North Avenue Bridge - 2007-2008 Bridge Replacement

2007-2008 Bridge Replacement

Due to age and the high cost of maintenance, a replacement fixed-span bridge was built through 2007–2008. The new bridge is a fixed-span, hybrid suspension/cable-stayed bridge of the same height but nearly twice as long in length. The original bridge was two lanes wide; the new bridge has four lanes plus sidewalks, helping to reduce congestion caused by the 40,000 cars/day that travel on the bridge.

James McHugh Construction Company served as the general contractor for the new bridge. Designs for the new North Avenue Bridge were constrained by several factors. An active railroad line runs along the eastern bank of the river, requiring a level-crossing or overpass. The banks of the Chicago River were to be kept open and utilized for further development. The United States Coast Guard required the bridge to have a vertical clearance of 18 feet (5 m). Finally, the new crossing, situated at the northern tip of Goose Island and having a clear view to the Chicago skyline, was required to be aesthetically pleasing.

Due to rising costs of steel, the construction of a new bascule bridge was ruled out. The length of the crossing proved to be too short for a pure suspension bridge, while a pure cable-stayed bridge would require a larger tower than preferred for the site. As a result, engineers turned to the concept of a hybrid suspension/cable-stayed bridge.

Prior to work on the new bridge, work crews constructed and opened a temporary bridge just to the north of the new bridge. The temporary bridge allowed for the relocation of utilities, construction of an improved river wall and uninterrupted substructure construction for the new bridge. The center span was constructed on three barges in the Chicago River, because of Coast Guard regulations that required the channel to be open at all times. After the center span was constructed, it was towed to the bridge, where 16 jacks lifted the span into place.

The new bridge was completed in 22 months, and cost US$25,000,000 to build. The bridge is anchored by four pairs of pylons, with each pylon weighing 95,000 pounds (43,091 kg). The pylons rest on 10 smaller micropiles that extend 90 feet (27 m) into bedrock. From the pylons, a total of 24 cable-stays support the approach spans, as well as the ends of the center span. A suspension cable, running from anchorages at the feet of the bridge to the tops of the pylons, supports the middle portion of the center span.

The deck of the bridge is post-tensioned to allow horizontal forces of the bridge to be transferred to anchor blocks at the ends of the span. It was constructed with 10 inches (25 cm) of high-performance concrete, and with a 2-inch (5 cm) latex overlay. The total thickness of the deck is only 4 feet (1 m).