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Interstory drift ratio associated with performance objectives for shallow‐buried multistory and span subway stations in inhomogeneous soil profiles

The interstory drift ratios (IDRs) associated with the performance objectives for the underground structure are not well defined. In this paper, four levels of performance objectives are defined for shallow‐buried subway station structure: operational, immediate occupancy, life safety, and collapse prevention. In order to develop IDRs corresponding to these performance objectives, 18 subway station structures were selected for this study. Pushover analyses were conducted using three‐dimensional finite‐element models considering two conditions of seismic loading: vertical and horizontal ground motions; and horizontal ground motion only. Shear‐displacement capacity curves of the 18 subway station structures were obtained, and IDR limits were defined for each structure based on the relationship between the capacity curves and performance objective. Statistical analysis of the results demonstrated that the IDR limits considering the vertical ground motion (characterized by the same frequency with horizontal component) are smaller than that without consideration of the vertical ground motion. Based on these results, the IDR limit of 0.05%, 0.21%, 0.46%, and 0.72% is assigned for the four performance levels, respectively, for rectangular frame underground structures. The performance evaluation applied to Daikai station based on the proposed limits is found to be consistent with the actual postearthquake damage observations. The proposed limits of IDR could provide some guidance to the seismic design of underground structures, and could form the basis for developing appropriate performance limits for seismic provisions in design codes relevant to underground structures.