Constant Ductility Inelastic Displacement Ratios Under Offshore Ground Motions: Effects of Hysteretic Behavior and Motion Components

This study investigates the influence of offshore ground motions on the inelastic displacement response of structures, aiming to enhance seismic design and hazard mitigation strategies for offshore environments. Within the displacement-based design framework, the inelastic displacement ratio spectrum (IDRS) serves as an effective tool for estimating maximum inelastic displacements from maximum elastic demands of linear single-degree-of-freedom (SDOF) systems. Recognizing the distinctive characteristics of offshore ground motions relative to their onshore counterparts, the study systematically evaluates the offshore IDRS using multiple hysteretic models. Notable differences between offshore and onshore IDRS patterns are identified, particularly in terms of horizontal and vertical components of offshore ground motions. Parametric analyses are conducted to examine the effects of moment magnitude, epicentral distance, site condition, seismic source type, and hysteretic behavior on the offshore IDRS. The results reveal pronounced discrepancies, especially in the long-period range, highlighting the limitations of directly applying onshore-based IDRS models to offshore seismic scenarios. To address this issue, a simplified empirical model is proposed to predict ductility-dependent inelastic displacement ratios under offshore ground motions, demonstrating improved predictive accuracy and practical applicability for offshore seismic design.