Damage-based strength reduction factor for seismic design of structures subjected to offshore ground motions

This paper focuses on damage-based strength reduction factor (SRF) of single-degree-of freedom (SDOF) systems subjected to an ensemble of 892 offshore ground-motion records from the Kyoshin network in the Japan Sagami Bay Region. Damage-based SRF spectra are statistically developed considering both the offshore ground-motion characteristics (such as seafloor stations, magnitude, epicentral distance, significant duration and mean period) and structural parameters (including initial period, damage level, postyield stiffness ratio, ultimate ductility factor and hysteretic behavior). The differences in damage-based SRF spectra under offshore and onshore ground-motion records are also investigated. The results showed that the effects caused by offshore ground motions on the estimation of damage-based SRF are negligible, while the influence caused by the ultimate ductility factor can reach up to approximately 50 %. Analytical estimates of damage-based SRFs for mean level, 10th percentile values and 90th percentile values in terms of period, damage index, and ultimate ductility factor are proposed for the aseismic design of offshore structures.