Seismic Action
The seismic design approach of underground structures is generally characterized by two different type of earthquake events: the maximum design earthquake (MDE) and the operating design earthquake (ODE) typically defined as:
- MDE: is the earthquake event that has a return period of several thousand years. It has a small probability of exceedance, approximately 5 % or less, during the 100 years facility life. It is aimed at public life safety.
- ODE: is the event for which recurrence interval is several hundred years; the probability of exceedance of this event is approximately 40 % during the facility life. It is aimed to guarantee full functioning of the structure. The response of underground structures should, therefore, remain within the elastic range.
In this case, two different type of artificial seismic accelerograms are taking into account for the evaluation of the seismic behavior of Mercato Santa Anita Metro Station, according to the study of seismic risk of the project area described in the previous paragraph and to the values shown in Table D: one seismic event characterized by a return period equal to 1000 years in order to represent the ODE (Figure 9) and another seismic event characterized by a return period equal to 2450 years in order to represent the MDE (Figure 10), according to GB50909-2014. The recording has been corrected with a low-pass filter at the cut-off frequency of 15 Hz using [2].