Seismic Response of Buildings with Soft Story

Abstract

Extending lifts` reinforced concrete cores to foundations usually restricts services and lobby functions of a building. A core supported on columns at core`s corners is considered a good alternative for sorting out this problem. However, this will cause an abrupt reduction in the lateral stiffness of the building below the cores level i.e. soft story vertical irregularity. Hence, large lateral displacement of the building is expected causing its instability and potentially results in its collapse. No caution at seismic response parameters (for instance, lateral story drift ratio) is found in the current codes for buildings having soft stories. The ratio between the lateral stiffness of a given story and that of a story / three stories above is the only parameter found in current codes, for example UBC and ASCE7 codes, to identify a soft story criterion. In this paper, the seismic response of a building due to change in the level at which the central core is planted and the distribution of the columns supporting it is investigated. Partial support of reinforced concrete cores via underneath columns is found to be an effective means of enhancing the lateral stiffness of the building and reducing its lateral side sway. This effect is extremely dependent on the level of the soft story i.e. the level at which the cores are planted. The phenomena of the UBC and ASCE7 codes for identifying the soft story in a building needs to be revised. Keywords: Soft Story; Seismic Loads; Vertical Irregularity; Drift Ratio; Concrete; Cores;