Project Overview
The Washington State Department of Transportation’s (WSDOT’s) vision for the SR 520 Montlake project near Seattle, Wash., was ambitious: reconnect the communities divided by the freeway, construct a safer and seismically sufficient eastbound approach to the new floating bridge, improve regional mobility, ensure vehicle and pedestrian safety, and enhance pedestrian mobility by connecting local trails and paths. Such a massive project required not only extensive coordination and logistical planning, but also the right building material to bring everything together. WSDOT, in partnership with Concrete Technology Corporation and engineering firm TYLin, adopted a wide variety of precast concrete products for the effort. The result was a three-acre landscaped lid structure, a regional shared-use path tunnel, a signature pedestrian land bridge, and a 1.1-mile, 40-span, base-isolated approach bridge.
Bridging Connections
Precast concrete was incorporated into the project from the conceptual stage, leading to significant cost and schedule improvements through accelerated bridge construction (ABC) techniques. The project includes four separate precast, prestressed concrete girder bridges, a pedestrian tunnel supported by precast concrete slab girders, and miles of prestressed concrete girders. Precast concrete columns and diaphragms were innovatively used for each pier of the 1.1-mile, 40-span approach bridge. Collectively, the SR 520 Montlake project deployed numerous precast concrete components: 513 girders, 80 columns, 39 diaphragms, and extensive stretches of traffic barriers.
The columns and diaphragms used on the project were particularly important because of the significant advantages they brought to the construction of the approach bridge. Complex geometry on the bridge’s approach columns, combined with the project’s strict aesthetic requirements, led to a flared column design with a smaller diameter at the base and a larger diameter at the top. Precasting the columns allowed for improved quality control while also eliminating the complex formwork efforts that cast-in-place concrete construction would have required. This shaved considerable time off the schedule, delivering crucial cost savings for the nearly $500 million project.
The lower portions of the approach bridge diaphragms were also precast and prestressed for ease of construction and to reduce costs. Because the approach bridge is base-isolated, the diaphragms were set on triple-friction pendulum bearings atop the columns. The isolation bearings were temporarily locked and stabilized during construction. Using the precast concrete diaphragms allowed the girders to be erected and the upper, cast-in-place portions of the diaphragm to be poured without temporary falsework. This approach resulted in further formwork savings and minimized both in- and out-of-water work. Beyond these advantages, the decision to use precast concrete was heavily influenced by the sustainability goals of the project team.
“Using various precast components throughout the project reduced the overall quantity of concrete needed, thus also limiting the project’s carbon footprint,” says Eric Bowles, sales manager for bridges with Concrete Technology Corporation. “Traditional formwork tolerances often result in thicker-than-required concrete members. But with tighter tolerances, the opportunity for chemicals and construction materials to leak from formwork is decreased, resulting in less waste and minimized environmental impact.”
Additionally, because the approach bridge was constructed using ABC techniques, work around the water was limited. This reduced impacts to the lake by preserving fish habitats and important natural resources. Precast concrete’s resilience and expected long service life also mean that travelers in the region can expect the SR 520 Montlake corridor to serve them well for years to come—all with the added benefit of structures that were designed, manufactured, and installed to withstand the harsh environmental conditions of the area.
“This project increases the capacity of the SR 520 east/west corridor over Lake Washington, improving the connection between Interstate 5 and Interstate 405,” Bowles says. “Using precast concrete resulted in reduced construction time with the promise of low maintenance costs and a long service life.”
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry since 2013. |