WA Bridge Shut Down Indefinitely
Late last month the Seattle Department of Transportation issued an emergency bridge closure on the high-rise portion of the West Seattle Bridge after discovering that previously inspected concrete cracks along the midspan had accelerated at a rapid and unanticipated rate.
“Even in the midst of a pandemic, the Seattle Department of Transportation has been closely monitoring our critical infrastructure. Last night, our engineers identified safety risks in our West Seattle high-rise bridge and are now taking swift action to protect the public by removing traffic from the bridge while next steps are assessed,” said Mayor Jenny Durkan.
About the West Seattle Bridge
Built in 1984 and envisioned to last over 50 years, the West Seattle Bridge encompasses a six-lane cantilevered concrete structure that measures more than 150 feet high; the main span extends 590 feet long, crossing the Harbor Island and the Duwamish Waterway at the point where it enters Elliott Bay.
According to reports, the bridge cost $150 million to build and a year after its construction, was given an Honorable Mention Award from the Consulting Engineers Council of Washington. In 2009, the structure was renamed the Jeanette Williams Memorial Bridge (as a secondary designation).
Over the years, the bridge has been reported to be “overused,” seeing roughly 107,000 drivers and 25,000 transit riders per day, in addition to adding on a bus-only seventh lane in the last decade. Officials have even gone on to report that the heightened usage could be a major reason for the accelerated cracking along the midspan.
While some were shocked to hear about the issue, The Seattle Times reports that SDOT has been inspecting the infrastructure every two years—as required by federal law—with engineers only noticing unusual crack patterns in mid-2019.
Prior to the 2019 inspection, the structure received a 5-merit, based on a scale of 1-9 for both superstructure condition and structural evaluation, which was reported to the National Bridge Inventory. However, the bridge also received a 69 out of 100 for its sufficiency rating, which is a federal metric that combines strength, traffic, environmental impact and navigation.
SDOT Director Sam Zimbabwe expands on the suspected culprits, stating that, “I think it’s mostly a load issue. But a lot of the load is the bridge itself, so it’s a combination of things.” Adding that an additional eastbound lane created uneven weight distribution and that with traffic, roughly 80% of the weight is the bridge itself.
In following the initial discovery, SDOT hired outside experts to evaluate the bridge who found that the superficial cracking within the girders was actually much more serious, and in an inspection conducted in March, the issues only worsened, leading to the bridge’s indefinite closure.
Upon the discoveries made, Zimbabwe adds that officials couldn’t identify if the rebar within the concrete showed corrosion as well, or if the post-tensioning cables are at risk.
According to reports, repairs on the structure will likely take months. John Stanton, civil engineering professor at the University of Washington says that the structure could be reinforced by carbon wrap or exterior steel bracing to tighten the bridge from the outside. Although, the latter is typically a last resort.
“Transit, freight and first responders will continue to have access to the Spokane Street bridge to ensure access to and from West Seattle. To the residents and businesses of West Seattle: I want to thank everyone for their flexibility and patience during this challenging time in Seattle’s history. It is a top priority to ensure safety and access to goods and transit, and we will be working as quickly as we can resolve this.”
The Federal Highway Administration is currently monitoring the situation.