THE SPIRAL, NEW YORK: SUPPORTING THE RIBBON OF GREEN

Image courtesy of BIG – Bjarke Ingels Group

Terraces encircling The Spiral create verdant outdoor space for occupants and a different floorplate at each level.

The fast-rising Hudson Yards district on Manhattan's West Side includes two towers using A913. Three Hudson Boulevard (FXFOWLE), reports WSP's executive vice president and director of building structures Ahmad Rahimian, uses grade 70 A913; its neighbor at 66 Hudson Boulevard, The Spiral (Bjarke Ingels Group) uses grade 65 A913 combined with grade 50 A992 in some perimeter columns. Three Hudson places a special emphasis on sustainability, with photovoltaic sunshades and green roofs among its attractions. The 66-story, 1,050-foot-tall Spiral carries the latter feature even further, giving occupants of each floor from level 7 to the roof a verdant terrace, positioning the setbacks at an angle so that park space appears to curl around the building as it ascends. The design creates different stress patterns at each floor, making the high strength-to-weight ratio of A913 the key to its feasibility.

WSP has relied on A913 steel since its work on the Hearst Tower in the early 2000s, using grade 65 for the signature diagrid columns. “One thing that we're primarily interested is the yield strength, how high we could go,” Rahimian recalls. As architects have aimed for ever-higher buildings, he has found grade 50 steel is not strong enough without excessive column bulk. “You need more than that on a 50 ksi,” he says; “you have to add plates. Now, if you go to 65, 70, or 80, you could be able to eliminate the plates and keep the column sections more compact; that's where they see the benefit.” Changes in market conditions and material availability, he adds, have made it easier to specify the high grades, especially as grade 65 A913 has caught up to the equivalent grade of A992. “Early on, 65 had a bit of a cost premium,” he says, “but eventually it seems nowadays there's not much of a cost premium.”

The Spiral offers its tenants a futuristic office environment where work space opens onto private gardens in the sky, framing interior areas near the terraces to allow for double-height zones and optional stairs between interconnected levels. Achieving this form amid high vertical loads and high wind shear encountered near the Hudson River means contending with large perimeter-column axial forces transferred from the steel frame (including an outrigger system in three primary zones) to the concrete core, complicated by floor-by-floor geometric variations and multiple two-story sloping columns, 10 degrees each, that allow for optimal perimeter-column placement while continuing load paths to the foundation. No two floors are identical in plan or in structure.

“It’s a structurally complex building, so the strength of steel is important in allowing us to deliver the geometries,” notes BIG associate Dominyka Voelkle. “Utilizing A913 for The Spiral gave us flexibility; at The Spiral, each floorplate is different. In order to achieve the geometric complexity, the high-strength steel was really the only way forward. So far, we can safely say that A913 gave us the building that we initially sought to build. It would not have been possible to do what we have done without A913. The high-strength steel also gave us large column spans, making the interiors more desirable for tenants. Furthermore, A913 allows us to step the setback of the building in a cost-effective way.”

“For The Spiral, the terrace condition required an adjustment of the structural frame at every floor,” she continues. “This required numerous custom built-up shapes. Most of the shapes in The Spiral are custom box sections, custom I-beam sections, and custom W-shapes. With high-strength steel, the dimensions of both standard/typical and custom shapes are less, which means taller ceiling heights, longer spans with smaller members, and more transparency through the building glass (since framework is less bulky).”

“We have this condition that we call the 'walking column,' which means that every floor of the building steps back by 2.5 feet,” Voelkle adds. “With high-strength steel, this complex structural moment was optimized in the reduction of element sections, which economizes the production of steel as well as structural member sizes. The structure is more slender than it could have been if executed with another material [or] steel type.” Reducing the size of steel members, she notes, also created cascades of benefits in nonsteel components of the building: “The use of high-strength steel allows us to reduce the size of the bracing core. One big concern we had was during the erecting of vertical structures–specifically, the efficiency of the bracing element in the center of the floorplate. A913 was economizing and optimizing, while also giving us more rentable area as well. A better-performing steel framework reduces the overall weight of the building by default–with the reduction of steel is the reduction of weight of the concrete, as the high-strength steel takes on more performance.”

Image courtesy of Binyan Studios and BIG – Bjarke Ingels Group

The double-height atria connected to terraces at The Spiral allows ample daylight and flexible interior design while complicating the building’s structural stress patterns.

Smilow points out how The Spiral requires large numbers of thick plates, large angled columns, nodes, extensions welded to flanges to allow bolting, and welded joints. “Sloped columns: we've been doing it for a long time, but of course the buildings are getting taller now, the loads are more significant, and all of this contributes to the larger welds, thicker plates, and also ‘the fact that we're able to reduce it by a significant percentage . . . there's a significant change in thickness, and at the top end, it means you could use the rolled shape versus building up box-shape outer plates. You can go further with the rolled shapes.”

“A913 facilitates complex geometries,” Voelkle summarizes. “With stronger steel, it's becoming more cost-effective to build atypical floors, introduce variety, and accommodate atypical structural conditions─such as increasing column spans where more open spaces are desirable.”

