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Cost-benefit analysis will inform City’s decision to repair or replace the West Seattle High-Rise Bridge

The initial structural analysis of the West Seattle High-Rise Bridge is complete, and our engineers, bridge consultants, and the Technical Advisory Panel (TAP) are all in agreement that repairing the High-Rise Bridge is still possible. 

Now we must decide whether the repair or a replacement option is the best path to restoring travel across the Duwamish.

Just last week, the TAP released an official statement sharing that the information they have reviewed indicates repairs appear feasible and recommend we continue to explore the repair option fully, along with various replacement options. This statement is in agreement with our bridge engineers’ and our bridge consultant WSP’s current analysis, and it marks the end of review about whether repair is possible. Now the critical question is whether repair of the High-Rise Bridge is advisable, relative to other replacement options available. 

We are currently undergoing a cost-benefit analysis to better understand and assess the full range of repair or replace concepts.

A cost-benefit analysis (CBA), in the simplest terms, is the formalization of a mental process we all regularly undertake – weighing the pros and cons of an action and choosing the one with the greatest net benefit, or least undesirable trade-offs. The crux, of course, is identifying which pros and cons are considered, and determining how much weight each attribute carries relative to others.

Our bridge consultants at WSP have been working on the engineering contours of a cost-benefit analysis for quite some time, so we are already several months into this work. This has resulted in the identification of a variety of broad concepts for what repair or replace could look like and a clear set of choices to be evaluated and compared through a cost-benefit framework. These are by no means complete designs, but serve as a starting point for weighing the pros and cons for these potential pathways forward.

Along with comparing the estimated dollar amount of each of these concept options, the cost-benefit analysis will also take into consideration timeline, travel capacity, environmental impact, and a variety of other proposed “costs and benefits”, or evaluation criteria attributes, explained in greater detail below.

Today, we are presenting these proposed evaluation criteria attributes to the West Seattle Bridge Community Task Force. We are asking them to provide feedback on the criteria to ensure the community understands and concurs that these are the correct costs and benefits for evaluating the viability of each repair or replace concept, and to help determine the weight each criteria holds in the cost-benefit analysis.

The cost-benefit analysis will be completed in early October and used to help the City identify the best possible solution to safely restore travel across the Duwamish Waterway.  

We initially anticipated reaching the repair or replace decision point in late summer, but a few additional weeks are needed to contextualize all we have learned, fully consider a tunnel option, and build in time for the valuable involvement and input of the TAP and Community Task Force.  

Even though the cost-benefit analysis and the decision point is anticipated to be in October, we are simultaneously advancing all options by searching for a team to lead the detailed design and engineering for a potential replacement and beginning stabilization construction on the bridge that is needed for all repair and replace options, saving months of time by overlapping these multiple workstreams. 

If the ultimate decision is to replace the bridge, the team we hire (at the end of the ongoing search just referenced above) to do the design is anticipated to carry out a full Type, Size & Location (TS&L) study to refine and identify the best alternative for the replacement. This TS&L study would build off the cost-benefit analysis findings by advancing replacement options that the cost-benefit analysis identified to have the most promise and value.

Across the six broad concepts of what repair or replace could look like, we have also worked with WSP to identify very rough and ever-subject-to-change years in which each option could restore some traffic across the Duwamish, and a broad window of service life, or lifespan, that each scenario could yield. 

After the rapid and extensive cracking in the High-Rise Bridge that occurred in a matter of days, we anticipated severe structural health issues and conservatively estimated that the repair pathway could yield little more than 10 years of additional service life. After extensive analysis, better-than-anticipated non-destructive evaluation (NDE) and core sampling results, positive instrumentation readings, and feedback from our Technical Advisory Panel (TAP), we believe the repair pathway could now provide at least 15 years of service life. The TAP will work to provide greater clarity on the assumed service life for repair options.  

