Dynamic response of bridges to near-fault, forward directivity ground motions

Eliot Bonvalot

Research over the last decade has shown that pulse-type earthquake ground motions that result from forward-directivity effects can result in significant damage to structures. The objective of this research is to use recent ground motion data to improve the understanding of the response of typical reinforced concrete and precast concrete bridges to pulse-type ground motions that result from forward directivity effects. Nonlinear, dynamic finite element analysis was applied to three bridges, and they generally survived forward directivity ground motions without significant damage to the columns. However, column flexural failure was predicted for one of them when subjected to two of the forward directivity ground motions. The bridge models often indicated distress at the abutments, including pounding, and exceedance of abutment strength limits. The response of bridges to forward directivity ground motions was found to be highly dependent upon the coincidence of the bridge fundamental period and the ground motion velocity pulse period. The severity of the demand is controlled by the ratio of the pulse period to bridge fundamental period. Analysis results showed that most of the damage in the bridge columns during forward directivity ground motions occurred at the beginning of the record in response to the velocity pulse. Therefore, a ground motion consisting of a sinusoidal single pulse may be sufficient to evaluate bridge performance for forward directivity ground motions. A study of the effect of foundation flexibility showed that not including SoilStructure-Interaction might lead to over-conservatism, especially for the FDGMs. Nonlinear SDOF analyses were performed, but they are not recommended in the case of forward directivity ground motions since the results were not consistent. However, the use of the acceleration response spectra to compute the expected response of the bridges was found to be quite successful for both non-forward directivity and forward directivity ground motions. A response modification factor must be used to include the inelasticity effect on the maximum base shear in the columns. Due to the variation in the acceleration response spectra with period caused by forward directivity ground motions, to amplify the spectra for design does not provide a reliable basis for representing near-fault, forward directivity ground motions. Depending on the importance of the bridge being designed or assessed, the appropriate approach taken with forward directivity ground motions should be carefully considered by the designer.

Dynamic response of bridges to near-fault, forward directivity ground motions

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