City Council briefed on West Seattle Bridge closure, repair plans and timing
The closure of the West Seattle Bridge on March 23 came as shock when the Seattle Department of Transportation (SDOT) revealed that serious cracks were determined to be so dangerous that the bridge had to be closed. In a first, a "social distance" tele conference for the Seattle City Council was briefed by SDOT on the status of the bridge, how the decision to close it was made and what the plans are to repair it and reopen it to traffic.
SDOT Director Sam Zimbabwe said that during the inspection process, once a year (more frequent than the normal every two year cycle) since 2013, showed that the cracks and their acceleration warranted closure. Since the closure he noted they've been providing almost daily updates on the status of the bridge. He noted the website where people can keep track www.seattle.gov/transportation/westseattlebridge
He said, "The Seattle Department of Transportation does not take chances when it comes to public safety."
The rapid announcement to close the high bridge "came as a result of our ongoing and close monitoring of the condition of the bridge."
The bridge according to Zimbabwe was intended to last 75 years. "During the inspections over the last several years there were no indications that the bridge was unsafe for ordinary use. Remediation would affect that. The normal maintenance we've done over the past few years did not indicate there would be any change to the normal use of the bridge."
SDOT first noticed cracks in 2013 and called in a consulting engineering firm. Concrete crack space monitors were installed. Those monitors link to a website to allow real time monitoring.
Advance analysis and load rating began in 2019 under federal guidelines.
Donohue described the bridges, "It is two, cast in place, segmental concrete box girder bridges, basically a reinforced concrete bridge, comprised of two very large box beams with a monolithic slab that ties across the top of those beams. He said, "to some extent bridges are made to crack. So we anticipate the concrete will crack over time and we perform design calculations and construct the bridge to accomodate that." He noted the the bridge is unique in terms of its span lengths, overall height and that it was built to our topography.
It was originally designed for three lanes of traffic in either direction and it has three lanes of traffic in the east direction and three lanes in the west bound direction.
"Every bridge in the National Highway System monitored by the FHWA and the state DOT is required to be inspected above water every 24 months. In 2013 we had a routine inspection for this bridge and that was the first time we noticed atypical cracking, beyond what you would normally expect for this bridge type. It triggered right away a close eye in that we hired an engineering firm here in Seattle to work with us in 2014, to study this bridge and see if we could determine what was causing this atypical cracking. The results of that study weren't particularly conclusive. They pointed out a number of things that could be causing it but the recommended action was to continue inspect on a more regular frequency, at least every year instead of every two years, and also to monitor the bridge. So that same year we installed crack width guages, with a consulting firm that does that sort of work, that link back to a website so we can monitor that data real time. That equipment was put at four of the distinct cracking locations we've observed."
From 2014 through 2019 they continued to perform the annual inspections and keep an eye on the crack width data. "We saw the cracking continue to grow but not at an alarming or accelerated rate. You can see from the data that the cracks expand and contract over time the amount was actually very very small at some thousandsth of an inch or less. In 2019 what changed is we also do something that is required by the Federal government that is called load grading of bridges. It's a specific calculation method to look at whether or not the bridge can handle very specific types of truck. So those trucks are specified in terms of axle spacing and weight by the federal government. Whenever they come out with a new truck that makes it into the federal load rating program, the government comes out with a mandate that says, 'everbody that has type of bridge or any type of bridge within a certain distance from the federal highway system you have to re load rate the bridge for these type of trucks. So in 2013 they rolled out new trucks and they gave us until 2022 to do the load rating. We had actually scheduled this bridge to do the new load rating later than 2019, but because we had this cracking issue we decided to move that load rating up to 2019. The way you do the calcuations for the load rating are very specific to the types of defects that we observed and if you see atypical defects you are required to do a more advanced analysis and we knew that was going to take more time, so we bumped it up in the schedule. Two things happened. We had these two parallel lines. We're doing this advanced analysis and load rating starting in 2019 and we're continuing to inspect the bridge just because we need to from an inspection program standpoint. We're also trying to gather more data to feed into the analysis so we can really define these cracked sections and understand how to accurately analyze them. Ulitmately what we did was built a large mobile linear elastic model and then a localized non linear model to really dive deep on these cracked sections and understand their behavior so we could do a possible load rating.
