Damage to CableStayed Bridge during 1988 Saguenay Earthquake. II: Dynamic Analysis

The November 25, 1988, Saguenay earthquake (ML=6.0) is the largest seismic event recorded in eastern Canada. The cablestayed Shipshaw bridge, which crosses the Saguenay river near Jonquire, Qubec, suffered significant structural damage during the earthquake. One of four anchorage plates connecting the steel box girders to one abutment failed, apparently due to the ground shaking. This paper is the second of two companion papers describing the circumstances of the failure. This paper deals with the dynamic analyses and testing of the bridge that were performed to confirm the cause of failure. Freevibration analyses were undertaken to determine the natural frequencies and mode shapes of the bridge. These results were verified by field vibration measurements. A nonlinear dynamic model of the contact friction at the tierod assembly was derived. The model was incorporated in nonlinear timestep dynamic analyses using horizontal and vertical accelerograms recorded near the site. The results of these analyses confirm the hypothesis that the Saguenay earthquake was the direct cause of failure.