Influence of ground motion duration on inelastic seismic response of subway stations in soft soils

Owing to the repeatedly observed long-duration ground motions (GMs), the duration effect is becoming increasingly critical for the seismic design and assessment of important infrastructures. In this study, dynamic analyses are performed to evaluate the influence of GM duration on the inelastic seismic response of subway stations. Based on the Daikai subway station in Kobe, Japan, a two-dimensional numerical model incorporating a concrete damage plasticity model and a soil nonlinear model is developed. Twenty-five spectrally matched records with different significant durations (D5-95) are selected for dynamic analyses. The responses are evaluated based on various engineering demand parameters (EDPs) including the internal force, drift ratio, element damage index, and the total damage of the structural member. The results show that for weak earthquakes (PGA = 0.15 g) the duration has a negligible influence, while for strong earthquakes (PGA = 0.45 g) the duration effect is particularly pronounced due to the significant cyclic degradation of stiffness and strength of the material. Compared to the location-dependent maximum internal forces and the element damage index, the maximum drift ratio and the total compressive damage index are suitable EDPs, since they correlate well with D5-95. On average, for every 10-s increase in D5-95 of strong GMs used in this study (PGA = 0.45 g), the maximum drift ratio of the side wall and center column increases by 0.12 % and 0.2 % while the total compressive damage index of the side wall and center column increases by 0.01 and 0.04, respectively. The duration effect is more pronounced for the center column implying that damage to the weak structural member is exacerbated by long-duration GMs. The increased column stiffness or elastic soil behavior may reduce the duration effect. This study highlights that strong GMs having similar amplitude and response spectrum with different significant durations can significantly affect the inelastic seismic response of subway stations.