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WA Bridge Begins Phase 2, Looks for Contractor

Wednesday, February 17, 2021

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Upon completing stabilization measures at the West Seattle Bridge, the city’s Department of Transportation is now entering phase two of the project, which will involve designing the full rehabilitation of the structure.

Seattle DOT reports that it will be bringing a contractor on board for this phase.

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.

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.

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.

Seattle Department of Transportation

Upon completing stabilization measures at the West Seattle Bridge, the city’s Department of Transportation is now entering phase two of the project, which will involve designing the?full rehabilitation of the high-rise structure.

After 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.

Temporary Fix & Sensor Technology

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 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, 2020, 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, Durkan and SDOT have formed 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 was expected to start later that 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.

In the plan announcement, SDOT reiterated that a cost-benefit study by engineering firm WSP was scheduled to be completed in early fall, which would help dictate if the city should repair the bridge or demolish it.

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.

The following month, in an effort to retore traffic to its 125,000 daily travelers faster, Durkan announced that SDOT would repair 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 reported at the time that SDOT would 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.

Rehab Progress

In December, SDOT reported that crews were removing work platforms from the West Seattle Bridge, marking the end of emergency measures to stabilize the structure and concluded preparations for upcoming repair efforts.

According to the Department’s blog, crews were able to successfully install and stress the post-tensioning system, use epoxy crack fillings, install carbon fiber-reinforced polymer support wrappings on sections of the structure and Pier 18’s bearing release and replacement.

Last month, after work platforms were lowered crews began offsite disassembling the platforms. In addition, crews also reported that they would be finishing the last round of coating applications on the post-tensioning brackets. The effort is slated to protect the new system.

Now, while teams enter the preliminary design milestone, SDOT has also launched the process of selecting a contractor who can aid in the infrastructure’s rehabilitation. While a contractor isn’t usually brought onboard a project until a final design is reached, SDOT believes that bringing in someone early will promote a better collaboration between designer and contractor so that innovative solutions can be obtained and construction schedules accelerated.

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.

Since the structure’s closure, SDOT reports that it’s completed over 190 traffic improvements in the area, 20 of which were identified through its community ballot process last summer. These projects work hand-in-hand to keep communities moving throughout the closure—with more on the way this year.

   

Tagged categories: Bridges; Bridges; Infrastructure; NA; North America; Ongoing projects; Program/Project Management; Project Management; Rehabilitation/Repair

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