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
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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