Why is it needed?
Table of Contents
The March 11, 2011 earthquake in Japan is a reminder of the importance of building structures so they can survive a major earthquake. Many scientists believe that the Pacific Northwest is overdue for the same type of seismic event that struck the Japanese coast (a subduction zone or "megathrust" earthquake) where sustained shaking of the ground can cause extensive damage to buildings and infrastructure. The recent Japanese earthquake was magnitude 9.0. The last megathrust earthquake in this region was the Cascadia earthquake in 1700, which had a magnitude 8.7 to 9.2.
In the event of a megathrust earthquake in Portland, it's possible that many of the existing Willamette River bridges would be un-usable for a period of time afterward. The fastest recent major bridge replacement in the U.S. took 13 months (following the I-35W Mississippi River Bridge collapse in Minneapolis in 2007). All of our existing Willamette River bridges were built before local seismic risks were well understood. Both the Burnside and Marquam bridges have had seismic upgrades in recent decades, but no upgrade can match the built-in strength of a new bridge designed to today's modern seismic standards.
The good news is that the new Sellwood Bridge was designed to the latest seismic standards and construction engineering. Bridge engineers took into account all the potential seismic conditions in the region and at the Sellwood Bridge's specific site. The new Sellwood Bridge was designed to address both 500 and 1,000-year recurrence period earthquakes. The seismic goal is for the new Sellwood Bridge to remain standing through an earthquake that is equal to the largest felt here in the last 1,000 years. The 500-year standard is for a smaller earthquake. In that case, the bridge would not only remain standing after the earthquake but would need only moderate repairs after the event.
A major earthquake in Portland is a scary scenario. We all depend on safe travel across the bridges in our region. Those that are built to modern seismic standards – such as the new Sellwood Bridge – offer the best hope for continued safe river crossings following a major earthquake.
Take a Sellwood Bridge Tour with Former Multnomah County Chair Ted Wheeler
The Sellwood Bridge was constructed in 1925 to replace the Spokane Street Ferry, which shuttled passengers across the Willamette River between Sellwood and West Portland. The bridge was designed by Gustav Lindenthal, a noted bridge engineer of the time, and--like the Ross Island and Burnside bridges--was built with funds from a $4.5 million local bond measure.
In response to public outcry at budget overruns on the Burnside Bridge, the Sellwood Bridge design was scaled back to minimize cost. With a construction cost of just $541,000, the scaled-down design resulted in a number of limitations. The bridge is extremely narrow: two lanes, no shoulders or median, and one 4-foot-wide sidewalk.
The Sellwood Bridge was the only four-span continuous truss highway bridge in Oregon and possibly in the nation. (A continuous truss requires fewer parts and is cheaper to construct than other bridge types.) It was also Portland’s first “fixed span” bridge across the Willamette (meaning it was high enough to avoid the need to “open” for river traffic). It was Portland’s first Willamette bridge without trolley tracks. Because it was not designed for the additional weight of streetcars, the structure itself was not as substantial as the city’s other river crossings.
In addition to these design limitations, the bridge also had topographical challenges. The west end of the bridge was constructed on fill material and was located in an area that is geologically unstable. The hillside above the bridge is slowly sliding toward the river, exerting pressure on the west end of the bridge. In fact, in the late 1950s, the hillside actually slid several feet toward the bridge. As a result, a section of the bridge had to be removed and foundations were reinforced. The west end interchange with Highway 43 was completely re-built in 1980. Since then, ground movement has caused the west end approach girders to crack. The bridge was also not designed to withstand a significant earthquake.
Multnomah County continued to take steps to prolong the safe use of the bridge until a long-term solution could be found. In June 2004 after the discovery of the cracks in both the east and west concrete approaches, cracks were restrained with steel clamps and the weight limit for vehicles traveling across the bridge was reduced from 32 tons to 10 tons. This limit caused the diversion of 94 daily TriMet bus trips (a loaded bus weighs about 19 tons), which formerly crossed the bridge. A 2005 engineering study recommended short-term safety improvements for the bridge, which resulted in epoxy being injected into cracks in the girders and columns. The county also inspected the Sellwood Bridge every 3 months to monitor the cracks and slope to ensure the bridge was safe to use.
Existing Bridge Deficiencies:
- Buses and trucks are restricted from using the bridge
- Narrow lanes
- Narrow sidewalk
- No shoulders
- No bike facilities and poor connections to trail system
- Bridge not designed to withstand earthquakes
- Tight turns at west end
- Unstable slope at west end