Damage to CableStayed Bridge during 1988 Saguenay Earthquake. I: Pseudostatic Analysis

An inspection of the cablestayed Shipshaw bridge near Jonquire, Qubec, following the 1988 Saguenay earthquake (ML=6.0) revealed the failure of one anchorage plate connecting the steel box girders to one abutment. This paper is the first of two companion papers describing the circumstances of the failure. The static aspects of the problem are considered in this paper, and the dynamic aspects of the analyses are reported in the second paper. The purpose of the investigation was to determine if the Saguenay earthquake was responsible for the failure of the anchorage plate. A site visit revealed damage to the concrete abutments, which suggested a longitudinal motion and pounding of the deck against both abutments. Local buckling of horizontal bracing members underneath the deck support this hypothesis. The results of a finite element analysis showed that the anchorage plates were subjected to high stress concentrations under dead load only, and that the stress increments induced by the earthquake were sufficient to cause the failure. Repair of the bridge with bolted connections (to eliminate stress concentration in the anchorage plate) is described.