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Damage, collapse potential and long duration effects of subduction ground motions on structural systems Capraro, Ilaria


Current code designs are based on design spectrum which does not account adequately for long duration shaking and therefore they do not reflect the duration effects of subduction earthquakes on seismic demands. The 2015 edition of the National Building Code of Canada (NBCC) probabilistically includes the effects of subduction earthquakes in developing the Uniform Hazard Spectrum (UHS), thus leading to increased interest in the impact of subduction ground motions on seismic demands for design. The research aim of this thesis is to investigate in detail the impact of subduction motions on design, particularly the effect of duration and the evaluation of the damage potential of the subduction motions. The evidence of structural damage observed during reconnaissance after the 2011 Tohoku and 2010 Maule subduction earthquakes was used to investigate the characteristics of ground motions that could be used as consistent indicators of damage potential. Characteristics considered were basic shaking parameters, as well as spectral parameters. Among the parameters considered, the constant strength spectrum appeared to be the best indicator. The effects of the long duration shaking due to subduction motions on the dynamic performance of structures was investigated using Incremental Nonlinear Dynamic Analysis (IDA). The response of several nonlinear Single-Degree-of-Freedom (SDOF) systems designed using a force-reduction factor of 5.0, covering a wide range of fundamental periods and ductility capacities was studied. The effect of duration was isolated by compiling two suites of spectrally compatible motions representing crustal and subduction earthquakes. Based on the drift ratios from IDAs, fragility curves were developed giving the probability of exceeding a prescribed drift ratio. The study clearly showed that the conditional probability of exceeding a prescribed drift for subduction earthquakes is higher than the probability associated with crustal motions. However, the magnitude of this effect depends on the fundamental period, the ductility of the building and the shaking intensity. In order to investigate how the above findings apply to a Multi-Degree-of-Freedom (MDOF) building, the same pair of input motions were used to run IDA on a 6-storey reinforced concrete (RC) moment frame. The resulting fragility curves confirmed the findings obtained with the SDOF analyses.

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