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Sandy River bridges on track, in more ways than one
Gantry cranes move beams into place without a work bridge.
Gantry cranes move beams into place without a work bridge.
More or less turning standard construction procedure on its head, the project team replacing the Sandy River bridges is setting the beams from the top down instead of the usual bottom-up method.
Prime contractor Hamilton Construction's new $1.2 million gantry crane can hook, lift and place steel beams up to 167 feet long and weighing up to 192,000 pounds.
The specialized crane, constructed on-site for the Sandy River bridges, is perfect for this project. It’s located in the delta with the Columbia River, where the wide and shallow basin fills with water from both upriver and down. The river basin is vulnerable to flooding from winter storms that send water hurtling down from Mount Hood, carrying debris that can back up against obstacles like bridge pilings and essentially create a dam. Conversely, the same shallow basin is vulnerable to spring surges from the Columbia River, as happened last spring, when rapid melting of Canadian ice packs turned the river below the bridges into a temporary lake.
By setting the beams from the air, the project team keeps traffic moving on the detour bridge, adheres to the logistics and environmental timetables and, most important, avoids the need for a work bridge during high-water season.
"On most replacement structures, building a detour bridge and a work bridge is standard procedure," said Ron Reisdorf, ODOT project manager for the Sandy River bridges. "We rarely use gantry cranes on ODOT projects, but we’re finding it a very useful addition to our toolbox. This solution protects both the fish in the river and the property owners nearby."
Though gantry cranes have been around since 1840 and used extensively in ship-building and manufacturing, this model, from North American Industries in Massachusetts, is decidedly high tech. It can be operated at the push of a button from the control platform or by remote control from up to 3,000 feet away.
"This is the first time Hamilton has used a gantry crane for bridge construction, and we are excited to see how it performs," said Wendell Snook, project manager for Hamilton Construction.
Though the crane does the heavy lifting, the process still involves the basics of human ingenuity and forethought and on-the-ground logistics. As always in infrastructure construction, the numbers are awe-inspiring: The tracks that guide the base of the crane out across the river extend for 840 feet, the same length as the bridges themselves. The two tracks must be aligned exactly 90 feet apart. The northside track had to be laid only inches away from the already-operating detour bridge.
Because the beams are steel, pile bucks — journey-level trade workers who specialize in all aspects of working with piles — are essential to the process. They had to cut off the excessive lengths of bolts on the existing detour bridge one by one to make room for the track. When the beams are spliced together, sometimes on the ground before placement and other times while suspended from the crane, ironworkers secure them with 550 bolts — top, bottom, back and front — first lining up all the holes with pins and then fastening them permanently with bolts.
Right now, the top-down beam setting process is in the set-up phase. Once actual construction begins, in January 2012, the entire substructure for the eastbound bridge will be laid and spliced in two months.