Opened in 1958, the Skyway had deteriorated and needed updating. In 2006, the Chicago Department of Transportation (CDOT) tackled the project in a $250 million (2½ times the total original cost), two-year project that included reconstruction of all overpasses and viaducts, modifications to toll plazas, reconstruction of the southern roadway, a new bridge deck, new lighting, and replacement of deteriorated structural steel. The steelwork was far more than cosmetic, with more than 500 tons of fabricated steel going into the bridge alone.
The work included removal and replacement of truss chords, diagonals, plumb posts, laterals, bracing, and floor beams. A particularly challenging aspect of the steelwork was that the city specified that bridge traffic not be shut down during steel replacement.
Hydraulics Keep Traffic Moving
CDOT devised hydraulics-based steel replacement procedures that allowed for continuous live traffic load on the bridge during work. “The only time we were allowed for traffic stoppage was 15 minutes between midnight and 5 a.m.,” says Ron Necco, Field Engineer for Danny’s Construction Co., Inc. (DCCI), the steelwork contractor on the project.
Rich Hill, DCCI Project Manager explains that while the procedures were devised by CDOT, DCCI had to “prove” them, so the brief closures occurred only during the first time for each type of hydraulic operation. According to Hill, “The hydraulics were perfectly reliable; no jack leaks, no pump problems, no hose or connector problems. In this hydraulic setup, a saddle is secured outboard of each end of the chord section to be replaced. The chord tension is then transferred to eight tie rods (2½-inch, 50 kips) by means of eight Enerpac 150 ton double-acting hollow plunger cylinders. The cylinders are supplied by an air-operated 10,000 psi pump, with needle valves used for load control.
The amount of tension applied to each chord was determined by the stress calculations shown on the original drawings, and they turned out to be pretty accurate,” says Brian Santosuosso, engineer for Wiss, Janney, Elstner Associates, Inc., (WJE) the engineering firm hired by CDOT to monitor the work. WJE installed load cells at the opposite ends of the rods from the hydraulic cylinders, with the outputs fed to a SensoTech readout and a laptop computer to handle data logging.
Four load cells were visible, and the four-channel data acquisition system, housed in an insulated job box, is shown below.
|Pier and support replacement required construction of temporary support structures on each side of the original support.|
A second data system handles the other four load cells. Sample rate is 16/sec. DCCI’s Hill says the several dozen 40-ft. box member chords being replaced were handled by splitting each along the top and bottom, replacing one half at a time, then installing cover plates on the top and bottom. Average time per chord is approximately 10 days, and, “Preparation is 90 percent of the job,” says Hill. Diagonals are handled in the same manner.
The two-speed 10,000 psi air hydraulic pump used to power the cylinders consumes at most 40 scfm from a standard 60-100 psi air supply. It incorporates two hydraulic relief valves, one factory preset, the other field-adjustable to limit maximum system pressure. The pump is of compact design, with a 10-gallon reservoir.
The double-acting hollow plunger cylinders used for the push-pull function are fitted with one male and one female coupler to ensure that the advance and retract ports are connected correctly. Each cylinder contains an internal relief valve for extra protection, and nickel-plated plungers with wipers help to resist the outdoor environment.
In another phase of the Skyway project, DCCI replaced some of the concrete pier and steel support structures with concrete legs. Here too, hydraulics helped shoulder the load. Four Enerpac 600 ton double-acting cylinders were used to raise an adjoining pair of truss ends 3/8 in. so that the original support structure could be removed. The original pier was then reconditioned and a new concrete support leg constructed.
The lift cylinders, powered by the same 10,000 psi pump used for the truss work, include internal stop rings to help prevent blowout, bearings to help absorb side loading, and plated plungers with wipers to resist weather and contamination. “The jacks worked perfectly,” says Joe Hodum, DCCI General Superintendent.
Chicago’s “can-do” attitude paid solid rewards on this job by leading the city to specify a hydraulics-based steel replacement procedure that kept traffic rolling.
Grainger offers a wide variety of Enerpac products, from the most basic lifting set for small jobs to custom configured systems for large scale lifts similar to that featured in this article.
Article courtesy of Enerpac