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Investigation of Near-Fault Ground Motion Effects on Substandard Bridge Columns and Bents
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Until recently, seismic design codes were generally based on research addressing far-field earthquake characteristics. As more near-fault ground motions have been recorded, it has been realized that special considerations may be needed for structures located in near-fault regions. The objective of this study was to investigate the effects of near-fault ground motions on substandard bridge columns and bents. In addition, the effect of combined horizontal and vertical near-fault ground motion was examined. To accomplish these goals, several large scale reinforced concrete models were constructed and tested on a shake table using near and far-field ground motion records. The first set of testing consisted of two identical substandard bridge bents, both of which had been constructed as part of a previous study. One of the bents had been tested under the Sylmar far-field record and the other was tested in the present study under the Rinaldi near-fault record. The second set of testing consisted of two substandard circular bridge column models, 6FN and 6FNV. The first model was subjected to the near-fault Rinaldi earthquake. A model similar to this column was tested in a previous study subjected to the El Centro far-field record. The effect of vertical near-fault ground motion was studied on 6FNV by including the vertical component of the Rinaldi ground motion in addition to the horizontal component. Because the input earthquakes for the test models had different characteristics, three different measures were used to evaluate the effect of input earthquake. These measures were peak shake table acceleration, spectral acceleration at the fundamental period of the test specimens, and the specimen drift ratios. For each measure, force-displacement relationships, strains, curvatures, drift ratios, and visual damage were evaluated. In addition, analytical studies were conducted for each of the test models. Through the experimental and analytical studies presented in this report, it was seen that regardless of the measure of input or response, the near-fault record generally led to larger strains, curvatures, and drift ratios than those of the far-field motion. It was also found that the residual displacements were small compared to those for columns meeting current seismic code requirements. From the vertical ground motion study it was seen that the combination of horizontal and vertical ground motions did not increase the column deformations. This was due to the fact that the peak lateral displacement did not occur at the same instance as the peak vertical force and the failure mode was dominated by lap splice bond slip.
Report No. CCEER-09-01