WEST SEATTLE BRIDGE CLOSURE: New memo examines how a collapse might unfold, and prevention recommendations

(SDOT graphic)

If all or part of the West Seattle Bridge collapsed – how exactly might that happen? SDOT has just released a new memo from its consultant WSP, part of the ongoing work to determine the bridge’s future (or lack of one). Here’s the 7-page memo (4 pages of text and 3 of graphics), which warns, “This bridge’s issues are unique, and we are not currently able to indicate the likelihood of any of the potential failure scenarios”:

(It’s also here, in PDF.) If you don’t have time to read the whole thing, here’s the heart of it, from the SDOT Blog post releasing the memo:

… The memo identifies 9 proactive steps to prepare for, and potentially prevent, these worst-case scenarios. We have already begun work on all of them.

… The steps to better understand and monitor the structural integrity of the bridge include:

1. Continue the daily visual inspections of the bridge

2. Implement an automated intelligent monitoring system that collects data in real time

3. Implement localized data logging using an automated system that will report total deformation across multiple cracks

4. Undertake non-destructive testing of select vertical post tensioned tendons

All these steps are underway. We have conducted in-person visual inspections of the bridge every day since March 20. We have nearly completed installation of the intelligent monitoring system that includes 8 high-resolution cameras, 16 movement sensors, and 52 vibrating wire sensors to monitor cracking.

Our structural engineering consultant has completed about 30 percent of the 100+ non-destructive tests we plan to conduct. This includes using ground penetrating radar to create an image of cavities and voids deep within the bridge concrete and identify whether there is any corrosion around the steel support tendons. We look forward to sharing more about this incredible technology and the important role it plays in a future blog post.

The steps to stabilize the bridge and potentially prevent bridge failure include:

5. Design and construct interim repairs at the distressed locations to arrest the crack propagation in the near term.

6. Repair the bearings at Pier 18 that are restricting thermal expansion and contraction movements of the structure.

7. Design, fabricate, and deploy temporary shoring to support the bridge in case of partial or multi-span superstructure collapse.

8. Evaluate full repair alternatives relative to the potential need for bridge replacement.

9. Design and construct full repairs if feasible or demolish the bridge and plan for a bridge replacement.

Meantime, as reported two weeks ago, there’s now an emergency-response plan for what would happen **if** a collapse seemed imminent – or close to it.

32 Replies to "WEST SEATTLE BRIDGE CLOSURE: New memo examines how a collapse might unfold, and prevention recommendations"

  • skeeter May 18, 2020 (12:39 pm)

    Just so I understand.  Is the “controlled demolition” contingency designed to make sure the Spokane Street Bridge does not get damaged?  Or asked another way — the Spokane Street Bridge is in the “Fall Zone” of the high-rise bridge.  Would a controlled demolition be able to be designed in a way that the debris would fall either straight down or to the south to protect the Spokane Street Bridge?  

    • BBILL May 18, 2020 (2:20 pm)

      You mean protect the “critical habitat,” the Chinook estuary below, I hope.

    • Vic May 18, 2020 (2:37 pm)

      As I understand, as I’ve seen this discussed on the blog many times already, a controlled demolition of which you speak would include supporting the parts that need demolition (aka shoring up) and then removing piece by piece. They can’t just let it fall/implode to demolish it for reasons you have stated (the low bridge and businesses underneath and in the fall zone.

  • Aerial Observer May 18, 2020 (1:01 pm)

    Thanks WSB for this very good (and very fast!) summary of the e-mail we all received earlier today. As it makes clear, SDOT is moving as fast as safely possible on the West Seattle High Bridge. There will be no quick fixes or easy solutions, no matter how much everyone might want them. 

    • WSB May 18, 2020 (1:22 pm)

      Well, not “all.” SDOT says 2,000 people are on its bridge-related email list. That leaves 98,000 or so West Seattleites who didn’t receive it, many of which read WSB, so that’s why I summarized it, with full credit to SDOT Blog (as linked above), which is where I saw it first. And as for “fast,” we’d usually be faster, but this was unexpected and I had to get through a few other things first. – TR

      • Aerial Observer May 18, 2020 (1:28 pm)

        My apologies for any confusion. My use of “all” was meant to include everyone on the e-mail list, not all readers of WSB. Your summary remains timely and excellent.

