The newly reconstructed North Avenue Bridge in Chicago is a hybrid cable-stayed/suspension bridge, the first of its kind in Chicago and one of few in the country. But the unusual combination of a cable-stayed bridge and a self-anchored suspension bridge isn’t the only reason the bridge is garnering attention. A support system of complex pylons help make it a site to see.

The 420-foot span bridge crossing the Chicago River consists of three segments. The east and west 84-foot approach spans are supported by the cable stays, and the 252-foot middle span over the river is supported by the suspension-cable system. The systems share two sets of 67-foot tall support pylons constructed of built-up rolled carbon steel plates. Weighing about 95,000 pounds each, the pylons support both the suspension and cable-stay systems. The systems work together to support a post-tensioned bridge deck that also acts as a strut to transfer the horizontal loads of the systems to large anchor blocks at either end of the bridge span. Each pylon foundation is supported by 10 micropiles extending 90 feet into bedrock.

Loads of support
Fabrication of the pylons was done off-site by BendTec Inc., Duluth, Minn. Each consists of two large elliptical pieces on the outside edges with a spacer plate between them. The ellipses were constructed from steel plate measuring 1 3/8 inches thick. BendTec subcontracted the forming of these ellipses to McCabe Steel, Stoney Creek, Ontario. McCabe performed the pressing and bent the plate into 20-foot sections. Then BendTec ran the pieces through a milling machine, put in the spacer plates and welded the pieces to the pylons.

Due to the large size and weight of the pylons, only one could be placed on a truck at a time to be transfered from BendTec’s facility in Duluth to the bridge site in Chicago. According to Scott Bergum, estimator and project manager for BendTec, once the pylons were unloaded from the trucks, they were standing vertically within an hour and set in place within three hours.

"It was a challenging project, something a little different," says Bergum. "We were dealing with plates from 3/8 inch thick all the way up to 1 3/8 inches thick on the ellipses. There were some critical parts on some of the cable locations where the cables come out of the pylons. We had to have the angle set just right so that they had perfect contact with the ends of the cable. So that was difficult."

Aesthetically inclined
Adding to the challenge of fabricating the pylons was the design by HNTB Corp., Chicago, that called for decorative, diamond-shaped openings on the upper part of the columns to allow light to shine through from below to illuminate the bridge at night. "It was fairly complicated fabricating," says Bob Meierhoff, president and CEO of BendTec. "It had a lot of artistic things involved in it like the openings for the lighting. It’s not just a straight column. It has a lot of intricacies that required a lot of different pieces to be made and put together."

Because the bridge is a combination of a cable-stayed and suspension, all of the connections for the cable stay portion had to be welded in and through the pylons. This required a lot of work on BendTec’s part of welding and fitting the components inside various parts of the towers. "It was a complex tower design," says Meierhoff. "There were a lot of pieces that were press braked and formed and then welded together. It would have been less expensive to use round pipe on the columns and just cut it in half, but the shape of the curved portions on the columns isn’t circular, it’s an elliptical shape. So it all had to be press braked."

"It was a fun project, I enjoyed being involved with it," says Bergum. "It was a challenge just from the weight side of things, so we had to be a little creative on how we moved things around and rolled things. But other than that it was really fun to watch it come together." FFJ