Several factors posed challenges for the builders of the replacement North Avenue Bridge over the Chicago River, but none of them more than the fact that the bridge was a hybrid of two types: a cable-stayed bridge to support the approach spans and a suspension bridge to support the center span.

James McHugh Construction Co., Chicago, called the bridge “a structure without precedent,” one that had to be designed from scratch because there were no precursors to adapt. The team performed extensive computer modeling to test each load- or stress-bearing element to make sure they would all work together in balance.

The team found they would have to build the two support systems simultaneously, because several points in the bridge were supported by both of them. The 252-ft center suspension span and the two 84-ft-long approach spans meet at a set of four 70-ft-tall support pylons. The four pylons, each weighing 95,000 lb, support the combined forces of the suspension and cable-stay systems. The two systems work together to support a post-tensioned bridge deck that acts as a strut to transfer the horizontal loads of the systems to anchor blocks at either end of the bridge.

McHugh was not allowed to stop traffic on the existing bridge, so they built a temporary bridge, but they had to position it carefully. The constructor was required to keep the road open to automobile traffic in one of the city’s busiest industrial districts and keep the Chicago River open to recreational and commercial boat traffic just north of a barge-turning basin.

Ultimately, McHugh built the 850-ton center span of the bridge on barges in the river, then floated it into place, raised it into its proper position with 16 jacks and attached it to the suspension hangers.

The bridge work hit a snag when two 36-in. gas mains had to be relocated. They ran parallel to the bridge through tunnels under the river. They had to be moved to accommodate the anchor blocks at the ends of the bridge. Because the two mains were the area’s primary sources, they could only be taken out of service one at a time. The result was a 121-day delay in the bridge’s construction schedule.

The construction team used the computer model to tell them not only the forces on the bridge in its final configuration but also the best sequence for connecting the pieces of the bridge. The team came up with a 90-step plan, and McHugh monitored the bridge’s geometry while hooking up the 24 cable stays, two 4.25-in. suspension cables and 24 vertical cable hangers. Each of the cable stays was adjusted multiple times, tightening it, measuring its length and monitoring its load.