Seismic Sufficiency Review Committee (SSRC)
Alaskan Way Viaduct
Seattle, Washington

 
Following the magnitude 6.8 Nisqually Earthquake on February 28, 2001, the Washington State Dept. of Transportation (WSDOT) reviewed the condition of the Alaskan Way Viaduct and found significant structural damage in the vicinity of Pioneer Square. Temporary bracing was locally erected to support the viaduct and traffic was restricted on certain lanes. WSDOT then assembled a group of 6 internationally recognized bridge engineers

to review the long-term performance of the viaduct and to advise the state on repair and replacement options. Mr. Grant was selected as a member of this Structural Sufficiency Review Committee (SSRC) to provide geotechnical earthquake engineering support to the team. The purpose of the committee was to assess the adequacy of the DOT's emergency measures to restore traffic on the viaduct and to determine the feasibility of seismically rehabilitating the viaduct or replacing the viaduct with a new structure. Mr. Grant's involvement included helping the committee establish appropriate standards for evaluating the seismic risk to the structure and then assessing the seismic hazard of the site with respect to the evaluation criteria. This assessment included determining the level of ground shaking experienced at the viaduct during the Nisqually Earthquake and then developing hazard curves for the viaduct relating peak ground acceleration to recurrence intervals for future earthquakes. Mr. Grant also assessed the liquefaction potential of the soils underlying the viaduct and estimated a threshold acceleration that would be required to result in widespread liquefaction beneath the site that could ultimately lead to a catastrophic failure of the viaduct. This threshold acceleration, along with the probability of occurrence of such motion, was used to determine an acceptable time window for accomplishing repairs to the viaduct. The bridge engineers on the committee used the results of these studies to determine appropriate strategies to seismically rehabilitate the viaduct. The committee then developed cost estimates for the rehabilitation as well as the replacement of the viaduct and presented their findings in a report to the State.