Rahimian distinguishes structurally between a tall office tower like The Spiral and some of the city's “Billionaire's Row” residential supertalls, predominantly of structural concrete, with profiles slim and light enough to be susceptible to swaying under wind forces, frequently requiring tuned mass dampers or liquid (slosh) dampers. In those conditions, “you want concrete, because there at the moment, you get the extra mass that you don't get from us from steel. From steel you get strength, which is fantastic, but it's much lighter material, so for that type of application, it's not the material of choice.” In a major commercial tower like The Spiral, on the other hand, with large open floorplates and extraordinary structural sophistication, A913 is increasingly the material of choice.

BUTTERFIELDS BRIDGE, IOWA: IT ISN'T JUST FOR SKYSCRAPERS

Photo courtesy of Brian Keierleber, Buchanan County Secondary Roads Department

The Butterfields Bridge in Buchanan County, Iowa, is the Western Hemisphere’s first small bridge to use A913 steel (grade 65).

The benefits of high-strength steel are not limited to major towers in major cities. Engineer Brian Keierleber, who heads the Secondary Roads Department of Buchanan County, Iowa, makes a clearcut case for A913 as a cost-saving choice on smaller projects as well. His department has built the Western Hemisphere's first bridge using beams of grade 65 steel; a manufacturer donated the beams, which were fabricated and coated by Iowa Engineering Processors, based in the county seat, Independence. The small but innovative single-span Butterfields Bridge on 310th Street just west of Overland Avenue in this rural county may herald a step forward in economical, resilient bridge design and construction.

Iowa's location between the Mississippi and Missouri Rivers means that it has numerous smaller rivers and creeks; with its geographical features including the Cedar River, Wapsipinicon River, Buffalo Creek, the Maquoketa River's south fork, and multiple smaller creeks, Keierleber reports, Buchanan County has 260 vehicular bridges. “Up 'til just a few years ago,” he says, “there were more bridges in Iowa than in the state of California,” he says. “There are counties in Iowa with over 400 bridges.” The state's bridge infrastructure is among the worst-maintained in the nation, he says, and the Federal Highway Administration's National Bridge Inventory backs him up, ranking Iowa first in the nation for its number of structurally deficient bridges, and second for the percentage (American Road & Transportation Builders Association). However, his county has a reputation for practical innovation; the choice of grade 65 A913 is grounded in no-nonsense cost-benefit calculations showing the lower amount of material more than offsetting the per-unit premium. “For the strength you're gaining,” he says, “you're gaining about 30 percent strength for about a 5 to 7 percent increase in price.” Lighter beams, he adds, are easier to handle with hydraulic excavators, an advantage in an area where cranes can be hard to come by.

Iowa's climate has a recent tendency toward “micro-bursts” of rain as well as winter conditions requiring ample use of sand and salt. “When it comes to taking care of the bridges,” he says, “we're playing around with a lot of different ways of handling these issues. I'm doing some deck sealing right now where I'm sealing the concrete; I've got numerous bridges with the galvanized steel beams and the galvanised rebar.”

For the Butterfields Bridge, Keierleber persuaded county officials that selecting Grade 65 steel would save not only on the material itself but on an auxiliary aspect of the project, the grade of the approaching road. With the stronger steel, “you can get by with shallower sections. Where the shallower sections come into play is that means you're not building the approaches up as high either. You still have to have the clearance between the stream and the bottom of your beam, and with the shallower beam, you're not building the road as high.” Because periodic flooding can submerge bridges of this scale, their design includes “fuse plugs, an area for the road to wash out before it takes out a bridge,” though closures still occur with severe floods (Keierleber recalls having to close 10 bridges during the 2008 flood of the Cedar and its tributaries). Galvanizing helps protect the steel under such conditions; since 80 percent of his county's roads are rock roads, salt is not as much of an issue as with pavements. Other new bridges in Buchanan are protected with a hot-dip galvanizing technique that submerges the fabricated H-piles, piers, girders, rebar, and other members in a bath of molten zinc, though this method was not used on the Butterfields.

Although Keierleber's research has suggested that fatigue can be a concern with stronger steels, “in the environment that we deal with, that is never a factor here,” he says. “I'm going to say that in 99 percent of the county bridges across the nation, that's not a factor. LA County may have issues on their bridges when it's at peak, but you're not going to have that in most of the nation.”

After working with the manufacturer on this bridge, he says, “since then I've learned so much more that I truly wish that I would have redesigned it and done it a little differently. I would still use the grade 65 steel,” but he would have “gone with a little lighter beam; closed my spacing down; and used the stay-in-place metal decking.” Keeping up with advances in both designs and materials, even after a project is complete, is Keierleber's way of leveraging the available resources, regardless of how much federal infrastructure repair funding eventually finds its way to the county. He disavows any claim to leadership in the field, but cheerfully acknowledges how his work is perceived: “We have a reputation here of looking for better ways of doing things.”

The A913 specification (grades 50 through 70) is now included in ASTM's A709 Standard Specification for Structural Steel for Bridges. The success of this initial use of A913 beams in Iowa augurs well for the proliferation of this steel as the U.S., through legislation and fund appropriations that are pending at this writing, stands poised to address its longstanding infrastructure deficiencies and bring bridges large and small into the current century.