Accelerograms obtained during the 1979 Coyote Lake, California earthquake are used to examine the response of a multiple-span, steel girder bridge to strong earthquake loading. The structure studied, the San Juan Bautista 156/101 Separation Bridge, is typical of many highway bridges in seismic regions of the United States. Although the bridge was not damaged, the strong-motion records are of significant engineering interest as they are the first to be recorded on such a structure.
An engineering seismology study suggests that long-period ground displacements at the bridge site were caused by Rayleigh waves. A three-second period, pseudostatic response of the superstructure is attributed to small amounts of differential support motion induced by the surface waves.
A time-domain technique of system identification is used to determine linear models which can closely replicate the observed bridge response. Using time-invariant models, two structural modes at 3.50 and 6.33 Hz, are identified in the horizontal direction. Each mode, having approximately ten-percent damping, involves coupled longitudinal and transverse motions of the superstructure. Time-variations of frequency and damping in the horizontal response are also identified using a moving-window analysis.
A three-dimensional finite element model which includes soil-structure interaction predicts several important features of the dynamic response of the bridge. The first two computed horizontal frequencies are found to be in excellent agreement with the observed responses provided the model's expansion joints are locked, preventing relative translational motions from occurring across the joints. Locking is confirmed by the observed deformations of the structure in the fundamental mode. Fundamental vertical frequencies of the individual spans, predicted by the finite element model, are in very good agreement with ambient vibration test data. Results of the strong-motion data analysis and the finite element modeling are used to recommend a plan for expansion of the strong-motion instrumentation array on the bridge.
