Mystic - Contractors have started to dismantle the control house of the historic Mystic River drawbridge, part of the third and final year of rehabilitation work.

State Department of Transportation Chief Inspector Brent Church said work on the 90-year-old bascule bridge, which began in December 2010, is on schedule to be completed by April. Total project costs to date are about $15 million, slightly above the original estimate of $14.8 million.

On Tuesday, crews from Maine-based Cianbro Corp. attached steel beam frames under portions of the control house. A 180-foot crane arm situated on a barge alongside the bridge was expected to lift the building off in five different pieces.

The building is being taken down for demolition to allow for a complete rehabilitation of the control system underneath - mechanicals, gears, machinery pit and electrical system. Other work has included or will include cleaning and repairing the bridge's structural steel and replacing lighting, traffic signals and gate. A stand-by generator for use of the bridge during power failures will also be installed.

"It's really the operating system for the entire bridge being replaced," Church said.

The control house, home to the bridge tender during months when the bridge is in operation, will be replaced in sections and maintain the exact footprint for historic preservation's sake, Church said. The replacement building has been constructed and awaits delivery.

Two of the largest gears on the bridge - 10,000-pound toothed half-moon shaped bull gears - will be cleaned and saved as possible show pieces for the towns of Stonington and Groton. They are being replaced with new gears.

Another large part of the project is rehabilitation of the counterweight containment systems, or ballasts, which will be cleaned and resealed. The counterweights are composed of 45-pound concrete blocks and weigh 286,000 pounds each.

Church said there are two 54-hour complete bridge shutdowns planned between January and April. Dates for the closures have not yet been scheduled. Alternating one-way traffic is slated to begin in early January.

The bridge was built in 1922, and Church said "we should have no reason to come back for the next 25 years."

g.smith@theday.com