Portland’s Burnside Bridge was designated as a lifeline route across the Willamette River in the event of an emergency, supporting traffic for emergency services, evacuation, and recovery immediately after a major disaster. Constructed in 1926, the Burnside Bridge was not designed with seismic resiliency in mind and the existing bridge is not expected to withstand a major seismic event.

Shannon & Wilson participated in a study to determine how the Burnside Bridge will respond to an earthquake, and to identify replacement or rehabilitation alternatives. The Burnside Bridge is a relatively complex structure sited in a dense urban area, and understanding how it will react to an earthquake is further complicated by the variable geology supporting the bridge. Our study used FLAC 2D numerical modeling to model the complex interactions of the subsurface soils and the structure itself.

Based on subsurface explorations, laboratory testing, and FLAC analysis, Shannon & Wilson provided Multnomah County with options for both retrofitting the Burnside Bridge to withstand a significant seismic event, and for constructing a new bridge that would be both compatible with existing infrastructure and economically sustainable long term.