Chart showing CBA Framework. Column headers are: are Scenario​, Option​, Description​, Potential to restore traffic in*, Approximate lifespan* **:​

Row one: Shore​, 1​, Temporary shoring​, 2022​, 3+ years​, 

Row two: 
Repair, 2, Repairs (carbon fiber wrap and external post-tensioning) with foundation strengthening​, 2022​, 15+ years​

Row three: Repair, 3​, 
Partial superstructure replacement with foundation strengthening​, 2022​, 15+ years​

Row four: Replace​ 4​, Full superstructure replacement with foundation strengthening​, 2025​, 50+ years​,

Row five: replace, 5, Full bridge replacement in same footprint​, 
2026​, 75+ years​, 

row six: replace, 6​, Tunnel replacement​, 2026​, 75+ years ​

* Dates and years are estimates and subject to change.​

** Lifespan estimates have increased as a result of better-than-anticipated NDE results, instrumentation readings, and other recent analysis, indicating an ability to go beyond 15 years of service life that is greater than initially anticipated

Each of the diagrams below are initial sketches of what repair and replacement concepts could look like and are by no means exact plans. They are provided only to identify a broad category of approach to help the community better understand and visualize a spectrum of potential options, and focus the cost-benefit work. As such, we are not prepared to talk about each of these concepts in greater detail at this time.  

The dates shared for potential reopening, as well as potential years of useable life stemming from that body of work, are also estimates and subject to change as we progress in our understanding of the High-Rise Bridge. Within each alternative there are variations that would affect both up front capital costs and ongoing maintenance costs. As we progress in the cost-benefit analysis we will be able to develop cost estimates to inform the decision-making process. 

Concept 1: Shoring to Restore Travel Capacity​ 

Concept Diagram showing what a K-Frame Shoring could look like on the West Seattle High-Rise Bridge
Concept Diagram showing what a Steel Truss Shoring could look like on the West Seattle High-Rise Bridge

A comprehensive temporary shoring system to return some travel capacity would add an external steel structure to the existing bridge to support traffic. There are a couple different approaches we could take to shore the bridge. Steel support beams could be added under the bridge called a “K Frame Shoring” (top diagram), or a Steel Truss Shoring system could be added along the top of the bridge (bottom diagram).  It is estimated that traffic could be restored in 2022 and likely add 3+ years of use to the bridge. 

Concept 2: Repair to Restore Traffic Capacity​ 

Concept diagram showing what post tension repairs could look like to restore the west seattle high rise bridge

Repairs would include similar, but more comprehensive, measures that we are taking to stabilize the bridge. Repairs would likely include more carbon fiber wrapping, new steel strengthening lines in the post tensioning system within bridge girders, and foundation strengthening. It is estimated that traffic could be restored in 2022 and has the potential to add 15+ years of use to the bridge. 

The results of recent analysis, better-than-anticipated non-destructive evaluation (NDE) and core sampling results, and positive instrumentation readings indicate that repairs could provide more than the 10 years of service life that we initially anticipated. 

Concept 3: Partial Superstructure Replacement with foundation strengthening 

concept diagram showing what partial superstructure replacement could look like on the West Seattle high-rise bridge

A Partial Superstructure Replacement would involve removing the center span of the High-Rise Bridge between piers 16 and 17 only and replacing it with a new steel box girder. This process would also involve some foundation work to ensure that the bridge is stable. It is estimated that traffic could be restored in 2022 and likely add 15+ years of use to the bridge. 

Concept 4: Full Superstructure Replacement with foundation strengthening 

concept diagram showing what a full superstructure replacement could look like
concept diagram showing what a full superstructure replacement could look like

A full superstructure replacement would involve replacing the three high-rise spans of the bridge between piers 15 and 18 while reusing the same foundation. The bridge could either be constructed out of concrete with a post-tension steel system like the current bridge (top diagram), or it could be replaced with steel box girders (bottom diagram). The foundations would also be strengthened during the superstructure replacement. It is estimated that traffic could be restored in 2025 and likely add 50+ years of use to the bridge. 

