The structural configuration with a soft first story proved to be very vulnerable and performed poorly during the past earthquakes. Like other many countries, brick infill masonry is used in Bangladesh as a nonstructural element. Its usages in upper stories and keeping building’s ground floor open result in lateral stiffness difference and cause soft first story state. The scarcity of land in Bangladesh has compelled to construct multi storied RC buildings with an open ground to be used as vehicle parking, stores or other facilities. This research committed to assess the seismic vulnerabilities of RC buildings with a soft first story, causes behind the collapse of soft first story during earthquakes, seismic performance difference with bare frames and sustainable approach to retrofit them. Seismic performance and vulnerabilities of the soft first story were assessed by the JBDPA guidelines of seismic evaluation, FEMA-356, BNBC-2015 and nonlinear static pushover analysis. Flexural moment magnification at the soft first story columns during earthquakes was determined. Sustainable retrofitting approaches to upgrade seismic performance and prevent catastrophe during earthquakes were proposed with cost analysis. This research found that, seismic behavior, ductility demand, inter story drift pattern and damage distribution of RC buildings with a soft first story were totally different than the RC buildings designed by only bare frame analysis. The soft first story suffered huge ductility demand, extreme inter story drift change and concentrated in severe damage. Magnification of flexural moment at soft story columns was detected as a variable entity. Retrofitting of soft first story was found different from conventional RC buildings. A combination of RC column jacketing and adding steel bracing proved to be effective to eliminate stiffness difference and control the excessive inelastic lateral drift.
Keywords: Soft story, Seismic performance, Nonlinear analysis, Retrofit.