Seismic retrofit program prepares bridges, overpasses for quakes

By guest blogger Tom Pearce

Steel column jackets, like these used in the seismic retrofit
of the SR 509 Puyallup River Bridge, will be installed
at 12 locations along I-5.
We’re going to have another major earthquake here. OK, I can’t tell you when or where it will be centered, but if you’ve lived around here for a few years, you know we live in a seismically active area.

When the big one hits, transportation infrastructure is going to be critical to the region’s recovery. We’re going to need the freeways and bridges, particularly from Joint Base Lewis-McChord to Seattle, to move supplies and materials to recover and rebuild. To prepare for this, in 1991 we started a seismic retrofit program to minimize and avoid catastrophic bridge failures.

As part of that program, we’re upgrading overpasses along I-5, from SR 18 to South 288th Street in Federal Way, and from Kent’s Military Road to South Rose Street in Seattle. We’re also just wrapping up a project at the M Street and I-705 overpasses in Tacoma. We’re going to strengthen freeway overpasses to help them better withstand earthquakes. How are we going to do this? Well, we’ll add:
  • Steel column jackets: These are steel plates wrapped around the columns that hold up overpasses and bridges to keep them from crumbling and collapsing in an earthquake.

  • Bolster supports: These widen the pier caps that hold up the girders supporting the roadway, making it harder for the girders to slip off in an earthquake.

  • Girder stops: These keep the girders from sliding side-to-side in an earthquake.

We’ll be working at 12 sites between Federal Way and Seattle, strengthening a total of 22 overpasses.  This work is important, but you’re going to see some impacts around worksites. Sidewalk, shoulder and single lane closures are necessary along the roads under I-5 where our contractors will be working.

Most of the work will be done at night, which could be noisy for nearby homes at a few of the locations. We have noise rules and work with each local jurisdiction to do everything we can to limit and mitigate the noise, but in some cases it will still be there. Our contractors will send flyers to homes that could be most affected before starting the work.

Major highways aren’t the only place we’re doing seismic retrofits. We’re also working on the Scatter Creek Bridge, east of Enumclaw. Every winter SR 410 closes at Crystal Mountain Boulevard. The Scatter Creek Bridge is important because it’s the only year-round westerly link for local residents, visitors and emergency vehicles.

Since the program began, we’ve completely retrofitted more than 280 bridges and done partial retrofits on more than 130 others. Even with this year’s work, we’ll still have more than 450 to go.

It takes time, and we’re doing the work as quickly as possible. But as sure as we’re going to have another earthquake, we’ll keep working to strengthen our bridges and overpasses.