TAKING STEEL TO NEW HEIGHTS IN THE WINDY CITY

Realizing the full potential of high-strength steel

Bank of America Tower Chicago is located between a multilevel thoroughfare, the Chicago River, and trunnion bascule bridges on the north and south ends of the trapezoidal site, which grows skinnier toward the north. The building’s sawtooth facade is supported by three trident columns rising 55 feet, giving onlookers mesmerizing views down the river.

As they searched for a way to make the distinctive, eye-catching design a reality, Clark Construction and Cives Steel Midwest, the structural steel fabricator, decided Nucor’s Aeos^™ high-strength steel would be the best option.

Aeos (Agile, Optimized, Efficient, and Sustainable) is North America’s only domestically produced A913 steel. Not only strong enough to meet the engineering demands for a tower of this size, this advanced steel also offered the flexibility and structural versatility needed to fulfill the project’s ambitious architectural vision.

“To increase the visibility as a pedestrian across the river, we did these large column transfers, which opened up the visibility but also gave the building a really unique defining architectural and structural characteristic,” says James Goettsch, chairman and co-CEO of Goettsch Partners, in a video posted on his company’s website. “By stepping the building, we were able to maintain a 90-degree planning grid throughout the project. These 30-foot setbacks along the river help to enhance the verticality of the building from the exterior. And then it provides a number of corner offices from the interior looking up and down the river.”

Typical uses of Aeos (available in 50 and 65 ksi strength) include columns, beams, and members of long-span trusses. Aeos 65, which was chosen for the Bank of America Tower Chicago project, is often used to reduce member weight, weld preheat, and time and energy spent on projects.

For welders, using Aeos can help simplify the process. “One advantage with (Aeos 65) is if they can eliminate the cover plates, the splices are much simpler for field erection,” says Brian Wessel, president of Cives Steel Midwest. “Lighter members also help eliminate the need for an assist crane. They can use the regular tower crane.”

Realizing cost-savings potential

Eric Fenske, an engineer with DeSimone Consulting, which participated in the 110 North Wacker project, said the design team originally used A992 50 ksi steel. “But then in talking with the contractor, they decided to bring up the option of (Aeos 65) steel for tonnage and cost-saving potential,” Fenske said. “Tall office towers, with their long spans, tend to lend themselves well to (structural) steel. For this project, A913 came into play for the columns to achieve steel tonnage savings, and also the outriggers. That was the main benefit for this project: significant steel tonnage savings. Another big benefit was that they were able to eliminate built-up column plating at some of the lower levels.”

The benefits realized were due to Aeos’ extremely high strength-to-weight ratio, which allows for greater architectural ingenuity—as well as greater cost efficiency for developers and fabricators.

Ease of design is another plus. “It’s just as easy to design with A913 as it is with 50 ksi steel,” Fenske says.

Seeing the benefits of made in America

Another advantage of using Aeos is it’s made in the U.S., Wessel says. “When Nucor started producing it, that’s when we started sharing with our customers that A913 steel is now being produced domestically,” Wessel says. “That eased a lot of customers’ minds knowing this.”

Aeos is produced domestically, which means mill order changes, long delays, tariffs, and foreign supply chain challenges typically are not a factor. As a result, it often means projects can be built faster and more efficiently.

“From a timing standpoint, there’s a significant benefit,” Wessel adds. “Rolling cycles are much more accessible with Nucor. Sometimes larger projects can accommodate longer lead times, but even then, there is often a late add that can cause issues with procuring from abroad.”

This also comes at a time when American-made products are a requirement for certain projects versus simply an option. With products such as Aeos, “the ‘domestic only requirement’ in specifications can be accommodated more easily,” Wessel says.