Seismic evaluation and reliability index of soft story buildings on flexible soils by using old and current seismic regulations in Mexico

A very important number of buildings with a flexible ground floor in Mexico City presented severe damages and collapses during the intraplate earthquake (Mw = 7.1) of September 19, 2017. The statistics of the damaged buildings showed that most of the collapsed structures were designed with a regulation in force in the late 70 ' s. As a response to the damages observed during this seismic event, the current version of the seismic regulation for Mexico City added a specific chapter for the analysis and design of structures with a flexible ground floor. This study assesses the seismic response of the existing buildings subjected to accelerograms recorded during the occurrence of this earthquake and the expected behavior of new structures designed with the current seismic regulations. To evaluate the methodology proposed in the code to design weak-story buildings, reliability indices of buildings designed with the regulation in force at the end of the 70 ' s and those obtained in buildings designed with the current regulation were determined. Soil-structure interaction played a very important role in the geographical distribution of the buildings with the greatest damage and also in areas that presented only slight damage. The soil-structure interaction was modeled as suggested in the current Mexican standards. The results show that the range of computed reliability indices in the structures built in the 70 ' s explain the collapses observed, and not in all cases, the current regulation leads to satisfactory values of the reliability index, particularly when including the soil-structure interaction effects.