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That’s no beam or column. A welder at Supreme Group puts the final touches on a composite panel. Every button on the surface is a weld for an interior connection rod. Photo provided by Supreme Group

In August the 58-story Rainier Square tower in Seattle topped out just 10 months after construction started. If this skyscraper had a conventional reinforced concrete core, the project probably would have taken far longer. But this tower had no traditional concrete core.

It instead has a kind of concrete-filled composite plate shear wall, with each building block having two hot-rolled steel plates connected with a series of rods that act as spacers (along with other roles in the composite design) as concrete is poured in between. It’s an ice cream sandwich of structural steel and concrete. Research around this construction method began years ago at Purdue University and is being funded partly by the American Institute of Steel Construction (AISC), which has dubbed it SpeedCore. If the building method takes off, it could affect structural fabrication in a big way.

“I will tell you, one of the things that was so important in getting us to where we are today was our partnership with the fabricator, Supreme Group, and erector Adam Jones (aka The Erection Co. Inc.) and the input they provided to develop the design into something that can be fabricated, built, and erected in a way that was effective—all that was critical to the outcome.”

This was said at AISC’s annual event, NASCC: The Steel Conference, by Ron Klemencic, chairman and CEO of Seattle-based Magnusson Klemencic Associates (MKA), the structural and civil engineering firm involved with the Rainier Square project. “And the outcome is we’re on schedule,” he added, “and the schedule was very fast. The whole point of SpeedCore is to go fast.”

The Rainier project’s initial construction timeline was 21 months, assuming a traditional reinforced concrete core would be used. But when the design shifted to SpeedCore, the timeline shrank to 10 months.

“Early in the job they were producing about a floor and a half a week,” said Charlie Carter, president of AISC. “By the end of the project, they were building four floors a week. That’s just outrageously fast.”

“Without a doubt, this is a game-changer for the entire steel industry as it relates to high-rise construction,” said Kevin Guile, president of Supreme Group LP, based in Acheson, Alberta, Canada. “We’re very proud to have delivered the first job and to help meet or beat the projected schedule. And without doing that, there’s no point. SpeedCore without the speed is just another way of building.”

Drive by a modern skyscraper being constructed with a concrete core and you’ll likely see the core towering above the steel framing that surrounds it. Building that concrete core takes more time than erecting structural steel. So to balance the schedule, the contractors start with the core first, then follow it with the structural steel framing. Eventually the steel framing “catches up” with the core as the building tops out—at least that’s the ideal.

“There’s a big lag [between the concrete core and steel framing],” Klemencic said, “and it’s that lag that we were trying to address in this new concept.” Initial research began about 15 years ago, with progress ramping up significantly during the past decade at Purdue University’s large-scale civil engineering lab.

The SpeedCore panels can be lifted and secured along with the steel framing around it, potentially cutting months off the construction schedule on the job site. With the panels secured, they’re then filled with a concrete (the “ice cream” in the sandwich, which for the Rainier Square project was closer to a grout) to a certain PSI.

In this simulation, the composite steel-and-concrete core is erected and the decking follows close behind. Image courtesy of Magnusson Klemencic Associates.

In his NASCC presentation earlier this year, Klemencic explained that outrigger truss connections with a concrete core can be onerous. A composite core with steel plate “essentially makes these connections steel to steel, and a lot of it can happen in the fab shop.”

The core itself at Rainier Square is the same size and dimension as if it were reinforced concrete, 40 feet wide by 90 feet long at the base (though the building tapers at the upper floors). The SpeedCore composite panels entail 0.5-in.-thick plate sandwiching 10,000-PSI concrete. The width of that “sandwich” varies from 21 to 45 in., depending on the panel location within the building.

The panel’s two steel plates are connected by 1-in.-diameter tie rods spaced at 12 in. on center. With 58 stories of composite core panels, the job required more than 350,000 rods, and each end had to be welded to a plate (creating the “sandwich”), for a total of 700,000 welds.

Each rod fits in holes in each plate, cut on the burn table. View a completed composite panel and you’ll notice the rod ends protrude slightly beyond the outside plate surface, and each one requires an outside circumferential fillet.

“We knew we’d be the pariah of all steel fabrication if we had looked to the workforce to weld these manually,” said Guile of Supreme Group. “It’s not fun work, and we would have burned our people out. So we looked to a variety of different solutions.”

Supreme has been involved in SpeedCore since the early days of research at Purdue University, years before Rainier Square was even being contemplated. “It was an opportunity for us to help learning institutions to advance structural steel research,” Guile said. “This was along with MKA, who was leading edge from an engineering perspective and was the brainchild behind the notion of SpeedCore.”

Fast-forward a few years, and Supreme had just finished delivering the Amazon sphere project in downtown Seattle, which went together smoothly and on schedule. “I believe that all set us up to be the first-choice steel contractor for the Rainier Square project.”

The collaborating parties—especially between MKA, Supreme, and Adam Jones—couldn’t treat this like a typical project. It was the first of its kind, after all. So before anything else, the collaborators developed a full-scale mockup of the composite panel. The general contractor, Lease Crutcher Lewis, started with just a plywood mockup (to vet the concrete work), and Supreme followed with a mockup of structural steel (to vet the erection process).

The mockups helped all parties solve problems from the get-go—like how exactly the grout-like concrete would be pumped into the panel cavity; how the fabricator would support and manipulate the panel structure in the shop; and how the 14-ft.-wide panels could be trucked to the job site.

“The mockup gave us an idea of how we would build this in the shop, and we worked with the erector on the alignment and field welding,” Guile said, adding that the shop altered the panel design in such a way as to provide quick alignment and adjustment in the field by the erector.

