Hollywood producers love a good sequel. But what about road engineers? While a successful movie's return squeezes extra profit out of a film franchise, a valuable highway follow-up cashes in on the lessons learned from previous work, yielding infrastructure that holds up over time. On the other hand, the engineering equivalent of a bad movie is a bloated boondoggle of a road that cracks under pressure.
In the case of Chicago's Wacker Drive, an epic rebuild more than a decade in the making, the effort so far hasn't been a flop. Based on lessons learned 10 years ago on a first phase, Chicago Dept. of Transportation is now pushing the life expectancy of the new Wacker to 100 years from 75 years in its second, final rebuild.
Wacker Drive—its cavernous lower level provided mood shots for such movies as “The Blues Brothers” and “Batman Begins”—wraps around Chicago's central business district like a manila envelope. The 1.2-mile-long, 20-ft-tall, 130-ft-wide, double-decker viaduct handles 60,000 vehicles daily, running west from Lake Shore Drive along the Chicago River. At the river's bend, Wacker curves south and continues for another half mile along some of the city's most expensive office real estate, including Willis Tower. It then connects to the Congress Expressway, the start of Interstate 290.
When CDOT wrapped up work on Wacker's east/west section, originally built in 1926 and rebuilt in 2002, the idea was to roll right into the north/south phase of the downtown road, which opened to traffic in 1955. It wasn't that easy.
The east/west section was the squeakier wheel, for sure, having already been about 30 years old when the north/south section was built. But the younger, 2,900-ft-long section was already showing its age.
“It was in pretty bad shape,” says Andrew Keaschall, structural engineer and project engineer at Alfred Benesch & Co., which designed the new north/south section from Randolph to Adams streets. San Francisco-based T.Y. Lin has the balance of the design work, which includes the Congress interchange.
Decades of thermal expansion combined with heavy salt had started to take its toll on the bridge's heavily reinforced concrete, a monolithic structure that was not originally designed to stretch and contract through the years, leading to cracking, crumbling and gaping holes around expansion joints. Now, the 13-in.-thick upper deck, consisting of 6,000-psi, high-performance, reinforced concrete with a 2-in. latex overlay, rests on 24-in.-deep ribs and column-mounted bearings. The deck also is post-tensioned in two directions.
Zero Tension
The tensioning, which compresses the deck to make it stronger, is the magic behind the 100-year lifespan. Throughout the deck, longitudinal tendons take 23 million lb, with transverse tendons stressed to 12.5 million lb. Although CDOT has specified a tension tolerance of 232 psi in the deck, Benesch is shooting for zero tension once cars, trucks and people hit the new road.
“Tension in concrete is what causes bridges to break down,” Keaschall explains. “Our goal with this is to get 100 years with almost zero substantial rehabilitation.” Keaschall says he accomplishes this by coordinating with traffic installers and carefully watching over the workers' shoulders, all of which takes extra time.
