First Look at WSLE’s High Bridge

Sound Transit's rendering of the proposed West Seattle Link Extension bridge over the Duwamish Waterway.

On July 18, the Seattle Design Commission met with Sound Transit to review its proposed design (pdf) for the West Seattle Link Extensions high bridge over the Duwamish and its landing at Pigeon Point. Although the cable-stayed bridge design had been spotted at a station planning open house event earlier this year, this meeting and design review provides the public its first real look into ST's preferred design for what may end up becoming a new defining feature of Seattle's southern skyline.

The Route

The proposed design was developed for Sound Transit's Preferred Alternative for the West Seattle Link Extension, or WSLE, which its Board selected in July 2022, and includes a high bridge south of the West Seattle Bridge connecting SODO to West Seattle via Pigeon Point. The preferred route takes over the SODO Busway south of SODO station, gradually climbing and curving over the Spokane Street Viaduct to reach the high bridge over the Duwamish. The route lands on Pigeon Point just south of the West Seattle Bridge and curves around the Point to Delridge Station adjacent to the Nucor Steel Plant.

Sound Transit's map of the route (pink) and potential surplus parcels after construction (green) from SODO to Pigeon Point.Climbing from SODO

Based on conceptual design plans from the original West Seattle-Ballard Link Extension (WSBLE) Draft Environmental Impact Statement (DEIS) published in 2022, the trains would climb from under a new roadway overpass at Lander Street to about 75 feet above the SODO Busway, and then cross about 50 feet over the Spokane Street Viaduct in a sweeping turn towards West Seattle. Although the WSBLE design documents indicate the route would dip slightly between the Spokane Street overpass and the high bridge over the Duwamish, these rendering appear to indicate a decision to keep the route level.

To build the elevated guideway, Sound Transit envisions using a mix of short-span elevated guideway structures and long-span bridge sections as it has done on each of the previous Link extension projects. The result will be a style of high-standing elevated guideway that's become familiar to the region.

Renderings of the SODO Guideway (By Sound Transit)

• Rendering of the elevated guideway rising and curving over the Spokane Street Viaduct.
• Rendering of the elevated guideway from East Marginal Way, facing east toward SR-99.
• Rendering of the elevated guideway over the Spokane Street Viaduct, facing east from the West Seattle Bridge.
• Rendering of the elevated guideway, facing west from the Columbian Way ramp to West Seattle.

The Duwamish Crossing

Once the train crosses SR-99, the mapped route becomes a seemingly straightforward crossing over the Duwamish Waterway and Harbor Island, with the bridge reaching nearly 175 feet above sea level to allow for the same vertical clearance allowed for ships passing under the West Seattle Bridge. Sound Transit also identified a need to avoid putting bridge piers in the Duwamish Waterway, necessitating a more complex long-span bridge structure.

Sound Transit considered four types of bridges:

• Segmental Box Girder, common to previous Link extensions and similar to the West Seattle Bridge
• Continuous Steel Truss, similar to the Ship Canal Bridge
• Cable-Stayed, a style not yet used on a significant bridge in the Seattle region, but used for transit bridges in Vancouver, BC, and Portland, OR
• Hybrid Extradosed, a combination of box girder and cable-stayed styles

Summary slide comparing bridge design alternatives. (click image for a full-resolution view)

Ultimately, Sound Transit chose to move forward with preliminary engineering on the cable-stayed design, resulting in a 1,690-foot-long bridge with a 970-foot center span flanked by two 360-foot spans. The spans would be supported by diamond-shaped pylons through which trains would thread the eye" of two 374-foot-tall needle-like tower standing on either side of the western fork of the Duwamish Waterway.

Of course, choosing such a design opened up another complication: they intrude on airspace limitations set by the Federal Aviation Administration, or FAA, to protect aircraft landing at Boeing Field. Sound Transit considered shorter towers reaching only 342 feet in height, but apparently was able to conduct an airspace study and coordinate with the FAA to get preliminary approval of the taller design.

Sound Transit's summary of potential impacts of airspace restrictions on bridge design.

The Design Commission found the cable-stayed bridge design striking and exciting, and an opportunity to join our peer cities of Vancouver and Portland in having this prominent style of bridge.

Renderings of the Duwamish Crossing (By Sound Transit)

• Aerial view looking southeast toward Mount Rainier
• The eastern pylon as viewed from the West Seattle Bridge
• Potential lighting options at night, viewed from above West Seattle
• Aerial view from above SODO, facing West Seattle
• View of underside of the bridge from Harbor Island, facing West Seattle
• Ground view from the Duwamish River, looking toward Elliot Bay.

Under the Bridge

In SODO and on Harbor Island, Sound Transit is planning to demolish a long swath of commercial and industrial properties in order to build the guideway leading up to the bridge. The Design Commission asked about the possibility of commercial buildings returning to the land beneath the guideway via redevelopment. Sound Transit's representatives noted that operations and maintenance is easiest when there is nothing below the guideway, but conceded that allowing redevelopment of the areas beneath the guideway might help alleviate some of the economic impacts associated with demolishing the long swath of buildings along the route.

Although Sound Transit didn't provide details regarding how land under the guideway in SODO could be restored or reused, they presented a fairly detailed concept for Harbor Island, including parkland, vehicle parking, and even a potential cafe space on the west end.

Sound Transit's rendering of plans for the under-bridge area on Harbor Island.Costs and Timeline

Discussion of the estimate costs to build the guideway and cable-stayed bridge were not on the agenda during the meeting with the Design Commission, but Sound Transit's 2024 Long Range Financial Plan, published October 2023, maintained the previous years' estimated cost for WSLE at just under $4 billion. Assuming the project as a whole remains affordable, Sound Transit expects to select the final route later this year and continue design through 2026, with construction slated to begin in 2027. Service between Alaska Junction and SODO would begin in 2032.

Our next post will review the landing at Pigeon Point and Sound Transit's plans to reshape the landscape there.