Project Overview
Carrying more than 100,00 vehicles per day, the Interstate 5 (I-5) bridge over SW 26th Avenue in southwest Portland, Ore., is an important thoroughfare for local travelers. When it became clear that the original six-lane bridge, constructed in 1959, had experienced significant deterioration, the Oregon Department of Transportation (ODOT) concluded that the structure had to be replaced. The bridge was also not built for seismic loading, generating additional safety concerns for users. Together with project engineer David Evans and Associates Inc. and PCI-certified precast concrete producer Knife River, ODOT designed a unique bridge replacement solution leveraging accelerated bridge construction techniques and 28 precast and prestressed concrete slabs to achieve the project goals.
56 Hours, One New Bridge
Given the large volume of daily traffic that navigates through the area, ODOT officials decided to keep all six lanes open during construction. This decision, however, meant a limited number of options were available for design. Traditional construction methods would likely require more than two years of staged traffic on I-5, undesirable temporary alignment geometry to stay within the right of way, and tightly constrained work zones.
To identify the most appropriate solution for the work, the ODOT team conducted a value engineering study and selected a design that would reduce the project’s impact on I-5 traffic to a single, 56-hour-long weekend closure. The concept involved the construction of a new, single-span bridge in the available space under the center span of the existing bridge. During the weekend closure, the existing bridge’s superstructure would be demolished, the roadway pavement above the new bridge would be installed, and traffic striping would be placed. This approach had the potential to generate many benefits for the project, but it also posed several challenges.
According to Joel Tubbs, PE, SE, Portland bridge practice lead in the Transportation Business Unit at David Evans and Associates Inc., one of the major design challenges was to select a structure that could meet the project’s horizontal and vertical clearance requirements both before and after construction. “Precast, prestressed concrete slabs were instrumental in addressing this challenge due to the structural efficiency of the section, and the fact that the prestressed slabs were of a constant depth for the full span length,” Tubbs said. “These characteristics maximized the clearances’ full width between abutments, minimized the structure depth, and provided for tremendous flexibility in accommodating local traffic and construction access.”
The team also had to contend with the horizontal and vertical constraints generated by the presence of the existing bridge carrying live traffic overhead. Without having access from above, the team could not use traditional methods of material delivery, placement, quality control, and more. Fortunately, the use of precast concrete slabs dramatically improved constructability, performance, and quality.
Furthermore, the use of precast concrete allowed the team to take advantage of accelerated bridge construction methods while preserving the existing bridge throughout most of the construction. The prestressed slabs were set on temporary supports adjacent to the existing bridge in three groups of 8 to 10 slabs each, with all slabs in each group tied together. Adjacent slab groups were also tied to one another. Each slab group was laterally pulled under the existing bridge so that the next group of slabs could be placed on temporary supports. The final lateral translation pulled all 28 slabs the rest of the way to their final position under the existing bridge.
“The completion of this project benefits the community and the traveling public in so many ways,” Tubbs said. “Beyond the time and cost savings afforded by precast concrete, we now have a new structure in place that’s designed to current static and seismic bridge standards while also providing for a smoother ride through this corridor in the future.”
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry since 2013. |