Precast Solutions - November/December 2008 - (Page 23) a set of complex, 160,000-pound prestressed and precast ribs that meet newer tsunami-zone infrastructure design requirements. That revision posed interesting problems for Keith Kaufman, Ph.D., Knife River’s chief engineer. He works primarily with a computer, but is not above simulating an outcome by creating an inexpensive balsa-wood model when the occasion calls for it. "I made a model to double-check calculations as to where to place the eyes," he recalled. Production of the heavier pieces proved no problem for the Harrisburg plant. However, shipment across the twists and turns of a mountain-range road between the plant and the beachfront bridge required extra planning. Another concern was how to securely lift a rib that would need to be shipped on its side instead of upright, as had been planned. Adding to the difficulty, ODOT allowed only one crane to be erected on the site, and it had to be positioned on a temporary work bridge and moved to the other side as needed. Eventually, the project involved many staff members with construction expertise, Kaufman said. Among them was Duane Downs, vice president and general manager of Wilhelm Trucking and Rigging, the firm chosen by Knife River to deliver the ribs to the site. "On a 10-point scale of difficulty, this one was a 9.8," Downs said. "It was very close to building a boat in a basement." Kaufman's calculations made use of the force on the post-tensioned rod running the length of the arch through its center of gravity. The numbers dictated placement of the eyes on the ribs. Yet the possibility of a mishap positioning the ribs for shipping and moving the ribs off the truck on site was still a concern. Kaufman solved the other issues with the ribs by relying on his wisdom, experience and, again, experiments with his model. By toying with the balsawood rib, some string and paper clips, he realized that once on site, all that was needed was a place where the rib could be set down briefly while still being supported from below the roll axis. He determined that a tower of stacked precast blocks, topped by a cushion of sandbags, would provide the crane operator a way to gently roll the ribs off the truck and then maneuver them into an upright position. He determined the placement of a few more eyes in critical locations to allow the crane to lift the ribs by first using one set of eyes, then moving the cables to a different set of eyes. Knife River began making the castings in January 2008. Wilhelm's drivers trucked the ribs – one at a time – on the 100-mile, four-hour trip to Spencer Creek. The unloading process required finesse from the Wilhelm drivers and skills from of the Campbell crane operators as the sandbags on top of the column predictably burst into cascades of sand each time a jumbo rib was set down then righted. Crews scrambled to fill and position additional sandbags for the columns for each unloading. The process was a stunning success for all involved. The project is slated for completion by the end of the year. "In spite of the limited site space and maneuverability NOVEMBER/DECEMBER 2008 End view, showing the rebar, arch-rib, post tensioning: two #8 bars, 28 #14 bars and 27 0.6-diameter strands jacked to 1,220 kips. | PRECAST SOLUTIONS 23
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