“The overall depth of the girder bridges is constrained at the site due to the impacts they create on the surrounding historic properties,” Wood says. “The use of the tied-arch structure allows us to increase the span without increasing the depth of the superstructures, compared to the conventional beam option. We minimized the cost by reducing the number of piers required from four for a conventional beam girder bridge to two with the use of the long-span tied arch.”

Building a Better Bridge

In a traditional arch structure, Wood says, the bedrock supporting the foundation resists both vertical load and horizontal thrusts from the arch. As with the original bridge in Norridgewock, because bedrock is about 25 to 50 feet below the riverbed, a tied-arch structure was used to transfer only vertical loads to substructure units.

The contractor places the concrete for the tie girder, the member that holds the ends of the arch together.

The tie girder, which uses conventional post-tensioning technology, ties both ends of the arch together, counteracting the thrust forces from the arch, Wood explains. At each end of the tie girder, a large concrete section serves as an anchorage block for the post-tensioning system and to connect the tie girder to the arch rib. Within the tie girder, eight post-tensioning ducts each carry 27 post-tensioning strands. The strands in the post-tensioning ducts are stressed and grouted — as is required in the specified construction sequence — to resist the thrust of the arch rib, Wood says.

The construction sequence itself is quite unique. Because of the bridge location, the construction team had to expedite the bridge building process so it was complete in one construction season. “The bridge is over the Kennebec River, which is known to have severe ice conditions, especially during the spring melt,” Wood points out. “We didn’t have a lot of time due to the ice conditions, so we had to develop a construction sequence for one construction season.”

Because a tied-arch structure cannot act as a fully-functional bridge until it is essentially complete, a construction sequence was developed to allow the arch to function prior to completion, Wood explains. “The use of precast floor beams, a staged post tensioning sequence, and placement of the diaphragms and deck after the removal of falsework were used to reduce the construction time,” he says. “We had to be able to build the bridge in a year.”

The project began in late summer 2008, when a temporary bridge structure was built parallel to the existing bridge. After completion of the substructures and the two approach spans in the spring of 2010, six temporary piers were put on each side of the actual bridge to hold it up. Falsework was placed on top of it, and then the floor beams, which were precast offsite, were laid out in place. “By pre-casting the floor beams offsite as opposed to casting onsite, it reduces the construction time tremendously” Wood explains.

Both the temporary falsework supporting the girder and the precast floor beams were part of the innovative construction sequence to allow the bridge to be built in one year.

Following the installation of floor beams, the construction team cast the end beams, arch-end connections and the tie girder, and the first phase of post-tensioning was put in place. Additional falsework was constructed and then the arches and transverse struts were built. After the removal of the falswork for the arch ribs and the transverse struts, hangers were installed, and the tie girder falsework was lowered place, which then allowed the bridge to operate as an actual structure.

The bridge had just opened to traffic. Full completion of paving and the finishing touches on the arches are to be done this fall.

At-a-Glance

Total length: 567 feet

Spans: 300-foot tied-arch span in center with two 128.5 New England Bulb Tee (NEBT) spans on either end

Arch: 60-feet high at the center

Hangers: 18 sets of hangers — nine per side, two at each location

Travel lanes and use: Carries two 12-foot travel lanes with a 6-foot shoulder and 5-foot sidewalk on one side of the road; on the other side is a 4-foot shoulder and 7-foot multi-use lane for snowmobiles in the winter and horses, bicycles, etc., in the summer

Norridgewock Bridge Project Unique Stipulations

Due to the historical nature of the Norridgewock Bridge Project, the bridge’s design had the following unique stipulations that had to be met, according to MaineDOT:

• Document the covered bridge in accordance with Historic American Engineering Record standards.

• Incorporate design features that are appropriate to the setting, through continued consultation with Maine Historic Preservations Commission.

• Design the replacement bridge in a collaborative process that includes Federal Highway Administration, MaineDOT, Maine Historic Preservation Commission and the town of Norridgewock. This team is called the “Signatories Committee.”