Before Chicago’s 41st Street pedestrian bridge was constructed in 2018, Lake Shore Drive, a busy eight-lane expressway that is flanked by a series of train tracks, kept South Side residents from easily accessing the Lake Michigan shoreline, one of the city’s greatest treasures. Anyone hoping to take a dip in the lake or a stroll on the beach had to go several blocks out of their way. It was an issue that the city had recognized several years prior.
In fact, the bridge, a 1,500-ft. span of massive tubular steel, is the result of “Bridging the Drive,” a 2003 international bridge design competition launched by the office of then-Mayor Richard M. Daley. Five bridges in total were constructed. Among them, the 41st Street bridge has garnered various awards, including the Midwest Project of the Year award from Engineering News Record as well as the 2019 Infrastructure Construction award from the Chicago Building Congress.
A project of such prominence called for the best and brightest in design, fabrication and erection. Even to a novice eye, it’s easy to tell that the sweeping bends of the S-curved bridge required a highly experienced team of engineers and fabricators to execute on the innovative design. F.H. Paschen, a respected infrastructure construction firm in Chicago, served as the general contractor while Hillsdale Fabricators out of St. Louis and BendTec Inc. based in Duluth, Minn., were enlisted to handle the complex task of bending and joining the massive sections of 36-in. and 48-in.-dia. pipe.
“Due to the nature of the project, there were a lot of moving parts and a lot of work that required different skill sets,” says Kenny Flowers, project manager at Hillsdale Fabricators. “The nice thing about this project is that all of the contractors and different entities understood the grand scope of the project, and we all worked well together. From the modeling and detailing to the fabrication, delivery and erection, it was a team effort.”
Hillsdale Fabricators, the steel fabricator for the 41st Street bridge project, had experience bending pipe on previous jobs, including for other pedestrian bridges in Chicago. But this project required a unique type of equipment that BendTec had actually produced in-house in the 1970s for the induction bending of large-diameter material.
As a pipe producer for power plants and oil and gas transmission, BendTec utilizes the process of induction bending on a regular basis – hence the need to produce their own specialized induction bending equipment in-house. Induction bending serves as a highly controlled means for bending tubular material with heat that is produced by using high-frequency electricity.
“Induction bending works by placing a piece of pipe through a coil that has an alternating current inside of it that creates a magnetic field,” explains Clint Zimpel, general manager and chief engineer at BendTec. “As long as the workpiece conducts electricity and has decent electrical resistance to the magnetic field, it will generate heat that you can control in small localized areas. It’s a very high-quality way of maintaining tolerances and accuracy.”
Benefits include minimized thinning and ovality of the workpiece, predictable material deformation and consistent mechanical properties based on controllable temperature profiles throughout the section being heated. Additionally, multiple radii can be used without the need for a die or mandrel.
For a project of such scale, BendTec’s induction bending technology was a necessity. Overall, BendTec’s team processed 1,160 ft. of 36-in.-dia. pipe and about 1,600 ft. of 48-in.-dia. pipe. The process, however, is a slow and methodical one – bending travels at 1 ipm. Producing 30 ft. of bend arc could take up to eight hours.
And that doesn’t take into account the time that’s required for the extensive non-destructive testing that is involved with making such massive bends. The project called for specific material that had to be qualified at the mill to meet tolerances laid out by the Chicago Department of Transportation (CDOT). After the bends were formed, they were inspected using ultrasonic thickness techniques to ensure that the walls had not thinned below the design minimum and that there were no cracks or defects in the finished bend.
“Bending pipe is an art form,” Flowers says. “Part of that art is guesswork, and not many people can do it successfully. Having BendTec on board took a lot of the guesswork and risk out of it. It’s what they do for a living.”
After all of the bent material had been quality tested, Hillsdale and BendTec were responsible for pre-assembling the various bridge components in their respective shops, which would later be transported to Chicago. BendTec handled the arches and shipped the remaining bent material to Hillsdale, including the pipe that was needed for the bridge’s deck and approaches.
“When you’re walking across the bridge, it may not be obvious to the typical pedestrian, but the concrete walkway required 36-in. and 48-in. bends to support all of that weight from below,” Zimpel says. “We made the induction bends and then shipped them down to Hillsdale so that they could lay out the decking and then weld it together at their facility.”
To complete the welds – at both BendTec’s and Hillsdale’s facilities – two welding processes were qualified based on AWS D1.1 specs: flux-cored and submerged arc. Weld joint design included an assortment of complete and partial joint penetration and fillet welds – all processes were carried out manually.
“When you’re dealing with something like a bridge where it’s curved and you’re working in multiple dimensions, it becomes a little bit more difficult to automate,” Zimpel explains. “Part of what made the manual welding acceptable, when considering labor costs, was sourcing the best possible material to minimize the number of welds required. Based on CDOT’s specs, everything had to be bought, made and melted in the United States. When you purchase the best quality steel available to minimize the number of welds required, that allows us to minimize overall labor costs and the time it takes for assembly.”
While Hillsdale and BendTec were finalizing the preassembly of the bridge, S&J Construction, the erector for the project, worked on securing permits for the final work that would be carried out at the jobsite. S&J also coordinated with Hillsdale and BendTec for the careful delivery of the sections.
“The bridge was mostly erected at night when traffic on Lake Shore Drive was slow, but S&J still only had a few hours to complete all of their work so that the city could open the lanes back up to traffic,” Flowers says. “S&J had a limited amount of time to piece the bridge together, so everything had to be carefully planned out beforehand.”
Leading up to the erection phase of the project, everything had to be perfectly coordinated – down to the last minute. The team even conducted trial runs to ensure that each section would be delivered to the correct side of the expressway and be oriented in the proper direction so that the crane could easily pick them up and piece them into place. Even the route to the jobsite had to be painstakingly mapped out.
“Figuring out how to get these massive pieces delivered was a bit of a challenge,” Flowers continues. “There are restrictions on permitted loads in different areas of the city, so more often than not, you can’t take normal streets. The biggest section was almost 25 ft. wide by 62 ft. long, and it weighed more than 84,000 lbs.”
From start to finish, the BendTec and Hillsdale’s portion of the project took a little under a year. And with F.H. Paschen at the helm as the general contractor, the overall project was a smooth one – even considering how many cooks, so to speak, were in the kitchen. Timothy Bea, senior project manager at F.H. Paschen, sums up the collaborative success that the bridge represents.
“From the planning and design carried out by Aecom, the lead design engineer; CDOT; and Trans Systems, the project’s construction manager, to the fabrication and delivery of the steel structure by Hillsdale and Bendtec and then finally to the erection by S&J Construction, the Bridge is a testament to the hard work and dedication put in by all the professionals involved in the project,” he says.
And although it was the first project for BendTec and Hillsdale to coordinate on, there is hope for future collaborations. In fact, the city plans to construct a sister bridge just a few blocks south of the 41st Street bridge.
“At BendTec, we’re typically working on power plants or other oil and gas type projects, so we truly enjoyed getting involved with this project – with something that’s a little bit different than our normal day-to-day work of transporting steam from one end of a pipe to the other end,” Zimpel says. “We didn’t get involved thinking the project was going to win any awards or anything, but it’s definitely been an exciting ride. It’s very gratifying to be a part of something like this that brings significant value to the people of Chicago.”