“What Does Your Building Weigh?”

Buckminster Fuller

Why Structure Matters

There is a short list of common structural materials: concrete, masonry, steel, and wood. Each of these has best fit applications that optimize structure for embodied carbon and cost. Each also has currently available lower-carbon options today. All four also have credible pathways to reach near zero carbon pollution with investment, innovation and – mostly importantly for the design and construction industry – demonstrating demand for lower carbon versions of these products.

Concrete and steel use accounts for roughly 12-15% of global carbon pollution, and most of the attention is rightly focused on these two as the largest sources of carbon pollution. Research and development of carbon-efficient manufacturing is bringing lower carbon versions each year, helping to meet low-carbon policies, such as in California or Massachusetts. Wood removes carbon pollution from the atmosphere and stores it during a building’s lifetime but has a very different challenge: forest management practices are a consequential part of the carbon math. Since we don’t yet have good data on the comprehensive impact of forestry, carbon calculations may be under- or overstating the impact of a given wood product.

Easy wins for all projects are available right now using all structural systems, described throughout this post. Since most structural masonry projects in the US use concrete masonry units (CMU), masonry has been combined into the concrete section since cement is the primary embodied carbon source of CMU.

The Structural Engineers (SE) 2050

The SE 2050 (Structural Engineers 2050) is helping lead the industry, developing many resources on embodied carbon, a basics guide, top ten things a structural engineer should know, case studies, rating system applicability, reductions, a page on getting started with embodied carbon, and more. The SE 2050 also has a quick calculator (ECOM) to estimate embodied carbon from structural materials, specific guidance on materials and specifications. A growing list of resources including strategies and timing, case studies and pathways, and material-specific strategies to reduce embodied carbon.

Structure and Carbon Pollution Basics

As the longest-lived component of a building (Post 09), the most critical carbon consideration with structure is that it provides a long service life for a building. Structural failure can lead to an entire building being demolished, wasting money and the carbon invested in the building. Adaptive reuse (Post 10) of a building depends on adequate structure that anticipates flexible needs – often through regular columns spacing and not-too-tight floor to floor height.

From an embodied carbon perspective, we know more about structure than any other building system. This is because a building’s structure is primarily composed of a short list of materials that are bought in great quantities in a single purchase. Because structural embodied carbon is so readily available, most analyses of whole buildings over-represent the embodied carbon impacts of structure. Most so-called Whole-Building Life Cycle Assessment (WBLCA) are limited to include only structure and envelope (excluding MEP, landscape, interiors, etc); this creates industry confusion and results in structure commonly understood  as the largest source of emissions. When including other emissions (Post 02), structure often accounts for between a quarter and a half of the total initial carbon pollution.

Steel, Concrete, Wood

The primary structural systems of steel, concrete, and wood are covered separately below. Two notes:

• Masonry can be made from cement (with low-carbon strategies similar to concrete), from fired clay (with decarbonization of heat being key), or include biogenic materials like hemp. Plant and vendor-specific embodied carbon data is available for many concrete mixes (though primarily in urban areas) and steel products, and some mill-specific data on masonry products exists.
• Embodied carbon for wood is much more complex and includes forest management practices, and this complexity has meant wood suppliers don’t generally have mill-specific data yet. Wood structures often have lower emissions than comparable structures of steel or concrete, but forests worldwide do not contain enough timber to construct all buildings out of wood, and clear cutting all forests would have significant environmental and ecological damage even if it was possible. Responsibly sourced wood structures, however, are an important strategy on the path to zero carbon.

Steel Manufacturing

The age of modern steel introduced the skyscraper, allowing unprecedented height and cantilevers with a minimum of material and weight. Steel is long-lasting when protected, flexible, and elegant. Even in timber structures, steel connectors are often critical for joining structural members; steel reinforcing bar is part of all modern concrete structures as well. It is also a great circular economy material due to profitable recycling despite a wide variety of alloys.

Steel’s embodied carbon comes primarily from converting raw iron ore into steel, and the two major methods of producing steel have a significant effect on resulting embodied carbon. A blast furnace within an integrated mill Basic Oxygen Furnace (BF-BOF) uses a high content of virgin material and produces first generation steel that is often higher quality; this process requires extensive use of carbon emitting fuel sources (often coal) to reach the high temperatures and chemical reactions required.

