Seismic fragility analysis of a two bay two storey underground metro station: a case study

Underground metro stations are increasingly being constructed in India after the commissioning of metro projects. It is very useful to know the vulnerability of the proposed and existing metro stations to seismic hazards so that they can be designed or retrofitted with appropriate protection if necessary. Fragility of a structure is the probability that a particular limit state is exceeded for a given intensity measure. In the context of performance-based design, risk and consequence-based design, and resilience-based design, one of the key steps is finding the fragility curves for the structure. Several fragility curves have been generated for the above ground structures, but there is a limited study available for the underground structures. In the present study, a case study involving two bay two storey underground metro station in weak soils supported on diaphragm walls and barrettes located in a moderate risk seismic zone in India is considered for the development of fragility curves. Seismic analysis of the metro station is performed using ABAQUS by employing a Python-based software framework to automate the process of model building and analysis. The peak ground velocity (PGV) is used as intensity measure (IM), and inter-storey drift ratio is used as damage measure (DM). Subsequently, nonlinear static pushover analysis is performed to identify the various limit state thresholds for the internal columns. The incremental dynamic analysis (IDA) is performed using recorded earthquake ground motions which match IS 1893:2016-based response spectrum corresponding to Seismic Zone III at bedrock level. The fragility curves for different limit states of the internal columns of the metro station are obtained from the IDA results. Subsequently, the vulnerability of the internal columns is assessed considering the seismic environment at the location of the station. It is concluded that the upper storey and lower storey internal columns have 2.9 and 5.8% probabilities of exceedance for life safety (LS) limit state for the maximum considered earthquake peak ground acceleration (PGA) level of 0.18 g.