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Inelastic response of SDOF systems subjected to subduction and crustal ground motions Mattman, Dominic Willy


Structural engineers are increasing turning to nonlinear static procedures to gain better insight into the performance of the structures they design. Key to these procedures is estimating the displacement demand of the design level earthquake. The Coefficient Method, from FEMA 356 (ASCE, 2000) and recently updated in FEMA 440 (ATC, 2005), estimates this target displacement. This method was developed through statistical studies of SDOF systems utilizing crustal ground motions. Therefore, to expand the applicability of the Coefficient Method to areas subjected to subduction earthquakes such as the Pacific Northwest of North America, this study examines the behaviour of SDOF systems with 6 different hysteretic models, 50 periods of vibration, 9 relative strength values and 88 ground motions (40 from crustal sources and 48 from subduction sources). Three of the hysteretic models included in the study exhibit strength degradation with one model incorporating cyclic strength degradation while the other two included in-cycle strength degradation. Furthermore, the frequency of instability of the in-cycle strength degrading systems was examined. The greatest differences between the subduction and crustal ground motions were observed in: the inelastic displacement ratios of the EPP systems (up to 3 times higher and convergence to the "equal displacement rule" at higher periods), the inelastic displacement of the cyclic strength degrading systems when compared to EPP systems and the maximum permissible relative strength value for the in-cycle strength degrading model. With the subduction records, a moderate increase in the frequency of instability of the in-cycle strength degrading systems was also noted. Negligible differences were seen in the inelastic displacements of the cyclic and in-cycle strength degrading systems provided the later remained stable. A simple SDOF model was used to compare the response of a system retrofitted only for strength with the response of the original weak, but ductile, system. The results showed an increased displacement in the retrofitted system unless there was a significant increase in strength, the negative slope of the response was not too steep and the system had a short period of vibration.

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