Portland's aging downtown bridges are not expected to withstand a major earthquake. Since 2016, Multnomah County has been working to create an Earthquake Ready Burnside Bridge.
The project is now recommending one bridge option to move forward for approval into design and construction: the Replacement Long Span. This option has several variations and could have a tied-arch design, a cable-stayed design, or a through-truss design.
The decision around specific bridge type will be made next year.
The type of lift span in the middle of the bridge will also be decided next year. This could be a bascule lift or a vertical lift.
The support structure above the bridge for this type allows for fewer columns underneath. This lowers the risk of damage during an earthquake by avoiding construction in unstable soils near the river.
The other options that were studied in depth were the Replacement Short Span, a Replacement with the Northeast Couch Street Extension over the river, and finally, an Enhanced Seismic Retrofit of the current bridge.
The Replacement Long Span is recommended because it has the fewest columns
in unstable soil, it's the least expensive, has more space under the bridge
in Waterfront Park, more space for bicyclists and pedestrians, fewest impacts to natural resources, and the least impact to the Burnside Skatepark.
All of our downtown bridges are vulnerable to an earthquake because of unstable soils near the river. The more we can avoid construction in these soils, the better.
The Replacement Long Span option only requires one set of columns in this area. The other three options all require many more. Fewer columns mean less construction in unstable soils, which makes the Replacement Long Span the least expensive option.
It also has the fewest impacts to natural resources.
Another benefit of fewer columns is more open space in Waterfront Park.
All of the replacement options would widen the portion of the bridge over the river, making space for wider sidewalks and bike lanes, as well as a physical barrier between cyclists and vehicles.
The Burnside Skatepark was built around support columns under the eastside approach to the bridge. The three replacement options wouldn't need columns in this area, allowing the skatepark to remain intact and usable after construction. By contrast, the Retrofit option involves replacing or reinforcing all the existing columns and would require demolition of the skatepark.
One of the impacts of all the replacement options is the removal of the existing historic Burnside Bridge. The Retrofit option would require replacing much of the eastside approach over the freeway and railroad tracks, but would preserve the trusses, control towers, views of the city, and other features of the historic bridge.
A final consideration for the Replacement Long Span is the changes it would have on current views. The support structure above the bridge that would allow us to avoid unstable soils underneath would also change views of the iconic Portland sign, downtown, and the Eastside from certain angles.
The exact design of the support structure and the movable portion
in the middle of the bridge have not been decided yet. Whether it's a tied-arch, through-truss, or cable-stayed design, a Long Span bridge could provide an opportunity for a more visually striking bridge.
Do you agree with recommending the Replacement Long Span option?
We want to hear from you.
Learn more and let us know your thoughts at www.burnsidebridge.org.