Low Bridge Projects

Low Bridge Projects

What's Happening Now?

  • In 2021 - 2022, we will make upgrades to the low bridge to ensure it continues to serve the West Seattle peninsula, surrounding communities and maritime and industrial businesses. This work includes structural rehabilitation, control system upgrades, and maintenance of the bridge's hydraulic drive system (lift cylinders). We're in the planning and design phases of these projects and are getting ready to bring contractors on board  for construction.
  • While we have limited details at this point, we anticipate the potential for some very short-term low bridge closures during construction. We will work directly with those businesses in the immediate vicinity to ensure minimal impacts to operations. We don't know the extent and timing of these impacts at this time given the stage where we are in these projects, but we'll share information, once known, with the community on this website, on our blog, and through our weekly email updates. A fundamental part of our planning for and constructing of low bridge improvements includes strong coordination with our partner agencies, maritime and industrial businesses, the broader community, and other City of Seattle and partner agency projects. Minimizing impacts to all is a critical priority regardless of the construction delivery method we choose. This will be reflected in the construction contract when we bring a contractor on board. 
  • We are evaluating environmental impacts and working to obtain environmental permits needed to conduct certain aspects of the work, particularly those activities that could potentially affect the Duwamish Waterway.   
  • With public safety as our top priority, low bridge access is restricted to ensure efficient emergency vehicle access across and around the bridge. For more information on the access policy, check out our low bridge access webpage.
  • The Seattle Department of Transportation (SDOT) has determined that the Spokane St Swing Bridge Horizontal Directional Drill Project will not have a probable significant adverse impact on the environment. As a result, SDOT issued a Determination of Non-significance (DNS) on January 11, 2021. This decision was made after review of a completed environmental checklist. Public comments on the attached DNS and checklist will be accepted until 5:00 pm, January 25, 2021, and the appeal deadline is February 1, 2021. Please see the DNS for more information on providing comments and submitting appeals. View Determination of Non-Significance and State Environmental Policy Act (SEPA) Checklist here.

Project Overview

The Spokane St Swing Bridge (also called the "low bridge") is an essential route for emergency vehicles, transit, heavy freight, and people biking and walking. It provides an important connection between the maritime and industrial businesses on the west side of the Duwamish Waterway to those on the east side, especially with respect to the marine cargo terminals. Due to recent bridge inspections and continued traffic demand since the closure of the West Seattle High-Rise Bridge, we are proactively taking measures to preserve the low bridge through a series of projects outlined on this page. 

We have no reason to think that the low bridge is in any imminent risk, but because the bridge now plays such an over-sized role in our transportation system, we have developed a forward-thinking plan to strengthen it further and have been taking numerous precautionary steps to monitor and care for this bridge since the closure of the high bridge.

We will make several improvements to the bridge in 2021 and 2022 to ensure that the bridge remains strong and in usable condition for those who rely on it. The region greatly depends on the low and the high bridges; with the largest of the two (the high bridge) out of service for a prolonged period, we are depending heavily on the smaller low bridge. It is critical that our team maintain the low bridge to the fullest extent during this time. We feel strongly that preparing for and conducting upgrades and repairs now, far outweigh the impacts the community and maritime and industrial businesses could experience if emergency repairs and unexpected maintenance are needed. We want low bridge upgrades to be planned, and not unexpected events.

Across all low bridge improvement projects, we're working to coordinate the timing of the construction with our agency partners and will be engaging those in the maritime and freight industries. More information, once known, will be shared on this website and through our weekly email updates (sign up here). You can also view bridge opening notifications on the Seattle Travelers Map.

The structure and system that opens the low bridge includes:       

  • two concrete spans that swing back and forth    
  • two pivot points where the spans rest on the bridge piers 
  • an electric control system that lets operators open and close the bridge and stop traffic

Each of these three parts will be rehabilitated  to extend the life of the low bridge and improve the resiliency of our system while the high bridge is closed. Thank you for your understanding and patience as we embark on this important work.

Structural rehabilitation graphic

Project Background

The low bridge is a critical crossing of the Duwamish Waterway, connecting West Seattle and Harbor Island with streets to SODO and Duwamish Valley neighborhoods, business districts, and Port of Seattle facilities. It was built in 1991 and opens for vessels about 1,500 times per year.

The bridge is made of concrete with two main sides (spans). The center span is 480 feet long, and when we need to open it for vessels on the Duwamish Waterway, the spans rotate 45 degrees to open instead of raising into the air like a drawbridge. To open the bridge, each bridge side "floats" on a steel barrel (called a cylinder) in hydraulic oil located on the center bridge piers. This allows for a smooth opening and closing each time. It is said to be the only bridge of its type in the world. 

For people traveling on the bridge, the roadway carries two lanes of traffic (one in each direction) and a 12-foot pedestrian and bicycle path. This is a popular route for many people biking to and from West Seattle.

