Seismic Analysis of Open Ground Story Framed Building

Abstract

The concept of open ground building (OGS) has taken its place in the Indian urban environment due to the fact that it provides the parking facility in the ground storey of the building. The cost of construction of this type of building is much less than that of a building with basement parking. Surveys of buildings failed in the past earthquakes show that this types of buildings are found to be one of the most vulnerable. The majority of buildings that failed during the Bhuj earthquake (2001) and Gujraat earthquake were of the OGS type. The collapse mechanism of such type of building is predominantly due to the formation of soft-storey in ground storey of the building. The sudden reduction in lateral stiffness and mass in the ground storey results in higher stresses in the columns of ground storey under seismic loading. In conventional design practice, the contribution of stiffness of infill walls present in upper storeys of OGS framed buildings are ignored in the structural modelling. Design based on such analysis, results in under-estimation of the bending moments and shear forces in the columns of ground storey. After Bhuj earthquake, IS 1893 code was revised in 2002, incorporating new design recommendations to address OGS framed buildings. According to this clause 7.10.3(a): “The columns and beams of the soft-storey are to be designed for the multiplication factor of 2.5 times the storey shears and moments calculated under seismic loads of bare frame”. The prescribed multiplication factor (MF) of 2.5, applicable for all OGS framed buildings, is proved to be fairly higher and suggests that all existing OGS buildings are highly vulnerable under seismic loading. The main objective of present study is the study of comparative performance of OGS buildings designed with various MFs using nonlinear analysis. As more realistic performance of the OGS building requires the modeling of stiffness and strength of the infill walls, hence they are also considered.