Tour of West Seattle Bridge reveals most recent explanation for its failure; And work proceeding means it's on track to open next summer
The Seattle Department of Transportation (SDOT) invited Seattle media for a tour of the West Seattle High Rise bridge on Thursday to show the work that's been done thus far to repair it and explain why it failed.
The tour included a look inside the box beam concrete girders of the center span of the bridge where SDOT showed off the new anchors for the new post tensioned steel cabling that has been added inside, plus the cracks, now filled with epoxy to seal them from moisture, and the wide array of sensors that adorn the girder interiors monitoring cracks, movement and more.
Speaking for SDOT Heather Marx Safety Program Manager for the West Seattle Bridge said the agency is "on schedule and on budget exactly as we expected, we're in really good shape." SDOT is just finishing with their 90% design review process, and they are "getting closer and closer to that day that we can start the actual rehabilitation construction," said Marx.
Why did the bridge fail? That question has had an evolving answer with multiple components but for the tour, engineer Matt Donahue, SDOT's Director of Roadway Services said. "Fundamentally, the post-tensioning strand, or the steel inside the bridge...Although they designed the bridge with the post tensioning standards of the time it turned out to be not strong enough for the bridge to carry out its useful life. What happened is that the post-tensioned strand stretched out over time and that changed the way the load moves through the bridge, in an area of the bridge, specifically the box girders, the floor of the girders..., in a way that those girders were not designed to take a load, and that's what caused the cracking. Over time the cracks grew and became worse. I view this as a maintenance and inspection success story. It's because of our maintenance and inspection program that we were on the bridge, as we're required to do, that we were able to catch that cracking and thus begin a program of analysis that led us to March 2020, when it became very clear that for the safety of the public and to eventually repair the bridge... That closure on March 23, was the same day that they closed the State. I knew it was the right thing to do but it was not an easy decision to make because I had a pretty good sense what the impact on the community was going to be."
In response to a question he also said that concrete "creep" a phenomenon that caused concrete to shrink and change over time as it dries. Concrete can in fact take up to 100 years to fully dry. This is also in part why concrete cracks.
The tour inside the bridge showed the gathered media the anchors for the new post-tensioned steel cabling that have been installed during the stabilization process, the Carbon Fiber Wrap that has been added to the walls to increase strength, the epoxy added to cracks to seal them against moisture that would, if left unchecked eventually reach the steel cabling embedded in the concrete and cause them to rust. Also displayed was an array of sensors on the walls and on the surface of the bridge that measure and report on even the most minor changes in cracks and on bridge "performance" in different temperature conditions. These sensor arrays will remain when the bridge returns to service in mid 2022.
Can't wait to be on that bridge again! Thank you Patrick Robinson for the great picks.
I don't know what part of the inspections equals a success story. It should never have gone so bad they had to close it down for yeais down for years on end.
It's just PR. Yeah, I laughed when they called it an "inspection success story" too. Monitoring seems like a pretty basic requirement. Starting an investigation and planning for fixes on the outset of the cracks being discovered so a fix doesn't take a full closure of over a year would be an inspection success story. I know people put out these sound bites for PR but if they actually believe this qualifies as a success, they need to be out of their job and replaced.