October 01, 2000|By FRANK D. ROYLANCE | FRANK D. ROYLANCE,SUN STAFF
Two rules for demolishing and replacing the Charles Street bridge over the busy Amtrak rail lines are crucial: Do it at night, and don't step into the headlights.
The $23 million Charles Street bridge replacement project is one of the most complex public works undertakings in Baltimore history, according to city public works officials. The railroad span is the most difficult part of the project. It crosses eight sets of tracks used by 190 trains a day - Amtrak passenger trains, CSX freights, MARC commuter trains and light rail cars.
"Anytime you do anything over a railroad, it adds a high level of complexity," said Daniel W. Zimmerman, project supervisor for the city's Bureau of Transportation.
Keeping the trains moving isn't the job's only complication.
Contractors working on the adjacent Charles Street spans across the Jones Falls Expressway must keep 75,000 cars flowing by the work on Pennsylvania Station, on bus, car and pedestrian traffic, and especially on neighborhoods north of the span. Businesses there are coping with detours that have rerouted 11,000 potential customers who drove by their doors every day before the roadblocks went up on May 21.
It's a big headache. "This is probably the highest-volume bridge we have ever closed entirely," says Frank Murphy, the city's traffic engineering supervisor.
Jim Berry is the project superintendent for Cherry Hill Construction, the job's Jessup-based general contractor. As weathered as the 89-year-old bridge he's assaulting (but much younger at 63), Berry is a construction veteran. He refuses to call this job difficult. "Call it challenging," he says.
Berry would dearly love to batter the old railroad span into a heap of rubble quickly, in place, with brute force. "Demolition of the bridge would be a 30-day job if it were out in a field," he says.
Instead, to protect the railroad property below, the contractors will spend the next 12 to 14 months methodically cutting up the bridge and hoisting the pieces out - some as heavy as 136,000 pounds. The lifting will be done by a 200-ton crane already perched on the old bridge deck.
To keep the trains running, the demolition work must be confined to the hours between 11 p.m. and 5 am. Those are the slow hours when Amtrak can shut off the power and divert any trains to othertracks.
"This really boils down to only four hours of useful work for the contractor," said William A. Geschrei, an associate with the project's engineers, Whitman, Requardt and Associates of Baltimore. "Light rail allows only 1 a.m. to 4:30 a.m..
And, to protect the tracks and the 13,200-volt "catenary" cable that powers the locomotives, the contractors have had to build a shield - a kind of portable steel tunnel that will be slipped under each 50-foot span whenever there is work scheduled overhead.
The bridge is a formidable structure. Built in 1911 to carry trolley cars, it crosses the tracks in four 50-foot segments, or spans. A shorter fifth span, over the light rail platform on the south end of the bridge, is already partly demolished.
Each span consists of a deck built with 22 steel girders embedded in reinforced concrete. The deck is topped by trolley rails set in paving bricks covered by asphalt. The deck is supported by concrete pillars, called piers, set between the tracks.
"They really overdesigned those things," Geschrei says. "Removing it is much more time-consuming due to the complexity of the bridge compared to a bridge today."
Inspectors found the bridge's steel girders and bearings rusted and its concrete deteriorating. Because it could not meet modern load standards, it had to be leveled.
Before they began cutting it up, Berry's crews had to strip the deck of its asphalt surface, then rip back the old trolley tracks and brick pavers. At the south end, they've begun working to isolate each girder in the deck, like the tines of a fork, by slicing through the concrete with a rock drill and a fiery oxygen lance.
Finally, with a diamond-studded cable saw that can take 2 1/2 hours or more to slice through one Concrete-encased girder, they're cutting each piece free and lifting it out. All the concrete and steel is eventually recycled, Berry said.
As each span is removed, the crane and the demolition team on the bridge will creep north to dismantle the next span. Construction crews will then move into the gap from the south, and start building the new bridge. There, too, work will be limited to the wee hours until the tracks are protected by the new bridge deck.
To establish foundations for each of the four new bridge piers, the contractors must sink five concrete columns 35 feet into the dirt and rock between the railroad beds.
For each of the 20 holes, a huge auger, 42 inches in diameter, will bore into the earth. Where it encounters solid rock, it will be replaced by a 42-inch circular hole saw - a giant, rotating cookie-cutter with teeth on its rim. Each open shaft, or caisson, will then be filled with concrete.