With the High-Rise Bridge closed and under repair until 2022, we are focused more than ever on both how the low bridge is operating and carrying traffic, and how the structure itself is functioning. In the next two years, we will make multiple upgrades to the bridge to ensure it continues to serve the West Seattle peninsula and surrounding communities for years to come. Through the construction of these upgrades, we will also do everything we can to minimize impacts to those businesses and maritime operations in the vicinity that rely on accessing the low bridge.

The low bridge is a City of Seattle facility and the responsibility of SDOT. We own, inspect, maintain, and/or operate nearly 280 bridges in the city of Seattle. A goal of our Bridge and Roadway Structures Program is to protect the public's investment, extend the service life of bridges, and provide safe travel across bridges for all modes. 

SPOKANE ST SWING BRIDGE PROJECT DESCRIPTIONS

Structural Rehabilitation Project

The Spokane St Swing Bridge ("low bridge") does not pose any imminent risk of closure or failure. We monitor it with in-person inspections at least once a month and a structural health instrumentation monitoring system. In 2021, we'll start making improvements to meet the current Federal Highway Administration commercial vehicle load standards. The rehabilitation improvements are a preventive measure to ensure the bridge is able to support vehicles and heavy freight in the years ahead. 

The work will likely include installation of carbon fiber wrapping, post-tensioning or a combination of the two. Wrapping the bridge spans' concrete makes it even stronger; post-tensioning means installing and tightening cables within each bridge span to better support the weight of vehicles.

We'll also be looking closely at the mechanical center-lock system that latches the two spans of the bridge together when it's in the closed position, as well as the condition of the bridge piers (the vertical support points). 

We're designing the rehabilitation work through mid-2021 and targeting construction to begin by the end of 2021. We are still early in the project, so we have limited details on the project at this point. Information will be shared on this website and through our weekly email updates (sign up here). 

Structural Rehabilitation Schedule            

  • 2020: Planning        
  • 2020 - 2021: Design         
  • 2021 - 2022: Construction  

Structural rehabilitation schedule graphic

Controls Project

The low bridge swings open and closed using the original electronic system of buttons, switches, and wires that are now about 30 years old. We already had a plan in place to replace this system in 2020, but then the high bridge was closed and we reassessed the project until we had a better understanding of the high bridge's condition. Ultimately, we decided to reroute the wires connecting the control tower with the motors that open and close the bridge off the high bridge - where they are today - to a new conduit under the Duwamish Waterway. Now we're recommencing with construction in 2021 as part of the West Seattle Bridge Program to give resiliency to the overall West Seattle bridge system and decouple the low bridge from the high bridge and prevent more unexpected closures.

The system includes computers that control the machinery that lifts and swings the spans and activates the gates that prevent traffic and people from crossing when it's open. It also includes the communication lines that connect the computers, control tower, and the moving parts to one another. Without making these updates now, we run the risk of complications in operating the bridge in the future.   

To complete the upgrades, we'll drill new conduits (like a small pipe) for the communications line beneath the Duwamish Waterway. These new conduits will be roughly 4-inches in diameter and about 20 feet below the bottom (the riverbed) of the Duwamish Waterway.

We'll then route the new communication cables into the new conduit and connect them to the new control system. The final step involves removing existing equipment and reinstalling new equipment for the control system.  Procurement, testing, and installation of the new communication and control system will take time and so we want to get that process started as soon as possible.

Existing

Controls project existing graphic

New Communications Lines

Controls project after graphic

Controls Project Schedule           

  • 2017 - 2018: Planning        
  • 2018 - 2021: Design          
  • 2021: Construction  

Controls project schedule

Lift Cylinder Project

Two large hydraulic cylinders, located on the east and west side of the low bridge, do the heavy lifting that allows the bridge to swing open for ships and boats in the Duwamish Waterway. Think of the cylinder as a pivot point where each span rotates out of the way of waterway traffic. Without the lift from the cylinders, we would not have a functioning swing bridge. 

In addition to the two active cylinders, the bridge has a third, spare cylinder in case one of the active cylinders needs to be repaired. In 2018, we removed the west cylinder and replaced it with the spare. Since then, we've inspected the west cylinder and expect that it will be repaired and ready for service in early 2021. In 2021, we'll install the fixed cylinder on the east side of the waterway. After the swap, we'll inspect the removed cylinder to replace seals and determine if any repairs are needed. 

To help make future maintenance more efficient and less impactful to those traveling on the bridge, we are exploring an option to redesign the lifting frame that we use to remove the cylinders from the bridge and thereby shorten any future closures for this work.

A photo of cylinder on the west side of Duwamish Waterway during removal in 2018.

Lift Cylinder Project Schedule

  • 2018 - 2019: Planning 
  • 2019 - 2021: Design  
  • 2021 - 2022: Construction  

Lift cylinder project schedule