By Thomas G. Zink, P.E.
Photos Courtesy of Gannett Fleming Inc. (www.gannettfleming.com)
Modern materials and precast concrete components have been blended with turn-of-the-century architecture to create a truly unique structure recently constructed in central New Jersey.
The George Street Bridge is one of several structures included in the New Jersey Department of Transportation’s (NJDOT) Route 18 Improvement Project in the City of New Brunswick. The project consists of the construction of a collector/distributor roadway adjacent to the existing mainline to separate local traffic from through traffic in an effort to reduce congestion and improve safety within the corridor. However, the vision for this project extended well beyond conventional highway considerations as the NJDOT recognized the role this project would play in enhancing the urban redevelopment currently taking place in the project vicinity.
Connecting a city to its waterfront
Supporting 85,000 vehicles per day, the existing Route 18 roadway separated the historic city from the underutilized parklands situated along the Raritan River. Working with its design consultant, Gannett Fleming Inc., NJDOT sought to provide the missing connectivity between the city and its waterfront properties. As part of the Context Sensitive Solutions initiative, NJDOT created a Community Partnering Team (CPT) that would help steer the project in a manner acceptable to all major stakeholders including city officials, Rutgers University, Johnson & Johnson and nearby neighborhood associations.
At the southern end of the project, the George Street Bridge serves as the entrance to the new collector/distributor roadway, carrying local traffic over a portion of the riverfront parkland. At its crest, a signalized T-intersection permits local traffic and pedestrians to safely cross above the express lanes. The resulting T-shaped bridge measures approximately 600-ft long by 60-ft wide for the main portion located above the park. A single 135-ft-long span forms the portion over the mainline.
During the design process, both steel and concrete alternatives were evaluated for the new structure. However, feedback indicated a strong desire from the community for the new bridge to blend in with the architecture of the surrounding structures, many of which were vintage concrete and masonry arch bridges. With CPT guidance, Gannett Fleming worked with NJDOT to evaluate alternatives that would be visually compatible with nearby architecture.
Concepts included conventionally constructed stringer configurations with ornamental “faux-arch” appurtenances, haunched steel plate girders and long-span precast concrete arches. Ultimately, the precast concrete arches were selected for their durability, reduced life-cycle costs and aesthetic compatibility with the surrounding structures. However, the single span over Route 18 required a steel multi-stringer configuration due to limited available structure depth and the need to flare the beams at the intersection.
A first: precast arches and lightweight cellular overfill
The precast concrete arches are supported by multi-column piers, with each pier consisting of three 6-ft-diameter drilled shafts with a cast-in-place (CIP) concrete cap beam. A multi-column configuration allowed the space below the bridge to be utilized for vehicular circulation, thereby maximizing useable park space. However, this presented a unique challenge to the design team, as the arches required about 12,330 cu yd of overfill material, and supporting such mass on relatively slender columns created difficult loading conditions, particularly with respect to seismic forces.
To reduce the structure’s mass, an engineered flowable fill consisting of a lightweight cellular concrete mix with a density of 30 lb/cu ft was specified for the overfill material. This significantly reduced dead load and seismic forces on the piers when compared with the use of a more traditional soil overfill weighing three to four times as much. The George Street Bridge is unique in that it was the first structure of its kind, combining precast concrete arches with a lightweight cellular concrete overfill.
Precast barrel arch construction method
The project was awarded to The Conti Group of Edison, N.J., in 2005. As one of the last elements constructed under this contract, the erection of the precast concrete arch components at the George Street Bridge did not commence until June 2008. Conti constructed the eight precast concrete barrel arches using the TechSpan system furnished by The Reinforced Earth Company of Vienna, Va. Each arch spans 66 ft and has a vertical rise of 20 ft.
A twin-leaf configuration minimized shipping dimensions and pick weights. Each barrel arch required 16 precast pieces comprised of 5,500-psi concrete. At the crown of the arch, the precast pieces were mated using a CIP concrete closure pour in a preformed keyway. The TechSpan components were designed as three-hinged arches using finite element software to compute horizontal and vertical forces in each piece. Helser Industries of Tualatin, Ore., fabricated the steel forms so that Precast Systems Inc. of Allentown, Pa., could cast the pieces.
