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SDOT Weighs Analysis of West Seattle Bridge

Tuesday, June 2, 2020

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Since the Seattle Department of Transportation issued an emergency closing of the West Seattle Bridge because of cracks in the midspan, several professional engineers recently issued a seven-page failure analysis, explaining next steps needed to support the degrading infrastructure.

Seattle Mayor Jenny Anne Durkan, along with SDOT, have also gone on to form two advisory boards that will help to decide best possible outcomes.

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).

SounderBruce, CC-BY-SA-4.0, via Wikimedia Commons

Since the Seattle Department of Transportation issued an emergency closing of the West Seattle Bridge because of cracks in the midspan, several professional engineers recently issued a seven-page failure analysis, explaining next steps needed to support the degrading infrastructure.

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 reported 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.

While 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. However, the latter is typically a last resort.

In April, the Federal Highway Administration was reported to have been monitoring the situation.

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.

Possible Outcomes

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.”

The more likely scenario, involving the partial collapse or complete failure of the bridge could, however, happen one of two ways. The first, involves symmetry where pieces of concrete detach and begin falling from the bridge at both critical locations simultaneously, eventually causing a portion of the main span bridge box girders to separate and fall as well.

On the other hand, an asymmetrical collapse is also possible, where concrete pieces would detach and fall at one location, causing a shorter portion of the main span bridge box girder to separate and fall.

In both scenarios, the columns could be damaged through the distribution of unbalanced loads, as well as the further damage to the back spans between Piers 15 and 16, and Piers 17 and 18. Additionally, the second mode of both scenarios would require the partial or complete demolition of the structure.

With their findings, the engineers have recommended the following mitigations needed in light of the partial collapse risk:

  • Continue daily visual inspections of the structure;
  • Implement an automated survey system that collects data in real time, with manual surveys in the near term until the automated system is functional;
  • Implement localized deformation data logging using an automated system that will report total deformation across multiple cracks;
  • Undertake non-destructive testing of select vertical post-tensioned tendons in the webs;
  • Design and construct interim repairs at the distressed locations to arrest the crack propagation in the near term;
  • Repair the bearings at Pier 18 that are restricting thermal expansion and contraction movements of the structure;
  • Design, fabricate, and deploy temporary shoring to support the bridge in case of partial or multi-span superstructure collapse;
  • Evaluate full repair alternatives relative to the potential need for bridge replacement; and
  • Design and construct full repairs if feasible or demolish the bridge and plan for a bridge replacement.

“It’s really just managing the fall zone, so that if pieces were to fall, the fall zone would cover less area,” Alex Pedersen, chairman of the City Council’s Transportation & Utilities Committee, said.

Interagency Task Force and Emergency Response Plan

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.

Confirmed TAP participants include: Stephen Dickenson, PhD, PE, DPE, New Albion Geotechnical, Inc.; Gregg A Freeby, PE, American Segmental Bridge Institute; Reggie Holt, PE, Federal Highway Administration, Office of Bridges and Structures FHWA Headquarters, Washington D.C.; Debbie D Lehmann, PE, Federal Highway Administration Washington Division Office; Barbara Moffat, PE, SE, Stantec; Scott Phelan, PhD, PE, SE, David Evans and Associates, Inc.; and Professor John F Stanton, University of Washington.

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.

“The state of the West Seattle High-Rise Bridge is complex and unique,” said Zimbabwe. “Through consistent inspections and an expanded intelligent monitoring system our talented team of engineers will have the data necessary to plan next steps.

“We are grateful to have the experts on our Technical Advisory Panel provide their review and insight to make sure we consider every possible avenue to reconnect West Seattle and the rest of the city. Equally important are the members of our West Seattle Bridge Community Task Force who will lift voices of the community so we can build a stronger, more connected Seattle.”

Together, the group has planned responses for three scenarios, involving immediate evacuation to be used if daily in-person inspections?indicate enough of a change, a one-to-five day notice if the monitoring instrumentation indicates enough of a change, and controlled demolition should the condition of the high bridge indicate the need for execution.

   

Tagged categories: Bridges; Bridges; Department of Transportation (DOT); Engineers; Failure analysis; Health and safety; Infrastructure; Inspection; NA; North America; Ongoing projects; Project Management; Quality Control; Rehabilitation/Repair; Safety

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