|Tall Order. Supplier built cableways in seven months. (Photo courtesy of FHWA)|
Flying high over the Black Canyon, the giant cableways are about to slide into two years of heavy construction for the $114 million bridge slated for a June 2008 opening. One of the “high lines,” as workers call them, has been in service since January, and the second is scheduled for final load testing and commissioning later this month.
Strung between the Nevada and Arizona approaches are two pairs of 3-in.-dia wire cables, each supporting a 13.5-ton trolley and load block assembly. Each block holds a nine-part line of 7⁄8-in.-dia wire rope with a maximum line speed of 400 fpm. The cables hang from two 330-ft-tall lattice-framed towers that can luff, or tilt, 7° laterally in each direction. Backstays and concrete foundations support the towers. The contractor will adjust slack in the cables three times during construction to change hook heights for placing steel tub girders and precast concrete sections that make up the 1,960-ft-long, 88-ft-wide and 277-ft-deep concrete deck arch-type bridge.
|(SOURCE: Obayashi Corp./PSM Construction USA Inc.)|
Each cableway will service an arch, according to Mike Motil, project manager for joint-venture contractors Obayashi Corp. and PSM Construction USA Inc. Before the team clinched the Federal Highway Administration contract in October 2004, officials proposed a cableway for “essentially all key activities,” says F. Dave Zanetell, FHWA project manager. Motil says there was “no other way” to build the bridge feasibly (ENR 7/12/04 p. 12).
However, finding two cableways that were up to the task proved to be almost as challenging as the bypass project itself. Cableways, along with their distant derrick cousins, are often considered dinosaurs of crane technology and can be difficult to locate. “They don’t get used very much,” says Robert C. Wild, crane safety specialist for the U.S. Army Corps of Engineers in Rock Island, Ill., and chairman of the American Society of Mechanical Engineers’ standards committee on cableways.
But when it comes to building bridges over deep canyons, cableways are an old method with a proven record. More than 70 years ago, it took 10 aerial cableways to build Hoover Dam, which still has a 150-ton unit in service. Modern cableways feature wireless controls and electronic load limiters.
Prior to bidding the job, Motil’s team went on a scavenger hunt, he says, and found a good-quality unit laying “in the weeds” at American Bridge Co.’s equipment yard in Coraopolis, Pa. The supplier had used the crane, built by hoist manufacturer Skagit in 1964, to erect West Virginia’s New River Gorge Bridge through 1977. After appearing on more projects, the crane was taken out of service in 1988.
Obayashi-PSM hired AB to refurbish the existing cableway and build a duplicate model. Design, fabrication, assembly and testing took seven months. The joint-venture pegs operating costs, including lease, set-up, labor, maintenance and dismantling, at $11 million.
uilding a 2,000-ft-long concrete arch bridge 900 ft above the Colorado River is turning into a tightrope walk for the builders of the $234-million Hoover Dam Bypass. Soon, the highway program will be in the hands of hoist operators manning two 2,500-ft-long cableways designed to carry up to 50 tons of material.