Spokane St Swing Bridge Rehabilitation Program

Updated April 9, 2024

What's Happening Now?

From April 19 through April 28, 2024, crews will reroute the control tower wires from the high bridge to a new conduit beneath the West Duwamish Waterway that connects to the motors that open and close the bridge. This operation will require closing the low bridge for people driving, biking, and walking for about one week. Read more about the upcoming work in our blog

Project Overview

The Spokane St Swing 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 bridge inspections and elevated traffic demand during the closure of the West Seattle Bridge, we are proactively taking measures to preserve the low bridge through a series of projects outlined on this page. 

Because the bridge played an out-sized role as the link between West Seattle and the rest of the city during the West Seattle Bridge closure, we've developed a forward-thinking plan to strengthen it further and have been taking numerous precautionary steps to monitor and care for this bridge since 2020.

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. Thank you for your understanding and patience as we continue this important work.

Structural rehabilitation graphic

Project Background

The Spokane St Swing 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.

Low bridge swings open for maritime traffic

The Spokane St Swing Bridge swings open for maritime traffic to pass. 

For people traveling on the bridge, the roadway carries 2 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.

Following the recent 2 ½- year closure of the West Seattle Bridge, we are focused on how the low bridge is operating and carrying traffic, as well as how the structure itself is functioning. In the next year, 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 continue to rely on accessing the low bridge.


Structural Rehabilitation Project — Completed October 2022

The Spokane St Swing Bridge does not pose any imminent risk of closure or failure. The bridge is routine inspected per Federal requirements. Due to the high demand on the low bridge during the West Seattle Bridge closure, we increased the frequency of in-person inspections at least once a month and installed a structural health instrumentation monitoring system that provides real‑time data of the bridge.

In October 2022, we completed rehabilitation as a preventive measure to ensure the bridge will continue to support vehicles and heavy freight in the years ahead. 

low bridge rehab graphic

We injected epoxy resin into any existing cracks and added carbon-fiber wrapping in several locations, on both interior and exterior surfaces. We wrapped sections of the bridge with carbon-fiber wrapping to strengthen the bridge, much like putting a cast on an injured arm or leg. After we added carbon-fiber wrapping to surfaces of the bridge, it's working in tandem with the steel already inside the bridge to increase bridge strength.  

carbon fiber wrap and bridge girders

Carbon-fiber wrapping can be applied inside and outside the girders to further strengthen the bridge, as shown here during phase 1 stabilization efforts on the West Seattle Bridge.

Controls and Communications Project

We awarded a contract to improve the control system that opens and closes the low bridge. Taurus Power & Controls, Inc. was the winning bidder, and has prior experience with movable bridges in Seattle. In 2023, we worked to procure components and developed the system off-site. We will test the system to ensure that it is ready for installation in April 2024.

The Spokane St Swing 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 the project was reassessed until we had a better understanding of the high bridge's condition. Ultimately, the project also included rerouting 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 West Duwamish Waterway. We will complete this work as part of the West Seattle Bridge Program to increase resiliency to the overall West Seattle bridge system.

low bridge control tower

The low bridge control tower, part of the low bridge's controls system.

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 component failures associated with operating the bridge in the future.

To complete the upgrades, we'll drill new conduit (like a small pipe) for the communications line beneath the West Duwamish Waterway. The new conduit 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, test them, and connect them to the newly installed control system. The final step involves removing existing equipment and reinstalling new equipment for the control system.


Controls project existing graphic

New Communications Lines

Controls project after graphic

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. In 2024, we'll install the rehabilitated cylinder on the east side of the waterway. After the swap, we'll inspect and refurbish the removed cylinder, replacing seals and determine if any other repairs are needed. 

To help make future maintenance more efficient and less impactful to those traveling on the bridge, we have redesigned and fabricated a new lifting frame that is used to remove the cylinders from the bridge; the new lifting frame will shorten any future closures for this type of work.

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

Turn Cylinder Replacement

The bridge span is opened and closed by turn cylinders that push and pull on the lift cylinder. Last October, we reinstalled the refurbished cylinder from the east pier that was damaged when the piston head became stuck in January 2023. Preparations to overhaul all four of the bridge’s hydraulic turning cylinders were already underway as part of our comprehensive repair and maintenance efforts, when the unexpected damage occurred, allowing us to complete repairs sooner than if we had been starting from scratch.

Our bridge maintenance crews, along with our team of design and repair experts, redeveloped a part of the turn cylinder so that this type of malfunction is much less likely to occur in the future. After we reinstall the refurbished cylinder we removed during the October 2023 low bridge closure, we will continue to rehabilitate the two remaining turn cylinders in the west bridge pier through this year and into 2025.

Crews removing the second turn cylinder from the pier house for rehabilitation in October 2023

Crews removing the second turn cylinder from the pier house for rehabilitation in October 2023. Photo credit: SDOT.

Hydraulic and Electrical Components Replacement

This project identifies and completes the next phase of major maintenance, replacement, and overhaul of the bridge’s electrical and hydraulic components. These components serve a variety of functions and help the bridge to operate. Parts eventually wear down either from ongoing use or by reaching the end of their intended service life. There are various parts in the bridge that have been in service for 30 years and are showing their age.

The component overhaul work will replace or repair these parts so that they can continue to function as originally designed, and can be readily replaced if needed, as part of our ongoing preventative maintenance work on this bridge.


  • 2020-2022: Community engagement
  • 2020-2022: Planning and design
  • 2022-2025: Construction


Greg Spotts, Director
Address: 700 5th Ave, Suite 3800, Seattle, WA, 98104
Mailing Address: PO Box 34996, Seattle, WA, 98124-4996
Phone: (206) 684-7623

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The Seattle Department of Transportation (SDOT) is on a mission to deliver a transportation system that provides safe and affordable access to places and opportunities for everyone as we work to achieve our vision of Seattle as a thriving, equitable community powered by dependable transportation.