This charming, masonry arch bridge in Tamaqua, Pa., was identified for a major rehab project due mainly to the condition of the existing sidewalks. Although the original stone masonry core was found to be worthy of rehabilitation, the retrofitted sidewalk structures were beyond repair, according to inspectors. It was also determined that the original stone arch core was a contributing element to both national and local historic districts. This designation necessitated the involvement of consulting parties in the project design, including the local Historical and Architectural Review Commission (HARC), which would oversee the design concept and provide input.
That meant the project team would have to figure out a way to replace the existing dilapidated sidewalk structures with a new system that was worthy to stand next to a thoroughly restored stone masonry arch from the 1890s, says Paul J. Rea, senior associate with Ammann & Whitney, the engineer for the project. This is why his team went with a precast concrete solution. “The design flexibility provided by precast concrete allowed us to develop a system that was fully functional and simple to construct, yet aesthetically pleasing and complimentary to the historic stone masonry arch.”
The structural system included prestressed, haunched concrete beams as the main focal point of the proposed new sidewalk structure. Arched variable depth shapes allow for a long-span look that complements the adjacent stone arches while exposing as much of the historic arch as possible. Earthy tones were chosen for the precast concrete pieces to blend naturally with original stone masonry.
The use of precast concrete also enabled the structure to achieve continuous action for live loads and superimposed dead loads while allowing for shorter beams to facilitate delivery and erection in a tight urban setting, he says. “The ability to customize precast concrete to certain project specific requirements allowed us to tailor a special solution to address all of the aesthetic demands and geometric complexities inherent to this out-of-the-box project.”
One of the most innovative features of the project was a scheme to allow the prestressed concrete beams to pass over the dry stack retaining walls and be supported on a solid, integral abutment while creating a faux concrete seat and end diaphragm at the dry stack wall to give the appearance that the beams were supported there and a part of the wall all along. “The members of the HARC and the Museum Commission really loved this approach since it looks historic while meeting current design standards,” he says.
All of the stakeholders on this project are pleased with how it turned out. “We set out to develop a restoration design that would rehabilitate the stone masonry arches while replacing the sidewalks with a new system that complemented and honored the bridge’s historic arch core,” he says. “With the use of precast concrete as part of the solution, I believe that we accomplished that goal.”