As we were gathering data for that analysis we could see the cracking patterns in the bridge start to change and accelerate. It got to the point that our consultant partner that was working on this had recommended going down to two lanes in either direction and as we were preparing, planning to do that and execute that from a traffic control standpoint, including outreach to the Mayor's office, City Council and everyone else that needs to know about this. The day we were having that briefing I got a call from our consultant partner and they changed their recommendation from restrcting to two lanes, to closure. That was on March 19. I had meeting with the leadership of SDOT on the 20th where we discussed lane closures and this changing recommendation and I spent the weekend digesting all that information, and then made the call to bring the consultant partner and engineering team up on to the bridge at 9am Monday morning.
This is what we saw when we got up there."
Councilmembers Herbold and Gonzalez expressed their concern about the lack of noitfication earlier but Donohue and Zimbabwe both explained that there was no real way to anticipate the rate of acceleration of the cracks or the call from their consultant. "We understand the frustration and concern about this and will seek from here forward will seek to have that dialog as early as we think possible," said Zimbabwe.
Gonzalez went further to complain that she "found out only a few hours before the general public," and said, "Their could have been more proactive engagement but that, "at a minimum the chair of the transportation and utilities committee and district representative for District 1," should have been told. "I'm disappointed that that proactive engagment did not occur and that it took this severe action for the department to acknowledge that it needs to engage city council members on issues of such significant import."
Zimbabwe explained again, that "the recommedation for closure did not come to us until Thursday March 19. It wasn't until Matt himself went up on the bridge on Monday morning March 23rd in the morning, confirmed what the recommendation was...and from that point once we have that level of recommendation we can't take the time to brief everybody and make sure we have all the plans in place. We have to take swift and decisive action."
Donohue said, "The decision to close on Monday. We saw rapid and unexpected growth (of the cracks) and because public safety is the number one priorty and for myself as a certified bridge inspector with over 20 years of experience. If you look at the blue lines in the photo that is growth that occured between the 5th of March, which was the earliest time I was on the bridge and the 23rd when we went out to see the bridge. That type of growth in a reinforced concrete structure is completely unacceptable. That's typically the type of growth you see over years not over weeks and days. So that drove me primarily as a bridge inspector and bridge manager for the city to pick up the phone from inside the girder and call Lorilei, my Deputy Director to alert Sam that we needed to close the bridge that day. That caused extreme concern with remaining vehicular or live load on the bridge at the risk of public safety."
He continued, "The other thing to note in the structural bridge world is that angular cracks of this type, that are diagonal are called 'Shear cracks'. That type of deterioration in a bridge will go until failure happens quickly and without warning, as opposed to other types of cracking that's not oriented that way. So this exacerbates the concern at this point. A failure mechanism that can happen very very quickly without the bridge deforming or changing shape before it actually gives way in addition to the weight of that crack growth.
They believe the bridge is currently stable, with traffic not on it.
Short term repair plans for the High Pass Bridge
SDOT hopes to return the bridge to normal operation as soon as possible while mitigating impacts for people and goods. They will seek interim repairs with a goal of restoring some traffic and accelerate major maintenance/repair to extend the life of the bridge by 10+ years.
They plan to keep inspecting the bridge during the repair phase to make sure it can handle its own "dead weight" not under load. So far they believe it is stable and that there's no danger to people using the lower bridge.
SDOT indicated that it might be possible to open a limited number of lanes shortly after the repair process is underway but they cannot determine that until they work out the shoring of the existing structure.
They are looking at different repair methods at the moment like "Design/Bid/Build" or PCCM (Percentage of Completion Capitalized Cost Method) to accelerate the design process and cut down on the lead time for materials which could become an issue.
The first task is to provide "shoring" or support for the cracked sections to make it safe for the contractor to be on the bridge to do more extensive repairs. Donohue said, "At this point we believe it's going to be carbon fiber wrapping with additional reinforcement at key connections. Carbon fiber wrapping is a woven carbon fiber mesh, a fabric that you can lay up on to the bridge then anchor down. You then coat it with a type of epoxy and when that hardens up it forms a shell that additionally reinforces and provides more integrity to the bridge." He explained they will likely combine that with post tensioning strand steel embedded in the concrete that was tightened up as part of the original construction and then locked down. We can still use that technique to put additional post tensioning strand either inside the hollow part of the girders or on the outside if necessary, depending on how the mechanics of those cracked sections ultimately work out.
