Behavior of Columns of RC Framed Buildings with Soft Ground Floor Subjected to Earthquake Load

Residential multistoried apartments with open ground floor for car parking are a common scenario in developing nations. Despite being identified as soft story buildings with high vulnerability against collapse due to earthquake load, construction of such buildings are being continued and seems to be unavoidable due to high socio economic demand. It has thus become essential to develop some guideline so that safer design of such buildings can be ensured. In the present paper, an investigation has been performed to study the behavior of multistoried buildings with soft ground floor subjected to dynamic earthquake loading. The structural action of masonry infill panels of upper floors has been taken into account by modeling them as diagonal struts. Finite element models of six, nine and twelve storied buildings are subjected to earthquake load in accordance with equivalent static force method as well as response spectrum method. It has been found that when infill is incorporated in the FE model, modal analysis shows different mode shapes indicating that dynamic behavior of buildings changes when infill is incorporated in the model. This indicates that for better dynamic analysis of RC frame buildings with masonry walls, infill should be present in the model. When the buildings are subjected to response spectrum loading, significant increase in column shear and moment as well as total base shear has been observed in presence of infill. In general, an increase of about sixty percent to one hundred and fifteen percent in base shear has been observed when infill is present on upper floors with ground floor open when compared to the base shear given by equivalent static force method. The study suggests that the design of the columns of the open ground floor would be safer if these are designed for shear and moment twice the magnitude obtained from conventional equivalent static force method.