Ductility demand spectra for marine structures characterized by the self-centering system with the flag-shaped model under offshore ground motions

Previous seismic designs of marine structures usually adopted onshore ground motion. However, the response of marine structures under onshore and offshore ground motions are marked differences. For this reason, this study investigated the ductility demand spectra of self-centering (SC) single-degree-of-freedom (SDOF) systems with the flag-shaped hysteresis model under offshore ground motions to guide the seismic design of marine SC structures. Based on the K-NET observation network, 192 groups (384 records in total) onshore and offshore ground motions were selected and divided according to the different ground motion characteristics and period components. Subsequently, a marine SC structure was simplified into the SC SDOF system, and the effects of the structural and ground motion characteristics on the ductility demand spectra were analyzed. The differences between onshore and offshore ground motions, as well as long-period (LP) and non-long-period (NLP) ground motions, were investigated. Finally, according to the statistical data, a fitting formula was established to estimate the displacement ductility demand of the marine SC structures. The results revealed that the ductility demand of the SC structure is significantly lower. The acceleration and energy of the LP offshore ground motion are higher than that of LP onshore ground motion, which increases the ductility demand of the marine SC structure. In addition, marine SC structures are also more sensitive to the characteristics of marine ground motion, which needs to be emphasized in seismic design.