High-Rise Bridge Replacement Study

Updated December 29, 2021 

What's Happening Now?

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We last provided an update about the planning study in November 2021, and since then we've completed our study of the different concepts relating to the long-term replacement of the West Seattle Bridge. Our core purpose for the study was to replace the West Seattle High Bridge to maintain long-term capacity, safety, mobility and access for West Seattle and the region. This core purpose helped us make decisions throughout the planning and concept evaluation process. We used that core purpose plus feasibility screening requirements and evaluation criteria to assess how well each of the replacement concepts stacked up to each other.

Read on for more information on the background of the study, what was examined and what's next.

Background

Historically, the West Seattle High-Rise Bridge is the City's most-used bridge arterial, carrying an average of over 100,000 travelers a day, including 19,000 bus riders. The concrete bridge was built in 1984 and has since been a major route for moving people and goods to and from West Seattle and providing connections with neighboring communities, such as the Duwamish Valley and SODO. In addition, the bridge and surrounding corridor are a critical connection with the Port of Seattle, serving as a path for goods and supporting the regional economy. In March 2020, we closed the high bridge in the interest of public safety. We made this decision based on regular inspections of the bridge, which showed rapidly growing cracks. Since the closure, we've wasted no time, working to repair the high bridge and to plan and build projects across affected neighborhoods that reduce impacts on local communities. Repairs are now underway, and we expect work to be complete in mid-2022.

Graphic showing bridge repair milestones

In November 2020, the decision was made to repair the high bridge after close consultation and analysis from city bridge engineers, members of the West Seattle Bridge Community Task Force, our Technical Advisory Panel, and countless others. In addition to repairing the existing bridge, we continued planning for an eventual replacement. Once contractors complete the necessary repairs to the existing bridge in mid-2022, we have full confidence it will last for decades. 

Because of the complex and lengthy process required to design and plan the eventual replacement, we completed a replacement bridge planning study well in advance so that we are ready when the current bridge nears the end of its service life around 2060.

Having completed the study, we now better understand the needs of the corridor, including bicycle, pedestrian, and transit options, as well as location opportunities. Having this planning study complete will also allow the City to respond much more quickly in the unlikely event of another emergency. 

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

What's a bridge planning study?

Our teams conduct bridge planning studies for priority City-managed bridges that are functionally obsolete or structurally deficient. Recent examples are the Magnolia Bridge and Ballard Bridge planning studies. The high bridge was not slated for a study until 2020, when rapidly growing cracks caused the emergency closure of the bridge. At that point the bridge was elevated as priority for long-term replacement and a bridge planning study.

Replacement concept locations studied

Our study focused on replacement location to determine which will best meet the needs of West Seattle and the region. Our team has researched and evaluated five replacement concepts within the vicinity of existing bridge, including:    

  1. A bridge north of the existing high bridge
  2. A bridge south of the existing high bridge 
  3. A bridge online with the existing high bridge 
  4. Underground in a tunnel 
  5. A hybrid bridge option (combination of the north and on-line bridge locations)   

Graphic showing the concepts

These location concepts were chosen for further study to span the range of practical replacement options that connect to existing structures, allowing us to explore and compare them to determine how we can best serve West Seattle and the region. 

When developing the concepts, we made the following base design assumptions for the bridge replacement: 

  • Maintain existing access and ramp connections 
  • 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 

Our feasibility screening focused on each concept's ability to meet these design objectives and constructability factors. Potential environmental/property impacts and conflicts with existing and planned infrastructure were also considered to inform opportunities for further evaluation.  

Evaluating the concepts 

Once we had some replacement concepts that passed the initial feasibility screening, we evaluated them against our criteria. Our evaluation criteria included: 

  • Mobility: Maintain long-term safety, access, operations and marine navigation of the multi-modal corridor as well as maintain capacity during construction
  • Construction activities: Consider the scale, duration and anticipated severity and likelihood of challenges arising during construction 
  • Equity: Limit temporary or permanent impacts to BIPOC and low-income communities, including cultural resources and living-wage jobs
  • Environment: Minimize impacts to the natural and build environment

Our evaluation found the On-Line and Hybrid Bridge Concepts to be the best performing. 

We found that 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. We also found that 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.  Process graphic

This graphic illustrates the process we used to arrive at our findings. We started with 4 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. 

Bridge replacement study area

What's next?

We finalized and documented our findings in a Replacement Planning Study Report in December 2021. As we get closer to the end of the repaired bridge's useful life, we will revisit the study and further refine the design of the future replacement bridge, including addressing decisions on the type and size of the bridge, pedestrian and bicycle access, how construction would be phased and what the bridge would need for its foundation and seismic requirements. Until then, we will continue to work towards the repair of the bridge and will keep planning for the current and future transportation needs of the city. 

Get involved

We're committed to keeping you informed of our progress on the West Seattle Bridge Program. At key milestones, we will be sharing our progress with the community.     

Materials