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What we’re doing to maintain and preserve the Spokane St Swing Bridge

Crews working to replace components for one of the multiple hydraulic pump systems which power the rotation of the bridge in October 2022. Photo credit: Greg Spotts, SDOT.

Update (December 31, 2022) 

Our crew of bridge engineers and technical experts have been in the bridge every day since the bridge was damaged during last week’s storm. Click here for a detailed update on the repairs we’ve completed so far.

Click here for resources to get around during the Spokane St Swing Bridge closure.


Safety for the traveling public is our number one priority. To keep our bridges safe, we continuously perform basic bridge maintenance including regular inspections, monitoring, preservation, and repairs. We also complete bridge earthquake retrofits and occasionally replace bridges, like the recent Levy to Move Seattle-funded Fairview Ave N Bridge Replacement Project.

The low bridge is staffed and monitored 24 hours a day, seven days a week. Planned closures are needed as part of the longer-term rehabilitation program and have been happening over the past year. We can alert travelers in advance to prepare for when these closures happen. Unplanned closures like the current one occur when the aging parts malfunction and need repairs. As of Tuesday, December 27, expectations are the closure will last a minimum of two weeks. Our crews have been working every day since the recent incident to develop a repair plan and schedule. This is a high priority, and we are working to correct the issue as quickly as we can.

While this work is underway, we’d like to share the low bridge work we recently completed and what’s planned for next year.

Structural rehabilitation – Completed Q4 2022

Earlier this year, we injected epoxy resin into existing cracks in the low bridge and added carbon-fiber wrapping in several locations on both interior and exterior surfaces. The carbon-fiber wrapping strengthens the bridge, much like putting a cast on an injured arm or leg.

When we add carbon-fiber wrapping to the surfaces of the bridge, it works in tandem with the steel already inside the bridge to increase overall bridge strength. 

Graphic showing the location of carbon-fiber wrapping and epoxy crack fillings in the Spokane St Swing Bridge. Graphic: SDOT.
Graphic showing the location of carbon-fiber wrapping and epoxy crack fillings in the Spokane St Swing Bridge. Graphic: SDOT.
Photo of carbon-fiber wrapping inside the bridge. Gray walls and work equipment are in the photo.
We reinforced the interior walls of the bridge’s spans by applying sheets of carbon-fiber wrapping. Photo: SDOT

Control and communications system upgrades – 2023

We use the control system to open and close the low bridge. The original system of buttons, switches, and wires is about 30 years old. Our work will include rerouting the wires connecting the control tower with the motors that open and close the bridge off of the high bridge to a new conduit under the West Duwamish Waterway. This work will increase the resiliency of the West Seattle Bridge system and help decouple the low bridge from the high bridge.

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 the low bridge is open. It also includes the communication lines that connect the computers, control tower, and the moving parts to one another.

By making these updates, we proactively address the risk of potential component failures associated with operating the bridge.

Graphic showing existing communications lines under the Duwamish Waterway.
Graphic showing new communications lines under the Duwamish Waterway.
Graphics of existing communications lines and locations for new communications lines. Graphic: SDOT

Lift cylinder replacement – 2023

Two large hydraulic lift 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 West Duwamish Waterway. The cylinder acts as a pivot point around which each span rotates to get 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 repair. In 2023, we’ll install a rehabilitated cylinder on the east side of the waterway. After the swap, we’ll inspect and refurbish the removed cylinder, replace the seals, determine if any other repairs are needed. Once this work is completed, the refurbished cylinder will go back into protected storage as a spare. 

Photo of a lift cylinder within the low bridge.
Photo of a lift cylinder within the low bridge. Photo: SDOT

Hydraulic and electrical components replacement – 2023

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.    

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