The fabrication team at Supreme poses in front of the last panel shipped to the job site. Photo provided by Supreme Group

Rainier Square still has floor framing, gravity columns, and other typical structural fabrication work. But the project’s core was, well, the core—those composite sandwich panels that would make up the building’s core and require the brunt of the welding.

Shop fabrication is usually much more cost-effective than field fabrication. That said, Seattle is in a seismic zone, which meant each composite panel splice would require extensive connection detailing between the panels.

To minimize the number of field welds, the project partners decided to make the composite panels as large as possible. Supreme purchased a new burn table that would allow it to cut massive plates into 14-ft.-wide by up to 40-ft. sections—big enough to minimize the field welding but small enough to be manipulated in the shop, onto the truck, and picked up by a crane at the job site. The heaviest panel weighed 36,000 pounds.

Once fabrication began in the shop, sequencing was especially critical off the burn table. As workers used a bridge crane to offload each massive plate, they stacked them strategically, ensuring that they had enough inventory buffer to keep production flowing, and yet also ensuring they did not bury a plate that would be needed immediately.

After the plates were cut and holes were made, the shop set up a variety of jigs that allowed for the base plate to be leveled and the rods to be installed with the top plate. Sounds straightforward enough, but because these plates were so large, stability became a challenge.

“As a fabricator, you need to be able to set them, weld them, flip them to weld the other side, and during all of this, the assembly has to be rigid,” Guile said. “We found, through finite element analysis, that the two plates with the rods in between were not as stable as we needed them to be. There was too much deflection. So we introduced an internal truss system.” The truss system served no purpose for final structural integrity, but it gave Supreme the rigidity it needed to manipulate and fabricate each panel section.

“Then of course you get to the greatest challenge,” Guile said. “How do you weld out 350,000 rods with a weld on each end?”

The company considered robotic automation but decided against it. The project had so many moving parts—not just the more than 500 core panels, but also the columns, beams, and deck plate surrounding the core and box columns that created the core’s corners. With all that in play, the company thought it would introduce significant risk if went with a static automated cell.

“The robot could no doubt do the work, but we would have had to move every panel to the robot station,” Guile said. “The material handling [of the large panels] would have become a bottleneck in our flow.”

The shop needed flexibility, including the ability to take the welding automation to the work. So in the end it developed a patent-pending mechanical orbital wire welding system that an operator can move from one weld to the next. The system has an arm that configures it to the correct location, after which the metal-cored arc welding gun orbits around the outside circumference, depositing a smooth fillet all the way around.

The average weld cycle took about 20 seconds. Interestingly, this was about the same speed that a human welder could accomplish the task, “but it allows us to avoid those ergonomic issues,” Guile said, “with a welder repeatedly hunching over to access the joint, all day, every day.”

With the system, the operator simply affixed the machine to the weld location, initiated the cycle, then removed the system and moved on to the next weld. The company developed seven of these welding systems, which meant that the operation could have many weld stations operating simultaneously.

Guile added that all these orbital welds did not create problematic distortion. In fact, the greatest challenges came not from distortion but from deflection, which was solved by those internal trusses that added panel rigidity.

“If you think about it, what is SpeedCore really doing?” asked Guile. “It’s moving nontraditional steelwork that used to be done in the field [by concrete contractors] and shifting it to the steel fabrication shop. The steel core itself is probably introducing two to three times more in-shop-fabrication man-hours than we would normally have with a traditional gravity-frame structure with a concrete core.”

SpeedCore moves more work to the controlled shop environment and away from the job site. Doing so does increase the structural fabricator’s slice of the pie. But as sources emphasized, because the overall construction cycle is so much shorter, the pie is smaller, which means owners and investors save a significant amount of money.

But as an industry, structural fabricators could stand to gain from that growing slice of the pie, especially if SpeedCore takes off as a preferred construction method for tall buildings. Structural fabricators that win SpeedCore projects have a significant amount of fabrication to perform, and more burn table work in particular might spread to area fabricators and service centers.

That said, for SpeedCore to take off, Guile said that collaboration among all stakeholders—the engineering firm, the fabricator, the curtain wall contractor, MEP (mechanical, electrical, plumbing), the erector, trucking and logistics, and more—is an absolute must. An error on a drawing or other small, seemingly inconsequential detail could snowball out of control in a hurry. After all, delivering an incorrectly fabricated composite panel is far costlier than delivering the wrong beam or column.

“Without our partners and that close collaboration,” Guile said, “this project wouldn’t have been the success it turned out to be.”

Any fabricator that implements continuous improvement knows the job is never finished. One can never theoretically eliminate all constraints. Freeing one constraint inevitably reveals another constraint elsewhere. Sure, those constraints should be less severe (at least ideally), but they are still there. Considering this, SpeedCore eliminates the concrete core constraint—so what’s next?

“If the core panels now can be put up so quickly, the next project will probably give us the opportunity to collaborate on developing a floor system that can keep up with it. And we’ve got some ideas.”

This circumferential weld secured the interior rods. The Rainier project called for 700,000 such welds.

That again was Charlie Carter, president of AISC. So what will that project look like? Carter said it’s too early to tell. “The system could be akin to the way ships are built, with double-walled construction along with bridge decks and other construction systems that already exist. Or maybe it’s a creative use of things we already do, but we can put them together in a way that makes the floors shippable.”

Carter again emphasized that these are all just conjectures at this point. Regardless, the success of SpeedCore could be a harbinger. The future of tall-building construction could look very different from how it does today, and, no doubt, the structural fabricator will play a pivotal role.

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Post time: Jun-23-2020
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