Originally built in 1929, the Kennebee River Arch Bridge in Norridgewock, Maine, began deteriorating, and showing signs of crumbling concrete resulting in a damaged pier cap and overhead cross members. The bridge also did not meet the demands of increased traffic due to its narrow travel way width. As a result, the Maine Department of Transportation, the Federal Highway Administration, and the Maine Historic Preservation Commission decided to demolish and reconstruct the bridge.
Keeping in mind the historic significance of the bridge, an extended timeline was put in place. The project was bid on and awarded to general contractor Reed & Reed, Woolwich, Maine, in May 2008. Plans consisted of replacing the original bridge constructed of reinforced concrete using four thru-arch spans with a 300-foot single-tied arch with a maximum height of 65 feet, and two 133-foot approach spans. The new bridge features 41 feet of roadway width, as well as 25 feet of vertical clearance—more than twice as much as the prior width and clearance. Strongly reflecting the community’s desire to create a modern historic bridge, the design incorporated key aspects of the old structure.
Most concrete arches are anchored to massive thrust blocks to prevent the ends from spreading apart, however, the concrete tied arch on the Kennebee River Arch Bridge project uses a post-tensioned beam to connect the arch ends. This posed a challenge, in which designer SEA Consultants, Augusta, Maine, provided specific guidelines for construction sequence and methods. Construction supports for the longitudinal tie girders and the arches were only allowed in certain locations. Also, the contractor couldn’t transfer any construction loads to any part of the permanent structure, requiring the installation of extensive temporary pilings in the river and temporary support beams beneath the bridge to support the construction.
The design of the project required the ability to span distances of 50 feet and carry loads up to 180 tons per support tower. A.H. Harris & Sons, Newington, Conn., provided a unique package that combined standard components with custom engineering and the only shoring system capable of handling the loads using four or six legs per tower. The complete structure consists of two side-by-side concrete arches connected by a series of lateral beams to be cast at a later date. To meet all these challenges, A.H. Harris developed a design using self-spanning steel form panels combined with custom wedge-shaped panels, custom arch panels, and the XPS-60 shoring system that supports 30-ton loads per leg. The self-spanning steel form panels allowed for the distances needed while the custom panels provided the desired shape for the concrete’s bottom surface. The shoring system supported the high loads at each support location with a minimum of legs, usually four per tower.
Choosing a shoring system relies heavily on its load capacity. The XPS-60 uses support towers with capacities of 60,000 pounds (30 tons) per leg in a typical shoring application. Reed & Reed senior project manager Ted Clark says, “We knew we could count on Harris to work with us to find a design that met our needs and our budget.” Scheduled for completion in fall 2011, the bridge is estimated to cost $21 million. With all the planning and design elements, the bridge will meet modern demands and serve as a historical landmark.
Learn more at www.ahharris.com.