Bridge Dynamic Tests: Implications for Seismic Design

A five span, 400 ft (120m) long, reinforced concrete box girder bridge supported on single column piers, which are pile founded, was subjected to extensive dynamic tests. The pullback and quick release method of excitation was used. Two D-8 crawler tractors were used to apply total release loads which had a magnitude of about one-quarter of the earthquake design loads. System identification techniques were used to extract the experimental in situ dynamic stiffness of the pier pile group foundations from the suite of field data. An independent geotechnical analysis of the bridge's pile foundations yielded pile group stiffness which were in acceptable to good agreement with the experimentally determined values. Moreover, this analysis indicated that for design level seismic loadings the pile group stiffnesses are quite deflection dependent. It was further shown that it is essential to include the soil-structure interaction effect in the analytical model if correct distributions of seismic loads are to be obtained.