Not Quite Your Grandfather's Steel
Is It Still Aspirational To Speak Of Green Steel?
Steel has been described as ideally suited to a circular or cradle-to-cradle economy, in which materials are durable enough that many structures, particularly well-maintained buildings and bridges, remain in use as long as possible, and reuse or recycling replaces the disposal and waste that characterize the late phase of products' life in a linear economy (World Steel Association). Steel production inevitably consumes large amounts of energy; its full energy and carbon footprints depend on how that energy is produced. General comparisons of the relative environmental burdens placed on the Earth by steel and its structural competitors (concrete, with its well-known heavy footprint largely due to Portland cement, and engineered timber, the relative newcomer widely considered practical only on certain scales) are a topic for other settings; suffice it to say that as a durable material made largely from recycled scrap in the U.S., steel in general is far greener than some assume. Over 90 percent of American structural steel has recycled content, making the U.S. the global leader in this metric.
Image courtesy of Binyan Studios & BIG – Bjarke Ingels Group
A series of green spaces wraps the 1,005-foot-high, BIG-designed Spiral tower, creating a double-height atrium at every terrace.
Because the two manufacturers of A913 available in North America have both moved to energy-efficient production methods drawing on renewable energy sources, and because steel products are fully and continuously recyclable at the end of their usable lives, there is a coherent argument that specifying A913 aligns with the goals of sustainable construction on both the single-building and the global levels.
Stine notes that from an environmental standpoint, “high-strength steel not only continues to allow you to use less tonnage to get to the strength that you need; the big benefit of things like A913 is the way it's produced.” Unlike traditional steel mills that use a blast furnace and basic oxygen furnace (BF-BOF) and a high proportion of iron ore, mills producing A913 use the electric arc furnace (EAF) method, fed by recycled scrap, and with a substantially lighter energy and carbon footprint than either BF-BOF or EAFs using direct reduced iron. The International Energy Agency ranks the iron and steel industry first in carbon dioxide emissions and second in energy consumption among heavy industries (IEA, 2020) but also describes scrap-based EAF as “60 to 70 percent less energy-intensive than the other routes,” BF-BOF and direct-reduced-iron EAF (IEA, 2021). Along with shifting the proportion of steelmaking from BF/BOF technology to scrap-based EAF, there has been speculation in Europe that decarbonization efforts can make further headway by replacing fossil fuels as reductants in direct-reduced-iron production with green hydrogen, which is currently costly to produce but is on a path toward cost competitiveness (Hoffmann et al.).
Stine asserts that “100 percent of every type of steel that we make is through electric arc furnace, which means everything is comprised of scrap.” BF-BOF is more prevalent outside the U.S.; “that is the carbon suck,” she says. “When you see the statistics of what's the global warming potential of the world steel industry, it's because of that blast oxygen furnace.” The industry is currently divided between the methods, with EAF users generating “a fraction of the greenhouse-gas emissions” of the older BF/BOF method. “We are committed to an EAF for everything that we do, and we are going to continue to bring that down to zero. We're going to have Net Zero steel in the future.”
This manufacturer has launched a Net Zero certification extending to all its metals, including those in the construction sector. The designation addresses Scope 1 and Scope 2 emissions under the 2015 Paris Climate Agreement (direct emissions from steelmaking and indirect emissions from electricity generation, respectively), while the firm is exploring potential reductions in Scope 3 (emissions attributed to upstream and downstream operations) voluntarily.
Steels outside the construction sector, incidentally, will soon have similar strength-to-weight ratios. “What's coming in the world of high-strength plate products would be something like A913, but a similar grade for plate,” Stine adds. “It'll have the same attributes.” The firm is building an all-EAF plate mill, aiming to produce the nation's thickest, highest-grade plate steel. “It's not just for bridges and buildings,” says Stine; “it's so important that so much funding for sustainability and renewable energy that Biden Administration is pushing so much for, wind and that offshore market, is going to be fed through this huge plate product that we're going to be bringing, to be able to build those towers as wind turbines. They will be able to continue to reduce our dependence on coal power.”
Although this new mill is in a coal-producing state–according to the Energy Information Administration, 69 percent of the state's electricity was still generated by coal-fired power plants as recently as 2020─the plan calls for virtual power purchase agreements with renewable projects elsewhere, supplying steel for offshore wind towers and solar farms and reducing its own Scope 2 attributed greenhouse-gas emissions at its new plant. “We have renewable energy through wind and solar that we're using to fuel our steel mills, and we will get there,” Stine continues. Like many claims of Net Zero status across different industries, she acknowledges, her company’s is based on a combination of renewable energy sources and carbon offsets: “Initially, it's the purchase of reputable high-level carbon offsets in the market, but that's step one. In the future, it will be doing things that we can do at our mills,” including carbon capture and underground injection, biochar carbon sequestration, and other methods. The proportion of renewable energy directly used in its plants is not yet public information, she reports, “but there's many different things that we're going to do every single day to be able to get to Net Zero on our own without the purchase of offsets.”
