Seattle Mayor Announces Plan to Fix Bridge
Last Thursday (Nov. 19), Seattle Mayor Jenny Durkan announced that the city had reached a decision regarding the repair or replacement of the West Seattle Bridge.
Because the approach decks are still in good condition, officials have decided that repairing the structure is the quickest, more cost effective option.
Bridge Closure Background
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 Seattle 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.
The goal is for normal traffic to return to the West Seattle Bridge by mid-2022.Posted by Seattle Met Magazine on Friday, November 20, 2020
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 in 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 span 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.
Work began on a temporary fix for the bridge in early July, which included wrapping sections with carbon fiber reinforced polymer (CFRP) and installing post-tensioning tendons.
SDOT detailed a multi-step plan for the emergency shoring that begins with reopening more than 100 traveler holes that were filled following the original construction.
The leftover holes are to be opened with water jets and aim to provide a place to fasten the hanging platforms the workers need to fasten the carbon wrap with epoxy. Carbon will also reportedly be attached inside the girders. That task is expected to start later this month and last 10 weeks.
The second stage involves stringing additional steel cables across the central span of the bridge—or post-tensioning. To anchor those cables, steel blocks will be fastened to the concrete underside. That phase is slated to be complete about two weeks after the carbon wrapping.
Then, Kraemer will proceed with released a stuck rubber bearing that’s on top of Pier 18, an issue that hinders normal thermal deck expansion and contraction.
In the plan announcement, SDOT reiterated that a cost-benefit study by engineering firm WSP is scheduled to be complete in early fall, which will help dictate if the city should repair the bridge or demolish it.
“Though our recent analysis indicates that repairing the West Seattle High-Rise Bridge may be possible, we still do not know how much repairs would cost, how long they would take, how many lanes could be restored, and whether repairs would last long enough to be a worthwhile investment,” the statement said.
It's that unknown price tag, as well as the unknown of where the money would come from, that prompted the emergency declaration as such a label might make funding easier, according to officials.
Weighing the Options
In September, the Technical Advisory Panel released an official statement sharing that the information they reviewed indicated repairs appeared feasible and recommended SDOT continue to explore the repair option fully.
The criteria for the cost-benefit evaluation included constructability, environmental impact, equity, forward compatibility, funding, maintenance and operations, mobility, multimodality, regional business impact, and seismic resilience and safety.
However, by October, SDOT announced that its analysis was ongoing, although it had tapped HNTB to design a tentative replacement. The company’s first tasks included completing its own study to determine the type, size and location of several bridge and tunnel replacement options.
In October, Durkan announced that solutions teams were discussing height requirements that either solution would need to meet. According to The Seattle Times, if the bridge was replaced, the new span might need to be even higher than the current 140-foot-tall span.
If the extra height is needed (which would be dictated by the Coast Guard), that would hike up costs by another $140 million as approach decks would have to be replaced as well as possible new columns. And, if a replacement is chosen that combined motor vehicles with a light rail, a complete corridor replacement would be required.
However, should the choice be made to repair the bridge, the city would be able to forego the height question for another 15-40 years, which, for some, is a point for the replacement column.
Repairing the West Seattle Bridge
In an effort to retore traffic to its 125,000 daily travelers faster, Durkan has announced that the Seattle Department of Transportation will be repairing the West Seattle Bridge, instead of replacing the entire structure.
Repairs are expected to extend the life of the bridge by between 15 and 40 years, using additional “post-tensioning” as its primary method for rehabilitation. The project is estimated to cost $47 million, only a fraction of what a $390 million to $522 million steel-arch replacement bridge would have been.
The decision follows Congress briefings to Durkan, which informed her that the federal government wouldn’t be acting on an infrastructure bill to make bridge grants available until mid- to late next year.
However, The Seattle Times reports that SDOT will still conduct a type, size and location study for an eventual replacement, which is expected to include bridge concepts and shallow immersed-tube tunnel.
In the meantime, SDOT Director Sam Zimbabwe says that the city has since hired WSP, its bridge consultant, to design the repair project following emergency stabilization earlier this year. Thus far, the firm is reported to have produced more than 400 pages of technical findings about the seismic qualities and internal girder forced of a repaired bridge.
Once the bridge is repaired, the SDOT has pledged to conduct full inspections twice a year with specialized worker-lift trucks, in addition to spending extra time on increased tensioning steel volume and to watchdog a full electronic network of monitoring devices.
The bridge is slated to return to full use by mid-2022, followed by the launch of collaborations with Sound Transit for the design build of a multimodal crossing by the early 2030s. The crossing will serve light rail, personal vehicles, freight and bicycles.