Electric arc furnaces (EAF) operate at lower temperatures than blast furnace methods and primarily utilize recycled steel as inputs. They produce steel with a lower carbon impact than blast furnaces generally because of the lower temperatures, the use of electricity which is rapidly decarbonizing due to cheap solar and wind, and the use of scarp steel which avoids the mining of raw iron. Instead of using scrap, iron produced by a Direct Reduced Iron (DRI) process can be fed into an EAF. DRI-EAF avoids or eliminates the fossil fuel inputs into the process, and has a clear carbon pollution advantage over BF-BOF.

The path forward for near-zero carbon steel includes renewable-powered mining, transportation, and the use of DRI-EAF process for virgin material and EAF recycling of scrap steel. Currently few companies invest in this supply chain and they will reap advantages as the world tends toward clean technologies.

Since building stock is expected to double by 2060, most steel is made from virgin material using blast furnaces. Blast furnaces produce slag as a by-product, which is used to replace cement content in concrete, lowering concrete’s carbon footprint.

Low Embodied Carbon Steel

Organizations such as the Sustainable Steel Buyers Platform, First Movers Coalition, and Steel Zero are providing demand for millions of tons of green steel with Responsible Steel certification providing guidelines. The World Economic Forum’s steel’s carbon intensity tracker showing more than a 20% drop in emissions intensity, with a goal for a 90% or greater reduction by 2050. Some companies are beginning to offer low or zero embodied carbon steel already.

Data on steel is widely available in Environmental Product Declarations (EPDs), but some caveats need to be mentioned. In most EPDs:

• A1 includes raw material extraction or recycled content through the steel mill production
• A2 includes transportation to the fabrication shop
• A3 includes work at the fabrication shop but excludes coatings. EPDs often include an industry average for A3 and, for certain custom shapes, this will underreport embodied carbon. Some EPDs exclude fabrication, making comparisons more challenging.
• A4 and A5 are typically excluded from the EPDs: A4 is straightforward (transportation to site), but A5 installation includes on-site welding (tied to local electricity carbon intensity) and very little data exists on energy use and carbon emissions.

Use procurement weight for estimating embodied carbon for steel; this is often around 10% higher than the steel weight within a building model because many elements are not modeled.

Steel Embodied Carbon Reduction Strategies

• Structural Efficiency:
  • Program arrangement and stacking can require fewer transfers and allow deeper, more efficient beams.
  • Reduce the weight of the building.
  • Heroic cantilevers and column transfers can significantly increase the total weight of steel required, adding to foundation concrete embodied carbon as well
  • Optimize bay spacing, using multi-spans, castellated beams, cambered beams, asymmetrical beams, and trusses to reduce total steel weight while carrying the same loads.
• Require EPDs from steel manufacturers, to compare at bid time
• A steel-core high rise often weighs less and has lower embodied carbon than a concrete core building
• Consider salvaged structural steel, which can save around 50% of the GWP compared to a new steel member
• Using higher strength steel (65 ksi or more) may reduce total embodied carbon by reducing total steel weight when strength (not stiffness) sets performance requirements. It can also reduce cost, since less steel is being procured
• Minimize site fabrication and on-site welding
• Buy low-carbon steel. Establishing a cost of carbon allows GWP (submitted within EPDs) to be compared at procurement time. Applying a carbon price can then be used to level bids toward meeting low-carbon goals. Newer technology is lowering the embodied carbon of steel
• Specify that relevant steel products comply with Buy Clean California’s maximum GWP limits, CLF’s baseline (see above) or other GWP limits
• Steel fireproofing has a noticeable impact, lower-carbon options are available
• Other strategies are in Architecture 2030’s Materials Palette

Low Embodied Carbon Concrete

Concrete is one of the most useful, durable construction materials on the planet, especially when paired with a small amount of steel (often rebar). When water is mixed with Portland cement and aggregates, a chemical reaction occurs (hydration) that binds the mix together to form concrete. The proportions of Portland cement, water, aggregates, and any admixtures determines the strength, cure time, workability, and other concrete properties.

