West Seattle Bridge closes unexpectedly on March 23, 2020.

  • By Phil Dougherty
  • Posted 5/27/2020
  • HistoryLink.org Essay 21034

On March 23, 2020, the Seattle Department of Transportation (SDOT) unexpectedly closes the West Seattle Bridge after rapidly expanding cracks are found in its support structure. Further investigation reveals that the damage is more significant than first realized, and within a month SDOT announces that the bridge will be closed for at least two years. The agency adds that the damage is so severe it's not clear whether the bridge can be repaired. Even if a repair is successful, engineers estimate the bridge will need to be replaced within 10 years, nearly 30 years short of its original projected lifespan.

Monitoring the Spread

The West Seattle Bridge, built over the mouth of the Duwamish River to connect Seattle with the western part of the city, consists of three spans totaling 1,340 feet long. The 590-foot-long center span, which passes over the Duwamish West Waterway, rises to 140 feet at its apex to allow boat traffic to pass underneath. The bridge was dedicated on July 14, 1984, and had a projected life expectancy of 75 years. It was built to handle six lanes of traffic (three eastbound and three westbound), but a fourth eastbound lane was later added. A smaller swing bridge, known as the lower bridge, opened below and immediately to the north of the highrise bridge in 1991.

SDOT typically inspected the West Seattle Bridge every two years. There were no concerns until 2013, when SDOT first noticed cracking on the center span's box girders, which are the main horizontal support beams below the roadway. The cracks were located on either side of the keystone, the section of the roadway in the middle of the center span that was poured last during construction. Initially, the cracks weren't a big concern -- minor cracking is considered normal and is expected over time. Nevertheless, SDOT began inspecting the bridge yearly and filling the cracks with epoxy as they appeared. In 2014, the agency set up equipment onsite to monitor the cracks remotely. The cracks continued to slowly spread until 2019, but that summer SDOT discovered they were growing more rapidly and in response began monitoring the bridge monthly.

The cracks continued to grow. On February 21, 2020, SDOT's engineering consultant recommended that traffic on the bridge be reduced from seven lanes to four in order to reduce stress on the bridge. SDOT continued to evaluate the problem but did not notify the city or the public. An inspection on March 6 showed that the cracks had spread further, and on March 19 the consultant told SDOT that its most recent analysis raised even bigger concerns that could lead to a recommendation to close the entire bridge.

An Unpleasant Surprise

On Monday morning, March 23, SDOT's Director of Roadway Structures, Matt Donahue, went inside the center archway of the bridge for a follow-up inspection. The previous cracks had been clearly marked with colored lines, and Donahue could see that some of the cracks had spread beyond the marked lines by as much as 2 feet in little more than two weeks. Shocked, he called the deputy capital projects director of SDOT, Lorelei Williams, as well as the director of SDOT, Sam Zimbabwe, and recommended that the bridge be closed immediately. Mayor Jenny Durkan (b. 1958) quickly approved the recommendation, and at 3 p.m. on March 23, SDOT announced that the bridge would be closed at 7 p.m. that evening.

Within a few days, SDOT provided historical details of the problem on its blog in order to assure an unpleasantly surprised public that it had been on top of it all along. Zimbabwe and Donahue also appeared at the Seattle City Council meeting on March 30 to explain what had happened and to answer questions. The council asked why it was not advised of the problem in late February when SDOT had first received a recommendation that bridge traffic be reduced. Zimbabwe responded that the cracking did not initially appear to be so serious that it would disrupt traffic. In its blog, SDOT added that the problem accelerated so rapidly that it didn't have time to draft a plan of action to review with the city and the community before it became necessary to close the bridge.

Long Road Ahead

The news got worse. On April 15, SDOT announced that the bridge would remain closed until at least 2022 in order to stabilize it and to then make repairs, if possible. Zimbabwe explained that SDOT had not yet determined whether the bridge could be repaired but warned that even if it could, a repair would provide at most an additional 10 years of life for the bridge. A front-page story in The Seattle Times on May 18 reported that the bridge's center span needed be shored up immediately to prevent a possible partial, or even total, collapse, which could also damage the adjoining spans. SDOT quickly issued assurances that a collapse was not imminent, but urged residents to sign up for online notifications with Alert Seattle (the city's  emergency notification system), "in the event the high rise bridge moves toward failure" ("Report: Time Running Out ... ").

The West Seattle Bridge had been subject to more than 35 years of normal wear and tear -- by 2020, an average of 100,000 vehicles and 25,000 transit riders crossed it daily -- and it had also been shaken by as much as 3 inches during the 2001 Nisqually earthquake. An early suspected cause of the cracking was the increased traffic on the bridge, particularly after the seventh lane was added. The location of the cracks, on the girders at the top of the bridge span and farthest from the support piers, was at a point where the bridge was under the most stress. The cracks ran along the sides and bottom of the girders in a symmetrical pattern on both sides of the span's keystone, which also suggested a weight issue. However, another suspect was soon identified: a possible design flaw in the bridge itself that underestimated the expected amount of "creep," a condition in which the concrete within the girders shrinks over time. The resulting volume loss allows the steel cables in the girders to slacken, weakening the concrete further and making it more susceptible to cracking.


Sources:

Matt Markovich, "West Seattle Bridge Closure:  What Happened, When and Why," KOMO News, March 31, 2020, website accessed April 23, 2020 (https://komonews.com/news/local/west-seattle-bridge-closure-what-happened-when-and-why); Mike Lindblom, "West Seattle Bridge Closure Already Causing Snarled Traffic on Bridge Below," The Seattle Times, March 30, 2020, website accessed April 23, 2020 (http://www.seattletimes.com); Mike Lindblom and Heidi Groover, "West Seattle Bridge Will Stay Closed Through 2020 and 2021," Ibid., April 15, 2020, website accessed April 23, 2020 (http://www.seattletimes.com); Mike Lindblom, "What Cracked the West Seattle Bridge?" Ibid., May 11, 2020, website accessed May 18, 2020 (http://www.seattletimes.com); Mike Lindblom, "Report: Time Running Out to Salvage West Seattle Bridge, " Ibid., May 18, 2020, website accessed May 18, 2020 (http://www.seattletimes.com); "What happened to the West Seattle Bridge?" March 28, 2020, Seattle City Council Insight website accessed April 23, 2020 (https://sccinsight.com/2020/03/28/what-happened-to-the-west-seattle-bridge/); Sara Davis, "West Seattle High-Rise Bridge Will Close Tonight at 7 pm," March 23, 2020, SDOT website accessed April 23, 2020 (https://sdotblog.seattle.gov/2020/03/23/traffic-advisory-west-seattle-high-rise-bridge-will-close-tonight-at-7pm/); "How We Caught the West Seattle High-Rise Bridge Deterioration," March 26, 2020, SDOT website accessed April 23, 2020 (https://sdotblog.seattle.gov/2020/03/26/how-we-caught-the-west-seattle-high-rise-bridge-deterioration/); Sara Davis, "Recap of March 30 Seattle City Council Briefing on West Seattle High-Rise Bridge," SDOT website accessed April 23, 2020 (https://sdotblog.seattle.gov/2020/03/30/recap-of-march-30-seattle-city-council-briefing-on-west-seattle-high-rise-bridge/).


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