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Girder Bridge Replaces Aging Maine Swing Span

Land view rendering of the new Richmond-Dresden Bridge
The new Richmond-Dresden Bridge will replace an existing swing-span bridge across the Kennebec River, in Maine. The new bridge will have an under clearance of 75 ft to allow even the U.S. Coast Guard's largest ice-breaking vessels to pass beneath it. Maine DOT

The Maine Department of Transportation will build a bridge across the Kennebec River offering 75 ft of clearance above the navigation channel to replace a dysfunctional movable bridge.

August 6, 2013—Although the bridge type is not unique, a new girder bridge across the Kennebec River in southern Maine will be notable for the host of issues it will resolve when it replaces the site’s existing crossing. Of prime importance, the new bridge will provide enough clearance for large river vessels without the need for a movable span, and it will be wide enough to truly accommodate two lanes of traffic, in contrast to the narrow, dodgy bridge that exists today.

The existing Richmond–Dresden Bridge (also known as the Maine Kennebec Bridge) is a movable truss bridge with two lanes that carries Route 197 between the towns of Richmond and Dresden. Constructed in 1931, the 20 ft wide bridge features a swing span to accommodate river traffic, including the large icebreakers that the U.S. Coast Guard uses to maintain navigation on the river in the spring. At 82 years old, the bridge has has been a valuable asset, but its obsolete design is presenting several safety and maintenance issues that make its replacement a priority for the Maine Department of Transportation (MaineDOT). “We’ve been studying this bridge for a long time,” says Nate Benoit, P.E., the project manager for the MaineDOT. “It’s an old bridge in poor condition.”

Among other liabilities, the existing bridge features a fracture-critical design and its swing span requires a great deal of maintenance. Operated by a bridge tender, the span recently became stuck in the open position for approximately 30 minutes when the bridge’s steel expanded in response to rising temperatures. A section of the bridge had to be removed for the span to close, Benoit says. Another issue with the bridge is that its narrow deck is made even narrower by the diagonal braces of the truss, and because of this larger trucks are forced into the middle of the bridge. “A box truck can’t drive on one side because it will hit these diagonal braces,” Benoit explains. “So if anyone wants to pass a box truck, usually the truck has to stop, and the vehicle coming the other way has to stop, and they’ve got to find a way to work around each other.”

While the existing crossing is not ideal, the MaineDOT determined in a 2006 feasibility study that the bridge could remain in operation during the construction of its replacement, an important factor given that the closest river crossing is at least 12 mi away. The new bridge is being constructed upstream of the existing one. Aligned on a skew, the new bridge will measure 44 ft from the centerline of the existing bridge at the latter’s western end and 197 ft from the centerline at the eastern end. “It was determined that the existing bridge had about 10 years’ worth of life left in it,” Benoit says. “So we went with the game plan that we could use the remaining life of it, including using it to carry traffic while we build the new bridge upstream, which is going to take about two years.”

The new bridge will be 1,478 ft long and 32 ft wide. Its two 11 ft wide travel lanes and two 5 ft wide shoulders will provide plenty of room for all types of vehicles to traverse the structure without issue. But perhaps the bridge’s most noteworthy feature will be its height. With a 6 percent grade from both ends, the bridge will offer 75 ft of clearance over the river’s navigation channel—enough room for even the Coast Guard’s largest (73 ft tall) icebreakers to pass beneath it. While a 6 percent grade on a bridge is significant, other portions of Route 197 have grades of as much as 7 percent, so the slopes will be compatible with the existing characteristics of the road, Benoit says. “Our biggest challenge was accommodating that 75-foot underclearance,” he says. “The bridge will be built onto a crest curve to maximize the clearance at the middle of the navigational channel.”

The seven-span bridge will have two concrete cantilevered abutments and six concrete hammerhead piers. One abutment will be founded on piles, and the other will be founded directly on bedrock. All but one of the piers will be founded directly on bedrock at depths of as much as 40 ft; the remaining pier will be founded on piles as a result of the soil conditions at that location. All of the piers will be in the river, their foundations to be constructed in the dry within sheet-pile cofferdams. Finger joints will be included at the westernmost pier and the eastern abutment, and the remaining piers and the other abutment will be fixed to effectively handle live loads on the structure. “We designed for ice, seismic, and wind loading, as well as the thermal movement as [the bridge] expands and contracts with temperature,” Benoit says. “Sharing those loads over three piers was certainly more efficient than having just one fixed pier.”

The bridge’s 86 in. deep welded plate girders of weathering steel will support a cast-in-place concrete deck. To maximize its life, the deck will be embedded with doubly coated reinforcing steel and will have an unusually thick (2 ½ in.) concrete cover. “Then on top of that, we have a high-performance membrane and then pavement,” Benoit says. “We wanted to have a bridge deck that could last 75 years.” Another unusual feature of the bridge will be a manhole located in the center of the deck that will open into a utility vault within the bridge. Several utility companies will have lines running through the bridge, and since the structure will be so long, the companies need to have a manhole in the middle through which to pull wires, Benoit says.

Another consideration of the project is a site on the western side of the river that was once home to Fort Richmond, which was constructed in 1721 and decommissioned in 1755. A fill slope will have to be formed over the historically important site to support a new approach road leading to the bridge on that side of the river. But before the fill could be placed, the Maine Historic Preservation Commission required the MaineDOT to fund an archaeological dig at the site. That work is now under way, but it is not delaying the bridge’s construction because it is not on the critical path. “The contractor is building the in-water piers first,” Benoit explains.

While many departments of transportation contract with an engineering firm to design projects of this size, the MaineDOT took the unusual step of designing this $18.6-million project in-house. “This was a pretty good achievement for the department,” Benoit says. “We mobilized our entire bridge program to do this project, and we gave all of our assistant engineers the opportunity to work on it.” The department hired Ed Caswell, P.E., M.ASCE, the principal and president of Caswell Engineering, P.A., an engineering firm based in Topsham, Maine, to serve as the senior designer on the project and to review all aspects of the department’s designs. The project is being funded by both a federal Transportation Investment Generating Economic Recovery grant and state dollars. Reed & Reed, Inc., a construction services firm based in Woolwich, Maine, is the contractor on the project.

Construction of the new bridge began July 1, and the contract requires that the structure be completed and the old bridge be demolished by December 31, 2015. Once finished, the project will provide the communities with a bridge they can rely on for decades to come. “The bridge will no longer have to open, people will no longer have to wait for boats to go through the navigational channel, and there will be no long-term maintenance associated with maintaining a swing span,” Benoit says. “It should be a relatively maintenance-free bridge with a 75- to 100-year design life.”



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