Plastic Energy Density-Based Time-Dependent Seismic Performance Evaluation for Underground Structures

Large underground structures such as subway stations are critical components of urban lifeline systems, but their seismic deformation and damage mechanisms remain not fully understood. This paper investigates the seismic response and damage mechanism of a typical two-story three-span underground frame structure in soft soil using three-dimensional nonlinear finite element analysis. The deformation transmission between the structure and surrounding soil is examined, the frequency response characteristics of the soil-structure system are revealed, and the energy dissipation and damage evolution are systematically analyzed. The results show that the predominant frequency of structural relative displacement is governed by both the free-field site and the structure itself. Soil deformation imposes forced displacement on the structure and suppresses its natural vibration. Seismic input energy is mainly dissipated by damping and plastic strain energy, with central columns and slabs as the primary plastic energy-dissipating components. The most severe damage occurs at the base of the lower columns. A plastic energy density-based damage index is proposed, which offers a supplementary perspective for time-dependent cumulative damage assessment of underground structures.