Chicago Skyway Toll Bridge High Bridge Corridor, Chicago

Using 4 million lbs. of steel and a temporary strongback scheme, the 45-year-old truss structure carrying the Chicago Skyway was rehabilitated without removing the live load. More than 50 nonredundant truss members were replaced while vehicles traveled above.

A Unique Link

The Chicago Skyway Toll Bridge is unique among Chicago bridges because of its height and length of its truss spans. The High Bridge portion of the Skyway consists of 34 truss spans with a total length of more than 6,000 ft.

The Calumet River Bridge, the centerpiece of the Skyway, consists of two cantilevered through-truss spans supporting a suspended span of 650 ft. The suspended span allows a horizontal clearance of 200 ft. and a vertical clearance of 125 ft. for the ship traffic on the Calumet River.

The Skyway is a vital link to the Chicago transportation system. It carries six lanes of traffic between the Indiana Toll Road and the Dan Ryan Expressway.

But for years the Skyway struggled with inconsistent revenue levels until recently when several changes in traffic patterns improved the revenue stream and revenue forecast, providing capital for the much-needed rehabilitation.

Significant Work Needed

A structural investigation was done. Findings showed corrosion and section loss of steel truss members, floor beams and stringers; members with below-standard load ratings; and warping of components of built-up truss members due to inadequate sealing pitch of the rivets. Warping was so severe on more than 50 truss members that complete replacement was required.

The city committed to a comprehensive capital plan, including a major rehabilitation of the high bridge, repair of the trusses and replacement of the deck.

Work Ensues in Traffic

The truss repairs were to be performed under full live load. The use of shoring towers would have increased project costs due to the heights and loads involved, so an alternative was developed.

The chord members to be replaced carried up to 800 tons of force, and temporary strongbacks were used to handle the load. The strongbacks were steel brackets bolted to the truss chords, high-strength-steel threaded bars and four jacks.

Loads were applied by four jacks simultaneously until the total theoretical member dead load was achieved. The procedure was monitored by load monitoring cells connected to portable computers under each jack.

The built-up steel sections were removed using a half-at-a-time approach. This provided a "belt and suspenders" safety factor under live load and helped hold truss geometry while new members were positioned.

Fabrication of new members was aided in large part by furnishing the detailer with original shop drawings in electronic format, and the contractors verified dimensions in the field.

Other repairs included the main truss gusset plate replacement and compression chord partial replacement.

The former involved providing shoring towers for support at top panel points, thereby releasing the bottom chord tension loads in the area of the main gussets.

For the compression members, rolled W-sections were temporarily bolted to the sides of chords to provide for sufficient lateral stability while the top and bottom plates and angles were removed and replaced. Members were strengthened as a result of this procedure, and there was no jacking.

The jury said, "They kept the roadway functioning while they did the rehab by using a couple of innovative systems. This was a smart engineering feat."

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