320 SOUTH CANAL (UNION STATION TOWER), CHICAGO: LESS BULK, MORE BALLET
Image courtesy of Goettsch Partners
The strength-to-weight ratio of A913 steel facilitates a Goettsch Partners design signature: open lobbies with no corner columns.
The 50-story office tower soon to open on the West Loop, part of the wider redevelopment of Chicago Union Station (whose headhouse is located to its immediate north), presents the city with a 1.5-acre park and a striking three-tiered addition to the downtown skyline. Its site, in return, has presented its architects and engineers with a formidable challenge: beneath the building lie active rail lines into and out of Union Station, an abandoned trolley line, and a sewer line draining the Eisenhower Expressway interchange. This is one of the last vacant sites in the Loop; there has been conjecture that its complexities scared other developers away.
Goettsch Partners–originally the Office of Mies van der Rohe, then inherited by Mies's grandson Dirk Lohan and serially renamed (Fujikawa Conterato Lohan Associates, FCL Associates, Lohan Associates, Lohan Caprile Goettsch Architects, and finally its current incarnation led by James Goettsch and James Zheng)–has perhaps the deepest local roots of any Chicago firm still in operation. Working with structural engineers Magnusson Klemencic Associates (MKA) and steel fabricator Zalk Josephs, Goettsch and colleagues selected A913 to execute this latest expression of the less-is-more philosophy, sparing nearly 20 percent in total steel tonnage on the columns.
From ground level, the building strikes a bold profile defined by V-shaped pairs of angled columns, three Vs each on its east and west sides, topped by a second level of paired Vs, so that the two stories above the ground-level Vs read as “WWW”: plausibly a gesture toward l'architecture parlante for the digital era, though the building is otherwise consistent with Chicago's Modernist tradition, with its resistance to obvious ornamentation. (The owner Riverside Investment and Development is using a W-over-V shape as a logo for the building.) The V columns themselves, however, are pure form-follows-function. As senior associate Peter Stutz of Goettsch Partners explains, they serve to transfer structural loads to as few points as possible. “We had some underground issues and sewer work and other things to work around,” he says, “with some existing site factors that drove us to design the columns to come down to points at the base of the building, rather than just bringing them all straight down.”
This feat of structural ballet relies on the superlative strength of A913 steel, including the nation's first grade 80 members in the upper perimeter columns. In a hybrid steel-frame/concrete-core design with setbacks and private terraces on floors 17, 33, and 49, creating a three-volume form with column-free floorplates, these slim columns hold the upper structure's loads and transfer them to a belt-truss system of grade 65 steel on the third to fifth floors, then to the V columns, box columns leading to the footings, and to bedrock. “The high-strength steel is more efficient in use at the columns, where the compressive strength is the most important factor,” Stutz says. “So we used it on a lot of the tower columns. It really benefited the project overall in a sense of optimizing the available square footage on the project; we had more slender columns, reducing the overall weight of the building as well.” The smaller column members and enclosures give the building a svelte profile and maximize fit-out space for the occupants, which include Bank of Montreal as ground-floor anchor tenant and law firms in several midrise floors.
The Vs from levels one to three, Stutz notes, are “the biggest, heaviest members on the project, and they start driving the construction logistics, and so really early on there was talk of, 'Well, are we going to use standard rolled shapes that are plated? Are we making some sort of custom shape?' ” They chose grade 65 Web Tailor-Made steel─“essentially like rolled plates,” says Stutz, “a similar process to wide flanges, but they come in the higher strength that you can't typically get in a 65 ksi. And so we were able to make some really unique box shapes that allowed us to shave on the weight, that then made the construction logistics and the sizing of the tower crane and all that a lot more practical and feasible.”
Image courtesy of Goettsch Partners
V-shaped steel columns on the east and west facades at 320 South Canal transfer structural loads to a minimal number of ground-level contact points.
