New Sellwood Bridge will be built to withstand a major earthquake

Recent world events remind us how important it is to build structures so they can survive a major earthquake. The new Sellwood Bridge is being designed to the latest seismic standards. Bridge designers will take into account all the potential seismic conditions in the region and at the site of the new bridge.


The 1989 Loma Prieta earthquake caused an upper deck section of the San Francisco-Oakland Bay Bridge to collapse onto the lower deck.

All the existing Willamette River bridges in Portland were built before local seismic risks were well understood. Two existing bridges (the Burnside and Marquam) have had seismic upgrades in recent decades. But no upgrade can match the built-in strength of a new bridge designed to today's seismic standards.

Engineers do not design bridges to survive a specific Richter level, a number that only describes the motions at the actual spot where the rupture occurs. Instead, bridge designers use seismic history based on the frequency and intensity of earthquakes felt at each specific site. The methodology is similar to designing for flood events. The new Sellwood Bridge will be designed to address both 500 and 1000-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 1000 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.

Bottom line: The new Sellwood Bridge (and the new TriMet light rail bridge) will be built to today's seismic codes and will be the local Willamette River bridges best designed to survive a major earthquake.

Read this technical memo for more information about seismic design for bridges.