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SDOT Hires Designer for Bridge Replacement

Thursday, August 27, 2020

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The Seattle Department of Transportation has selected HNTB Corporation to design the West Seattle Bridge replacement, though the timing of that replacement remains unclear as the department is still completing an evaluation on whether to repair or replacement the bridge in the immediate future.

The department issued an RFQ in early June, three months after the bridge’s abrupt closure in March.

West Seattle Bridge Closure

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.

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.

Sounder Bruce, CC-SA 4.0, via Wikimedia Commons

The Seattle Department of Transportation has selected HNTB Corporation to design the West Seattle Bridge replacement, though the timing of that replacement remains unclear as the department is still completing an evaluation on whether to repair or replacement the bridge in the immediate future.

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.

Temporary Fix

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.

Cost-Benefit Analysis

A few weeks ago, the Technical Advisory Panel released an official statement sharing that the information they have reviewed indicates repairs appear feasible and recommend SDOT continue to explore the repair option fully.

The criteria for the cost-benefit evaluation includes constructability, environmental impact, equity, forward compatibility, funding, maintenance and operations, mobility, multimodality, regional business impact, and seismic resilience and safety.

Currently, the analysis framework is considering six options, which include:

  • Concept 1: Shoring to Restore Travel Capacity - A comprehensive temporary shoring system to return some travel capacity would add an external steel structure to the existing bridge to support traffic. There are a couple different approaches that could be taken. Steel support beams could be added under the bridge called a “K Frame Shoring,” or a Steel Truss Shoring system could be added along the top of the bridge. It is estimated that traffic could be restored in 2022 and likely add three or more years of use to the bridge.
  • Concept 2: Repair to Restore Traffic Capacity - Repairs would include similar, but more comprehensive measures to stabilize the bridge. Repairs would likely include more carbon fiber wrapping, new steel strengthening lines in the post tensioning system within bridge girders, and foundation strengthening. It is estimated that traffic could be restored in 2022 and has the potential to add more than 15 years of use to the bridge.
  • Concept 3: Partial Superstructure Replacement with Foundation Strengthening - A partial superstructure replacement would involve removing the center span of the high-rise bridge between piers 16 and 17 only and replacing it with a new steel box girder. This process would also involve some foundation work to ensure that the bridge is stable. It is estimated that traffic could be restored in 2022 and likely add more than 15 years of use to the bridge.
  • Concept 4: Full Superstructure Replacement with Foundation Strengthening - A full superstructure replacement would involve replacing the three high-rise spans of the bridge between piers 15 and 18 while reusing the same foundation. The bridge could either be constructed out of concrete with a post-tension steel system like the current bridge or it could be replaced with steel box girders. The foundations would also be strengthened during the superstructure replacement. It is estimated that traffic could be restored in 2025 and likely add more than 50 years of use to the bridge.
  • Concept 5: Full Replacement in Same Footprint - For this route, SDOT is examining replacement options that are in the same footprint or alignment as the current bridge. Different bridge options include a steel-truss bridge or a cable-stayed bridge. It is estimated that traffic could be restored by 2026, and the bridge would likely be useable for more than 75 years.
  • Concept 6: Tunnel Replacement - A replacement tunnel would go under the water, in roughly the same footprint of the existing bridge. None of the current bridge, including foundations, would be reused in a tunnel scenario. It is estimated that traffic could be restored by 2026, and the tunnel would likely be useable for more than 75 years.

The analysis aims to be complete by early October, and SDOT reiterated that it is still simultaneously searching for a design/engineer team for a potential replacement, as well as beginning the current stabilization of the bridge.

What Now

SDOT said that bringing in a design firm now will ensure that the city is in a place to pivot quickly should the cost-benefit analysis show that an immediate replacement is the way to go.

“Restoring travel capacity across the Duwamish is our top priority” said SDOT Director Sam Zimbabwe. “Everywhere possible, we have simultaneously pushed forward multiple bodies of work to expedite progress for the communities in and around West Seattle, the Duwamish Valley and the whole Puget Sound region. Today’s announcement of our selection of HNTB to design the eventual replacement of the West Seattle High-Rise Bridge epitomizes these efforts."

While SDOT’s analysis is ongoing, one of HNTB’s first tasks will be to complete its own study to determine the type, size and location of several bridge and tunnel replacement options.

“HNTB is honored to work with the City of Seattle to plan for the replacement of this critical piece of transportation infrastructure,” said Kevin Collins, PE, HNTB Seattle office leader and vice president.

“The first step is selecting the right solution. HNTB and our partner firms are committed to working with SDOT, the Community Task Force and Technical Advisory Panel to help the City develop, evaluate and advance alternatives to quickly restore mobility in this corridor and alleviate the impacts of the bridge closure on our underserved neighborhoods and regional economy.”

   

Tagged categories: Bridges; Bridges; Contract awards; Health & Safety; Health and safety; NA; North America; Safety

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