  • WSeattleite May 18, 2020 (1:22 pm)

    Thanks for the excellent summary WSB! I feel that SDOT is doing the best they can and I appreciate the regular updates. 

  • Craig May 18, 2020 (2:00 pm)

    Remember a few years ago when we thought an earthquake would take out the Viaduct, but the WS bridge was stable? Those cracks during an earthquake will be enlarged for sure. Glad to see a contingency plan for how the bridge will fall when/if we get another Nisqually quake. 

    • Boop May 18, 2020 (9:55 pm)

      It ain’t gonna fall straight down in an earthquake!

    • Ex-Westwood Resident May 19, 2020 (2:49 pm)

      IIRC, the cracks were first discovered 7 years ago, in 2013. That would be TWELVE (12) years AFTER the Nisqually earthquake.

      All bridges and raised roadways were inspected rather vigorously after that quake if you remember, and there was NO mention of ANY damage to the high or low bridge due to the 6.8 quake. So the quake in 2001 had NOTHING to do with the current demise of the WSB. Instead, imho, poor design, coupled with substandard materials and a good bit of graft got us a bridge that was “supposed” to be a 75 years bridge, but lasts less than half that.

      Amazingly, bridges like the Brooklyn Bridge, Golden Gate, and many others, have been around for YEARS longer than the WSB, and they are still standing.

  • Recall Durkan May 18, 2020 (2:21 pm)

    Thanks WSB. I feel SDOT should be taken to task for being too lazy to fix this bridge in 2013. They are NOT doing a good job and they need to be reminded every day how this crisis is going to harm our community. Seattle leadership is too lazy to keep a bridge open and too incompetent to fix it. Those who cheer on SDOT are part of the problem. Its not surprising Seattle cant keep a bridge operational when constituent standards are so low as to cheer for 30% complete. Stop fiddling while the bridge sits empty. Fix the bridge Durkan. Get to work!!

    • BBILL May 18, 2020 (4:08 pm)

      In case you missed it, the West Seattle Bridge was properly maintained, the cracks were repaired. “properly” in so far as “best practices” were being followed.

      • Chemist May 18, 2020 (9:34 pm)

        No, the crack filling/epoxy is not any sort of structural repair and the consultant reports made it clear that they’re just excluding the elements to reduce further deterioration due to open cracks.  From back in the Jan 2019 BDI report – “STRUCTURAL CONDITION AND SAFETY  Hopefully the cracks do not present a concern from a load capacity or safety viewpoint, regardless the cracks do pose a threat to the long-term performance as they allow for the ingress of moisture, chlorides, and oxygen which can accelerate
        corrosion of the reinforcing steel
        . Therefore, it is recommended that all cracks in the bottom flange and the vertical webs be sealed with epoxy injection. In addition, stiffening procedures should be considered, as epoxy injection will not significantly reduce the segment flexibility. Increased flexibility resulting from the existing cracks is currently a driving mechanism for continued crack growth. If stiffening procedures are not implemented, epoxy injection may be a re-occurring procedure for the life of the

        • BBILL May 18, 2020 (10:24 pm)

          Yes, the epoxy injection was performed as recommended.

          • Chemist May 19, 2020 (12:29 am)

            But were the cracks repaired or just covered up?

    • Jon Wright May 18, 2020 (4:30 pm)

      Another great example of laziness is to huff and puff with simplistic, banal sloganeering (i.e., “Get to work!!”) rather than bothering to educate oneself about the technical details of a complicated situation.

    • AMD May 19, 2020 (7:02 am)

      Every key position in 2013 has turned over since then (SDoT Director, city council members, mayor, etc.) so I’m not sure what punishment you’re asking for and for whom.  There is an argument to be made that some level of delay/unpreparedness could have been precisely BECAUSE of that turnover, since each person would be playing catch up and learning about the city’s many roads, bridges, and sidewalks anew every time a position turns over.  

