Hydraulics Boost Productivity
Early in the development of the Chicago expressway system, planners identified a link to the south as a vital component. City officials decided to build a toll road, only to find that they lacked authority to do so.
The Chicago “can-do” attitude took over. They discovered that the city did have authority to build a toll bridge, and that there was no limitation on the length of approaches. What resulted is the Chicago Skyway, a 7.8-mile elevated expressway, but it is classified as a half-mile toll bridge with approaches totaling 7.3 miles in length. Opened in 1958, the Skyway had deteriorated and needed updating. The Chicago Department of Transportation is presently tackling that in a $250 million (2½ times the total original cost), four-year project that includes 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 is far more than cosmetic, with more than 500 tons of fabricated steel going into the bridge alone. That work includes removal and replacement of truss chords, diagonals, plumb posts, laterals, bracing, and floor beams. A particularly challenging aspect of the steelwork is that the city specified that bridge traffic not be shut down during steel replacement.
Hydraulics Keep Traffic Rolling
The Chicago Department of Transportation (CDOT) devised hydraulics-based steel replacement procedures that allow 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.
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 hydraulic operation. According to Hill, “The hydraulics were perfectly reliable; no jack leaks, no pump problems, no hose or connector problems.”
In this setup, a saddle is secured outboard of each end of the chord section to be replaced. The chord tension is then transferred to eight Williams rods (2½-inch, 50 kpsi) 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 are visible, and the four-channel data acquisition system, housed in an insulated job box, is shown at right. 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 are 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. Hill, whose company operates nation-wide, spoke well of the Chicago Ironworkers (Local #1) who are doing the work.
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-gal. 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 is in the midst of replacing some of the concrete pier and steel support structures with concrete legs. Here too, hydraulics shoulder the load. Four Enerpac 600 ton double-acting cylinders are used to raise an adjoining pair of truss ends 3/8 in. so that the original support structure can be removed The original pier is 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 prevent blowout, bearings to 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.