An engineering team from the University of Washington (UW) evaluated the seismic vulnerability of the Alaskan Way Viaduct, located in Seattle, Washington. This report presents the evaluation of a typical three-bay unit that was designed by The City of Seattle Engineering Department (SED). The evaluation team performed response-spectrum analyses and nonlinear analyses for the fixed-base condition. The team considered a widely used soft-soil spectrum and worst-case, site-specific spectra. Wherever possible, the UW team evaluated the vulnerability for each failure mode following procedures proposed by the Applied Technology Council and by researchers at the University of California, San Diego.
The evaluation team found that the vulnerability of the Alaskan Way Viaduct exceeds that of bridges built to current standards. The vulnerability is a result of a combination of two factors: (1) the design ground motion would strongly excite the viaduct; and (2) many of the structural components lack the ductility required by current standards. The following deficiencies were identified as the most critical.