    • Todd Martin May 19, 2020 (9:36 am)

      I agree

  • Symmetric has 2ms May 18, 2020 (4:09 pm)

    The report graphics are particularly astonishing – showing  a Symmetric or an  Asymmetric failure.   I had hoped that the east and west approaches to the bridge could be saved in case of failure. But the graphics show that everything gets torqued no matter what type of failure occurs.  

  • JJA May 18, 2020 (5:09 pm)

    Shoring is required for a controlled demo – you can’t just start taking pieces out of the middle of a bridge like this without propping up the sides.  As I read the article/report, it sounds like the experts believe there may not be enough time to get that shoring in place before the main span collapses. I think it’s going to fall . . . and I think the city thinks so too. 

    • WSJ May 18, 2020 (6:10 pm)

      I tend to agree, with every update that comes out, the situation gets a bit more clear. Professional engineers don’t say things like “oh man, this thing is so f***ing f***ked.” until Friday afternoon beers at the pub, off the record, but I’m guessing that’s what the chatter at wsp is like. 

      • Jethro Marx May 18, 2020 (7:58 pm)

        The report is great and all, but what we’re all wondering is, how many of their engineers have said, “?+#$ no, I ain’t going out there!” when it’s time to install the monitoring equipment? And maybe we could kill two birds with one stone and have one of those morale-boosting flyover jets shoot a missile at it. Morale, lifted! Bridge, fixed by failure?

    • Tsurly May 18, 2020 (6:11 pm)

      There is one important thing missing from all the bridge failure discussion; if it does catastrophically fail, where are the best places to watch it.

    • Jon Wright May 18, 2020 (6:41 pm)

      I seems to me that having “potentially prevent these worst-case scenarios” as one of your objectives sure doesn’t sound especially optimistic!

  • ACG May 18, 2020 (5:39 pm)

    I am assuming the 99 interchange is far enough East from this scenario that traffic on that corridor will not be impacted by a collapse?

    • BBILL May 18, 2020 (10:26 pm)

      For the purposes of being out of the danger zone, Highway 99 is very far east of Pier 19.

  • Bradley May 18, 2020 (7:39 pm)

    No pedestrians or parking should be allowed underneath the bridge or East/West approaches until the entire failed structure can be safely demolished. The businesses underneath the main span near the marina need to be relocated. If that monstrous structure suddenly comes down, those underneath who are inside structures or on foot would likely be killed.    

  • MBF May 18, 2020 (7:43 pm)

    I really hope the powers that be realize that there is only one way on and off Harbor Island. Whether it’s construction, destruction, or collapse, the pinch point to/from Harbor Island runs directly under Pier 18 and well within the “collapse zone”. Terminal 18 is one of the busiest cargo terminals in the Puget Sound area, and there’s 3 major petroleum storage facilities that supply fuel to not just the public, but Metro, fire, police, government, airports…basically, everything in the greater Seattle area depends on the facilities on that island. If the bridge goes down, there’s going to be much, much wider ramifications that just getting to/from West Seattle. 

  • Wseattletite May 18, 2020 (11:36 pm)

    Think tension, not compression.  There is a quick solution to make it what it always should have been.   The materials and infrastructure are in place to make it happen.  8 months.  100 year bridge with light rail.  Yes, I am an  engineer.  8 months. Better bridge than before and carrying the future of transport.  Come on SDOT, you will barely be done with you “studies” by then.  Listen to  those who have seen it happen. 

    • bolo May 19, 2020 (11:31 am)

      Wseattletite, care to share more info on your 8 month bridge fix?

  • tk May 19, 2020 (7:39 am)

    re: MBF- “Only one way on and off Harbor Island”- What about the SW Spokane bridge on the east side of the island? It is a direct route off of Harbor Island, very busy with commercial traffic, busses, etc. going to downtown, I-5, & all points east of Harbor Island. It would not be directly impacted by an upper WS upper bridge collapse  or demolition (except, more traffic off of the island).

  • Scott Collins May 19, 2020 (6:22 pm)

    I too would be interested in the details.  This sounds like the vaccine that will be available in July.  Wishful thinking.

Sorry, comment time is over.