The West Seattle Bridge Community Task Force, formed as part of set of advisors to Mayor Jenny Durkan, came closer to end of their mission on Oct. 21. They reviewed both the Cost Benefit Analysis (CBA) that has taken more than 200 days to develop and a newer idea, a tied arch super structure replacement plan that was only announced last week. 

SEE THE FULL MEETING VIDEO HERE

After a brief review of bridge stabilization measures still underway and a look at the Reconnect West Seattle implementation plan with Sara Zora

the focus of the meeting shifted to the new super structure replacement idea, brought to SDOT's attention by HNTB, who have been selected to build the repair or replacement choice made by the Mayor. That decision which was supposed to happen on the 21st is now delayed.  

Two alternatives stood out in the CBA.

Alternative 2, Repair and Alternative 4, Accelerated Super Structure replacement. Repair which could put traffic back on the bridge by 2022 and could last between 15 and 40 years. It would cost $916 million total with $47 million upfront. SDOT said in the CBA that "We are not confident in how long the repaired bridge would last - it has already belied the expectations set when it was built. It could be difficult to secure maintenance funding on an annual basis. This option had a lower seismic performance compared to replacement alternatives. This alternative would ultimately require replacement and closing the bridge again."

Alternative 4, Accelerated Super Structure Replacement (this took the place of the former Alternative 4) would cost just over a billion dollars total with $383 million upfront. It would put traffic back on the bridge by 2023.

 SDOT stated in the CBA "There would be significant impacts on mobility with this option, which requires a longer bridge closure. Funding a larger up-front capital cost could be challenging, and permitting and regulatory issues could impact the timeline. Any opportunity to condense schedule would improve Alternative 4’s performance and reduce impacts on mobility." 

Neither of the best performing solutions would permit the inclusion of light rail.

Ted Zoli, Structural engineer with HNTB said he's been involved in numerous emergency bridge replacements during his 30 year career and that West Seattle's situation is "eerily similar" to many others. He explained in his review of the CBA that On alignment replacement vs off alignment replacement was usually the better choice if speed is the top priority.

In making his comparison to a similar project HNTB completed in Lake Champlain in 2011 he pointed to the need to be aware that the low/swing bridge opens 1800 times a year, where so such issue was in place in the other project. He also made the observation that "Seattle has an earthquake every 20 years so we are about due."

The Lake Champlain bridge is longer at 2000 feet vs what would be 1300 feet in West Seattle but it was completed in an 18 month time frame in part due to fast track permitting and the ability to both demolish the former structure while building the replacement at the same time. That kind of permitting is not a certainty in Seattle.

Zoli shared a professional produced animation (click the link or see below) during the presentation depicting the process by which the damaged section would be removed and the new super structure replacement would be installed.

Zoli explained that the new Accelerated Super Structure replacement option offered a number of advantages. Specifcally the potential to eliminate in-water work, low navigation impacts to float out the damaged section once removed and lowered using heavy lift techniques and float in via barge the replacement sections. He also mentioned the reduced impact on fisheries. Zoli said there would be reduced hazmat impacts, the strengthening of the foundations of the bridge could be minimized or eliminated and an opportunity for faster Coast Guard permitting might happen.

The plan would get underway once EIS work and permitting work was complete (Zoli noted it was done in six weeks in the Lake Champlain project) and benefit from an accelerated schedule. It would reuse existing piers and foundations and involve cutting and removing the damaged center span in a single piece then lowering using massive cranes to a barge where it would be floated out. While that work was underway, the replacement arch would be constructed off site then floated in. The piers on both sides of the river would be lowered and the new arched structures would sit atop delta frames that "allow the superstructure for the approaches to be redundant and extra safe," said Zoli. The resulting section would weigh 40% less that the existing bridge. 

They would aim for a 100 year service life.

The concept is not yet fully vetted no has it been shared with other stakeholders but will be presented to the Muckleshoot tribe this week. 

In the discussion that followed the Port Seattle's Peter Steinbrueck said he has serious doubts regarding funding saying, "Miracles can happen but they are very rare." He also expressed doubts around the duration and number of shut downs of the lower bridge. Zoli said he expected it would require four 24 shut downs to accommodate the float out and float in process.

Zoli was adamant that this project is more likely to get funding since "There's not one project in the U.S. more important right now."

SDOT's Greg Izzo said that if the repair option was the choice the bridge would have a similar seismic response to the bridge before it was closed. But also noted that the certainty of the repair was not clear.

Izzo went through the other choices which performed poorly in the attributes model used to rate them. In the discussion that followed Peter Goldman pointed out serious flaws in SDOT’s estimates on the immersed tube tunnel (Alternative 6) noting that other tunnels have been built faster at lower cost and pointed out that the future is linking the region together. "I absolutely get that it’s scary as hell going into the ground there. We need the right numbers and to not dismiss it. It will be easier to sell it to the federal government if we can present an integrated solution," noting that it's the only choice going forward that permits a repaired bridge to stay open as the tunnel is built and the only one that could accommodate light rail. 

Adding light rail would mean three bridges over the Duwamish at some point in the future.

Steinbrueck said he favors moving forward with either repair or the new super structure replacement to pursue funding.