Concept 5: Full Replacement in Same Footprint​ 

concept diagram showing what a steel truss bridge replacement could look like
concept diagram showing what a cable stayed bridge replacement could look like

A full replacement could take many shapes. For this cost-benefit analysis, we are examining replacement options that are in the same footprint or alignment as the current bridge. This means that the “approach spans,” or the roadways leading to the bridge, would not be changed or replaced, as this introduces even more time and expense to the work. We would explore different bridge options, including a steel truss bridge (top diagram) or a cable stayed bridge (bottom diagram). It is estimated that traffic could be restored by 2026, and the bridge would likely be useable for 75+ years 

Concept 6: Tunnel Replacement 

concept diagram of what a tunnel replacement could look like.

A replacement tunnel would go under the water, in roughly the same footprint of the existing bridge. None of the current bridge, including foundations, would be reused in a tunnel scenario. It is estimated that traffic could be restored by 2026, and the tunnel would likely be useable for 75+ years.

The Cost-Benefit Analysis is an opportunity to compile all the information we have gathered for each repair or replace concept and evaluate them based off a unified rubric of evaluation criteria.  

We have identified ten key evaluation criteria. Though all are important, these criteria will not hold equal weight in the analysis.  

Of the ten criteria we are putting forward today, we are asking the Community Task Force to help us identify which are the most important to the community at large and how they should be weighed in the analysis. To aid in this effort, each Community Task Force member will complete a matrix worksheet that will be returned by the August 5th meeting. We will also conduct similar exercises with our TAP and SDOT engineers. 

Here are the ten criteria we have identified to evaluate each of the repair or replace concepts:  

  • Constructability Will the contractor be able to build this repair/replace concept given site constraints and schedule? 
  • Environmental What kind of temporary and permanent impacts will this repair/replace concept have on the Duwamish River and surrounding area? Can we build it within the mandated in-water work windows?​ 
  • Equity  How will this repair/replace concept contribute to equity in West Seattle and the Duwamish Valley?  How will it impact historic and treaty rights of local Native Peoples?  How will it impact and/or alter communities of color from accessing cultural and community hubs?     
  • Forward compatibility  Will this repair/replace concept be compatible with Sound Transit light rail? Will it restore traffic capacity (weight and quantity) to the desired levels? Will it meet waterway navigational needs?  
  • Funding What funding will be available, and what will the potential funding burdens be on local resources and communities? Is this repair/replace concept eligible for federal, state, local, or emergency funding?  Estimated capital costs and a Bridge Life Cycle Cost Analysis will be conducted for each alternative, too.  
  • Maintenance and  operations What will this repair/replace concept need over its lifespan in terms of operations, maintenance and inspection? 
  • Mobility  How will this repair/replace concept contribute to the movement of people and goods and overall access?  How will it impact vulnerable communities (seniors, people with disabilities and others) from accessing social service needs such as meals, healthcare services, case management and other vital services?   
  • Multimodality  Does this replace/repair concept facilitate or improve the movement of people and goods by all modes? How will it impact current local and regional traffic?  
  • Regional business  How will this repair/replace concept impact businesses as it pertains to the local and regional movement of people and goods? How will it impact the ship channel? Will construction affect business access?   
  • Seismic resilience and safety What seismic standards will the repair/replace concept meet? How will seismic upgrades be incorporated into the design?   

Cost is a factor for all options and is at the heart of the cost-benefit analysis. These attributes assign the benefit portion of the cost-benefit analysis; cost, time and the cost of time, will be overlaid for the analysis and conclusions. 

After receiving input from Community Task Force members and TAP members in early August, we will finalize the evaluation criteria for the cost-benefit analysis. Meanwhile, we will continue to develop cost estimates for each repair or replace concept and gather the pertinent information needed to evaluate each concept. The cost-benefit analysis will be complete in October and used to inform the City’s decision to repair or replace the bridge.  

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