Frederick Avenue Bridge
Designers for the Frederick Avenue Bridge in Baltimore faced a challenge becoming more common today: The existing two-span, closed-spandrel wall-arch structure, built about 1911, needed replacement, and the community and Maryland Historic Trust wanted the new bridge to closely resemble the original design. To satisfy this request, designers used modern-day techniques with precast concrete forming to replicate the shape of the original arched design.
“Rather than design and construct another arch, it was decided to use prestressed concrete beams and provide an arch façade to give the appearance of the existing arch,” explains Mike Izzo, bridge department manager for Whitney Bailey Cox & Magnani LLC (WBCM). Historic concrete railings also were included in the $14-million project.
Precast concrete was chosen primarily to avoid having to shore the formwork needed to construct the arch, as the bridge crosses both an environmentally sensitive area, the Gwynns Falls, and tracks for the CSX Railroad. “Attempting to construct shoring over the CSX span would have been extremely difficult, if not impossible,” he says. Even so, designers had to create a system to span the tracks with the precast concrete components without shutting down service for an extensive period.
The two-span bridge features sixteen 112’1”-long precast concrete bulb-tee girders (63 inches deep and 28 inches wide) and 12 arched ribs, with three pieces comprising each of the four arches. The ribs consist of precast concrete box beams tapered from one end to the other, with tapered voids inside and large galvanized-steel connection plates at each end.
“The arches were designed to be larger at the springline (end) since the member sees more load at this location,” explains Izzo. The primary loads taken by the arch are the vertical dead load of its own weight and the arch wall, which increases closer to the springline. The arches are not designed to support vehicular live loads as they are a façade only. As the dead load increases approaching the springline, the axial compressive load also increases.
To fabricate the arch pieces, the precaster cast them in an upright position due to the epoxy-coated rebar that had to project from the top of each piece, explains Dan Cuccurullo, draftsman for NPP. This proved challenging, because not only are the bottoms of the pieces arched, but so are the tops. Each epoxy-coated rebar stirrup was bent to a different shape to accommodate the taper in each piece. “There was a large amount of rebar, as the pieces were not prestressed.” The plate connections, weighing between 700 and 3,300 pounds apiece, were cast into the beams.
The arches were shipped on flatbed trailers in an upright position, as they could not be laid flat at their fabricated size with the rebar protruding from the end. Special mounting blocks were fastened to the trailers to secure the pieces during transit.
Another challenge came at the site, which was extremely tight, with limited access, explains Ryan Surrena, senior project manager at Joseph B. Fay Co., the general contractor. Fay designed and constructed a temporary causeway over the Gwynns Falls to gain access to Span 2, over the CSX Railroad tracks, and to create additional room so the precast concrete arch ribs could be erected. An approximate 20- by 20-foot area was required to safely install the foundation for the shoring towers needed to erect the arches.
The restrictions also meant a smaller crane couldn’t move around the project with ease, so a Manitowoc 4100 crane was set up in two stationary positions, one in each span, for the arch erection. Meetings with CSX Railroad at the beginning of the planning process ensured the track closure lasted less than four hours, during which the center portions of the arch ribs were connected.
Each connection for the arches had to be field-drilled, with the bigger connections requiring 60 holes while the smaller ones required 24, says Cuccurullo. After the pieces were connected, Fay’s crews formed and made closure pours to make the three arch pieces appear to be monolithic.
The project is on schedule to be completed in January 2015. “Overall, the community has been pleased with the look of the new bridge mimicking the old one,” says Surrena.
All the precast components were manufactured by Northeast Prestressed Products LLC, Cressona, Pennsylvania.
Structural Precast Elements:
• Twelve arch ribs pieces (three pieces per arch, weighing 18 tons each).