|SMALL WINDOW Pre-assembled leaf sections were floated in by derrick and bolted in place.|
Contractors erecting a 5,000-ton steel and concrete bridge in southern Virginia beat the clock as they attached the second of two pre-assembled 250-ton leaves and raised it on Aug. 8. The feat allowed them to reopen the busy Atlantic Intracoastal Waterway with time to spare on their 72-hour channel-closure window.
The five-lane bridge at Great Bridge, in Chesapeake, Va., works like a fine watch. Tolerances are tight. Steel was shop drilled for 60,000 2-in. bolts to a tol-erance of 0.03 in. by a fab-ricator in Wisconsin for bolt-up on site.
The overhead counterweight, hydraulically-driven, rolling thru-truss bascule bridge opens when below-deck pistons thrust the outboard ends of the leaves up. As the outboard end goes up, teeth on curved girders on the back corners of the leaves engage with teeth on flat, stationary tracks, walking the leaves back 6 ft as the tips rise into the air.
The fabricator, PDM Bridge LLC, Eau Claire, Wis., used numeric computer control software to keep connections true. "This entire bridge went though progressive assembly in the shop," says Ray Iesalnieks, PDMs project manager. "The counterweights were assembled as one piece, then broken down for shipment."
The meeting faces of the four curved, 38-ton segmental box girders, the cast toothed racks and the girders supporting the assemblies were mated in the shop to ensure that they meshed smoothly. "Each is machined to 0.006-in. tolerance," says Tom Fulton, project superintendent for erection planner and contractor Tidewater Skanska Inc., Norfolk, Va. Castings are attached with high-strength, high-torque 112-in. turned bolts.
|BIG PUSH Precise connections speeded assembly at joint near piston.|
Positioning the fixed tracks on either side of the waterway was critical, says Fulton. "Once you set your track girders youve locked in your distance across the river. They had to be perfectly aligned."
Tidewater Skanska site-assembled the counterweights and shored them in closed position to receive the leaves, which were assembled in a waterfront equipment yard about two miles away, transported on the hook of a 350-ton barge-derrick and swung into place.
"When we brought both leaves down to mate them at the finger joint, each 110-ft-long leaf was within 18 in. in plan length and within 12 in. in alignment," Fulton says.
|60,000 bolt holes were pre-drilled by fabricator in Wisconsin for assembly in Virginia.|
The U.S. Army Corps of Engineers began planning the project in 1984, says Andrew Spendlove, the Corps resident engineer. The $20.3-million bridge construction phase, started in November 2001, should be done in February. A companion, $6.2-million, old-bridge demolition and roadway realignment contract was awarded to Tidewater last month and is set to finish in July 2005.
Spendlove says designer URS Corp., San Francisco, chose the overhead counterweight design because a high water table made below-grade counterweights too costly. He says it efficiently shifts the load as the bridge opens, reducing wear. "You would expect a little creaking and cracking, but its just silent," says Fulton. "And it moves fastopens and closes in a minute and a half," he says.