Evaluation of different cases of soft story formation for mid-rise RC buildings

Soft story irregularity is one of the main reasons of the building damage during past earthquakes and has been mentioned in almost all reconnaissance reports. Soft story due to increased story height is a recognized subject but soft story may also arise due to abrupt changes in amount of infill walls between stories, which are usually not considered as a part of load bearing system. This study investigates soft story behavior due to increased story height, lack of load bearing infill walls at ground story and existence of both cases using nonlinear static and dynamic response history analyses. Mid-rise reinforced concrete buildings are considered due to their high portion in existing building stock. Displacement capacities at Immediate Occupancy, Life Safety and Collapse Prevention performance levels and story drift demands of the regular and soft story models are determined. Soft story behavior due to change in story height and/or infill amount is evaluated in view of displacement capacities, drift demands and structural behavior. It is observed that, soft story due to infill walls may be as damaging as soft story due to increased story height.