At our 11th official West Seattle Bridge Community Task Force Meeting, we learned about HNTB’s “Rapid Replace” option.

This rendering shows the rapid replace option proposed by HNTB. Credit: HNTB.

Summary

  • We discussed important developments related to the West Seattle Bridge in other blogs last week, including Automated Enforcement on the Low Bridge and the Cost-Benefit Analysis. These items were also discussed in this week’s meeting. 
  • This blog is dedicated to describing the Rapid Replace option proposed by HNTB, and we have also provided a link to the Community Task Force meeting
  • The proposed Rapid Replace would use the existing pile foundations of the West Seattle High-Rise Bridge and shorten the existing piers. New twin arch spans made of steel would rise above the refurbished, shortened bridge piers. The proposal features lighter, replaceable components, corrosion protection, and displacement tolerance from seismic shocks. 
  • We are very optimistic about this possible Rapid Replace approach, but there are still risks that we are taking into consideration. There is much more discussion ahead. 
  • You can see a video of the Rapid Replace concept here
  • A full conversation with the design lead for the Rapid Replace concept can be seen here

View the meeting by clicking on the above image. 

We want to pursue the most efficient, cutting-edge and novel approaches to solving the West Seattle Bridge challenge affecting our city, region, and state. 

We’ve assembled a team of experts to help us identify innovative approaches that improve upon the six bridge repair or replace archetypes identified in the cost-benefit analysis (CBA).  One such proposal is a “Rapid Replace” concept put forward by HNTB that could restore travel as early as 2023. 

A similar approach and expedited timeline designed by HNTB was recently used for a successful bridge replacement across Lake Champlain.  

Since the closure of the West Seattle High-Rise Bridge, we’ve worked to simultaneously advance all efforts to urgently pursue both repair and replace pathways, while mitigating traffic and environmental impacts on the ground. That is why, as far back as June, we posted a Request for Qualifications (RFQ) to start our search for a team to design a potential replacement for the West Seattle High-Rise Bridge while we continued working towards a possible repair and necessary stabilization. After careful vetting and review, we selected infrastructure design firm HNTB to lead this effort and finalized their contract in late October.  

The design work being done by HNTB for the replacement of the West Seattle High-Rise Bridge is necessary in either a “repair, but replace later” or “pivot immediately to replacement” scenario. 

HNTB has proposed an innovative “Rapid Replace” option for the West Seattle High-Rise Bridge that builds and improves upon the superstructure replacement archetype included in the CBA, also referred to as Alternative 4.  

Since this Rapid Replace concept was only recently introduced by HNTB to SDOT and the infrastructure design firm was under contract in late-October, we were not able to include this concept in the final CBA without significantly delaying its timely release to the public.    

There are plans for this option to be evaluated at a high level and measured against the same CBA attributes in order to develop a similar level of detail and risk evaluation of the new Rapid Replace option. 

HNTB’s National Bridge Chief Engineer, Ted Zoli, has a global understanding of the greatest bridge challenges of our time. He noted that the West Seattle High-Rise Bridge challenge is, in his opinion, the greatest bridge challenge and opportunity our nation is facing right now. There is no other bridge in America closed right now that served as many passengers as this bridge, and served so many critical economic engines for a region.  

HNTB has presented an innovative approach that’s been successfully used elsewhere  to build bridges faster than transportation agencies previously thought possible. HNTB’s goals with this concept include construction that is faster, more durable, more affordable, and lower risk.  

As part of our continued commitment to clear communication and transparency, we’re sharing our consideration and education process around the Rapid Replace concept with you now, despite the fact that we’re in the very early stages of learning about the opportunities and potential challenges this option presents.  

Some of the opportunities Rapid Replace presents include using the existing pile foundations of the West Seattle High-Rise Bridge and shortening the existing piers. New twin arch spans made of steel and efficiently fabricated in tandem with superstructure demolition would rise above the refurbished, shortened bridge piers.  

Optimistically, this approach could shave up to three years off the time to open a bridge replacement from the traditional, conservative, linear delivery method and timeline used in the CBA for Alternative 4 (superstructure replacement), which could mean an opening date sometime in 2023.  

