Photo Courtesy Chicago Transit Authority
The bridge's upper deck carries commuter rail; the lower deck carries pedestrians and vehicles.

The Chicago Dept. of Transportation will have just nine days in late April to replace a large section of a double-deck drawbridge spanning the Chicago River, mirroring an early March operation to avoid a lengthy disruption of rail operations atop the two-leaf structure.

Beginning April 26, CDOT will suspend rail operations to replace a cantilevered leaf, or bridge arm, on the 91-year-old Wells Street Bridge, one of 40 fixed-trunnion bascule spans in Chicago and one of two that features a lower level for vehicles and pedestrians and, above it, commuter rail.

The project marks the first time CDOT has replaced large sections of a bridge servicing rapid transit, an undertaking it says is necessary due to extensive deterioration of the steel truss structure.

During the shutdown, crews will dismantle the north leaf of the 345-ft-long, 72-ft-wide bridge and replace it with a 500,000-lb prefabricated section, barged up the river. Crews of up to 60 will work around the clock on 12-hour shifts to complete the work by May 5, says Johnny Morcos, CDOT project manager.

Though CDOT replaced a south leaf in the same amount of time in March, "it's nerve-racking," says Dipal Vimawala, vice president with project engineer AECOM, Chicago. "Bolt holes for new and existing steel need to perfectly align. We've got 50 to 60 connections per gusset plate and tolerances are only a sixteenth of an inch."

Contractors must take into account wakes, which can cause work barges to move. "Normally, conditions like that wouldn't be as much of an issue," says Vimawala. "Here, we can't afford much time for misalignment."

Because adjacent streets aren't available for staging, the entire operation, including materials delivery and removal, will rely on marine equipment, says Sven Bosold, project manager with Walsh Construction Co., Chicago, which is undertaking the project in a joint venture with Chicago-based II In One Contractors. In addition to a pair of barges supporting Manitowoc 999 and Link-Belt 248 cranes, operations will require materials barges, scrap barges and barges for new and existing leafs, the movements of which must be carefully choreographed to ensure the river remains open to traffic.

Tricky Work

The operation is part of an effort, begun last fall, to repair or replace the majority of the bridge's steel, including roadway decking, stringers, bracing and framing elements, according CDOT's chief engineer, Daniel Burke, who notes that several of the span's historic components, including a pair of bridge houses, also are undergoing renovations. While the vehicular deck has been closed for a year, CDOT couldn't suspend rail operations for that amount of time because 70,000 passengers ride the track daily.

Instead, CDOT will have only days to remove the north leaf, secure its replacement with 10,000 bolts, install new rail ties and track work, and equip the structure with a new rail power system.

In addition to expediting fieldwork, the decision to prefabricate the 29-ft-long, 72-ft-wide, 29-ft-high truss allowed CDOT to ensure all connection points conformed to specs, says Morcos. Munster Steel, Hammond, Ind., constructed the A572 grade-50 steel section on a dry barge over six weeks.

To prepare for its installation, crews spent two weeks erecting three sets of 28-ft-high shoring towers beneath a counterweight serving the north span. The two-leg towers will ensure remaining sections maintain their horizontal positions once the river arm is removed. "Bascule bridges are like teeter-totters, with the counterweight continuously balancing the leaf," Vimawala explains. "If you remove substantial amounts of weight from the bascule, the remaining sections will tilt up, absent shoring to support them."