Seismic fragility of open ground storey RC frames with wall openings for vulnerability assessment

Reinforced concrete (RC) buildings with open ground storey (OGS) are characterized by accumulation of large lateral displacement at the ground storey. This is due to drastic reduction in relative stiffness of the ground storey compared to the upper stories, a phenomenon known as soft storey effect. Openings present in the masonry infill walls reduce both the lateral strength and stiffness of the fully infill RC frames. It is a general perception about OGS buildings that openings present in the infill walls reduce the stiffness of upper storeys, and thus, offsets the soft storey effect. In the present study, this issue is investigated by carrying out a simplified performance assessment of low- to mid-rise masonry infill RC frames with different infill configurations followed by fragility analyses. It is observed from fragility analyses that there is practically no influence of openings in masonry infill walls of OGS frames on lateral load behavior of such frames. OGS frames with any bay and storey configuration, even with large openings in infill walls, remain highly vulnerable to earthquakes. A new representation of fragility, Fragility Flow Plot, is proposed, independent of discrete damage states, where results of the fragility analyses can be shown for different parameters, such as, natural period of vibration, number of bays and storeys, and openings. The present study contributes towards quantification of seismic fragility and vulnerability of OGS building frames and establishes an initiative for seismic fragility based design of OGS frames using the Fragility Flow Plots.