Abstract: High-rise buildings with open ground storey have stiffness irregularities as visualized during earthquakes. This paper has carried out an extensive numerical investigation to find out the column capacity of soft storey as well as their seismic vulnerability. Few finite element (FE) models of multistoreyed buildings have been developed and analyzed under equivalent static earthquake loading. Infills on upper storeys have been modeled as equivalent diagonal strut while ground storey is free of infill. Linear and nonlinear analyses are carried on FE models to investigate and compare the column capacity of open ground storey. Pushover analysis shows that the total moment and shear of open ground storey columns are significantly higher than their capabilities. This indicates that the soft ground storey columns are over stressed due to moment and shear magnification in presence of infills on upper storeys. Conventional equivalent static force method (ESFM) is incapable of predicting these behaviors resulting in significant under-design of the columns of open ground storey which led to the collapse of many such buildings in the past earthquakes. Thus, unexpected soft storey failure occurs before reaching to its desired drift. Recently few codes have suggested to magnify moment and shear of columns of soft ground storey by 2.5 times the moment and shear of same column as obtained by conventional ESFM. Finding of this study shall lead us to better understanding of the behavior of high-rise buildings with open ground storey and safer design of such buildings.

Keywords: RC framed building, infill, soft storey, pushover analysis

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