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When the going got tough

Turner Center Bridge nears completion

By Thomas Densford, P.E.

Bridge replacements have occurred throughout time for many reasons, but a unique motive to replace the bridge between the communities of Turner and Greene may hold the title of simplest and most succinct: the 90-year-old span was too narrow for two buses to travel on it in opposite directions. In “bridgespeak,” the structure was functionally obsolete.
 
The design-build project, which is nearing completion, was notable for its use of a haunched steel girder design. The project is also one of the first design-build bridge replacements commissioned by the Maine Department of Transportation.
 
In early 2012, the Maine DOT released a Request for Interest looking for a design-build team to replace the 400-foot-long Turner Center Bridge over the Androscoggin River. The job went to local Woolwich-based contractor Reed & Reed, with Fay, Spofford & Thorndike as lead designer.
 
In August 2014, the communities celebrated the official opening of the new bridge with a ribbon-cutting ceremony. Officials and residents from the neighboring towns, MaineDOT, and the design/construction team gathered to stretch a red ribbon across the bridge. The ribbon was cut by construction Superintendent Chris Whittemore’s son, Ethan, and a small parade crossed the new bridge. Gill Busby and Mary Charbonneau, who live next to the bridge, were the ceremonial first drivers across.
 
The new bridge design uses two-span, horizontally curved haunched steel girders with a total length of 480 feet. The haunched girder design, coupled with the vertical curve on the bridge, minimized the extent of the approach work and reduced project costs while satisfying the clearance above the design flood level. The substructure was designed for ice loading due to the ice flows that are common in the river at this location. The foundations consist of spread footings on tremie seals that extend to bedrock and required cofferdams up to 35 feet in depth.
 
An additional challenge was the need to keep traffic flowing throughout construction: the closest crossing is 20 miles away, so a detour was not possible. To meet schedule, the team prepared plans in stages and obtained approval for the early release of individual bridge components for construction. This approach, coupled with a fast track design, allowed the contractor to begin work on critical path bridge components and enabled the work to be sequenced around the in-water work windows.
 
The new bridge ties into the approach roadway near the existing abutments, providing the necessary river waterway opening. It was also essential that the abutment and approach design satisfy the environmental commitments regarding minimal disturbance of the protected riparian areas in the river. The design minimized right-of-way impacts and grade changes at the approaches while satisfying abutter concerns about drainage.
 
Three-dimensional (3D) modeling was essential to the design process, and required by the bridge design code for a large curved bridge. Modelers developed nine bridge and approach alignment alternatives. The team then used 3D modeling software to evaluate cost, clearance, constructability, and environmental impact. In this way, the FST/Reed & Reed team efficiently evaluated and selected the preferred alternative. The chosen alternative was further developed using approximate methods to optimize the girder flanges, with final verification using 3D finite element modeling. The approach used industry-standard software intended for straight girder bridge design, modified to account for curvature effects. The novel design approach and girder optimization was presented by FST at the 2014 World Bridge Symposium in Toronto.
 
The new bridge is in use. The existing steel trusses have been removed, and the final phase of the wing wall construction is underway. The project is to be completed in December 2014, and is already a nice addition to a scenic river crossing. n
FMI: For more information about the project, go to the “News & Events” tab at www.fstinc.com. Tom Densford, senior principal engineer at Fay, Spofford & Thorndike, may be reached at tdensford@fstinc.com.

 

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