By Thomas G. Zink, P.E. | Photos courtesy of Gannett Fleming Inc. (www.gannettfleming.com)
To appreciate precast concrete’s contribution to the George Street Bridge project in historic New Brunswick, N.J., one look at the outstanding architecture of the finished structure will suffice. A completed project photo shows how precast concrete design plays a big role in delivering classic architecture and community connection to a riverfront park.
The main goals of the New Jersey Department of Transportation (NJDOT) improvements to Route 18 were to facilitate the flow of traffic, reduce congestion (85,000 vehicles per day), and improve road safety. But in addition to serving its primary transportation functions, the new George Street Bridge goes beyond the call of duty in contributing to the quality of life in the city.
Using the new bridge to connect the city with its waterfront in the most acceptable way possible to the Community Partnering Team (CPT) was part of NJDOT’s Context Sensitive Solutions Initiative. The CPT was comprised of city officials, Rutgers University, Johnson & Johnson and nearby neighborhood associations, all of who wanted to see a pleasing pedestrian link to the underutilized parklands along the Raritan River.
Both steel and precast designs were evaluated
During the design process, both steel and concrete bridge solutions were evaluated. Community feedback indicated a strong desire for the new bridge to blend in with the vintage architecture of the surrounding structures, many of which were concrete and masonry arch bridges. With CPT guidance, design consultant Gannett Fleming worked with NJDOT to evaluate alternatives that would be architecturally compatible.
Ultimately, precast concrete arches were selected for their durability, reduced life-cycle costs and architectural aesthetics. The single span over Route 18, however, 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 used for vehicular flow, 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 loading, 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 typical 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 to combine precast concrete arches with a lightweight cellular concrete overfill.
Precast barrel arch construction method
The Conti Group, general contractor for the project, erected the precast arches for George Street Bridge in June 2008. 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. Helser Industries of Tualatin, Ore., fabricated the steel forms so that Precast Systems Inc. of Allentown, Pa., could cast the pieces.
To expedite placement using one crane, the arch pieces were 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 reused at the next span, allowing Conti to complete one arch barrel in two days.
Steel tie rods installed between adjacent pier caps stabilized them against excessive deflections during construction and ensured a proper fit for the precast spandrel panels 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.
Precast MSE retaining wall panels were used in all four approaches and 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 the city’s 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 were fully detailed in the bid documents as CIP concrete elements. However, by utilizing adjustable forms, Conti achieved all desired aesthetics and met profile requirements using precast 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 technique to be more cost effective than CIP construction.
Full-scale mock-ups were required for various structural and architectural components, including form-lined MSE panels. 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. (see Precast Solutions magazine, Fall 2011, for more details).
A level playing field: comparing steel and precast solutions
The total cost of the George Street Bridge was $11.6 million and includes $9.6 million to construct the 36,000 sq ft of precast concrete arch structure over the waterfront park; $2 million was spent to construct the 7,860 sq ft of steel girder/concrete deck structure over Route 18. This equates to a unit price of $266 per sq ft for the precast arch bridge and $261 per sq ft for the steel span.
This finding is particularly significant, as both the steel and the concrete portions of the bridge were built at the same time, by the same contractor and on the same site, and utilized the same architectural surface treatments. These similarities provided a unique opportunity to evaluate the cost of providing a “non-conventional” structure type to that of a more traditional bridge 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 requiring 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 precast 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.
Thomas G. Zink, P.E., is vice president of Gannett Fleming Inc.