A hybrid cable-stayed/suspension bridge, the first of its kind in Chicago, is being built over the north branch of the Chicago River at North Avenue. This project replaces the former North Avenue Bridge — a 99-year-old, inoperable bascule bridge which was demolished earlier this year. Designed by HNTB Corp., the new, wider, fixed-span structure will accommodate two lanes of traffic in each direction instead of narrowing to one lane over the river as the old bridge did. The new bridge will also feature sidewalks on both sides.
In a city with a long heritage of innovative bridge design, the new North Avenue Bridge will be a notable addition. "Chicago has this marvelous heritage of innovative bridges, dating back to the turn of the 20th century," says Brian Steele, a spokesman for the Chicago Department of Transportation (CDOT). "The new North Avenue Bridge will be in the top five, perhaps top three architecturally significant bridges in Chicago. In terms of newer era bridges, it will rival the Damen Avenue Bridge (a bascule bridge replaced by an arch bridge in 1999)."
James McHugh Construction Co., Chicago, is the general contractor for CDOT's $25-million, multiphase project. Construction began in mid-2006 and is scheduled for completion around Thanksgiving.
"This bridge is one of the most complicated structures we have built," says McHugh Project Manager Carlos del Val Cura, a 20-year construction professional (13 years with McHugh). "It is very challenging."
Featuring a unique hybrid of a self-anchored suspension bridge and a cable-stayed bridge, the North Avenue Bridge will be one of the first such hybrid systems in the country, states Mike Gould, McHugh vice president of infrastructure construction.
A hybrid structural system was selected for the new structure to meet various geometric and clearance constraints, provide for future utilization of the riverbanks adjacent to the structure, and provide for an aesthetic structure, according to CDOT Project Manager Johnny Morcos, P.E.
"Space at North Avenue is limited because a railroad crossing is located just east of the bridge," he explains. "And due to Coast Guard regulations, we have to maintain a certain vertical clearance. The previous North Avenue Bridge was a bascule bridge with very high trusses, and we didn't want to build something like that again due to the cost of steel. Also, an operable or movable bridge was not required in this area because the river traffic here is mostly recreational."
CDOT carefully looked at the options of constructing a pure cable-stayed bridge or building a pure suspension bridge. "However, a massive tower would have been required for the cable-stayed bridge, and the length of the river crossing didn't warrant a pure suspension bridge," says Morcos. "So, we turned to the idea of building a hybrid suspension/cable-stayed bridge."
North Avenue Bridge Components
The new North Avenue Bridge will stretch 420 feet from anchor block to anchor block and will be 78 feet, 6 inches wide. It will consist of three segments — a 252-foot-main span and two 84-foot approach spans. The middle portion of the center span over the river will be supported by suspension cables and hangers, and the remainder of the center section and the east and west approach spans will be supported by cable stays (a total of 24) arranged in a fan or semi-harp configuration.
Both of these systems share two sets of 67-foot-tall support pylons (which rise 50 feet above the deck) constructed of built-up rolled steel plates. The pylons — each weighing approximately 95,000 pounds — are extremelycomplex, since they are required to support the combined forces of the suspension and cable-stay systems. Each pylon foundation is supported by 10 micropiles extending 90 feet into bedrock.
The cable-stayed and suspension systems will work together to support a post-tensioned bridge deck that also acts as a strut to transfer the horizontal loads of the systems to the massive anchor blocks at either end of the bridge span. The bridge deck is being post-tensioned and constructed with 10-inch high-performance concrete to provide greater durability and strength characteristics. And once all the bridge spans are in place, a 2-inch latex overlay will be placed over the entire deck, according to Morcos.
CDOT says the configuration of the hybrid cable-stayed/suspension bridge allows for an open, airy bridge. It also creates the maximum amount of clearance over the river, as required by the federal government. At a little over 18 feet, the new bridge will have the same clearance as the movable one it replaces.
Reconstruction of the North Avenue Bridge is quickly taking shape, with more than 70 percent of the structure's concrete (approximately 3,500 cubic yards) in place. "This has been a fast-moving project," Morcos says.
Despite extensive utility relocations and obvious winter weather constraints, the project has moved steadily forward. During a late July jobsite visit by Construction Digest, construction crews were busy setting formwork and rebar for the center span, which measures 109 feet long by 80 feet wide, according to McHugh Quality Control Engineer Nick Bostic. A week later — during a two-day pour (Aug. 2-3) — workers placed approximately 378 cubic yards of concrete into the center span.
"On the first day of the pour, we used two of our pumps to do the outside concrete beams and sidewalks," says Bostic. "Then, on second day, we used just one pump to pour the actual deck. The deck is 10 inches deep, and the sidewalk and beams on the outside are 3 to 4 feet deep in places."
The center span was built just off-site — on three barges tied together along the east bank of the Chicago River. "Building it off-site was more complicated, but it was safe and from a schedule point of view it was the best way we found to get it done," says del Val Cura. "Also, there is a requirement from the Coast Guard that the channel has to be open all the time, so we couldn't work in the channel."
With assistance from two tow boats, the center span is scheduled to be floated into position for erection between the east and west approach spans. "We will float it down river into the middle of the bridge." says del Val Cura. There, workers using two launching trusses, combined with 16 jacks, 100 tons each one, will lift the span into position.
Actual construction of the new bridge comprises only the last portion of this signature project. The project has required numerous other operations, including building a temporary vehicular and pedestrian bridge over the river to keep traffic moving while the permanent structure is being built; relocation and adjustment of utilities (including relocation of two 36-inch gas lines crossing the river); river wall improvements; and extensive foundation work to accommodate the new bridge.
McHugh spent the first nine weeks of the project building the temporary bridge. The city structured the project so that North Avenue will remain open to traffic throughout construction. The temporary bridge has one lane in each direction, the same as the old bridge. One sidewalk also remains open.
A Team Effort
Reconstruction of the North Avenue Bridge is no easy task."As with any project of this magnitude, it requires a team effort with a lot of different personnel and trades," Morcos says.
The team includes URS Corp, resident engineer, and McHugh's primary subcontractors Perdel Contracting Corp., Aldridge Electric Inc., Thatcher Engineering, and Accurate Steel Installers Inc. Fabricators include BendTec Inc., Acme Structural Co., Clodfelter Bridge & Structures International Inc. and Dywidag-Systems International USA Inc.
Key CDOT project personnel include Bulent Agar, P.E., construction manager, and Moira Coughlin, resident engineer. For James McHugh Construction Co., other project personnel include Peggy Blaney, assistant project manager; Ray Marino, quality assurance/quality control engineer; Lisa Laws and Alan Courtney, project engineers; Tom Reidy, concrete superintendent; Larry Staal, superintendent; Don Zirbel, engineer; and Bill Giovanetti, safety manager.
URS Corp.'s resident engineers are Richard Dolan and Alison Smith.