The University of Washington (UW) team reviewed the Washington State Department of Transportation (WSDOT) report titled "Earthquake Analyses of the Alaskan Way Viaduct" and performed an independent assessment of two typical sections of the structure. Additional analyses were performed to investigate the influence of some factors that were not considered in the WSDOT report.
The input motion and geotechnical characteristics assumed in the WSDOT report were consistent with the information available to the WSDOT and to the UW. However, the paucity of information available regarding the seismological risk and the subsoil conditions precluded the possibility of reliably estimating the input motion, foundations stiffnesses, foundation capacities, and potential for liquefaction. Inspection of the structural plans suggested that timber-concrete spliced piles in the section of the structure built by the WSDOT might be particularly vulnerable.
The elastic dynamic models generated by WSDOT and those constructed for this study were found to give comparable natural periods in the first three modes. Those in the higher modes differed because of the disparate ways in which the structures were modeled. However, the higher modes provided only a small portion of the total response, so the differences in calculated response were small.
For the WSDOT designed part of the structure, g-ratings and dynamic code ratios were established by assuming that the reinforcement would reach its yield strength. The present study found the structure to be generally weaker than did the WSDOT study. Some of the ratings showed a consistent relationship with those given by the WSDOT study, while others showed considerable scatter. Regardless of the resolution of the discrepancies, both analyses indicated that the demands on structural members would be likely to greatly exceed their capacities. The main shortcomings in the structure appeared to be inadequate confinement steel and development lengths that were too short. Because no distress was observed after the 1965 Seattle earthquake, these calculations are undoubtedly conservative. However, the response of these brittle details cannot be predicted reliably without further investigation. The University of Washington team is proposing further study to verify the seismic safety of the structure.