|BUILT TO LAST Oregon bridge used 400 tons of stainless steel rebar and may last 120 years (Photo courtesy of Oregon DOT)|
An unusual arch-hinged Oregon bridge with 400 tons of stainless steel reinforcing bar in its deck and T-beams is all but complete. The 773-ft-long link over Haynes Inlet Slough is expected to last 120 maintenance-free years, or more than twice as long as average, says Frank Nelson, Oregon Dept. of Transportation bridge preservation manager. Until recently it was the largest bridge in the U.S. to use stainless steel rebar.
Nelson says the agency had used stainless steel rebar on two small bridges previously but now plans future use on coastal spans susceptible to corrosion from the salt air.
The $12.5-million Haynes Inlet replacement bridge has two sets of three-span, two-hinge, cast-in-place concrete deck arches varying in length from 198 to 239 ft, says James Bollman, ODOT project designer. ODOT awarded a $15.5-million contract to Hamilton Construction Co., Springfield, Ore., in 1999 for a November 2003 completion. Challenges pushed the schedule to March 2004, but all that is left to do now is striping.
"The schedule was built around when we couldnt do work," says Eric Hill, Hamiltons project manager. Crews were not permitted to impact nearby oyster beds and could not work from the water between Nov. 15 and June 15. Marine traffic had to be allowed to pass with 24-hour notice. Heavy rain in 2002 also caused mudslides at the south end.
The contractor built two sets of falsework from either end to transfer loading into the arches and to support the deck. The arches were post-tensioned in three phases to compensate for the nonuniform loads on their ribs. Twenty-four sets of stainless steel hinges, each weighing 10,000 lb, were embedded into the arches for seismic considerations. The contractor used microsilica concrete for the arches for further anti-corrosion. "Microsilica is used often in deck overlays and decks, but this project used it in the whole superstructure," says Bob Sherman, Hamilton general manager.
One footing was redesigned from pile-supported to a spread footing after rock was found 20 ft shallower than expected. That brought a months delay, says Hill, but Hamilton and ODOT worked together on redesign. Another change came when crews planning to remove the old bridge piers found 3-ft-dia boulders. The original plan to build cofferdams and remove the piers became a wire sawing subcontract to American Concrete Cutting, Springfield, Ore. Overall, change orders cost $2 million.
Carpenter Technology Corp., Wyomissing, Pa., stepped in to supply $1.6 million of stainless rebar after one supplier went bankrupt and another was dropped from consideration over a "buy American" clause. "The bridge got us into the business in a big way," says Rick Trate, Carpenter managing director of stainless steel rebar. "It was a learning curve." John Magee, Carpenter senior metallurgist, notes that "the strength is typically 60 ksi; we had to increase that by 25%" plus triple the ductility.
The steel came in maximum 30 ft lengths but the quantity was 80% of that required for other steels, says Bollman. It cost about 13% of the total job. Stainless steel can cost up to five times more than average steel, but life-cycle costs make up for it, says Trate.