Interstate 405 and SR522 Ramp

Earthquakes pose substantial risks to transportation infrastructure in Washington State. As part of its bridge preservation program, WSDOT uses seismic retrofit of bridges to mitigate the potential risks associated with these events.

The purpose of the Seismic Retrofit program is to minimize and avoid catastrophic bridge failures by strengthening bridges and structures to resist future earthquakes.

The first step in seismic retrofit is to perform an engineering analysis to determine if an existing bridge can resist a design level earthquake. Computer models are used to apply a force to each bridge pier. This is also call a “Push-Over” analysis. The capacity of the bridge pier is then compared to the demand of the design level earthquake forces.

A total of 921 bridges are part of the Bridge Seismic Retrofit Program. These bridges are located on state routes basically west of a line drawn through the center of Washington State (west of Wenatchee and Yakima). Retrofit priorities are based on seismic risk of a site, structural detail deficiencies, and route importance.

Bridges in the Seismic Retrofit Program as of July 2008

Source: WSDOT Bridge Office

The 2005 Transportation Partnership Account (TPA) provides $87 million to complete projects for 172 “High” and “Moderate” risk bridges in the Puget Sound vicinity. The work began in 2007 and will be completed in eight years.

WSDOT has collaborated with federal, state and local agencies to determine how the remaining seismic retrofits should be prioritized. The conclusion was to focus on the bridges on Interstate 5 from the McChord Air Force Base near Lakewood to the I-5 and I-90 Interchange in Seattle. Retrofitting these bridges along Interstate 5 will provide a systematic plan that will begin to provide an earthquake resilient route that could be used to speed a recovery following a major seismic event.