The US National Ready Mix Concrete Association has suggested a pathway to carbon neutral concrete. The Global Cement and Concrete Association also has a plan to carbon neutrality by 2050, with country-specific plans, including the Portland Cement Association of the US that proposes reduction strategies that rely heavily on carbon capture and storage instead of cement substitutions. Some near-zero carbon products are in testing or undergoing commercial scale pours already.

Two realistic pathways for decarbonizing concrete are being simultaneously pursued:

1. Electrification of the cement manufacturing process and capturing the CO₂ from the chemical reaction. This is incremental as cement manufacturing plants are built and rebuilt.
2. Replacing nearly all cement with substitute cementitious materials that perform similarly enough. This second pathway is already in progress and includes more opportunities each year.

Concrete’s GHGs: Mostly Cement Manufacturing

The creation of Portland cement results in 6-8% of global carbon emissions. Roughly half of the emissions from Portland cement are from fossil fuel energy use (heating to 1,450°C) and the other half from the chemical reaction that creates it (CaCO₃ converted to CaO and CO₂), resulting in 8 or 9 tons of CO₂ for every ton of cement.

Cement is responsible for 80-90% of the emissions for concrete. For this reason, improving the embodied carbon of concrete starts with reducing the amount of Portland cement required, often using supplementary cementitious materials (SCMs) that have similar properties. Fly ash (a byproduct from burning coal), blast furnace slag (from virgin steel production), finely ground raw limestone (Type 1L), limestone calcined clay (LC3), and other pozzolans (such as ground glass) can be substituted for a significant amount of cement. Some SCMs are less expensive than cement, resulting in cost-neutral embodied carbon reductions. The free, online EC3 tool has thousands of concrete mix EPDs in the database to help compare mixes, suppliers, and regional impacts.

Concrete Embodied Carbon Reduction Strategies

• Design to reduce building weight. This strategy is effective for all structure types since concrete foundations support most buildings. Reducing column transfers and utilizing lighter and more efficient structural systems, like post-tensioned slabs. Instead of thickening slabs for transfers, using a ribbed or waffle slab can also be effective.
• Type 1L cement (incorporates more raw limestone to replace cement clinker) is now increasingly standard across the US, while limestone calcined clay cement (LC3) and others are emerging cement substitutes.
• Consider importing different aggregates that allow for better mix consistency and performance, which requires less cement (as long as transportation carbon is also considered). Since hydration is a chemical reaction, nuances in aggregates can require more or less cement to be used.
• Maximize use of supplementary cementitious materials (fly ash, slag, pozzolans) and carbon sequestering additives like Zeolite. Slag and Fly ash can increase time to strength; many mixes won’t be fully loaded for months or longer and can easily use a 56-day or 100-day time to reach strength.
• Use performance based concrete specifications instead of prescriptive – instead of specifying a minimum cement content for each mix, specify the performance parameters such as strength and shrinkage, and allow the concrete suppliers to build mixes to meet performance criteria with lower carbon mixes.
• Set total global warming potential (GWP) targets for concrete mixes by strength lower than the National Ready-Mix Concrete Association 2022 baselines , Carbon Leadership Forum baseline or typical GWP limits.
• Don’t limit SCM content beyond code requirements. The concrete building code, ACI318, only limits the quantity of SCMs for concrete exposed to de-icing chemicals.
• Other strategies in the Rocky Mountain Institute’s Concrete Solutions Guide and Architecture 2030’s Materials Palette.

Over the next few years, widespread options include reducing cement through several techniques describes below. Several formulations eliminate cement by using other materials, including C-Crete and Sublime.

Design-phase discussions between architect, structural engineer, contractor, and potential concrete suppliers are essential to establish project goals. LMN has found that we can reach 30% below the National Ready Mix Concrete Association baselines on nearly all projects across the US with no cost increase. This is accomplished by reviewing all mix types and the reasonable upper limit of SCMs for each mix. Many SCMs increase concrete workability, but can take longer to come up to strength and at higher levels can be more difficult to finish. For many mat slabs, shear walls, some floors, and other elements that will not need full strength at 28 days, allowing a longer cure time increases the upper limit of SCM use and the final strength of the concrete. Coordination not just each mix, but each pour type with the schedule for pouring, stripping, and loading often uncovers many opportunities for high-SCM mixes.

