Roll with the Changes. In the FF Journal

In a city brimming with daring architecture, Millennium Park injects new meaning into urban innovation.

The centerpiece of a $475 million redevelopment project is the Jay Pritzker Pavilion, the boldly expressive concert venue that is reminiscent of architect Frank Gehry's groundbreaking Guggenheim Museum in Bilbao, Spain. Of the 11,000 people in a capacity crowd, it might be the 4,000 in the actual seats who get the short end of the deal. The other 7,000 can stretch out on the pavilion's Great Lawn under the distinctive trellis of interwoven structural steel tubing.

Projecting some 625 ft. to the south out of the trademark Gehry stainless steel ribbons that crown the stage, the trellis stands 60 ft. high and spans 325 ft. wide east to west. Comprised of a network of mild steel architectural tubing ranging from 12 in. to 20 in. OD, the trellis spans the entire width of the lawn with no central supports, relying instead on careful engineering and joint positioning to maintain structural stability.

Practically, the trellis serves as a lattice to hang the speakers for audience members on the Great Lawn. Aesthetically it pulls the audience together within a wide-open green space, while doing away with the poles or support columns that might otherwise obstruct views of the stage.

"It certainly is an artistic element," says George Wendt, president of Chicago Metal Rolled Products (CMRP), the company that curved the tubes in the trellis. "It creates space that people feel enclosed in and complements the stage."

Early involvement counts

The pavilion opened on July 16, 2004. However, CMRP was brought into the loop in the early planning and design stages about five years before that, Wendt says. CMRP rolled the tubes over a three-month span in 2003, basically as the pavilion was being constructed.

The tubes, which are 40 ft. to 65 ft. long, all were roll-bent on a single plane by CMRP on what was then a new three-roll Roundo roll bender. That single-plane feature illustrates just how instrumental CMRP's early involvement in the project was--and how money talks, even in a $475 million project.

Back in 1999, Wendt was approached by John Zils, senior structural engineer at the Chicago office of Skidmore, Owings & Merrill, who worked with Gehry's firm on the project and whose past masterpieces include Chicago's Sears Tower and the John Hancock Center. Zils wanted Wendt's input: What did he think? Was the design going to work economically?

"We like to design structures and come up with ideas that are not only responsive to the architecture but also have a practical sense to them, that can be built and fabricated properly," Zils says. "It's very normal for us during the design to meet with, if it's a steel structure, some fabricators or potential fabricators . . . to vet the design a little bit--to see what their input is, what their suggestions might be and how we can maybe improve what we're doing."

Gehry originally had called for curving each pipe in two planes and changing the radius of each arch numerous times. After consulting with his estimators and engineers, Wendt was up front: Those things could be done, but they were going to cost more.

"We suggested from a fabrication viewpoint, and from the viewpoint of curving the steel, that it would be easier if we could curve each pipe in one plane," Wendt recalls. "We could do what he wanted, but it would be simpler, less expensive and save time if we could curve the pipe in one plane, and also have each pipe that we curved be a true, or constant, radius and let the radius change at each nodal junction, where the pipes cross each other." That kind of feedback was exactly what Zils was looking for. His team took Chicago Metal's input right back to Gehry's group, who then concocted a new plan: Instead of curving the pipes in two planes and arching them up and over the lawn vertically, why not bend them in one plane and lay them down at an angle, creating the illusion of being bent in two planes?

It worked. The reworked design preserved much of the same look, but it also saved CMRP time, saved the organizers money and "was a significant contribution to the project," Zils says.

Gehry's office also adopted Wendt's recommendation to sharply reduce the number of different radiuses in each arch. That move saved money without sacrificing aesthetic appeal. "We were able to quite substantially reduce the number of different radiuses and still achieve the same basic profile," Zils says. "It wasn't exactly the same, but for all intents and purposes, it was similar."

Ardent simplicity

Still, producing 570 tons of curved pipe to the required specifications was no walk in the park for CMRP. The 98-year-old fabrication business has seen its share of precision projects, including the rail that carries the retractable roof sections at Milwaukee's Miller Park, but Millennium Park was special.

For one thing, to do the work quickly and at a competitive price, CMRP chose to bend the tubing cold, a rarity for 20-in. OD pipe. Most pipe that large is worked after heating induction. CMRP actually purchased the Roundo, one of the largest section benders operating in the United States, and installed the tooling before it won the job. "We thought we could use it for other jobs, but we joked that this is the Millennium Park machine," Wendt says.

For another, all the pipes had to be free of distortion and meet surface architecturally exposed structural steel (AESS) quality standards--no scratch that so much as caught a fingernail passing over it was allowed. Doing so required the shop to keep its tooling clean and well maintained. Other than that, CMRP turned to its in-house know-how to bend the pipes to the desired radius, which ranged from a steep 100 ft. to a shallow 1,000 ft.

"This is a little bit more of an art than a science," Wendt says. "When you roll these things, there's a considerable amount of skill required from the operator."

In addition, much of the work had to be done just-in-time, particularly for what those involved in the project called the X-node--the portion of the trellis that intersects and actually fits behind the stainless elements that billow up from atop the stage. "As they were putting those on, they had to stop for a minute and get those pipes in there and then finish it," Wendt says. "We expedited a few pipes to go there so they could keep going on the stage."

Fabricating exactness

Wendt was careful to give credit to Acme Structural, the Springfield, Mo.-based fabrication house responsible for creating all the TYK joints holding the trellis together, as well as cutting and welding the tubes. "In general, logistics was a challenge," he says. "We had to deliver pieces to [Acme], who had to deliver them to the site in a timely manner so they could build the trellis. The logistics had to be worked out to keep everybody busy.

"The good news is, working with Acme Structural, the pieces at the job site fit together," he adds.

Matt Vienhage, Acme vice president of sales and engineering, says he never had any worries about that. Well, almost none. "I was pretty sure it was going to work," Vienhage says with a smile. "We did have contingencies if they did miss, whatever needed to happen."

No need to worry. The Acme crew took all those pains in-house, going so far as to ask their counterparts at CMRP to tighten the radius on some of their pipes "because it was easier for them to open a piece up than it was to close a piece," Wendt adds.

Acme cut the numerous necessary holes in the pipes, mostly with oxy-fuel torches but also with plasma torches. However, the company's big job was to fabricate and weld the many 5-ft. to 6-ft.-long TYK joints where the pipes intersect, named thus because of the letters they form when the pipes come together.

The trick unique to the Pritzker Pavilion trellis was that the radiuses of the pipes meeting at joints are all different, meaning that the extensions on each TYK joint also had to be bent to those exact radiuses "because you had to have the radius start from the center of the intersection to the pipe," Vienhage says. Each joint also had to be rotated at a special angle to accommodate the pipes it was joining, Vienhage notes.

To install the pipes, the erectors stood on portable towers and adjusted each TYK joint up or down as necessary. Once they had three rows of arches up, they would verify them and then field-weld the joints so that the erectors were always two rows ahead of the welders. Many of those joints called for full-penetration welds, one small factor in meeting what Zils called the project's key challenge--ensuring the trellis's stability.

"It's a huge column-free space with quite small members," Zils says. "There's always the potential of individual pieces buckling and also for the entire structure to collapse on itself. This became the crucial issue in terms of the design.

"It's one thing to erect a building with columns and beams. It's another to erect a structure that has no precedent. That's what we were doing." FFJ