Precast Concrete Arches Provide Perfect Solution to George Street Bridge Challenges
By Dave Chartock – Photos courtesy Rataj-Krueger Architects, Inc.
Historic communities often face a challenge when rebuilding their aging infrastructure: mproving structural integrity without compromising the aesthetics of the historical surroundings. This was the challenge facing officials in New Brunswick, N.J. recently when planning for $12.1 million improvements to the aging George Street Bridge as part of the city’s Route 18 Improvement Project. The solution turned out to be a series of precast concrete barrel arches and headwalls that fit in perfectly with the surroundings of the
The long-span precast concrete arches also served as the solution to replacing steel, thereby providing a higher durability and lower life cycle costs for the bridge.
The entire project is part of a $200 million rehabilitation of the two-mile Route 18 corridor north of the Route 1 interchange, explained John McCleery, a project manager with the New Jersey Department of Transportation (NJDOT), the project’s owner.
Thomas G. Zink, a structural engineer with Gannett Fleming Inc., the project’s South Plainfield, N.J.-based design consultant, said the purpose of the project was to reduce the number of accidents that occur on that stretch of Route 18. This was accomplished by providing a collector-distributor road adjacent to the Route 18 northbound roadway. The collector-distributor road allows local traffic to be separated from express lane traffic, thereby easing congestion for through traffic and limiting access to the mainline roadway.
NJDOT viewed the project as both a traffic management project and as a vital part of the urban redevelopment. “They recognized that the existing roadway separated the residents of the city from the riverside parks situated between Route 18 and the Raritan River and that the proposed improvements would further add to the separation since the collector-distributor road would widen the overall footprint of the roadway,” said Zink. For this reason, NJDOT decided to take a context-sensitive design approach to the project by implementing a Community Partnering Team (CPT) to get the surrounding community involved with the project to help steer it in a manner that would be acceptable to all major stakeholders, including the City of New Brunswick, Rutgers University and nearby neighborhood associations.
The George Street Bridge
Zink pointed out that the existing George Street Bridge was a curved ramp bridge consisting of a combination of chorded steel beams and concrete slab spans that carried traffic over Route 18 and northbound into the City of New Brunswick. During preliminary design, it had been determined that the entrance to the proposed northbound collector-distributor road would have to be located near the existing George Street ramp. This meant that the existing curved bridge would have to be replaced with a T-shaped bridge that could carry the collector-distributor road above the riverfront park and provide access to and from the city with a signalized intersection on the bridge.
Zink said early designs for the new bridge included continuous steel plate girders to maximize span lengths. The community objected because it wanted the new bridge to blend in with other surrounding structures, most of which were concrete arches. This required some research on the use of long-span precise concrete arches in lieu of steel. Although the precast concrete arches would require more substructure units, it was determined that the high durability and low life cycle costs associated with this design appealed to NJDOT.
“Early designs for the precise concrete arch bridge placed the arches on top of the wall piers. This was later revised to a multi-column pier configuration to allow for the construction of a one-lane access road below and parallel to the proposed arch structure. By placing a partition on the access road under the arch bridge, the usable park space was maximized,” Zink explained.
The proposed collector-distributor road was designed with a 3,937-foot horizontal radius so the precast concrete arches could be chorded along the length of the bridge. The bridge also had to accommodate a vertical profile curve with an entrance grade of +2.6 percent and an exit grade of -4.9 percent. These geometric requirements were achieved by providing stepped pier caps that were trapezoidal.
Zink added that the channels were cast in the top of the pier caps to accept the concrete arch pieces. Each concrete pier cap is supported by three 6-foot-diameter drilled shafts, which minimized excavation. A soldier pile and lagging retaining wall located between the park access road and the Route 18 northbound roadway hides two of the three shafts from view. The exposed shafts to the east of the access road were cast square from the ground line to the pier cap to allow form liners to be used to simulate the appearance of masonry columns.
The combination of the structure’s height and weight presented another design challenge for the piers, Zink said. The arches required nearly 12,330 cubic yards of overfill material. To reduce dead load, a lightweight cellular concrete material with a density of 30 pounds-per-cubic-foot was used instead of a soil overfill weighing three to four times more. This also substantially reduced the computed lateral seismic forces.
The bridge features eight 66-foot-long arch barrels with a rise of 20 feet. These arches were designed as three-hinged arches using finite element software to compute horizontal and vertical forces in each precast arch piece. Helser Industries of Tualatin, Ore., fabricated the steel forms so Precast Systems Inc. of Allentown, N.J., could cast the 128 arch pieces. Exterior pieces included an integrally cast collar to provide edge support for the precast concrete spandrel panels.
The precast concrete portion of the bridge is 593 feet long and 60.7 feet wide. The concrete strength of the arches is 5,500 psi. In addition, a total of 128 precast spandrel panels were required, the largest of which is 26 feet high and 9 feet wide. The panels were held in place by MSE straps that were temporarily suspended in place while the concrete cellular overfill was being poured.
According to Mustafa Gok, a project manager with Conti Enterprises Inc., the project’s South Plainfield, N.J.-based general contractor, the design required erection of a minimum of four arch pieces in order to be a self-supporting and stable arch system. After several meetings with project team members, it was determined that steel towers on wheels would be used under the first two arch segments for support, and once four arch segments were erected, the system would be stable. Once stable, the towers on wheels were removed and a single crane could be used to erect the other 12 arch segments. The erection technique saved substantial time and money, enabling eight arch pieces to be erected each day.
The Reinforced Earth Company Ltd. (RECo) of Vienna, Va., was selected as the designer and supplier of the precast arch and stabilized earth components. Sherif Aziz, RECo’s Mid-Atlantic regional manager, said the George Street Bridge is unique in that the arches are the first in the world to receive lightweight concrete fill around and above them. The behavior of the arch during the backfilling operation was different from those built with conventional fill, requiring additional steel reinforcement at the crown.
In addition, Aziz said the forces in the precast pieces were computed using a specific software program and a mesh generated by a RECo in-house program. “The short-term deflection of the arch had to be calculated and monitored during erection,” said Aziz. “The accuracy in calculating the deflection resulted in a perfect fit for the spandrel panels that had to be placed with precision to have the Random Cut Stone architectural finish line up.”
Furthermore, Aziz noted that the arches sit on pier caps cast on top of drilled piers. To make sure that these piers do not spread out under the horizontal force applied by the arch at its bases, tension rods were installed at each arch span during erection. The heavy precast concrete arches, which have an end collar architectural feature produced by this system, required them to be lifted and rotated into position in one step directly from the truck bed instead of using traditional double handling of the units. This required extensive coordination between RECo and the erection teams, resulting in a two-day erection cycle for each complete barrel.
RECo contracted with Precast Systems to fabricate the 128 precast panels. “RECo supplied us with the form for the panels,” explained Robert DeMaio, plant engineer. “Chris Clements, project manager and Hieu Tran, engineer for RECo, worked closely with our plant personnel during the initial form set up and early pours.”
The Route 18 Improvement Project, which began in May 2005, is scheduled to be complete by late 2009.
David S. Chartock is a freelance writer based in New York City. He writes for both trade and national business publications. He can be reached at [email protected].