| August 02, 2011 |
Making History Again
When a historical bridge in Maine had to be demolished and rebuilt, the question was how to do it and still preserve a landmark
By Tina Grady Barbaccia
When the historical bridge in Norridgewock, Maine, had deteriorated to a point where it had to be either demolished or rehabilitated, a delicate situation presented itself. The “concrete bridge” as the locals called it was part of the community’s character and charm — and part of the town’s emblem. No one wanted to see it razed, but the bridge needed to provide a safe and efficient river crossing. This made for a very sensitive situation, but one that was able to be worked out. The bridge opened to traffic this summer.
The design needed to be “both contemporary and fitting for the setting and the history of the site,” according to Wayne Frankhauser, project manager for the covered project. The challenge was clear: A bridge had to be designed in a way that was similar to the existing structure but incorporating as many modern design and construction techniques as possible.
Complicating plans further was the fact that the bridge would be built with federal and state money. The Federal Highway Administration (FHWA) and Maine Department of Transportation (MaineDOT) would have to agree on a plan. Because of the historic significance of the bridge and historic buildings close to it, federal law requires that the Maine Historic Preservation Commission also had to be in agreement, according to MaineDOT. After conducting a study from 1998 to 2004 to evaluate four crossings — both east and west of the current covered bridge — MaineDOT and FHWA came to the conclusion that although rehabilitating the bridge was an option, it was recommended that the current bridge be removed “due to its poor condition, narrow width and outdated height.” A new bridge would be built on the site.
Creating a Worthy Landmark
In 2004, the FHWA, MaineDOT and the Maine Historic Preservation Commission came to an agreement on the bridge’s design plan in a document known as a Section 4F statement. (See “Norridgewock Bridge Project Unique Stipulations” sidebar.)
After all the stakeholders in the project had come to an agreement on the bridge design selected, they engaged the community with both a public meeting and an informal survey. When asked their preference about the two design options, the response was divided, with neither design being a clear winner.
The plans began to move forward. “Though we know [the] decision won’t please everyone, it’s time to move forward with construction plans,” Frankhauser said after the selection of the bridge design. Earle G. Shettleworth Jr., director of the Maine State Historic Preservation, fully supports the design decision.
He notes that the covered bridge in the current condition is “a bold statement of the modern age . . . one of Maine’s most significant 20th-century bridges.” However, with the two options that met the project’s purpose and need, “one embellishes what is otherwise a fairly standard girder bridge,” Shettleworth says. “The second uses a 300-foot center-arch span employing the latest design concepts. I firmly believe that the arch option is the only one that will result in a bridge [with a] design both functional and noteworthy, creating in the process a 21st-century engineering landmark worthy of this historic crossing.”
Combining Yesterday, Today and Tomorrow
When the bridge was first constructed in 1870, it was built as a 538-foot-long bridge with an arched portal and a gambrel roof. Laminated wooden trusses were later added to the inside of the bridge to increase the capacity. The 1870 bridge was lost to a flood and replaced in 1928. The 1928 structure was an eight-span, 600-foot-long bridge with four 100-foot concrete tied-arch spans and four concrete T-beam approach spans, considered one of the most advanced designs of the time.
Now, the historic structure is again incorporating some of the latest in bridge design technology. A 300-foot concrete tied-arch with New England Bulb Tee (NEBT) approach spans was chosen for the design as the preferred alternative, to minimize substructure cost while mitigating for the loss of the existing historic concrete tied-arch structure.
The concrete tied-arch for the new $21-million bridge is the first modern concrete tied-arch — to the best of all the stakeholders involved in the bridge’s knowledge – built on the East Coast in the past 50 years, says Keith Wood, P.E., senior project engineer for Kleinfelder|SEA, the consultant to MaineDOT on the project and the firm that conducted the preliminary study and the final design on the chosen bridge site.
“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 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.
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.
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.”
• Produce a design that is both contemporary and fitting for the setting and the history of the site, by seeking ways to:
° visually enhance the setting,
° reflect its history,
° maintain compatibility with the character of adjacent historic properties, and
° prudently use transportation funds.
• Erect a plaque and/or interpretive panel(s) depicting the covered bridge, its history and its significance at a location near the site of the bridge.
• Produce a model/diorama depicting the history of river crossings in Norridgewock.
• Publish an illustrated booklet documenting the transportation history of the Norridgewock area.