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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 900 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 March 2013
Source: WSDOT Bridge Office
SR 99 Aurora Ave. Bridge Approach
span piers – Design Visualization of retrofit
Seismic Retrofit Investment
Since its seismic retrofit program began back in 1991, WSDOT has invested nearly $100 million to strengthen bridges to withstand earthquakes. This includes a portion of the 2005 Transportation Partnership Account (TPA) funding package that dedicated $87 million to retrofit bridges in Central Puget Sound. The TPA funded work began in 2007 and will be complete by 2015.
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 Joint Base Lewis McChord near Lakewood to the I-5 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.
Bridge engineers will perform a seismic analysis of each bridge to determine the exact scope of the retrofit. The analysis looks at all the bridge elements above the foundations and compares the capacity of those elements to the forces applied by an earthquake (the demand placed on the structure). Generally, if the capacity to demand ratio is less than 1.0, then the bridge element is retrofitted.
The most common type of retrofit of most bridges includes adding steel jackets around the columns and adding more concrete-and-steel reinforcing to the pier caps (also known as a “bolster”). WSDOT uses the American Association of State Highway and Transportation Officials (AASHTO) adopted 1,000 year return period (7% probability of exceeding in 75 years) to determine the forces (stress) the bridge must resist.