Weekend Reads | Evaluating the West Seattle Bridge Repair One Year Later

by Kevin Schofield

This weekend's read is a report published by the Seattle Department of Transportation (SDOT) last month on a question that may be top of mind for a bunch of South Seattleites: How's the West Seattle Bridge doing?

The bridge, which was closed in March 2020 after cracks in the long span over the Duwamish River began rapidly increasing, was repaired over the next two years and reopened to much fanfare and relief in September 2022. SDOT's new report, which was issued by a bridge construction consultant hired by the department, summarizes the data on how the bridge is holding up after one year of use.

As anyone who has ever built or owned a concrete structure probably knows, concrete often cracks; in fact, cracking is considered "normal." Over its lifetime, concrete will continue to dry, harden, and shrink; it also expands and contracts with temperature and weather. All of that leads to some inevitable cracking, most of which is considered harmless and inconsequential to its structural strength. But there are two kinds of cracks in concrete that engineers particularly worry about: first, ones that are deep enough to expose the steel rebar inside the concrete to air and moisture that would eventually lead to rust and corrosion; and second, "shear cracks" that signal that there are forces being applied to the concrete that are greater than what it was designed to withstand. As with all concrete bridges, the West Seattle Bridge saw some minor cracking that was being monitored and sealed to ensure the steel rebar was protected, but what shut it down was the discovery of shear cracking on both sides of the central span that rapidly increased and could be interpreted as leading to an eventual failure of the bridge.

The central part of the bridge was supported by several massive steel cables that ran the length of the span and were pulled taut and anchored on both ends. But over time, the cables stretched a bit, placing more of the load on particular parts of the concrete. Also, the way the bridge was being used by traffic changed (more lanes and heavier traffic), which changed how the load was transferred into the bridge structure. Engineers determined that these were factors contributing to the shear cracking. The repairs were twofold: Several additional steel cables were added to the existing ones to provide more support (and the existing ones were tightened up), and the shear cracks were wrapped in carbon-fiber sheets to give them some additional protection from the elements and strength to resist further cracking.

SDOT also added an electronic monitoring system to the bridge to provide 24/7 surveillance of the structure and the existing cracks, so SDOT will know immediately if anything changes. The data from the monitoring system also lets the department validate its own analytical model of the bridge and how it's behaving. The West Seattle Bridge, even after repairs, is not a static, immovable thing: It continues to expand and contract with the temperature, sway in the wind, and shake in earthquakes. Bridges (and buildings) are designed not to hold still, but instead to be flexible enough to withstand all of those forces without breaking. SDOT has a theoretical computer model of what the newly repaired bridge should do based on the weather, the traffic load, and other factors. The data from the monitoring system lets them calibrate that model and also validate that it's accurately predicting the bridge's real behavior. The monitoring system has instruments that track the temperature inside and outside the bridge, along with the overall movement, expansion, and contraction of the bridge. It also has sensors sitting on top of the shear cracks that can detect if the cracks widen or if the concrete on either side of the crack "slip" sideways across each other.

So, what did the one-year report say? Pretty much all good news. The movement of the bridge in the past year has been less than before the repairs were done, suggesting the repairs did indeed strengthen the structure (so far). And the report says there have been no unexpected movements in the cracks. Further, SDOT's computer model has accurately predicted the actual motion of the bridge, across four seasons of Seattle weather.

Over the past year, SDOT has also conducted frequent physical inspections of the bridge, both inside and outside. As part of the repair work, it added new inspection platforms and new internal lighting to allow for better up-close inspections. And this past June, the department conducted a full bridge inspection, including using the "under-bridge inspection truck" — a truck that parks on top of the bridge and lowers a platform down over the side so workers can inspect the underside up close. The inspectors found "no signs of distress" on the bridge. They did find some minor maintenance issues as a result of the repairs: some minor, expected cracking in the newly-poured concrete; a bit of water leakage around the holes they cut in the bridge to get repair equipment inside; and a bit of corrosion in places where the repair work scraped the protective coating off bits of steel cable.

SDOT intends to continue its frequent inspections for one more year, at which point, if all continues to go well, they will switch to the federal Department of Transportation requirement for physical inspections every two years. They will, however, continue to keep the 24/7 electronic monitoring system in place so they will know immediately if something starts to change.

West Seattle High-rise Bridge — One Year Since Opening Monitoring Summary

Featured image by Ian Dewar Photography/Shutterstock.com.