Earthquakes and Structures

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Isolator displacements of a bridge with non-uniform local sites accounting for spectrum compatible spatially varying ground motions

  • Ali I. Karakas (Department of Civil Engineering, Karadeniz Technical University)
  • Received : 2023.10.04
  • Accepted : 2024.11.09
  • Published : 2025.01.25
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Abstract

This study aims to provide valuable insights into the significant effects of varying local soil conditions, particularly non-uniform distributions, on isolator displacements in bridge structures. To achieve this, earthquake-induced maximum displacements in lead-rubber isolators of a box-girder highway bridge are systematically examined through a comprehensive parametric analysis. The research employs nonlinear time history analyses, utilizing an optimal set of artificial spatially varying earthquake ground motions (SVEGMs) that match the response spectra of different local soil classes. Additionally, wave-passage and incoherency effects, as well as local soil conditions, are incorporated in the generation of SVEGMs to ensure robust modeling. A custom-developed code, seamlessly integrated with a bridge structural analysis program via an application programming interface (API), automates the parametric studies. The numerical results reveal that, while wave-passage and incoherency effects are negligible for relatively short bridges, local soil effects significantly influence isolator displacements. Maximum isolator displacements increase as much as 358% when weak soils are located at critical points, such as piers adjacent to abutments, with the most pronounced impacts occurring at the abutments. In contrast, assuming a uniform distribution of the weakest soil type for analysis can conservatively reduce displacement variations by up to 17%, though this approach often leads to over-conservative and economically inefficient isolator designs. Moreover, increasing the height of pier columns on weak soils decreases isolator displacements at that support by approximately 33%, while simultaneously increasing displacements at neighboring supports by around 22%. These results underscore the importance of considering non-uniform soil distributions to achieve both safety and cost efficiency in seismic isolation design.

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