The Northwest Corridor Bridge project in Marietta, Ga., was in the planning stage for more than 15 years. The goal of the project, which was finally completed in 2018, was to add a managed-lane solution that would relieve congestion in this fast-growing metropolitan area and increase traffic capacity along a 29.7-mile corridor.
When the project was awarded in 2013, it was the first design-build-finance project procured by the Georgia Department of Transportation (GDOT), says Alan Kite, structure design lead. The project was led by a construction joint venture that financed up to 10% of the project’s cost at GDOT’s discretion.
Along with the budgetary issues, the job site faced many logistical obstacles, including horizontal and vertical restrictions, the need to protect sensitive natural environments, and a constant flow of traffic on the southbound interstate. “Minimizing impacts to the adjacent wetlands, utilities, and traffic while constructing piers was a major challenge on this project,” Kite says.
The design had to overcome these obstacles, while still providing an economical and practical construction approach, says Bill Little, design manager for Parsons, the engineer of record for the project. “After collaboration between the design team, fabricator, and erector to address potential transportation, stability, access, and erection issues, we determined that using prestressed concrete girders provided the optimal solution.”
Precast concrete girders were selected because of their high durability, low maintenance requirements, excellent quality, and low cost compared to other construction materials, Little says. “In addition, the prestressed concrete girders provided better transportation options, lead times, and lower procurement costs compared to a deep steel plate girder option for these spans.”
One project, 30 bridges
The project scope included 30 bridges, all constructed with precast concrete beams. The mainline viaduct bridges consist of prestressed concrete bulb-tee beams supported on single-column hammerhead piers, with bridge lengths ranging from 1250 to 5981 ft to span roads and railroads, highway on- and off-ramps, streams, and wetlands.
Prestressed concrete beams were used throughout the project, including bulb-tee or AASHTO beams on all secondary bridges. “Using precast concrete elements for pier construction allowed for a faster construction process, with most bridges going up over the course of a single night with only partial lane closures,” says Timothy M. Williams, chief engineer with Archer-Western Contractors.
The successful delivery of this project demonstrates the benefits of a design-build delivery method that could transform the way Georgia approaches bridge project planning in the future. “Parsons, GDOT, and several other state and private agencies are working together as a team to ensure that the quality, schedule, and financial performance of this project set the bar for future design-build mega-projects both in this state and around the country,” Kite says. “This project will improve safety and mobility for the entire interstate corridor.”