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CCEER-20-04: Evaluation Of The Domain Reduction Method Applied To Broad-band, Near-fault Earthquake Ground Motions With Inter-code Comparisons
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This report summarizes the results on the evaluation of the Domain Reduction Method (DRM) applied to broad-band near-fault earthquake ground motions. Two different nonlinear finite element codes (ESSI and OpenSees) were utilized for inter-code comparisons. The DRM model is a modular two-step algorithmic approach that allows complex three-dimensional wavefields generated with a geophysics model to be appropriately introduced to drive the response of local coupled soil-structure systems. As the first step, a large-domain geophysics model, which simulated earthquake source and wave propagation, was first developed and analyzed in SW4, a fourth-order finite difference code, to simulate various earthquake events ranging from simple source small magnitude earthquakes to more realistic vertical strike-slip earthquakes for magnitude 6.0 and 7.0 events. The mesh size of this model was refined so that the frequency resolution of computed ground motions could be modeled up to 5 Hz. As the second step, a reduced-size model (i.e. DRM model) was developed for the ESSI and OpenSees finite element programs to represent a localized region in the large regional-scale domain. Material properties and energy dissipation in the DRM model were identical to those in the geophysics model. Computed ground motions from the first step model were extracted and interpolated to drive the DRM model in the second step. Comparison of computed surface motions was made between the DRM and SW4 model at the same spatial location on the ground surface and exceptionally good agreement is obtained for these comparisons. This suggests that the DRM has been appropriately implemented and the DRM boundary works well in the presence of large ground displacements and permanent ground offsets in the near-field. In addition, the evaluation of the DRM was extended to simplified two-dimensional (2D) input motions using a narrow three-dimensional model. Good agreement was also obtained between the DRM and SW4 models under the 2D input motions. Furthermore, four representative planar steel moment frame buildings (3-, 9-, 20-, and 40-stories) supported on mat foundations were introduced to the soil islands of the DRM model. The DRM models of the buildings were developed in both ESSI and OpenSees with identical idealizations and assumptions. Nonlinear response history analyses were then performed for the DRM models including buildings on the soil islands to investigate the influence of soil-structure interaction (SSI) and complex seismic waves. For comparative purposes, a reference analysis was conducted for the same building models under fixed-base conditions and pure translational motion excitations. Inter-code comparison was also made between the ESSI and OpenSees models for the DRM and fixed-base models in order to allow a cross check on the model results from each code, provide confidence building in terms of the correctness of the finite element input files, and the appropriateness of the implementation of the DRM in both ESSI and OpenSees. Finally, sensitivity analyses regarding the influence of the width of the soil island on the response of the buildings was also investigated. Satisfactory agreement was obtained for the comparisons of the results from nonlinear pushover static and dynamic earthquake time history analyses for the same model in ESSI and OpenSees, which suggests that the DRM models of the four buildings and soil island as well as the corresponding fixed-base models have been correctly developed in ESSI and OpenSees. The comparisons between the DRM and fixed-base models show that although the SSI and complex incident waves might have a slight influence on the maximum peak inter-story drifts (IDs) of the four steel moment frame buildings, they could cause significant overestimation or underestimation of the peak IDs of certain stories of the building models. The observed difference depends on the relative locations of the analyzed domains to the fault, i.e. characteristics of the motions in these domains. These results are far from complete since the analysis is only performed at three different locations of the large-scale geophysics model. Further analysis is required to confirm this observation at more locations and different earthquake magnitudes and sources. These results contribute to the advancement of efforts to improve seismic risk assessment of structures with consideration of the complex wave incident directions and soil-structure interaction. The verification of the DRM in ESSI and OpenSees provides engineers with a reliable and powerful tool to pursue risk-informed, performance-based designs for structures. With the completion of the comprehensive evaluation of the DRM-based code coupling, further study and evaluations will incorporate the effects of the nonlinearity of soil, dimensions of the soil islands and larger earthquake events to provide increased insight and improved design guidelines especially for structures located in the near-fault region.
Report No. CCEER 20-04