Reliability Based Seismic Design of Open Ground Storey Framed Buildings

Open Ground Storey (OGS) framed buildings in which the ground storey is kept open without providing any infill walls and mainly used for parking, are increasingly common in urban areas. Vulnerability of this type of buildings has been exposed in the past earthquakes. OGS buildings are conventionally designed considering a bare frame analysis, ignoring the stiffness of the infill walls present in the upper storeys, which under-estimates the inter-storey drift and the force demand in the ground storey columns. To compensate this, a multiplication factor (MF) is introduced by various international codes while calculating the design forces (bending moments and shear forces) in the ground storey columns. Present study focuses on the evaluation of seismic performances of OGS buildings designed with alternative MFs through performance-based design approach using a probabilistic framework. The probabilistic seismic demand models and corresponding fragility curves for all the selected OGS buildings are developed for different performance levels. Reliability curves are developed for the OGS building frames against the seismic hazard associated with maximum seismic zone of India (Zone-V of IS 1893, 2002). Similar analyses are also carried out on bare frames and fully infilled frames for reference. It is found from the present study that the application of MF only in ground storey, as suggested by many literatures and design codes (including Indian standards), is not an appropriate solution for design of OGS buildings as it leads to vulnerable adjacent storey. This study proposes an effective scheme of MF for design of OGS buildings that yields acceptable levels of reliability index.