied carbon. Going above and beyond code limits, there’s a balance between providing future flexibility and providing material in the building that we don’t need.”

HOK’s Hendel points out that “it’s not just about finding the lowest weight for your structure” in every situation. Early in his career, after assuming that the most lightweight structures would also be environmentally optimal, he discovered that “weight may be in direct conflict with another consideration.” Curtain wall, with its multiple elements, is often high in EC; “if you’re able to go with a lighter-weight structure, but it results in a really deep floor... you can add a little bit of weight but cut your floor depth down by a foot and take out a foot of curtain wall for every floor of a building. That’s a tremendous amount of material.”

Photo courtesy of LaGuardia Gateway Partners/Jeff Goldberg

Figure 2. Pedestrian bridge connecting headhouse to island at LaGuardia Terminal B.

New York’s LaGuardia Airport Terminal B, transformed in 2022 from an outdated, congested facility to a spacious multiple-award winner, offers a salient example of precise attention to embodied carbon, note Hendel and Meola of HOK (part of a design-build team also including WSP, Thornton Tomasetti, Skanska Walsh, Meridiam, and JLC Infrastructure). The poor flood-plain soil beneath the airport required a bed of long piles extending some 180 feet into the ground, even taller than the building itself, reports Meola. Designing the foundations of the structure (a concourse headhouse linked to islands by 450-foot pedestrian bridges above active taxiways; see Fig. 2) and determining the appropriate amount of material, in the midst of a complex construction process that could not interrupt airline operations, called for precise optimization studies.

“A pile is essentially a step function,” Hendel says; “if a pile has a 300-kip capacity and you have 350 kips on that capacity, you need two piles, but those two piles are really underutilized, and that’s a waste of material and money. We did all these studies where we looked at the bay spacing, the thickness of the slab on the first floor—which was going to need to be a structured slab spanning between foundations, not resting on grade—and did an optimization to reduce, significantly, the number of foundations required.” Skanska’s fact sheet notes that design optimization, including the pile design, reduced the project’s overall EC by 10 percent. “The design of the pedestrian bridge,” Hendel continues, “was driven by reducing the overturning moment at the concourse end, and there were some gymnastics that we did there in order to get the number of piles down.... It’s all underground; you don’t see it; but after we started on our design, we cut the number of piles by 40 percent.”

This optimization process was one aspect of an all-hands-on-deck sustainability effort by the firms working with the LaGuardia Gateway Partners consortium, addressing both embodied and operational carbon. Terminal B is the world’s first air terminal to receive a LEED Gold rating under version 4 (which recognizes EC reduction and material selection based on EPDs), and its accolades to date include the first-ever Envision Platinum award from the Institute for Sustainable Infrastructure.

Another recent project where structural engineering optimized material efficiency is the expansion of an eight-story 1975 concrete-frame office building in the Penn Quarter section of downtown Washington, D.C., 600 Fifth Street NW (Figures 3-4). The goal was to give the Rockefeller Group/Stonebridge partnership a trophy-class commercial building that makes the most of the site’s location between Capital One Arena and the National Building Museum, with superb public transportation options as well as views and amenities. Adding four new steel-framed floors (three office floors and an occupied penthouse) and a new lateral system, reports Cropper, Thornton Tomasetti (working with New Haven-based design architects Pickard Chilton and Houston’s Kendall/Heaton as executive architects) moved the building’s side core to a more central position and recreated its parking ramp, all without requiring any new foundations or below-grade construction–a particular challenge, since the building sits atop the Red Line tunnels of the Washington Metro system (in its previous incarnation, it served as Metro’s headquarters for nearly 50 years). “A tear-down scenario was studied,” Cropper says, “but ultimately the project schedule, budget, and sustainability goals drove a transformation approach–in essence, a conversion from office to ‘better office.’”

Photo courtesy of 600 Fifth

Figure 3. Rendering of 600 Fifth Street NW, Washington, D.C., on completion, with four new steel-framed floors added to an existing eight-story commercial building.

Photo courtesy of Thornton Tomasetti

Figure 4. Steel bracing added to the existing concrete frame at 600 Fifth Street NW.

The team achieved carbon-footprint savings by retaining most of the existing structure (removing the concrete roof), adding floor area and terraces with steel framing, and incorporating supplemental steel framing and fiberglass-reinforced plastic where needed for strengthening. A series of steel braces supplemented the existing lateral system, tuned to meet stiffness and strength requirements without exceeding the existing foundation’s capacities. Existing pile testing was conducted to justify increased capacity of the existing foundations, which required no strengthening. “The goal was to use as much of the existing building’s capacity as possible and add structure only where necessary,” Cropper recalls.

The revamped building will have open-air terraces on every other floor, a shared tenant amenity space on the 12th floor, and a public ground-level park. Energy-efficient features include a new curtain wall and a dedicated outdoor air system with Minimum Efficiency Reporting Value 13-or-greater air filters providing 100 percent fresh air. The new 400,000-square-foot building, on track to earn LEED Gold and WELL ratings, topped out last May and is 50 percent preleased at this writing, anticipating a 2026 delivery date.

