The Alaskan Way Viaduct, which runs along the Seattle waterfront, is highly susceptible to damage during earthquakes. Previous studies have shown that extensive soil liquefaction could cause collapse of sections of the Viaduct through a complex mechanism that involves the seismic performance of a nearby seawall. Occurrence of such collapse events would be expected to occur at some time after the initiation of earthquake shaking, thereby providing some period of time in which motorists could potentially be kept from entering the Viaduct. The purpose of this project was to evaluate the feasibility of instrumenting the Viaduct to (a) provide warning of potentially unsafe situations, and (b) reduce post-earthquake closure times by speeding the process of evaluating the condition of the Viaduct following earthquake shaking.
Because of the numerous and significant sources of uncertainty in the prediction of collapse, a probabilistic approach to the warning system problem was adopted. This procedure accounted for, to the extent possible given available information, uncertainties in ground shaking characteristics, uncertainties in soil properties, uncertainties in seawall behavior, uncertainties in Viaduct foundation behavior, and uncertainties in structural response. The results are expressed in estimated collapse probabilities for various warning system triggering levels. Selection of triggering levels will require balancing the costs and benefits of successful and unsuccessful collapse predictions.
Structural analyses of specific sections of the Viaduct were used to develop recommendations for performance monitoring instrumentation. Proposed response levels for inspection and inspection/closure are also presented.