To expedite their placement and to eliminate the need for a second crane, each arch piece was lifted from the delivery truck and rotated into position in a single step. Adjustable-height hydraulic shoring towers provided temporary support at mid-span until the first four pieces were placed. Once the pieces were self-supporting and stable, the shoring towers were removed and relocated for reuse at the next span. Work continued on each barrel until all 16 arch pieces were installed. This placement method allowed Conti to complete arch barrel in two days.
Steel tie rods were installed between adjacent pier caps to stabilize them against excessive deflections due to unbalanced thrust loads developed during construction. This was done to ensure a proper fit for the precast concrete spandrel panels that were placed on the arches to contain the backfill material. The spandrel panels were cast with a cut-stone form-liner finish and were held in place with metal straps similar to those used on mechanically stabilized earth (MSE) walls. The project specifications provided limitations on the differential height of the overfill material between adjacent arch valleys to minimize differential thrust forces.
Precast concrete was also incorporated into other components of the structure. MSE retaining wall panels were used in all four approaches. Similar to the spandrel panels, the MSE panels were cast with form-liner finishes. Edge chamfers were eliminated from the precast panels to minimize the visual appearance of the panel joints, which further enhanced the faux-stone textures that replicate traditional masonry. Custom spray-on color, based on mock-up approval, was applied to the precast stone finish to give the George Street Bridge its authentic “turn-of-the-century” architectural stature.
Precast replaces CIP parapets
Conti opted to precast the bridge parapets as well. The parapets contained numerous architectural details and were fully detailed in the bid documents as CIP concrete elements. However, by utilizing adjustable forms in the precasting yard, Conti was able to achieve all desired aesthetics and met profile requirements using precast concrete parapet modules. Even though the bridge was constructed on a crest vertical curve, the parapet modules were fabricated and placed so that all pilasters and simulated balustrade patterns were plumb in their final condition. Although such details were not conducive to standardization and numerous parapet module variations were required, Conti found this precasting method to be more cost effective than CIP construction.
The bid documents required the contractor to construct full-scale mock-ups for various structural and architectural components, including formlined MSE panels, simulated balustrades, metal fencing and sidewalk treatments. The mock-ups helped the contractor identify fit-up and alignment issues and find ways to adjust construction techniques to improve quality while expediting construction. Final production work was not permitted until the mock-up panels were approved. After approval, the mock-ups served as a performance standard for all production work.
A level playing field: comparing steel and precast solutions
The total cost of the George Street Bridge was $11.6 million. This includes $9.6 million to construct the 36,000 sq ft of precast concrete arch structure over the waterfront park and $2 million to construct the 7,860 sq ft of steel girder/concrete deck structure over Route 18. This equates to a unit price of $266/sq ft for the arch bridge and $261/sq ft for the steel span.
This finding is of particular significance as both the steel and the concrete portions of the bridge were built at the same time, by the same contractor, on the same site and utilized the same architectural surface treatments. These similarities provided a unique opportunity to validly evaluate the cost of providing a “non-conventional” structure type to that of a more traditional design. On a square-foot basis, the precast concrete arch portion of the bridge was just under 2% more expensive to construct than the steel portion of the structure. However, without need for a paint system, bearings or joints to maintain, the life cycle cost of the precast concrete arch bridge is substantially less than a steel bridge alternative. The durability of the precast solution and its substantially lower life-cycle costs made this a very attractive solution for a very modest increase in initial cost.
A unique application, the use of long-span precast concrete arches with a lightweight cellular concrete overfill represents an engineering innovation that proved economical and exceeded the expectations of the client and the community. In addition, this structure was awarded the Eugene C. Figg Jr. Medal for Signature Structures at the 2010 International Bridge Conference for a “single recent outstanding achievement in bridge engineering that through vision and innovation provides an icon to the community for which it was designed.”
Thomas G. Zink, P.E., is a vice president of Gannett Fleming Inc. and serves as regional bridge practice manager in the northeast as well as the manager of the transportation division in Mount Laurel, N.J. With 20 years of experience in bridge design and rehabilitation projects, Zink was the lead structural engineer for NJDOT’s Route 18 Reconstruction Project.