Workers already have removed substantial numbers of rivets from those sections, replacing them with temporary bolts on angles connecting floor beams to trusswork. "Rivet removal is time-consuming," says Bosold, "Now, work will proceed more quickly when the time comes to remove those beams.

Once CDOT closes the bridge, operations will duplicate the March game plan for the south leaf, following the same hourly schedule. "You have a lot of things happening simultaneously," says Burke. During the earlier job, workers dismantled rail track on March 2, the same day steelworkers, suspended on lifts from barges, began dismantling the leaf using torches to sever the chords and diagonals. As crews continued their work on the steel, cranes deposited 15-ft-long sections of old track onto scrap barges.

The leaf was ready for removal that evening, a procedure that required situating a barge beneath it, then pumping water out of barge compartments to buoy it.

"The operation calls for imparting buoying forces under the leaf—pumping out sufficient amounts of water to impart an upward force equal to the dead weight of the leaf," says Burke.

After the leaf was removed, workers docked the barge east of the bridge and began prepping gusset connection points on the truss' lower chord. On March 4, the barge containing the replacement moved into position from the west, the leaf supported by four 10-ft-by-10-ft, 20-ft-high shoring towers at a height approximating that of existing sections.

"Hydraulic jacks then made adjustments to account for fluctuations on the river," says Bosold.

Crews spent a day connecting new and existing members. "We pinned bolt locations, then started bolting once we were aligned," says Burke. By March 7, the leaf was self-supporting. On March 8, ODOT lowered the north leaf so that crews could install the new rail bed. "We literally had masses of workers laying tracks and ties and ensuring everything aligned," says Morcos. "That took the better part of three days." Work was completed early on March 11, in time for the Chicago Transit Authority to resume rail operations during the morning rush hour.

The original plans called for replacing only the bottom chords of the north and south leaves, an operation that would have suspended rail operations for 15 weekends over the course of a year. "Those members were in the worst shape" says Vimawala. "The lower deck received large amounts of snow and salt that would wash down and collect on the chords, eventually causing extensive corrosion."

When it became clear that resuming rail operations on Monday mornings would prove problematic should crews encounter unforeseen conditions over the weekend, CDOT elected to remove entire sections. "We wouldn't have been able to access and inspect some interior angled members prior to construction," says Morcos. "A weekend might not have provided the time needed to make repairs we hadn't anticipated. We didn't want to be in the position of, 'Here's Monday, and we can't resume rail operations.' "

Replacing entire sections also reduces the potential for ad-hoc repairs to the bridge once the one-year renovation program is complete.

"It's been the most challenging project of my 20-plus-year career with CDOT," says Burke. "From planning through construction, the amount of thought, effort, coordination and scheduling involved has been immense."

By John Gregerson