"We need to be careful in our design that is does not impact the box, the theoretical box around the federal navigation channel beneath the bridge in the Duwamish. If you cross into the plane of the horizontal limits of the navigation channel or if you change the clearance of the bridge it triggers a federal permit through the coast guard that could add significant time to the path if you can't stay outside that box."
Donohue did not offer a timeline on either the initial shoring or completion of the process and also offered no timeline on when the repairs might be complete since so much is unclear regarding the assessment of what must be done.
Short Term repair plan for the low level/ Swing Bridge
SDOT plans to continue weekly inspections and monitoring, then complete a load rating project for this bridge. Up next would be the completion of the pedestrian gate replacement, complete the controls upgrade project then complete the rehabilitation of the Pier 6 and Pier 7 lift cylinders. The lower level bridge suffered a failure of one of the lift cylinders in 2018 and a replacement had to be brought in.
Traffic Management Plan
Adiam Emery with SDOT addressed the traffic management plan.
She said the primary consideration at this time is that it is critical to maintain unfettered access for first responders to and from West Seattle in the midst of a public health crisis.
Emery explained that the bridge closure is similar to the closure of the former Alaskan Way Viaduct but with fewer re-route choices. Previously nine lanes (combining both the high and low bridges) have been reduced to two lanes, for more than 100,000 average daily trips. The viaduct closure had years of planning, 40 joint community briefings and dozens of press events. Not the case with the bridge.
Commuters must now take either the 1st Ave South Bridge or the South Park Bridge to get to and from downtown.
Emery said the new traffic signal at Highland Park Way SW and SW Holden is now operational. She said they are adding traffic count stations as well at that intersection. Emery was asked by Councilmember Herbold about frequency of monitoring the traffic count stations.The first day after the closure Emery said SDOT saw 15,000 cars use the lower level bridge, "which really stressed the emergency responders." She indicated they are data driven and might be able to open up the bridge for more open use in the "late evening" but they don't yet have enough data points. She emphasized the lower level bridge is "Very essential especially given the time we are in," for emergency responders.
Despite signage up on the low level bridge telling people the bridge is restricted to emergency responders, freight and mass transit, "We've seen a lot of people violating those restrictions," said Zimbabwe.
"If we can't get people to stop using the lower bridge then nobody will be able to use the lower level bridge," said Zimbabwe.
Emery said they have formed a task force with King County Metro and the Port of Seattle to strategize how the city can handle the impact of the closure.
Timelines are not possible at this time
Zimbabwe said "I'm extremely reluctant to offer timelines at this time," but did say the repairs would almost certainly go beyond the pandemic closures now in effect, indicating that they would need to address traffic remediation "once things return to normal. Since the federal government's timeline for this extends through April currently and could be extended that means at least a month or more to effect interim repairs.
Have Vashon Ferries go to…
Have Vashon Ferries go to Coleman AM/PM
The physics of shear stress…
The physics of shear stress...instead of pulling apart or crushing in a straight line of force, the shear stresses imply "twisting" forces, and or combined with "bending" forces.
Either the bridge is over-rated for load capacity, or, the traffic in practice exceeds the design loading. Seattle has a long recent history of failing projects...creating bottlenecks. Design is a large portion of State's Authority. and the way money is thrown about, politics is ugly on these transportation issues.
The first thing to be fixed, is The Blame.
I agree with the earlier…
I agree with the earlier comment that the lower bridge should be available for all traffic outside of rush hour. Makes no sense to have time limits when not needed.
Could bikes use the High…
Could bikes use the High Bridge? Bikes could go both ways on the West Bound lanes from 1 st Avenue where you would Exit or Enter the High Bridge. There would be many ways to Enter and Exit on the west Seattle side. This sure would keep the bike riders safer going over stop lights and railroad tracks + be out of the way of vehicles on the lower area. Maybe there would be more people riding bikes if they could use the High Bridge. This plan would be while their figuring out what to do for repairs, or even while their working under the Bridge repairing it. Let's be... c r e a t i v e!
I think you could allow cars to use the lower bridge outside rush hour times. It doesn’t make sense to make someone drive across the 1st ave bridge on a Sunday morning for example