Type 1L cement is increasingly standard in many states, the importation of better aggregates can reduce embodied carbon, and  areas while the availability of SCMs varies. In some locations innovative technologies are available or in development to reduce the embodied carbon of concrete: CarbonCure, Carbicrete, Solidia, Fortera, .

Reinforcing bar (rebar) is also a noticeable portion of concrete embodied carbon. For our models we often ask the structural engineer for the rebar weight per volume of concrete, since they can estimate that early in design.

Note that many supplementary cementitious materials (SCMs) are waste products from coal burning (fly ash) and iron production (slag). As we reduce coal burning for electricity (Post 06) and virgin iron production as part of the Circular economy (Post 09), we will need to rely on others. Research is ongoing into alternatives covered above, including alternative cement production and new SCMs like recycled ground glass pozzolans.

Wood / Mass Timber Basics

Wood is a beautiful, biophilic material and is often used in buildings for those reasons. It also naturally removes carbon dioxide from the atmosphere as it grows and stores it as carbon within wood fibers and soil.

Many smaller buildings and most residential structures use wood studs and trusses, made of small members like 2x4s and 2x6s. These smaller members can be laminated together to create mass timber structure, which allow the aesthetics and carbon sequestration of wood to be used as a structural system for large buildings. Since mass timber is built up of small lumber members, it provides a market for small-diameter trees from tree farms and forest thinning to prevent wildfires.

Mass timber is allowed as the structural material for buildings up to 18 stories or 270’ tall in the International Building Code, so nearly all new buildings have the potential to be timber buildings. While there is not enough timber in US forests to build every building out of wood, storing significant amounts of forest-sequestered carbon in building is an important strategy towards a carbon neutral built environment.

Wood structures were discouraged in building codes after great fires destroyed many cities around the turn of the 20^th century, but contemporary testing and building codes show that modern wood structures meet life safety requirements. Mass timber members are sized to provide a sacrificial char layer on the outside that may char in a fire, but protect the structural portion of the timber beneath it. With modern sprinkler systems required in tall buildings, it’s unlikely the entire char layer will ever be needed.

Vertical mass timber structure is primarily glulams, while floors and roofs (and sometimes exterior wall panels) are primarily Cross Laminated Timber (CLT) or Dowel-Laminated Timber (DLT), but other products such as Nail-Laminated Timber (NLT), Glue-laminated Timber (Glulam), and Mass Plywood Panels (MPP) are also used.

Designing and Costing Wood Structures

Mass timber structure have many design possibilities, including hybrid structures that combine timber with concrete or steel to get the best out of each material while reducing and storing embodied carbon. Currently most mass timber buildings include steel or concrete cores for earthquake and wind lateral support and many mass timber floors also use a concrete topping slab to provide leveling and acoustic deadening for the space below.

Costing mass timber is also more complicated: timber columns and ceilings are often exposed, providing more volume and apparent height within each space compared to buildings with dropped ceilings to hide ductwork and other services.  This can reduce the number of additional finishes a project needs (reducing embodied carbon in the process), but also requires designers to be cognizant of ductwork, plumbing, and electrical runs that are now visible. Most mass timber buildings also have different optimal column spacing and spans compared to steel and concrete, which requires early consideration by design teams. Thankfully, bay sizing tools can aid in initial design decisions.

Wood’s Carbon Footprint is Complicated

Trees naturally change airborne CO₂ into O₂ and C that is stored in wood fibers, roots, and soil at a fraction of the cost of mechanical carbon sequestration, while also providing habitat, stormwater management, and many other ecosystem services. Globally forests currently store roughly 400 Gigatons of elemental carbon, which corresponds to nearly 1,500 Gigatons of CO₂ (CO₂ weighs 3.7x more than C). Recall that our remaining carbon budget to stay below 1.5C is 340 to 400 Gigatons of CO₂. Using some of this carbon stock within buildings while growing more presents an amazing opportunity.

