Effect of weak and soft storeys on seismic performance of reinforced concrete frames with unreinforced brick infills

Reinforced concrete buildings with unreinforced brick masonry infills are common in current building stock in all parts of India, irrespective of seismic zonation. The scarcity of land for parking in multi storeyed buildings together with importance to architectural features is resulting in provision of open ground storey. The presence of open ground storey in a multi storeyed building can lead to either soft storey effect or weak storey effect or even both and this increases the risk of a building in higher seismic zone areas. For safety of open ground storey structures in higher seismic zones, IS 1893:2002 [1] suggests increase in design forces for the columns in ground floor. To understand the increase in seismic forces on ground floor columns, a regular G+3 story structure, which is assumed to have open ground storey, is considered. A comparative study between three types of arrangements; Type I: Structure with infill walls in all floors, Type II: Structure with open ground storey, Type III: Structure with open ground storey and columns designed for increased forces. It is observed that the structure with open ground storey (Type II) has poor performance compared to the structure without open ground storey (Type I). And the structure designed for increased forces (Type III) has no better performance than the open ground story (Type II). Apart from this study, the same structure is assumed to have weak storey at different locations and a comparative study is carried out between the performances of each. From this study, it is observed that the presence of weak storey at lower floors of the structure causes poor performance than its presence in upper floors.