Evaluation of the duration effect on the seismic fragility of subway stations considering different damage state thresholds

Although the influence of duration of ground motion on the seismic response of aboveground structures is clearly recognized, its influence on underground structures remains unclear. To this end, this study performs incremental dynamic analyses under both short and long duration ground motions, to quantify the significance of the duration effect on the seismic fragility of subway stations. A two-dimensional soil-structure system is established on the basis of the Daikai subway station, consisting of an elastoplastic soil model and a concrete damage plasticity model. A set of thirty spectrally matched ground motions with varying significant durations (D_5-95) are employed. In particular, using the center column total compressive damage index (D_TCD) and peak inter-story drift ratio (IDR) as structural demand measures (DM), the percentage difference in fragility curves between long and short duration is evaluated by accounting for six suits of damage state thresholds. Correlations between the two DMs and D_5-95 show that ground motion duration affects significantly the seismic fragility of subway stations. Overall, the duration effect is not detected in the minor damage state and becomes more pronounced in the collapse state, suggesting that the duration effect increases as the damage state threshold increases. The median collapse capacity for long duration ground motions is up to approximately 60% or 37% lower than that for short duration ground motions, when a peak IDR or D_TCD are adopted, respectively. The results of this study highlight the great importance of properly considering duration when selecting earthquake records for seismic fragility assessment of subway stations.