One variable in the use of grade 80, says senior associate David Tayabji of MKA, was material availability. “There are some schedule implications when you go to 80: it comes from Luxembourg, so there's the additional shipping time you have to consider. You have to hit certain rolling cycles, so on this project, to get the steel to Chicago it goes through the Ship Canal, and the Ship Canal, I believe . . . closes in the winter, so the steel had to get to the fabricator in Wisconsin before the shipping season closed, and so it was really important to have the discussion early about, if we're using 80, then what does it mean for the schedule? When does the design need to be complete? When do they need to order the steel? And so what we actually did on this project: we designed all the columns in 65, 70, and 80, because we also wanted to see how the market would respond, and the market said 80.” Goettsch and MKA are covering their bases the same way on another project in Denver, he notes, preparing for market conditions and logistical challenges by specifying all three high-strength grade options.
Goettsch managing partner Joseph Dolinar identifies the “ground-floor lobby experience” as a signature feature of the firm—the Web Tailor-Made plates used for the V columns allow for cost-effective construction of the large open space. Stutz notes that the structure “allowed us to use 40-foot-tall, single, light, thin, supported glass, which really creates a dramatic open experience between the sidewalk experience and the interior lobby.”
“I would say the hallmark of a Goettsch office building is we don't have corner columns, and that actually necessitates some fairly large moment connections through these columns,” adds Tayabji. “They actually made them largely bolted connections; they were end-plated so that the beam stub cantilever bolted to the column, so that really minimized the amount of welding both on-site and in the shop.” Dolinar points out that the firm generally strives to minimize field welding, an expensive and unpredictable aspect of construction. “Just because of weather conditions [and] controllability, the shop is a much better location for the welding, and the fabricators and the connection-design engineers are pretty good at developing those details in such a way that the welding, if it is needed, is in the shop, and then it's just pretty much straight-up bolting in the field.”
Clear onsite communications with Clark Construction, Zalk Joseph, and Chicago Steel, Dolinar recalls, led to a useful onsite revision. In the trussing system between the tower columns and the V columns, “a simple 90-degree rotation of the members makes a big influence on all the connections and how the pieces come together in the field, and that's one spot where the construction team really helped drive the design there. Our original design had the members rotated one direction, and they asked for a simple 90-degree rotation, and it really changed everything for them . . . made all the connections a lot cleaner.”
This is not the first Goettsch-MKA collaboration where A913 has provided a solution to site constraints, Stutz adds. The 150 North Riverside building (2017) occupied “a very narrow site spanning between some underground Amtrak and Metra train lines and the Chicago River, and so there was really no space available to bring down columns all the way to the ground, so we designed a core-supported building that cantilevers into a truss up above, and the entire building is supported on the core at the base, so that allowed us to bring it down to a very narrow footprint.” This slender-cored, tuning-fork-shaped building, maximizing park space on the site, went up before grade 80 A913 was readily available and made extensive use of grade 70. At 110 North Wacker (2020) near a riverwalk, distinguished by trident-column elements with a family resemblance to 320 South Canal's Vs, the team's Bank of America tower is also structured to accommodate subterranean logistical challenges and uses grade 65 A913, eliminating built-up column plating and saving tonnage. “All of the easy sites had been taken,” comments Tayabji.
Finnigan points out that her firm also manufactures A913 with an EAF. The product “has a specific supply chain, and so it's possible for the end user to really drill down into what the carbon impact is of that material from a mill-specific route, and so going based on that versus the industry averages, you can see some really great returns on the carbon footprints of your project, so you would get a return from not only the reduction in weight, but also by drilling into the mill-specific EPDs [environmental product declarations], for example, you'd be able to see what the specific global warming potential is of that material. Aside from that, there are also benefits that come from the rest of the supply process; when it comes to material being transported to site, you can see some reduction in cost. When it comes to constructability, using a higher-strength steel will not only reduce the weight of material in your project, but in some cases, it can also lead to a reduction in, for example, the capacity of the crane that might have to be used on site, so having less material can have all sorts of knock-on effects that can also bring value to the project.”
Catchphrases like “green steel” are more meaningful when supported by emissions information such as global warming potential (GWP), the Environmental Protection Agency's metric for calculating and reporting emissions in ways that allow meaningful comparisons. One manufacturer's EPD provides detailed calculations of direct and indirect impacts; the other has an EPD covering the mill where its A913 is made plus a plant that makes non-A913 hot-rolled structural steel. (The AISC has also posted an older categorical EPD for U.S.-made hot-rolled structural steel, averaging data from three producers, two of which do not make A913; it is valuable for analyzing the GWP of the fabrication process but not as specifically informative as the manufacturers' EPDs.)
All factors considered, both of the available A913 products are an advance on its predecessor A992, and on other structural options, for any project guided by an awareness that the design/engineering/construction sector's steadily improving environmental performance needs to continue and accelerate in the coming years.
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