|MAIN ARTERY Suspension bridge's corroded wires (below) compose a helical shape that cannot be spliced together for rehabilitation.|
A complex load transfer operation involving installation of supplemental cables on a 72-year-old two-lane suspension bridge may be the first of its kind in the U.S. Consulting engineers for Maines Dept. of Transportation had discovered extensive degradation on a main cable that forced bridge load limits and a fast-tracked replacement project.
The Waldo-Hancock Bridge carries U.S. Route 1 over the Penobscot River between Prospect and Verona Island. It has an 800-ft-long center span and 350-ft-long side spans. The main cables rest on saddles in two truss towers and are anchored at each end in concrete blocks under approach ramps. The 10-in.-dia helical cables are composed of 37 strands; each strand contains 37 galvanized wires.
"The bridge was built during the Depression for $850,000 and they got the most bridge for the money," says Jamey A. Barbas, senior project manager for MDOT consultant Parsons Corp.s New York City office. "Helical strands are stronger and can stretch longer spans than parallel wires. [But] you cant splice helical wires together for a fix."
A $5.3-million rehabilitation program revealed in July that several hundred wires had broken in center span sections of the southern main cable, forcing a 12-ton load limit (ENR 7/28 p. 9). The bridge had been rated for a 50-ton limit. A 1.8 safety factor was verified after tests showed extensive corrosion, wire brittleness and low capacity. The rating is well below a comfort zone of 2.5, says Barbas.
Piasecki Steel Construction Corp., Castleton, N.Y., had rehabilitated the north cable last year. For the south cable, MDOT in August hired Pittsfield, Maine-based Cianbro Corp. under a $4-million emergency contract to be completed by Nov. 1. "Although [Piasecki] wanted to do it, this problem required an accelerated schedule that interfered with other commitments," says Philip D. Roberts, MDOT resident engineer.
Williamsport Wirerope Works Inc, Williamsport, Pa., rushed production of 16 reels of 2-in.-dia galvanized helical 91-wire strands. "We had to design a strand that could be made as quickly as possible, so we used one wire size instead of three or four," says Thomas E. Secules, Williamsport structural products project manager. "We completed the work in about nine weeks, or twice as fast as normal." Each reel contained over 1,800 ft and the job cost more than $400,000.
Cianbro completed two new concrete anchorages with up to 30-ft-long anchor rods. Crews installed continuous runs of strands on new saddles bolted and welded on new base plates atop cable bents and main towers. Workers placed two groups of four strands 12 ft above each main cable. Each strand weighs 4 tons. A rope pull was walked across, connected to a 7/8 -in. pull cable, then winched back across and connected to the strand, which was fed through a tensioner holding back about 15,000 lb to smooth the pull.
"We hooked and rehooked one strand per day on average," says Archie J. Wheaton, Cianbro project superintendent. "The strands were connected to anchor rods; then we set the sag." The new auxiliary cables are connected to existing double suspender cables by 11/8 -in. steel rods, then tensioned with 30-ton jacks, bringing the new cables about 3 ft from the main cables.
|CABLE CRAWL A rope pull is walked acorss the bridge, connected to a pull cable and winched back across to install the supplementary cables.|
"We used hydraulic winches to pull the cables and hydraulic jacks to tension them," says Alan D. Fisher, Cianbro chief structural engineer. "There was no heavy equipment, only 40 workers." Work finished Oct. 26. The new cables now support 50% of the dead load. The bridge now carries 40-ton loads with a 3.2 safety factor. Project officials know of at least one suspension bridge, in Portugal, that was strengthened by supplemental cables, but know of no others in the U.S.
"This is a temporary fix to maintain existing traffic patterns," says Roberts. MDOT plans a replacement to be completed in 2005. Tallahassee-based Figg Engineering Group will design the new bridge, estimated to cost more than $50 million. The historic bridges future is in doubt and demolition is expected.
(Photos courtesy of Cianbro)