Thursday, August 31, 2017

Carbon Group Post 4: Do you know how to achieve a carbon-efficient steel-framed building? Your fabricator does.

A common misconception within the Architecture/Engineering/Construction community is that structural steel is a carbon-intense, “dirty” product which sabotages the natural environment by utilizing large amounts of mined content.  In reality, the steel structure being fabricated for your current project has a decent chance of containing metal from a car similar to the one you were driving when you were 16.  
Figure 1 - The structural steel life cycle Credit: American Institute of Steel Construction

Domestic structural steel and rebar are produced with an average of 90% recycled content.  The process in which they are produced transitioned in the late 1970s from the Basic Oxygen Furnace (BOF) process to the Electric Arc Furnace Process (EAF).  This shift resulted in a 24-fold increase in productivity - from 12 hours per ton of steel to one-half of an hour per ton.  Steel products are also transported efficiently in the US, with the majority of steel being transported via barge or rail rather than truck.

Some lighter gage material, like decking and light gage studs, as well as Hollow Structural Sections (HSS) are made via both EAF and BOF.  The BOF process utilizes approximately 30% recycled content.   It is important to be able to differentiate between production processes for all types of steel used on your project when accounting for the carbon impact.  (In this post, as well as the carbon working group white paper, the term “carbon” is used to mean “carbon-dioxide equivalent.”) Versions 2 and 3 of the LEED rating system assume a 25% recycled content for all steel products as a default unless documentation via mill-specific data is provided.  In LEED v4, mill-specific recycled content data can be used to achieve the Leadership Extraction Practices option of the Building Product Disclosure and Optimization credit. 

The cradle-to-gate Environmental Product Declarations (EPDs) released by the American Institute of Steel Construction (AISC) in 2016 for fabricated hot-rolled structural shapes, fabricated HSS, and fabricated plate material provide a full accounting of material sourcing, production, transportation to fabrication shop, and labor and processing in the shop. However, only fabricators who were members of the Institute when it was produced (or have since joined and submitted environmental data) are able to legally use these EPDs to account for carbon content on their structures.

The EPD for Fabricated Hot-Rolled Structural Sections gives results for the impact category of Global Warming Potential (GWP) as 1.16 tons CO2e per ton of steel produced, with approximately 85% of the total impact coming from the furnace production process.  With the material production impact being the dominant contributor to the total impacts, it is important to be sensitive to overall material use when lessening carbon impacts is the overall goal.  
Figure 2 - Comparison of structural steel frame in the Empire State building as constructed in 1931 vs. now.  Carbon emissions numbers include material production only.  Credit: American Institute of Steel Construction

Material, however, is only one side of the coin regarding overall sustainability.  An argument can be made that designing an erection-friendly structure can also lessen carbon impacts by reducing schedule and saving weeks of labor in the field, although this has yet to be quantified by an official EPD or Life Cycle Analysis (LCA).   

In order to find the “sweet spot” where material efficiency, up-front cost, life-cycle cost, and resiliency come together, one needs to discuss specific project goals with the structural steel fabricator as early as possible – preferably as a preconstruction partner.  The fabricator will be able to educate the design team on local material supply, desired connection schemes, how material can most efficiently run through their shop, as well as be efficiently sequenced in the field. They can aid in implementing green goals such as overall material reduction (both with a material efficient structure and by exposing structure to reduce finishes), material reuse, and integrated process credits. 

The Steel chapter of the upcoming White Paper “Structures and Carbon” describes and compares production processes used domestically.  Specific strategies to produce a carbon-efficient structure are presented.  We look forward to your reviews and comments!

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