9:14 AM: The West Seattle low bridge has cracks- but don’t panic.
That’s one of the low-bridge updates SDOT has just posted.
From the post:
… It is normal and expected that all concrete bridges will eventually form cracks which help relieve stress from this movement. Small cracks in the Low Bridge have been closely monitored and tested to ensure that they do not affect the bridge’s structural integrity.
Fortunately, these cracks are only a few inches deep and are not progressing at rates remotely like the sudden crack growth that led us to close the High-Rise Bridge. We are confident that the Low Bridge’s crack depth, growth patterns, and type do not indicate that the bridge is unsafe for live traffic. …
However, SDOT says it’s taking steps to be sure things don’t get worse:
*Monthly inspections (federal standard, SDOT says, is every two years)
*Lowering the speed limit from 30 mph to 25 mph
*Banning low-bridge access “for the largest and heaviest trucks at 70+ ft and weighing over 207,000 pounds, about two-and-half times the maximum weight allowed on highways under state and federal law without a special permit to drive over the legal weight limit (the technical term for this is Over-Legal 2 or OL-2 class trucks). This will only affect about a dozen trips per week, and will not affect any emergency vehicles, buses, or freight trucks within legal weight limits.
*Carbon-fiber wrapping “as a precautionary measure” – we asked SDOT communications director Michael Harold for the timeline; his reply, “We are working with our consultants and contractors, as well as FHWA, to finalize plans and then move forward with haste, but don’t have a specific timeline to share at this time.”
Read all the details on SDOT’s site here.
1:53 PM: In our original exchange with SDOT this morning, we asked for any available related reports. So far they’ve sent this letter (and the linked document) written by acting roadway structures director Matt Donahue on June 30th about the load rating:
The attached PDF is the draft load rating summary table that we recently received from Jacobs… the difference between page 1 and page 2 is the amount of strength calculated using the Resal Effect Method… page one is for 0.9(Resal Effect) and page 2 is for 1.0(Resal Effect). As you can see from the differences in the Rating Factors (RF) the result is very sensitive to only a 10% change in the Resal Effect. Note that load rating calculations were made using live load test data.
This sensitivity indicates that there is likely real world behavior and additional strength capacity in the bridge that is not captured in the load rating calculation. As such I intend to handle management of the bridge with administrative methods that include the following:
Reducing the inspection frequency from every two years to:
Real time monitoring with Structural Health Instrumentation that takes crack width gauge readings every 20 seconds, reduces them to an average over 5 minutes and then reports off of the bridge to an interactive website. Similar to the WSHB SHMI this system will be set with alarm thresholds that contact RS engineering staff 24/7/365 via cell phone with any issues
Monthly arm’s length inspection of the interior girder surfaces at crack locations
Yearly inspection of accessible exterior girder surfaces via UBIT
Reduction of the speed limit over the bridge to 25 mph along with installation of flashing speed monitoring signs
Close coordination with SDOT Commercial Vehicle Enforcement to make sure that any permit applications for OL1 and OL2 vehicles receive the appropriate level of review by Roadway Structures
Engagement with WSDOT Commercial Vehicle Enforcement to coordinate receipt of permits they issue for loads terminating within Seattle so that we can contact operators that may try to use the low bridge for permit loads without an application to SDOT CVE due to their lack of knowledge of the need to do so
Outreach to the Commercial Vehicle Freight Board and other commercial carrier stakeholders to make sure that they are aware of the situation
Maintenance of the centerlock shimming system to reduce the impact of loads transiting the bridge
Implementing a strengthening program to return RFs to ≥ 1.0 that will likely involve addition of a combination of post tensioning strand and CFRP for completion by the end of 2022.
Meantime, a commenter asked about other bridges around the city and we noted that City Councilmember Alex Pedersen (the council’s Transportation chair) had requested an audit three months ago. We asked about the status. His response sent by a staff member via email:
ADDED 8:19 PM: From another exchange tonight with SDOT, they told us:
To clarify, there are no new cracks. The cracks being discussed have been there since the late 1990s. We observed some growth during an inspection last summer, but since then we have conducted numerous tests which verified that the cracks are only a few inches deep, do not penetrate through the girder walls, and are not growing. As you know, it is normal and expected that all concrete bridges will eventually form cracks which help relieve stress, and there is are no indications that this type of crack should affect the Low Bridge’s structural integrity.
The cracks are mentioned in this March document from the load-rating process.
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