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The SR 520 West Approach Bridge extends about 6,100 feet east from Seattle's Montlake neighborhood, over Union Bay and Foster Island, then into Lake Washington proper. The 1960s-era bridge is founded on pile-supported columns, and is located 4 to 8 miles from two active fault zones. The Washington State Department of Transportation (WSDOT) determined that the bridge is vulnerable to earthquakes. Construction is currently underway on the northern half of a replacement bridge (West Approach Bridge North).
Subsurface conditions along the bridge alignment consist of 5 to 80 feet of very soft peat and clay, over very dense/hard sand and clay. The new bridge will be supported by 8- to 12-foot-diameter drilled shafts that extend about 50 feet into the underlying very dense/hard soil. To reduce seismic demands on the bridge superstructure, WSDOT has decided to incorporate a base-isolation system into the bridge design.
Three-dimensional dynamic soil-structure interaction (DSSI) analyses were used to design the bridge. The design required substantial characterization of the soil dynamic behavior using both laboratory and in-situ testing. The analyses made use of earthquake time histories spectrally matched to conditional mean spectra. This led to reduced, and more realistic seismic demands on the structure, compared to conventional uniform hazard spectrum matching.
Base isolating the bridge and performing DSSI analyses allowed the design team to reduce shaft/column diameters and reinforcement. The design team estimates that the base isolated design could represent a net savings of around $60 to $90 million compared to a conventional design.
Bio:
Jeremy N. Butkovich, P.E., is a Senior Principal Engineer for Shannon & Wilson, Inc., For the past decade, Jeremy has worked extensively on seismic analysis and design of large infrastructure projects, including the Alaskan Way Viaduct and Seawall Replacement Project, the Evergreen Point Floating Bridge replacement, and the Port Mann Bridge replacement (British Columbia). Jeremy's areas of expertise include numerical modeling of liquefaction and ground improvement; nonlinear site response analyses; and soil-structure interaction.
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LocationBest Western Executive Inn (View)
200 Taylor Avenue North
Seattle, WA 98109
United States
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