As we prepare to reopen the West Seattle bridge in mid-2022, we’re also analyzing viable options to replace the bridge long-term when the time comes.

West Seattle Bridge at sunset. Photo Credit: Madison Linkenmeyer

Summary 

  • In November 2020, we made the decision to repair the West Seattle High-Rise Bridge (high bridge). 
  • We also continued planning for an eventual replacement and analyzed five replacement options using a detailed set of criteria. 
  • We found the on-line and hybrid replacement options (more on those below) to be the best performing in our evaluation based on these criteria. 
  • In the coming months, we will finalize and document our findings into a Replacement Planning Study Report. 
  • We continue to work towards reopening the high bridge in mid-2022 and will keep planning for the current and future transportation needs of the city. 

In November 2020, we made the decision to repair the West Seattle High-Rise Bridge (high bridge).  

From the beginning, Mayor Durkan made clear that her goal was to identify the pathway that provides the highest degree of certainty for a safe and rapid restoration of travel with the lowest level of impact to communities in and around the Duwamish Valley, the city, region, and state. After close consultation and analysis from SDOT’s structural team, members of the Community Task Force, SDOT’s Technical Advisory Panel composed of leading experts in their fields, and countless others, it became clear that repair is the best way to safely restore this critical connection as quickly as possible. That work is now underway with the bridge scheduled to be reopened to traffic in mid-2022. We are confident that the repairs will ensure the bridge’s service for decades to come. 

We also continued planning for an eventual replacement. 

West Seattle Bridge timeline 1984 to present.

 
We began researching potential locations for a replacement around the existing bridge. We’ve been evaluating these long-term replacement concepts, using practical criteria aligned with our core purpose, which is to:  

Replace the West Seattle High Bridge’s main span and side spans to maintain long-term capacity, safety, mobility and access for West Seattle and the region

The replacement planning study is limited to the section of the bridge that encompasses the long concrete spans between Pier 15 and Pier 18. 

The five replacement concepts we analyzed were: 

  1. North Concept: A bridge north of the existing high bridge 
  2. South Concept: A bridge south of the existing high bridge 
  3. On-line Concept: A bridge on-line with the existing high bridge 
  4. Tunnel Concept: A tunnel underground  
  5. Hybrid Concept: A hybrid bridge option, which is a combination of the north and on-line bridge locations. 
The five replacement options we analyzed shown on a map.

Our goal with each option was to: 

  • Maintain existing access and ramp connections 
  • Meet minimum design standards for a 45-mph roadway (grades, sight distances, etc.) 
  • Maintain existing road width  
  • Maintain existing navigational clearances of the Duwamish waterway (horizontal and vertical)  
  • Use demolition and construction methods that are cost effective and minimize impacts to community members 
We examined existing conditions and future considerations in the bridge vicinity to understand potential conflicts with existing and planned infrastructure in the area. Photo credit: SDOT.

Evaluating the concepts 

We did an initial feasibility screening to focus further design efforts on the most feasible options – that is, the options that meet both design objectives and constructability factors, such as potential environmental/property impacts and conflicts with existing and planned infrastructure. 

We then evaluated each option using our criteria: 

  • Capacity and mobility: We acknowledge the need for a multi-modal roadway, the needs of marine traffic, and the continued safe operations of the surrounding area 
  • Construction activities: We take into account the scale, duration and anticipated severity and likelihood of challenges arising during construction 
  • Equity: We want to minimize impacts to Black, Indigenous, and People of Color (BIPOC) communities, including their cultural resources and livelihoods 
  • Environment: We want to minimize  impacts to the natural and built environment, and reduce or eliminate any impacts to planned facilities such as those owned by Sound Transit, the Port of Seattle, the railroad, and King County Metro 

We found the On-Line and Hybrid Bridge Concepts to be the best performing 

The four bridge concepts – North, South, On-line and Hybrid – meet the core purpose and design objectives and can be constructed to maintain travel, but all would require lane closures during construction. The North and South Concepts do not appear to offer mobility benefits that offset their higher impacts and construction duration. The Tunnel Concept falls short of the core purpose, does not fully meet initial design objectives, and has greater scope and impacts that would decrease mobility throughout the corridor. 

This graphic illustrates the process we used to arrive at our findings. We started with four location concepts (North Bridge, South Bridge, On-line Bridge, and Tunnel), and screened those for feasibility. We found that all three bridge concepts met the core purpose, design objectives and would be feasible to construct. The Tunnel had some significant challenges, and that’s why it wasn’t explored further. We then refined the bridge concept designs and added a Hybrid Bridge Concept. We evaluated those concepts and landed on two that performed best and merit further study when the time comes. 

What else did we learn? 

Anyone who lives in the area or has traveled to and from West Seattle in the last year and a half could tell you that the bridge closure has had immense impacts for many people. We take this to heart. We understand that people are impacted during a bridge closure and know that maintaining access and mobility during construction of an eventual replacement is what we’ve learned to be essential – and was a key driver in our concept evaluations. On that note, we found that resiliency in the corridor is crucial. Having more than one way in and out of West Seattle in this area is vital now and will be in the future. This study will provide important information for the future when the bridge replacement planning continues in earnest. 

In the coming months, we will finalize and document our findings into a Replacement Planning Study Report.  

We estimate that the repaired high bridge will last approximately 40 more years. In the coming years,  we will revisit the study and further refine the design of the future replacement bridge. This could include the type and size of the bridge, pedestrian and bicycle access, how construction would be timed and what foundation and seismic requirements the bridge would have.   

We continue to work towards reopening the high bridge in mid-2022 will keep planning for the current and future transportation needs of the city. 

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