However, removing trees also releases some of the carbon from soils, much of the harvested timber doesn’t actually make it into timber products, and we don’t have a clear picture yet on these net impacts. While the US grows more timber than is harvested each year, each acre’s carbon balance is based on local forestry practice.

The diagram below shows the significant carbon stored in soils across the globe; if forestry practice is not careful, harvesting timber can emit much more carbon than it sequesters by releasing below-ground carbon. Modern mapping tools can help, with the goal of having forest-specific data on carbon emitted and sequestered based on forestry practices. The Biomass Carbon Monitor uses satellite data (GEDI is one example) to understand net above-ground stock in forests over time, but below ground carbon flux is still not monitored rigorously.

A study by Woodworks (and industry association) compares wood structures to steel and concrete.

Wood Embodied Carbon Reductions

As mentioned in Post 07, understanding the carbon emissions from bio-based materials like wood are less straightforward than other materials like steel or concrete. There are three ways that timber can help with embodied carbon:

1. A1-A3 Reduction: Mass timber forestry activities, harvesting, and manufacturing can emit less carbon than comparable steel or concrete buildings, but improper harvesting may result in larger impacts. This will be difficult to show until more forest- and mill-specific wood EPDs become available.
2. Carbon storage: Mass timber always stores sequestered carbon in the fibers, which can be included in the A1 module. While carbon storage varies by species, 1 ton of mass timber product has sequestered 1.84 tons of carbon dioxide when the dried carbon weight is around 50% of the product as discussed in Post 08. Per ISO 21930, biogenic carbon must sum to zero in cradle to gate LCA, but LMN’s viewpoint is that if the structure is designed to last 100 years or longer, it provides carbon storage service; the Moura Costa method provides one way to account for long term but impermanent carbon storage.
3. Forest carbon: carbon sequestration occurs in forests, beyond the harvested tree. Soil, roots, and other plants store carbon. They can be significantly impacted by forest management. EPD accounting omits the forest carbon flux, so the Upstream Tool can help to understand the impacts of procuring from different forests. More research and data is needed to inform decision-making when comparing wood structure to other materials and comparing forest practices; one estimate of global, consequential emissions suggests large net carbon emissions from forestry.

In order to realize carbon sequestration at scale, our industry will need to demand wood for sustainably managed forests and take care not to use illegally cut wood or forced/coerced labor; Buildings As a Carbon Sink and an important rebuttal discuss the supply issues.

Conclusions/Remaining Questions

Structural design is one of the important and straightforward ways to reduce the carbon emissions of new buildings right now, with decision points occurring from early system selection through procurement. Research into steel and concrete show promise for drastically reducing impacts over the next decade or two. Wood can remove carbon emissions from the atmosphere, sometimes resulting in a carbon-negative structural system, and comes with aesthetic benefits as well.

How quickly will demand rise for low-carbon products as developers, owners, architects, engineers, and contractors require them?

How quickly will steel and cement production decarbonize toward zero?

When will wood products have mill-specific EPDs widely available?

How will forest practices be incorporated into LCA practices?

Thanks to our external collaborators and peer reviewers
Erica Weeks, Hastings; Jacob Dunn, ZGF; Laura Lindeman, CPL; Dave Walsh, Dave Walsh Consulting; Shana Kelly, KPFF Consulting Engineers; Don Davies, MKA; Catherine Cai, MKA; Isabella Stahl, MKA; Dianna Gonzalez, DLR Group; Jill Porretta, KL+A Engineers and Builders; Kevin Brooks

LMN Architects Team
Huma Timurbanga, Justin Schwartzhoff, Jenn Chen, Chris Savage, Kjell Anderson, Jeremy Schoenfeld

Posted: 02/08/2023
Edited: 04/10/2026

The text, images and graphics published here should be credited to LMN Architects unless stated otherwise. Permission to distribute, remix, adapt, and build upon the material in any medium or format for noncommercial purposes is granted as long as attribution is given to LMN Architects.

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