The city of Bellevue is making significant investments in public transportation to make it easier for citizens to traverse the region. That includes extending its local light-rail system to connect two major metropolitan city centers. This process included the design-build of the E360 guideway bridge, a 0.86-mile structure adjacent to the highly-traveled Washington State Department of Transportation State Route 520.
“With a design-build delivery, construction cost and schedule are always top priority, or else we would not have won the original bid,” says Catherine Hovell, of T. Y. Lin International. Her team’s winning design also had to ensure the design was compatible with future expansion goals to accommodate the adjacent roadways.
They chose a precast concrete design early in the process to address these constraints. “Using precast simplified designs sped up project delivery, increased quality of materials, and reduced the impact of construction on the neighborhood,” she says. “For a project like this, located next to a major freeway and employment hub, these benefits were significant towards positive public opinions on the project.”
The final design features 31 spans ranging from 125 to 182 ft, and four parallel wide-flange girders, ranging in depth from 74 to 100 in. Typical spans are 95-in. girders spanning 140 ft.
During the design phase, the plan was optimized to minimize the number of foundations required, which allowed the design team to eliminate five 9-ft shafts from the original design. A 65-ft post-tensioned straddle bent was also eliminated by using the two 182-ft spans made continuous over a fixed pier. “This adjustment opened space under the guideway for a new highway off-ramp to be constructed in the future,” Hovell says.
Of the 128 prestressed concrete girders fabricated, 80 were the same size and length, with only minor changes in prestressing strand design, which helped accelerate delivery and minimize trades on-site. Consistent girder and pier designs were used to increase fabrication quality and further expedite construction.
That speed, quality, and efficiency was seen as a major benefit to the project, according to Eza Agoes, deputy project director for Downtown Redmond Link Extension (R200) . “E360 ’s use of precast bridge concrete panels eliminated the use of time-consuming, labor-intensive formwork that would otherwise be needed to deliver a cast-in-place concrete deck for the bridge superstructure construction,” Agoes says. “Additionally, the precast concrete panels were placed faster on-site, minimizing the number of traffic closures needed for the bridge deck construction.”
To further increase construction speed, precast, prestressed concrete stay-in-place deck forms were used on the guideway structure. The structure was laid out in a manner that will allow construction of a future slip ramp between the columns. “The construction speed and minimal disruption would not have been possible without the use of long-span prestressed concrete girders,” Hovell says.
Using precast concrete also created a safer jobsite, Agoes adds. “It allowed considerable work to be done off-site, reducing safety exposures to construction workers and the public around the construction site.”