Laurel Christensen Chądzyński, sustainable design leader at Dyer Brown and Associates in Boston and director of engagement for the nonprofit organization Mindful Materials, sees parallels between material-reduction efforts and the Lean manufacturing philosophy (derived from Toyota Motor Corporation’s kaizen team system), which defines any element of a work-flow process that is not valuable to end-users as waste and targets it for elimination. Applying such a process to the elements of architectural EC, she notes, may take straightforward forms like preferring locally produced materials to minimize shipping, all other factors being equal, though reducible energy and carbon may not always appear economically visible. “The carbon impact is still an externality; there’s not a fee for a higher-carbon material,” she says. “We’re at the stage in the market where for many products, you’re still paying a green tax for a lower-embodied-carbon material. So if anything, I think owners might be hesitant to embrace those lower-embodied-carbon options, unless it’s already a priority to them to lower their carbon footprint on a given project.”

Chądzyński points to the ECHO Project (on the verge of publishing public-facing documents at this writing) as a step toward resolving certain persistent anomalies in carbon-data reporting. At a recent event in Boston, she recalls a presenter discussing a building with a noticeable “discrepancy between what was modeled and what actually was delivered to the site... they had modeled a 16 percent reduction in embodied carbon for the overall project, and what they ended up getting was 6 percent.” Such differences between modeling and execution, she observes, are frequent enough to require organized attention; more communication about such cases is a key step toward bringing modeled and measured figures closer. Considering “the variability in the aggregate and the mix of concrete” from local suppliers, compared with steel delivered from a consistent source, she adds, she would expect “less of a discrepancy between what’s modeled and what’s delivered” with steel structures.

Riffing on past AIA president Carl Elefante’s widely quoted observation that the greenest building is the one that’s already built, Chądzyński offers the corollary that “the lowest-embodied-carbon material is the one that’s already on site.” Since her firm often works on existing buildings and is the on-call architect for upgrades to some 75 buildings in downtown Boston, where the average lifespan of a corporate office interior is five to seven years, she and her colleagues use EC measurements (from the EC3 database and the Revit plugin Tally) to show clients the scale of the offset between repurposing and demolishing materials at the end-of-life stage for a building or space. “If we’re not able to keep existing material in place,” she adds, “we try to partner wherever possible with manufacturers who have take-back programs and coordinate with them to reclaim the material.” After another project where an effort to reclaim and repurpose materials from a renovated office interior was stymied by contractual and timing considerations, she says, “we realized we have to start these conversations way earlier,” leading to “growth of a huge network here in Boston that’s focused on material reuse as a way to reduce embodied carbon.” She headed a CLF/Dyer Brown/Structure Tone/Mass Design Group team that produced the Boston Deconstruction and Material Reuse Roadmap (Christensen et al.).

EC is not the sole consideration in a project, she acknowledges, recommending that anyone responsible for material choices consider “the concept of regrettable substitutions,” where an undesirable material is replaced by one that has been less thoroughly studied and may be toxic in ways not yet understood. “Carbon is massively important, but there’s also a huge need for supply-chain transparency so that we know that we don’t have forced labor in the supply chain [or unknown] human-health impacts upstream and downstream, not just in the installation phase but the ecosystem impact.”

CONCLUSION

As information about EC proliferates among architects and clients, perception of risks and investments becomes clearer, and progress toward zero carbon, a position Cropper and others regard as imperative, can accelerate. “We’ve got to be zero carbon and drop the ‘net’ from that,” he says, recognizing that Net Zero claims bolstered by carbon offsets are no substitute for true decarbonization. Reducing EC is a necessary strategy, he finds, not a panacea but a component of action on multiple fronts: “We can’t necessarily design our way out of the climate crisis, and we can’t design our way out of having embodied carbon in buildings and in other structures. There’s a balance and synergy between making good design choices, being efficient with the material, and wider decarbonization efforts.”

Though the business axiom “what gets measured gets managed” (usually attributed to consultant Peter Drucker) is often misapplied in other domains, in the field of EC management it is fundamental, and the movement from early recognition to measurement—amid a chaotic-looking array of measurement methods—represents a form of progress. Measurement is essential to the story, yet it is not the whole story, commentators agree. The fragmentation of the construction industries and the complexity of making coherent measurements in appropriate contexts contribute to the challenges of carbon accounting. The rewards for taking on this responsibility and incorporating embodied carbon management into practice include the knowledge that a building not only meets regulatory or ratings requirements, but also contributes to a built environment that is less of a global problem and more of a solution.

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Bill Millard is a New York-based journalist who has contributed to Architectural Record, The Architect's Newspaper, Oculus, Architect, Annals of Emergency Medicine, OMA’s Content, and other publications.