SDOT to Solicit Engineers for Bridge Replacement
At the beginning of the month, the Seattle Department of Transportation issued a request for Statements of Qualifications from qualified engineering firms to design a replacement for the West Seattle Bridge.
According to the request, the contract for the project is estimated to cost between $50 million and $150 million.
West Seattle Bridge Closure
In late March, SDOT 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.
Built in 1984 and envisioned to last more than 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 Times reported that SDOT had 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.
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.
In April, the Federal Highway Administration was reported to have been monitoring the situation. At the end of the month, SDOT announced that it had selected Wisconsin-based Kraemer North America to complete Phase 1 of bridge stabilization work. Work for the first phase involves the repair of lateral bearings on Pier 18.
In Phase 2, Kraemer plans to add temporary external structures—otherwise known as “shoring”—and will determine the third phase if long term repairs are feasible.
By mid-May, the SDOT blog reported that it had been installing a new intelligent monitoring system on the structure, consisting of additional movement sensors, crack monitors and monitoring cameras. The monitoring instrumentation is expected to improve the Department’s understanding and tracking of the health of the West Seattle High-Rise Bridge with more precision and in real time.
Around the same time that the sensors were installed, Durkan requested a failure analysis of the structure so that emergency preparedness could be improved.
Although the report was initially due by the end of April, on May 15, Greg Banks, PE SE; Lee Marsh PhD PE; Bob Fernandes, PE SE; Kare Hjorteset, PE SE; and Chad Goodnight, PhD PE, released their findings, pointing out the need for an evacuation of a long “fall zone” from the Pigeon Point greenbelt to Harbor Island, in addition to possible support solutions for the 150-foot-high main span.
While the engineers did not provide odd estimates of a potential collapse or when it could happen, they did go over two possible outcomes involving the event of a total failure or partial collapse, and another possibility involving the progression of cracks which would then cease once the bridge stabilized itself.
According to the report, “The bridge is currently exhibiting progressive crack growth at two critical locations (Joints 38) of the four quarter points of the twin-box main san between Pier 16 and Pier 17. This is where the first failure mechanism has appeared. While a progressive failure does not mean collapse is imminent, it does illustrate an unintended redistribution of forces within the bridge that could lead to further damage.
“The cracks, without any mitigation, could stop, and the bridge could redistribute load until internal forces stabilize. However, it is not considered likely as the bridge will continue to creep (slowly deform under static load) over time and thus continue to crack.”
Until some or all mitigation efforts are implemented, Mayor Durkan and SDOT recently announced the formation of a Technical Advisory Panel and a West Seattle Bridge Community Task Force.
The task force includes the City of Seattle, King County, Washington State, Port of Seattle, Northwest Seaport Alliance, the United States Coast Guard and the U.S. Army Corps of Engineers. If necessary, in the event of a collapse a unified command will be led by the Seattle Fire Department, SDOT, the Seattle Police Department and USCG.
The two advisory boards are slated to play important, complementary roles in the city’s collective effort to push ahead with the best path for the high rise bridge, in addition to mitigating the impacts of the closure on West Seattle, the Duwamish Valley, and surrounding communities.
What’s Happening Now
SDOT has recently issued an SOQ for engineers to describe their qualifications and experience in regard to designing a partial or full replacement of the West Seattle Bridge. Specifically, engineering teams must prove that they’ve completed at least three government-owned bridges, each worth at least $200 million.
If awarded, the engineering team would oversee both the possible demolition of the West Seattle Bridge and construction of the new infrastructure. The repairs or full replacement would have to be capable of serving roughly 125,000 daily travelers and be able to withstand a “2,500-year event” due to its location next to the east-west Seattle fault.
“Whether we rebuild the bridge would be decided in the next six weeks,” said Heather Marx, Director of Mobility for SDOT. “How to replace it would not be decided in the next six weeks.”
While the solicitation doesn’t mean the city has abandoned bridge repair options, the SOQ does span for potentially 10 years, should the bridge be repaired but requires a new bridge later. Should the city decide to immediately demolish and build a new bridge, however, Marx hopes that a few years would be shaved off the contract.
Other possible solutions to the bridge issue include a shallow immersed-tube tunnel, or a gondola. The West Seattle Bridge Community Task Force is slated to host meetings on the on the infrastructure’s repair or replacement in the near future.