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Evaluation of soil-structure interaction effects in the dynamic response of instrumented bridges based on seismic records Fraino, Miguel


The main objective of this study is to gain a better understanding of soil-structure interaction (SSI) and how it affected the response of the bridges under earthquake excitation. Detailed studies of signals recorded at key locations of a bridge –i.e. the bridge deck, pier base, and free field– were conducted. A total of 6 instrumented bridges subjected to 12 earthquakes were selected for the analysis, focusing on the behavior in the transverse direction for these particular cases. The first step of the evaluation process was the investigation of the modal properties by means of a system identification process. Then response spectra were calculated for all the records, and the effects of SSI were determined by comparing the acceleration spectrum of the free field motions with the spectrum of the bridge motions recorded at the foundation slabs or on the pile caps. The frequency contents of the signals were compared based on the Fourier amplitude amplifications of the records. The analysis of the comparisons of column base vs. free-field and column base vs. deck allowed to highlight behaviors related to SSI effects. The possible variability of the fundamental transverse period as a function of the amplitude of shaking with time was investigated using wavelet analysis techniques. The results from both response spectra and Fourier spectra analyses showed clearly that the free field motions are not always de-amplified at the foundation due to the soil-structure interaction effect, as it has been generally accepted. It was also demonstrated that for the same site and bridge, amplification or de-amplification varies from one earthquake to another. For almost all cases in this study the time-frequency analysis results showed that the peak response at the deck corresponds to the natural transverse period. This observation does not hold true only for one case, where the dynamic behavior was highly affected by the approaching embankments.

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