This Rapid Replace design has the potential to achieve high levels of efficiency and reduce construction timelines by using several innovative methods: 

  • Parallel processes for both demolition of the existing bridge and fabrication of the new steel bridge, which means that both demolition and fabrication of the new structure can happen at the same time.  The arches and approach girders can also be fabricated independently and in parallel. Traditionally, these things can only happen one after the other. 
  • Avoidance of in-water construction work and reduced environmental impact. 
  • Off-site fabrication of the twin arch spans, which means the arch spans would be built off-site and then shipped in on a barge. There is potential to do this work locally, which adds additional layers of efficiency.  
  • Building on-alignment, which means the structure would be built on the same support structures and path of the current bridge. 
  • Replacing the superstructure only, rather than completely demolishing the existing structure and replacing the columns and approaches. This would enable us to leverage the previous investment in the existing pier structures and foundations.  Further, the existing and re-used piers and foundations have the potential to be given a longer life by removing the existing span and fabricating a lighter-weight bridge (about half the weight of today’s span over the Duwamish) for them to hold up, reducing the weight on the piers and providing better seismic performance and prolonged design life. 
  • Streamlined permitting and decision-making. 

The construction concept calls for the creation of the new bridge arch (the area of the bridge with the curved structure above it, shown in the above rendering) off-site. This means: 

  • The existing, compromised bridge structure would be demolished and removed by barge.   The remaining portion of the concrete box girders together with the tops of the main piers would be removed, effectively reducing the height of the main piers.   
  • The spans over land would be replaced with steel girder approaches, and delta frames (the triangular shaped extension of the steel girders), which are designed to support the arches during lifting and in the final configuration. 
  • In parallel with the erection of the approach girders and delta frames, the arches will be assembled off-site on barges. The last step in steel erection is floating in the arches and lifting them from the delta frames using strand jacks. New approach spans would then be joined to the new arch. 
  • Once all the steel is erected, deck construction can be completed and the bridge opened to traffic. 
     
Here is a video of what the process for replacement would look like using this option. 

As we’ve mentioned before, safety is our number one priority with all of our projects, including the West Seattle Bridge and all related work.  

This Rapid Replace concept provides enhanced safety. The proposal features lighter, replaceable components and corrosion protection. Because of the lower weight and seismic isolation, the design has long-term durability and seismic stability.  

Further, the new design could improve navigation clearance by about ten feet, and there would be a wider expanse of that maximum height, providing easier navigation for vessels. A new bridge span will also reduce long-term maintenance costs.  

We’re optimistic about this possible Rapid Replace approach, but there are still risks that we are taking into consideration and need more study.  

Risks include, but are not limited to: securing adequate funding, permitting, working in a constrained site, and successfully performing an engineered demolition of the existing superstructure.  

We are taking those into full consideration, especially as we fulfill our role as trusted stewards of taxpayer dollars, and work in service to our partners and community members’ potential questions and concerns. 

There are additional discussions to be had, community feedback to be gathered, and studies to be completed, and we look forward to continuing our dialogue with our partners and community members. 

The CBA was an invaluable document as we understand the risks and benefits of options to repair or replace the West Seattle High-Rise Bridge and restore travel across the Duwamish Waterway. The CBA makes clear that, when all other things are quantified and monetized, this decision really comes down to priorities of various stakeholders and, in turn, their tolerance for certain risks. There are real risks with each pathway. 

We want you and your neighbors to know that we are working diligently, efficiently, and in partnership with the community and technical experts to support the Mayor in making the best possible decision for our community. It is important for us to continue working towards each pathway – repair and replace – so that we are ready to move forward quickly once the Mayor makes her decision. 

Using this time to make the best decision at this juncture does not delay any necessary work. Since we closed the West Seattle High-Rise Bridge back in March, our contractor Kraemer North America has been working six days per week to stabilize the bridge and perform other critical work necessary in either a repair or replace scenario. We have not lost any time, and as we continue to explore options like this rapid replacement option, we understand the urgency of this work and are making every day count in getting full mobility restored. 

In the coming weeks, Mayor Durkan will share direction on the path forward that provides the safest, fastest solution to reconnect West Seattle, with the highest degree of certainty and lowest level of impact to communities in and around the Duwamish Valley, the city, region, and state.  

To learn more: