Stress relief: Crews reach critical milestone by releasing damaged bearings on West Seattle High-Rise Bridge, marking forward-progress for all repair or replace pathways.

Watch our video that explains the Pier 18 release work above! 
Photos included in the video can be found here

Summary 

  • Within weeks of closing the West Seattle High-Rise Bridge, we have had crews on top, underneath, and inside the bridge working around the clock, six days a week, to stabilize the center span. 
  • We reached a critical milestone in this work this week as crews “released” damaged bearings on Pier 18, one of the bridge’s four main support structures. We previously discussed the importance of this work on Pier 18 in a blog in April
  • Lateral bridge bearings distribute pressure and allow the bridge to move in response to traffic loads, normal concrete creep and shrinkage, thermal variations, or even an earthquake. 
  • The lateral bearings on Pier 18 were compressed and bulging, locking together two critical parts of the bridge which normally independent of each other.  
  • Our bridge engineers worked with the contractor to determine the best way to “release” the bearings so they are no longer compressed.  
  •  Over the next couple of weeks, we will place the new bearings, and then pour the new concrete to hold them in place, setting the stage for future repair or replacement of the bridge. 

Within weeks of closing the West Seattle High-Rise Bridge, SDOT has had crews on top, underneath, and inside the bridge working around the clock, six days a week, to stabilize the center span. 

All told, crews have spent the past seven months working on or inside the bridge nearly every day to make it stronger and keep our options open. As we gear up for winter, when colder temperatures and increased moisture can cause bridge elements to expand and contract, we’ll continue to monitor the bridge with the intelligent monitoring system installed back in May

We reached a critical milestone in this work this week as crews “released” damaged bearings on Pier 18, one of the bridge’s four main support structures. 

The bridge’s four main support piers hold up the three highest spans of the bridge – the part that goes over the Duwamish River, between Harbor Island and West Seattle. Pier 18 is the support structure on the east side of the high-rise span, towards Downtown Seattle. 

Imagine you have a bad back. There’s a tightness there, and you don’t know where it came from. All you know is that you need to release the stress to find relief. For the West Seattle High-Rise Bridge, a similar kind of relief came this week, as crews released damaged bearings on Pier 18. 

This critical milestone helps to ensure that all pathways forward – repair now and replace later, or pivot to replace immediately – remain viable. It does this by reducing stress on other structural elements of the bridge, and could help prevent further cracking as we gear up for winter weather.  

The bearing release is part of a series of vital steps we’ve taken to stabilize the bridge since it closed in March. As the important discussion about repairing or replacing the bridge continues, crews are continuing to focus on measures that need to occur regardless of whether the bridge will be repaired now or if we will pivot immediately to replacement. This includes carbon fiber wrapping and installation of a post-tensioning system, both of which will wrap up later this month. 

What are lateral bridge bearings and why are they important? 

The lateral bearings are cylindrical pieces of neoprene (a synthetic rubber used to make everything from tires to stress balls to industrial equipment). The bearings act like rubber cushions that separate sections of the bridge. These bearings are not unlike the discs that separate vertebrae in your spine – and they can bulge, just like spinal discs. 

Lateral bridge bearings distribute pressure and allow the bridge to move in response to traffic loads, normal concrete creep and shrinkage, thermal variations, or even an earthquake. Size-wise, the lateral bearings are a pretty small feature of the bridge, but they play a big role in allowing the bridge to move in response to traffic, temperature changes, or even earthquakes. 

Like other joints on the bridge, these bearings absorb shock and manage movement. During a typical morning commute, when there was heavy traffic on eastbound lanes of the bridge and light traffic on the westbound lanes, the bearings responded to the uneven distribution of vehicle weight and allowed some movement in the bridge.  

Even now when there isn’t any traffic on the bridge, the bearings absorb movement from gusty winds and temperature changes that cause concrete to expand and contract. 

 

Graphical user interface, diagram

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This is a cross-section of the West Seattle High-Rise Bridge, showing where Pier 18 attaches to the bridge structure. The top triangular figure is the median (or road divider) on the roadway surface. The blue areas to the left and right of the bearings are the “box girders”, which are the hollow tube-like structures that form the skeleton of the bridge structure. The thick dark gray area near the bottom is the top part of the support pier, also called the “column cap”. The thin orange lines labeled “B” show where the restrained lateral bearings are.

Bridges are designed to take a lot of stress. They need to stand up to earthquakes, weather, traffic vibrations, and natural changes in concrete and steel. Like many materials, concrete and steel expand in warm temperatures and contract in the cold. 

One critical way that bridges handle that stress is with joints between sections of the bridge. There are different kinds of joints between different sections, but they all work to help make the bridge more flexible and create a buffer between parts of the bridge that move in different ways. This is important because it helps isolate stress points and prevent the bridge from tearing itself apart as different sections push and pull on each other. 

The lateral bearings on Pier 18 were compressed and bulging, locking together two critical parts of the bridge which normally independent of each other. This means that the bridge could not move as it should. Instead, the bearing was creating additional pressure on the surrounding area and affecting the bridge as a whole. 

An “annular space”, or gap, should have existed between the bridge and the lateral bearings, relieving the “force”, or pressure, transferred between the bridge and bearing on a daily basis. Pier 18’s lateral bearings, however, did not have an annular space, causing the bearings to bulge. This impacts the motion of the whole structure, causing tension where it is not intended. This problem likely contributed to the cracking in the center of the bridge. 

Our bridge engineers worked with the contractor to determine the best way to “release” the bearings so they are no longer compressed.  

This involved building a temporary platform to perform the work. Then, specialized equipment was used to precision-demolish the concrete surrounding the bearings. Over the next couple of weeks, we will place the new bearings, and then pour the new concrete to hold them in place, setting the stage for future repair or replacement of the bridge.  

We look forward to keeping you updated on Pier 18 and ongoing West Seattle High-Rise Bridge stabilization efforts. In the meantime: