Project Overview
The 70th Avenue East Bridge project in Fife, Wash., replaced an outdated bridge that was delaying future construction on State Route 167. It was the first of six projects to support the Puget Sound Gateway Program, which will provide essential connections between the ports of Tacoma and Seattle, Wash., ensuring people and goods can move reliably through the region.
The Washington State Department of Transportation (WSDOT) chose precast concrete for this project because the material is durable and cost effective, and accelerates construction. During the design phase, WSDOT worked closely with the design-build contractors to make the final design even more efficient.
The original plan included prestressed concrete girders with intermediate piers, including one placed in the median of the heavily used Interstate 5 (I-5). However, the team at Guy F. Atkinson Construction offered an alternative technical concept that would adjust the bridge alignment and lengthen the girders to span I-5 without the need for intermediate bridge piers.
Unexpected curvature
The final design features 10 of the longest precast concrete girders ever used in bridge construction. The 222-ft-3-in.-long girders were manufactured just a few miles from the bridge site by Concrete Technology Corporation, using a high-strength, lightweight concrete mixture, which allowed the transport loads to stay within the maximum allowable vehicle vertical forces. This design choice was necessary to ensure that transportation to the bridge site would be possible on specialized variable-axle trucks.
“The choice of single long-span girders was made to meet the site’s geometric conditions and to eliminate the need for a pier in the freeway median,” explains Kevin Dusenberry, senior project manager at Jacobs. “Using WF100G girders eliminated the need for falsework of any kind over the freeway and greatly reduced the amount of freeway and lane closures required.” The elimination of a pier in the median also significantly reduced the project’s carbon footprint and its environmental impact on the adjacent wetlands.
This long-span approach saved months of construction time and eliminated the need for multiple disruptive closures and realignment of I-5 during construction. “The bridge spans five lanes in each direction and accommodates the ultimate build-out of the freeway in the future,” Dusenberry says.
Early in the project, the team had to adapt to an unexpected acceleration of the timeline to meet the fabrication schedule for the girders. “Jacobs worked closely with WSDOT for special ‘over-the-shoulder’ reviews and with Atkinson Construction and Concrete Technology to complete the girder design in record time and girder fabrication on schedule,” Dusenberry says.
In August 2020, the ten girders were hoisted across I-5 to form the foundation of the new bridge during two overnight I-5 closures. After the girders were erected the team discovered that nine of them had picked up a horizontal curvature, causing them to be out of plumb at the bearings. “The lightweight concrete girders were more flexible than anticipated, which allowed them to curve and twist more than standard-weight girders would have,” Dusenberry explains.
The girders had been braced into this configuration, and most of a false deck had been placed between the bottom flanges of the girders. To fix the problem, the contractor replaced all the bracing with adjustable braces and incrementally moved individual girders while adjusting the bracing into the final configuration. “All the girders were adjusted to be well within tolerance, and there was no damage to the girders as a result of the adjustments made,” says Dusenberry.
Most importantly, the girder adjustments were safely completed over live traffic on I-5, and no lane closures were necessary to resolve this issue. WSDOT has since made a change to Washington State’s standard girder plans to implement a 5-ft-1-in.- wide top flange to offset the flexibility issue on future projects using lightweight concrete. |