Effects of Axial Compression Ratio of Central Columns on Seismic Performance of a Multi-Story Underground Structure

High axial force in central columns induced by strong vertical earthquake component is one of the most important factors in failure of subway stations. In this paper, the pushover method is adopted to simulate nonlinear behaviors of a multi-story subway station in Shanghai with different axial compression ratios through the general purpose finite element code ABAQUS. To simulate concrete material degradation during an earthquake accurately, the concrete damaged plasticity model was selected to trace damage characteristics of central columns. The softening index was used to describe the damage of the overall structure. Numerical results showed that in the structure level, the increase of axial compression ratio leads to the remarkable decrease of the structure ductility. The value of the softening index become less, which implies that the range and magnitude of damage to the structure become less either. However it should be noted that damages may be concentrated on certain members of the structure, such as central columns which are commonly considered to be the weakest but most critical part of an underground structure. In the member level, numerical results also showed that the increase of axial compression ratio led to small eccentric compression failure of central columns and accelerated the development of their compression damage.